A full-scale wood compression failure mode in-situ non-destructive micro-characterization method

Abstract

This invention provides an in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood, relating to the field of wood testing technology. The invention pre-sets multi-level compression ratio target values, performs step-by-step axial compression tests on full-scale wood samples, and after each compression level, refines the observation section through progressive grinding. A full-domain scan is then performed to construct an initial failure feature distribution map. Local high-magnification imaging is then performed on crack propagation paths, fiber fracture zones, and interlaminar separation regions at high magnification. The relative displacement and fracture surface inclination of the crack sides in each failure region are extracted, and the local failure degree value and its failure increment value between adjacent compression ratios are calculated to determine the trend of failure mode changes. This invention achieves continuous, non-destructive, and microscopic-scale tracking of the same full-scale sample during multi-level compression processes, enabling precise quantification of the failure evolution process.

Description

Technical Field

[0001] This invention relates to the field of wood testing technology, specifically to an in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood. Background Technology

[0002] Full-scale timber is widely used in building structures, bridge engineering, and heavy timber structures, and its axial compression failure behavior is a core indicator for assessing the load-bearing safety of structural members. However, as a naturally anisotropic material, timber exhibits complex and diverse compression failure modes, often accompanied by the coupled evolution of multiple damage forms such as crack initiation and propagation, fiber breakage, and delamination.

[0003] Traditional methods of characterizing damage mainly rely on cross-sectional observation or visual inspection of the damage surface after macroscopic mechanical tests. These methods cannot perform in-situ, non-destructive microscale tracking of the same specimen during compression loading, and it is also difficult to accurately identify the starting position and evolution path of the damage feature region under different compression rates. In addition, existing microscopic observation techniques are mostly for small-sized standard specimens. For full-size components, due to the limitations of specimen size and field of view of observation equipment, destructive sampling or surface reprocessing is often required. This not only destroys the original state of damage, but also makes it impossible to continuously obtain damage information at different compression stages on the same specimen.

[0004] The information disclosed in the background section is only intended to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide an in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A non-destructive in-situ microscopic characterization method for the compression failure mode of full-scale wood, comprising the following steps: Step 1: Preset several different compression ratio target values, and conduct multi-level axial compression tests on full-scale wood samples in order of increasing compression ratio target values. After each level of axial compression test, use sandpaper of different grit to refine the observation section of the full-scale wood sample step by step until the observation section reaches a smooth state without burrs or scratches. Step 2: Set the low magnification range and the high magnification range. Within the low magnification range, perform full-area scanning imaging on the observation section of the polished full-size wood sample to obtain microscopic images at each low magnification. Then, identify the damage feature areas in the microscopic images to form an initial damage feature distribution map. Step 3: Based on the initial damage feature distribution map, scan the marked damage feature area within a high magnification range to generate several local high-magnification images. Obtain the relative displacement and fracture surface dip angle on both sides of the crack from each local high-magnification image, and calculate the local damage degree value of each local high-magnification image. Step 4: According to the next level of compression rate target value of the current compression rate target value, conduct the next level of axial compression test on the full-size wood sample. Repeat the above grinding, observation and calculation operations, record the local damage degree value of the next level of compression rate target value, and calculate the damage increment value of the local damage degree value relative to the previous level of compression rate target value. Based on the damage increment value, determine the changing trend of the failure mode of the full-size wood sample under each level of compression rate.

[0007] Furthermore, a full-scale scanning imaging of the observation section of the polished full-size wood sample was performed within a low magnification range to obtain microscopic images at various low magnifications. The specific steps are as follows: A preset magnification threshold is used to divide the low magnification range into a high magnification range, and an equal gradient magnification is set in each of the two magnification ranges. The specific steps for low-magnification scanning of the whole field of view imaging within the low magnification range are as follows: Place the polished full-size wood sample on the stage of the stereomicroscope, and conduct observation and imaging from the lowest magnification level within the low magnification range; during observation, first complete the initial focusing and imaging of the upper left corner vertex area of ​​the observation section, and then control the stage to move horizontally step by step according to the preset scanning path, focusing and imaging field by field of view, and the horizontal step distance is the result of multiplying the horizontal width of the field of view by the preset scaling factor; after the stage moves to the right end of the observation section, complete the vertical step by multiplying the vertical height of the field of view by the preset scaling factor, and then scan horizontally in the opposite direction, repeating the cycle until the full coverage imaging of the entire observation section is completed; All gradient magnifications within the low magnification range are scanned and imaged using a low-magnification scanning method to generate full-domain microscopic images of the observation cross-section at each low magnification.

[0008] Furthermore, the damaged feature regions are identified in the microscopic images to form an initial damage feature distribution map. The specific steps are as follows: Define the types of failure characteristic regions in the wood compression observation section, specifically including crack propagation paths, fiber fracture zones, and interlaminar separation regions; Based on the full-domain microscopic image corresponding to the lowest magnification within the low magnification range, the distribution range of crack propagation path, fiber fracture zone and interlaminar separation region is initially identified. Then, in the order of progressively increasing magnification, the full-domain microscopic images of each gradient magnification are switched sequentially to refine and iterate the various damage feature regions initially identified. The detailed features of crack propagation direction, fiber fracture zone morphology and interlaminar separation region outline at different magnifications are corrected. Finally, based on the full-domain microscopic image of the highest magnification within the low magnification range, the boundary calibration of various damage feature regions is completed. The location of each failure feature region is determined based on the crack propagation trajectory line, fiber fracture boundary line, and interlaminar separation region boundary curve, and an initial failure feature distribution map containing multiple failure feature region types is generated.

[0009] Furthermore, the destructive feature areas of the marker are scanned within a high magnification range to generate several local high-magnification images. The specific steps are as follows: Extract the geometric center coordinates of each damage feature region in the initial damage feature distribution map, arrange the geometric center coordinates in the order of the distribution of each damage region, generate the planned movement path of the stage, align the geometric center of each damage feature region with the center of the microscope optical axis in sequence and complete the focusing, so as to realize the positioning of all target damage regions. After completing the area positioning and alignment, switch the microscope magnification to the high magnification range and retrieve several preset gradient magnifications within the high magnification range; at each high magnification, with the center of the microscope optical axis as the reference point, and combined with the exclusive extension direction of different damage feature areas, move the field of view segment by segment according to the preset step distance and acquire images to obtain several local high magnification images. Among them, the extension direction of the crack propagation path is the direction of the crack propagation trajectory line, the extension direction of the fiber fracture zone is the direction of the long axis extension of the fracture surface boundary line, and the extension direction of the interlayer separation region is the direction of the maximum span of the separation region boundary curve.

[0010] Furthermore, the relative displacement between the two sides of the crack is obtained from each local high-magnification image. The specific steps are as follows: Displacement measurements were performed on each local high-magnification image, and the relative displacement was extracted using the corresponding measurement method based on the type of damage feature region corresponding to the image. When the image corresponds to the crack propagation path in the damaged area, the edge lines on both sides of the crack are identified in the local high-magnification image. Using the pre-calibrated crack propagation trajectory line as a reference, measurement lines are laid out perpendicular to the crack propagation trajectory line. The measurement lines intersect with the edge lines distributed on both sides of the crack at the left edge measurement point and the right edge measurement point. The vertical distance between the left edge measurement point and the right edge measurement point is measured as the relative displacement on both sides of the crack. When the image corresponds to the damaged feature region as a fiber fracture zone, the gray-scale boundary line between the fracture surface and the unfractured surface is identified in the local high-magnification image and used as the fracture surface boundary line. Measurement lines are laid out along the direction perpendicular to the fracture surface boundary line. These measurement lines intersect the fracture surface boundary line and the unfractured surface boundary line at the first boundary measurement point and the second boundary measurement point, respectively. The vertical distance between the first boundary measurement point and the second boundary measurement point is measured and used as the relative displacement of the fiber fracture zone. When the image corresponds to the destructive feature region as the interlayer separation region, the upper and lower boundary lines of the interlayer separation region are identified in the local high-magnification image. Measurement lines are laid out along the direction perpendicular to the upper and lower boundary lines. The intersection of the measurement line with the upper boundary line is the upper boundary measurement point. The intersection of the measurement line with the lower boundary line is the lower boundary measurement point. The vertical distance between the upper boundary measurement point and the lower boundary measurement point is measured, and this vertical distance is taken as the relative displacement of the interlayer separation region. Traverse all local high-magnification images to obtain the relative displacement of each damaged feature region.

[0011] Furthermore, the dip angle of the fracture surface is obtained based on each local high-magnification image. The specific steps are as follows: The dip angle of each local high-magnification image was measured one by one, and the dip angle of the fracture surface was extracted by a differentiated measurement method according to the different damage feature regions corresponding to the images. When the image corresponds to the crack propagation path in the area of ​​damage features, the intersection line between the crack propagation direction and the observation section plane is identified in the local high-magnification image, and the tilt angle of the intersection line relative to the direction of the observation section plane is measured as the fracture surface dip angle at the corresponding position. When the image corresponds to a fiber fracture zone, the intersection line between the fracture surface boundary line and the observation section plane is identified in the local high-magnification image, and the tilt angle of the intersection line relative to the direction of the observation section plane is measured as the fracture surface dip angle at the corresponding position. When the image corresponds to the destructive feature region as the interlayer separation region, the intersection line between the plane where the interlayer separation region is located and the observation section plane is identified in the local high-magnification image. The tilt angle of the intersection line relative to the direction of the observation section plane is measured as the dip angle of the fracture surface at the corresponding position. Iterate through all local high-magnification images to obtain the fracture surface dip angle corresponding to each image acquisition location.

[0012] Further, the local damage level of each high-magnification image is calculated, and the specific steps are as follows: For the same region of damage features, extract the relative displacement and fracture surface dip angle corresponding to all image acquisition locations within the region; Calculate the average relative displacement at all sampling locations in the damaged feature area, and divide the average relative displacement by the extension length of the damaged feature area to obtain the average displacement per unit length. Calculate the average dip angle of the fracture surface at all sampling locations within the damage characteristic area, and obtain the tangent function value of this average dip angle; Multiplying the average displacement per unit length of the same damage characteristic region by the tangent function value of the average tilt angle yields the corresponding local damage degree value.

[0013] Furthermore, based on the incremental damage values, the changing trends of the failure modes of full-scale wood specimens at various compression rates are determined. The specific steps are as follows: After loading a full-scale wood sample to the preset next-level compression ratio target value and completing the acquisition of relative displacement and fracture surface inclination angle, calculate the local damage degree value of each damage characteristic region under the current compression ratio target value. Subtract the local damage degree value corresponding to the previous level of compression ratio target value from the local damage degree value corresponding to the current compression ratio target value to obtain the regional damage increment value corresponding to each level of compression loading. Record the proportion of the area of ​​all destruction feature regions with a destruction increment value greater than 0 to obtain the evolutionary activity level, and preset the evolutionary activity level threshold. When the evolutionary activity level is greater than the evolutionary activity level threshold, it is determined that the damage of the full-scale wood specimen is in the accelerated evolution stage, and the cumulative damage across the entire area is rapidly aggravated. When the evolutionary activity is greater than 0 and less than or equal to the evolutionary activity threshold, the full-scale wood sample is determined to be in the local evolution stage, with only local characteristic areas generating new cumulative damage and the overall destruction evolution being gradual. When the evolutionary activity level is equal to 0, it is determined that there is no new damage accumulation in each level of damage characteristic area, and the damage state of the full-scale wood sample has not been updated.

[0014] Compared with the prior art, the beneficial effects of the present invention are: This invention achieves continuous, non-destructive, multi-stage preservation and observation of the failure state on the same sample by pre-setting multi-level compression ratio target values ​​and performing axial compression of the same full-size sample in ascending order. Combined with progressively refined grinding of the observation section after each compression, this invention enables full-domain scanning imaging of the ground observation section at low magnification and constructs an initial failure feature distribution map. This allows for in-situ, non-destructive, and continuous observation of full-size wood samples during multi-level compression. It can accurately locate the spatial distribution of crack propagation paths, fiber fracture zones, and interlaminar separation regions, providing a complete, accurate, and traceable spatial reference for subsequent microscopic quantitative analysis, significantly improving the availability and consistency of the data.

[0015] This invention also performs local high-magnification imaging of the identified damage feature areas under high magnification, and quantitatively extracts the relative displacement and fracture surface inclination on both sides of the crack, thereby calculating the local damage degree value. This elevates the damage morphology from a qualitative description to a quantifiable microscopic damage index. By continuing to compress at each level and repeating the observation and calculation, the damage increment value between adjacent compression rates is obtained, thereby judging the changing trend of the damage mode. This allows the local damage evolution process of the same sample at different compression stages to be accurately quantified and dynamically tracked, and the stage characteristics of accelerated and localized damage evolution can be identified. Attached Figure Description

[0016] Figure 1 This is a schematic flowchart of the in-situ non-destructive microscopic characterization method for full-scale wood compression failure mode according to the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0018] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0019] Example: Please see Figure 1 The present invention provides a technical solution: A non-destructive in-situ microscopic characterization method for the compression failure mode of full-scale wood, comprising the following steps: Step 1: Preset several different target compression ratio values, and conduct multi-stage axial compression tests on full-scale wood specimens in ascending order of the target compression ratio values. After each stage of axial compression test, pause the compression and maintain a stable load. After the specimen deformation stabilizes, proceed to the next stage of compression until all target compression ratio values ​​are completed. After each stage of axial compression test, use sandpaper of different grits (e.g., 800 grit → 2000 grit → 3000 grit → 5000 grit → 7000 grit) to progressively refine the observation section of the full-scale wood specimen from low to high grit. Low grit sandpaper is used to initially remove rough protrusions and burrs from the section, while high grit sandpaper is used for further refinement, gradually eliminating scratches and smoothing the section. Each time the grit sandpaper is changed, sanding is done in the same direction until the observation section reaches a smooth state without burrs or scratches. In this embodiment, the compression ratio represents the ratio of the axial deformation of a full-scale wood sample to its original gauge length. The purpose of presetting multiple levels of compression ratio target values ​​is to divide the failure process into several key deformation intervals based on the mechanical characteristics of typical wood compression failure stages, ensuring that each level of compression ratio can capture the change in the failure mode of the full-scale wood sample. The specific correspondence between each level of compression ratio and the morphological evolution stage of full-scale wood is as follows: when the compression ratio is in the range of 0.5% to 2%, it is the elastic steady state stage, at which time the wood cell wall undergoes elastic deformation, and there is no microscopic damage. When the compression ratio is in the range of 2% to 5%, it is the damage initiation stage, where some wood cell walls develop microcracks, and the wood begins to show microscopic damage. When the compression ratio is in the range of 5% to 15%, it is the crack propagation stage, where the wood shows macroscopic cracks, and the cracks extend along the direction of the wood fibers, and fiber fracture zones and interlaminar separation gradually form. When the compression ratio exceeds 15%, it is the unstable failure stage, where the damage rapidly intensifies, and the wood tends to macroscopically fragment.

[0020] Step 2: Set the low magnification range and the high magnification range. Within the low magnification range, perform full-area scanning imaging on the observation section of the polished full-size wood sample to obtain microscopic images at each low magnification. Then, identify the damage feature areas in the microscopic images to form an initial damage feature distribution map. In this embodiment, a full-scale scanning image of the observation section of a polished full-size wood sample is performed within a low magnification range to obtain microscopic images at various low magnifications. The specific steps are as follows: A preset magnification threshold is used to divide the low magnification range into a high magnification range, and an equal gradient magnification is set in each of the two magnification ranges. The specific steps for low-magnification scanning of the whole field of view imaging within the low magnification range are as follows: Place the polished full-size wood sample on the stage of the stereomicroscope, and conduct observation and imaging from the lowest magnification level within the low magnification range; during observation, first complete the initial focusing and imaging of the upper left corner vertex area of ​​the observation section, and then control the stage to move horizontally step by step according to the preset scanning path, focusing and imaging field by field of view, and the horizontal step distance is the result of multiplying the horizontal width of the field of view by the preset scaling factor; after the stage moves to the right end of the observation section, complete the vertical step by multiplying the vertical height of the field of view by the preset scaling factor, and then scan horizontally in the opposite direction, repeating the cycle until the full coverage imaging of the entire observation section is completed; All gradient magnifications within the low magnification range are scanned and imaged using a low-magnification scanning method to generate full-domain microscopic images of the observation cross-section at each low magnification.

[0021] The measured diameter of wood fibers is approximately 20-50 μm, the width of annual rings is approximately 1-5 mm, and the width of cracks is approximately 5-30 μm. At a certain magnification, such as 50x, the microscope's field of view is sufficient to cover complete annual rings and macroscopic cracks, while clearly distinguishing the morphology of individual wood fibers. Therefore, the preset magnification threshold is set to 50x. Magnifications below 50x are low magnifications, used to cover the entire area of ​​full-scale wood and pinpoint damaged areas. Magnifications above 50x are high magnifications, used to focus on localized damage in full-scale wood and provide microscopic data.

[0022] It should be noted that during scanning imaging, the scanning starts from the upper left corner of the observation section, and a serpentine scanning path is planned. The preset scale factor is set to 75%~85% to ensure that there is a 15%~25% overlap area between adjacent fields of view, which is used to achieve seamless image stitching. The first row of the full-scale wood sample is scanned horizontally from left to right until the right boundary of the field of view exceeds the right end of the observation section. Then, it moves vertically down by one step distance and scans in reverse from right to left.

[0023] It should be noted that: first, a full-coverage imaging is performed at the lowest magnification to generate a full-field microscopic image of the observation section at the lowest magnification. Then, the process is switched to the next magnification and the scanning imaging process is repeated until the full-field scanning of all preset gradient magnifications within the low magnification range is completed. From low magnification to high magnification, the field of view is gradually reduced to ensure that sufficient observation details are provided while covering the main damaged areas.

[0024] In this embodiment, the regions with destructive features are identified in the microscopic image to form an initial distribution map of destructive features. The specific steps are as follows: Define the types of failure characteristic regions in the wood compression observation section, specifically including crack propagation paths, fiber fracture zones, and interlaminar separation regions; Based on the micromechanical mechanism of wood compression failure, the failure characteristic regions on the observed cross section are defined into three categories: Among them, the crack propagation path refers to the linear crack area extending along the direction of wood fibers or interface, and its typical form is a narrow band crack with a clear direction. Among them, the fiber fracture zone refers to the band-shaped or sheet-like area formed by the collective fracture of wood fibers due to the concentration of compressive stress, which has a significant gray difference from the undamaged substrate. Among them, the interlayer separation region refers to the delamination region that occurs along the wood annual ring interface or cell layer interface, which is characterized by a loose and porous image texture.

[0025] Based on the full-domain microscopic image corresponding to the lowest magnification within the low magnification range, the distribution range of crack propagation path, fiber fracture zone and interlaminar separation region is initially identified. Then, in the order of progressively increasing magnification, the full-domain microscopic images of each gradient magnification are switched sequentially to refine and iterate the various damage feature regions initially identified. The detailed features of crack propagation direction, fiber fracture zone morphology and interlaminar separation region outline at different magnifications are corrected. Finally, based on the full-domain microscopic image of the highest magnification within the low magnification range, the boundary calibration of various damage feature regions is completed. In the full-domain microscopic image at the lowest magnification, based on the gray-level gradient abrupt change regions and texture anisotropy features, a region growing algorithm is used to initially delineate the initial distribution range of all suspected damage feature regions, and they are initially classified and labeled into three categories: crack propagation paths, fiber fracture zones, and interlaminar separation regions. After completing the initial classification and labeling, the geometric boundary coordinates of each marked damage feature region in the lowest magnification image are mapped to the next adjacent full-domain microscopic image at the next higher magnification. Subsequently, the geometric boundary coordinates mapped from the previous magnification are corrected in the high-magnification full-domain microscopic image. The steps for correcting the crack propagation path are as follows: The centerline coordinates of the crack region at low magnification are mapped to a high-magnification image as an initial reference line. In the high-magnification global microscopic image, the pixel positions of the crack's two edges are extracted. The local midpoints of the two edges are calculated point by point to form a discrete midpoint sequence. Local linear fitting is performed on the discrete midpoint sequence to generate an updated crack propagation trajectory. The difference in deflection angle between the updated crack propagation trajectory and the initial reference line in each local segment is compared. When the deflection angle exceeds a preset deflection angle threshold, the updated crack propagation trajectory replaces the original crack propagation path. The preset deflection angle threshold is determined based on accuracy requirements. A lower value indicates higher correction sensitivity, where even minor fluctuations in trajectory direction are interpreted as crack deflection, triggering crack path updates. This is suitable for research scenarios with extremely high crack path accuracy requirements. A higher value indicates lower correction sensitivity, triggering correction only when the crack undergoes a significant directional change. This is suitable for rapid engineering evaluation scenarios.

[0026] The steps for correcting fiber fracture zones are as follows: In the lowest magnification full-domain microscopic image, the Canny edge detection operator is used to extract the initial gray-level boundary points between the fracture zone and the unfractured substrate. Connecting each initial gray-level boundary point forms the initial fracture zone boundary line. Along the direction perpendicular to the initial fracture zone boundary line, several measurement points are selected at equal intervals. At each measurement point, a measurement line segment is drawn along the normal direction of the boundary line. The point with the largest gray-level change rate is recorded on each measurement line segment as the precise boundary point to replace the boundary point on the initial fracture zone boundary line. Precise boundary points are obtained on all measurement line segments, and all precise boundary points are connected to form a precise boundary. The precise boundary is mapped to the full-domain microscopic image at a higher magnification. Based on the originally selected measurement points, another measurement point is inserted between the center positions of the lines connecting adjacent measurement points, and the precise boundary is recalculated. The above process of inserting measurement points, mapping, and calculating boundaries is repeated until the full-domain microscopic image at the highest magnification is obtained.

[0027] The steps for correcting the interlayer separation region are as follows: In the full-domain microscopic image at the lowest magnification, for the interlayer separation region, the local binary mode operator is used to calculate the texture feature value in the interlayer separation region. The pixel point with the abrupt change in texture feature value is used as the initial contour point. The initial contour points are connected to form an initial contour curve. Several control points are equally spaced on the initial contour curve. At each control point, a search line segment is drawn along the normal direction of the contour curve. The point with the largest absolute value of the difference of the local binary mode feature values ​​is calculated on each search line segment and used as the precise contour point to replace the corresponding initial contour point on the initial contour curve. Precise contour points are obtained on all search line segments and connected to form a precise contour. The precise contour is mapped to the full-domain microscopic image at a higher magnification. Based on the originally selected control points, a control point is inserted from the center position of the line connecting adjacent control points, and the precise contour is recalculated. The above process of inserting control points, mapping, and calculating boundaries is repeated until the full-domain microscopic image at the highest magnification is obtained.

[0028] The location of each failure feature region is determined based on the crack propagation trajectory line, fiber fracture boundary line, and interlaminar separation region boundary curve, and an initial failure feature distribution map containing multiple failure feature region types is generated.

[0029] Step 3: Based on the initial damage feature distribution map, scan the marked damage feature area within a high magnification range to generate several local high-magnification images. Obtain the relative displacement and fracture surface dip angle on both sides of the crack from each local high-magnification image, and calculate the local damage degree value of each local high-magnification image. In this embodiment, the marked destructive feature area is scanned within a high magnification range to generate several local high-magnification images. The specific steps are as follows: Extract the geometric center coordinates of each damage feature region in the initial damage feature distribution map, arrange the geometric center coordinates in the order of the distribution of each damage region, generate the planned movement path of the stage, align the geometric center of each damage feature region with the center of the microscope optical axis in sequence and complete the focusing, so as to realize the positioning of all target damage regions. Extract the geometric center coordinates of each damaged feature region, and select the geometric center closest to the upper left corner vertex of the observation section as the starting node of the planned movement path to meet the physical constraints of the initial reset position of the microscope stage and reduce the idle travel time. Mark the starting node as a path node, and regard the geometric centers of all other damaged feature regions as unvisited nodes and include them in the unvisited node set. Take the starting node as the first current node, and calculate the Euclidean distance from the current node to each unvisited node in turn. Select the unvisited node with the closest Euclidean distance as the next target node to be visited, remove the target node from the unvisited node set, add it to the path node set, and then set the target node as the new current node. Repeat the above steps until every node in the unvisited node set is included in the path node set. At this time, the node order in the path node set is the planned movement path.

[0030] After completing the area positioning and alignment, switch the microscope magnification to the high magnification range and retrieve several preset gradient magnifications within the high magnification range; at each high magnification, with the center of the microscope optical axis as the reference point, and combined with the exclusive extension direction of different damage feature areas, move the field of view segment by segment according to the preset step distance and acquire images to obtain several local high magnification images. Among them, the extension direction of the crack propagation path is the direction of the crack propagation trajectory line, the extension direction of the fiber fracture zone is the direction of the long axis extension of the fracture surface boundary line, and the extension direction of the interlayer separation region is the direction of the maximum span of the separation region boundary curve.

[0031] The preset step distance is lower than the horizontal and vertical step distances used for observation at low magnification. This is because a larger step distance is required at low magnification to quickly cover a large area, while at high magnification, the goal is to obtain high-resolution images to extract relative displacement and tilt angle. The preset step distance is set to 60% to 70% of the actual field of view size to obtain sufficient microscopic morphological data while ensuring local coverage.

[0032] It should be noted that the high magnification range is greater than 50x and does not exceed 500x. Within the high magnification range, the observation field of view is reduced to allow for precise local measurement of the identified damaged area, providing sufficient microscopic resolution to extract microscopic parameters. The calculation of the local damage degree depends on the relative displacement and the fracture surface dip angle. The relative displacement is typically on the order of several micrometers to tens of micrometers. If the magnification is below 50x, the measurement error will be too large, failing to meet the requirements for quantitative analysis. Conversely, if the magnification is above 500x, the field of view will be too small, and a single image will not contain enough structural references. Therefore, the high magnification range is set between 50x and 500x.

[0033] In this embodiment, the relative displacement between the two sides of the crack is obtained from each local high-magnification image. The specific steps are as follows: Displacement measurements were performed on each local high-magnification image, and the relative displacement was extracted using the corresponding measurement method based on the type of damage feature region corresponding to the image. When the image corresponds to the crack propagation path in the damaged area, the edge lines on both sides of the crack are identified in the local high-magnification image. Using the pre-calibrated crack propagation trajectory line as a reference, measurement lines are laid out perpendicular to the crack propagation trajectory line. The measurement lines intersect with the edge lines distributed on both sides of the crack at the left edge measurement point and the right edge measurement point. The vertical distance between the left edge measurement point and the right edge measurement point is measured as the relative displacement on both sides of the crack. It should be noted that the relative displacement on both sides of a crack reflects the degree of crack opening in the normal direction under compressive load, i.e., the normal separation distance of the crack wall due to compressive load. This quantifies the severity of local deformation of the crack under compressive stress and is used to determine whether the crack is in an expanding state. When wood fails due to compression, the crack mainly expands along the fiber direction or interface direction. The material on both sides undergoes lateral extrusion or displacement under compressive stress, leading to an increase in crack width. Since the crack expansion trajectory line is accurately located in advance at low magnification, setting up the measurement line based on it ensures that the measurement direction is always perpendicular to the crack direction, thereby eliminating the interference of tangential slip on the opening measurement.

[0034] When the image corresponds to the damaged feature region as a fiber fracture zone, the gray-scale boundary line between the fracture surface and the unfractured surface is identified in the local high-magnification image and used as the fracture surface boundary line. Measurement lines are laid out along the direction perpendicular to the fracture surface boundary line. These measurement lines intersect the fracture surface boundary line and the unfractured surface boundary line at the first boundary measurement point and the second boundary measurement point, respectively. The vertical distance between the first boundary measurement point and the second boundary measurement point is measured and used as the relative displacement of the fiber fracture zone. It should be noted that the relative displacement of the fiber fracture zone reflects the separation or displacement distance along the normal direction of the observed section on both sides of the fiber fracture zone under compressive load. It is the local opening or compression deformation of the fiber fracture zone in the direction perpendicular to its boundary. In the microscopic image, the fiber fracture zone appears as a grayscale boundary line between the fracture surface and the unbroken substrate. The steps to obtain the grayscale boundary line are: performing grayscale processing on the local high-magnification image to obtain a grayscale image containing the grayscale values ​​of each pixel; the fiber fracture zone forms a rough damaged surface due to fiber breakage and cell wall collapse. Its light reflectivity is lower than that of the surrounding undamaged, smooth substrate. Therefore, in the grayscale image, the fracture zone area appears as... The current area is a dark region with low grayscale values, while the unbroken substrate appears as a bright region with high grayscale values. At the boundary between the dark and bright regions, the grayscale value changes drastically within a very short spatial distance. To capture this change, the Sobel edge detection operator is used to calculate the grayscale gradient of each pixel in the grayscale image in the horizontal and vertical directions. The square root of the sum of the squares of the horizontal and vertical grayscale gradients is then calculated to obtain the gradient magnitude of each pixel. A gradient magnitude threshold is preset, and pixels with gradient magnitudes greater than or equal to the threshold are selected. These pixels are the boundary points on the grayscale boundary line. After obtaining the gradient magnitude, a histogram is drawn based on the gradient magnitudes of all pixels. Histograms are classified binaryly using the Otsu method. An initial threshold is calculated based on the maximum inter-class variance criterion. This initial threshold is multiplied by an adjustment coefficient to obtain the gradient magnitude threshold. The adjustment coefficient is determined based on histogram features. When the histogram exhibits a clear bimodal characteristic (i.e., the ratio of the distance between peaks to the peak width is greater than 1), it indicates high overall image contrast and significant differentiation between boundary points and other pixels. However, some boundary points located in crack tips, fracture edges, etc., may be missed. Therefore, to improve the recall rate of boundary points, the adjustment coefficient is set to less than 1 to lower the threshold and retain more weak edge information. When the histogram exhibits a single peak or a fusion of two peaks (i.e., the distance between the peaks is greater than 1), the adjustment coefficient is adjusted accordingly. If the ratio of the peak distance to the peak width is less than 1, it indicates that the overall image contrast is low. The gradient magnitude of the true boundary point overlaps with the gradient magnitude generated by natural textures and noise. In this case, the calculated initial threshold will cause a large number of false edges to be falsely detected as boundary points. Therefore, in order to improve the accuracy of boundary detection and suppress false detections, the adjustment coefficient is set to be greater than 1 to increase the threshold and filter noise interference. When the ratio of the peak distance to the peak width of the histogram is equal to 1, it means that the initial threshold can sufficiently separate the boundary points, and there is no need to adjust the initial threshold. In this case, the adjustment coefficient is equal to 1. The measurement line is laid out along the direction perpendicular to the boundary line of the fracture surface. This direction is the main deformation direction in which the fiber fracture zone opens or shifts.The measuring line intersects the boundary lines of the fracture surface and the unfractured surface, respectively, to obtain two boundary measuring points. The perpendicular distance between these two points represents the normal relative displacement of the fracture zone at that location. This eliminates the interference of tangential slip and ensures that the measured displacement directly corresponds to the normal separation caused by compressive stress.

[0035] When the image corresponds to the destructive feature region as the interlayer separation region, the upper and lower boundary lines of the interlayer separation region are identified in the local high-magnification image. Measurement lines are laid out along the direction perpendicular to the upper and lower boundary lines. The intersection of the measurement line with the upper boundary line is the upper boundary measurement point. The intersection of the measurement line with the lower boundary line is the lower boundary measurement point. The vertical distance between the upper boundary measurement point and the lower boundary measurement point is measured, and this vertical distance is taken as the relative displacement of the interlayer separation region. It should be noted that the relative displacement of the interlayer separation region reflects the opening or displacement distance of the upper and lower separation surfaces along the normal direction of the observed section after delamination and debonding of the wood along the annual ring interface or cell layer interface under compressive load. In local high-magnification images, the interlayer separation region appears as two boundary lines: the upper boundary line is the top edge of the separation surface, and the lower boundary line is the bottom edge of the separation surface. The grayscale and texture differences between the two are obvious. Measurement lines are laid out along the direction perpendicular to these two boundary lines to ensure that the measurement direction is consistent with the main deformation direction, thereby eliminating projection errors caused by changes in the curvature or inclination angle of the boundary lines.

[0036] Based on the above, all local high-magnification images are traversed to obtain the relative displacement of each damaged feature region.

[0037] In this embodiment, the dip angle of the fracture surface is obtained based on each local high-magnification image. The specific steps are as follows: The dip angle of each local high-magnification image was measured one by one, and the dip angle of the fracture surface was extracted by a differentiated measurement method according to the different damage feature regions corresponding to the images. When the image corresponds to the crack propagation path in the area of ​​damage features, the intersection line between the crack propagation direction and the observation section plane is identified in the local high-magnification image, and the tilt angle of the intersection line relative to the direction of the observation section plane is measured as the fracture surface dip angle at the corresponding position. It should be noted that the fracture surface inclination angle of the crack propagation path reflects the spatial inclination of the actual crack propagation direction relative to the plane of the observation section under compressive load. The observation section of full-length timber is a polished two-dimensional plane, while the crack inside the timber is a narrow fracture surface in three-dimensional space. When this fracture surface is oblique to the observation section, there is an angle between the trace it leaves on the observation section and the plane of the observation section. This angle is the fracture surface inclination angle. The larger the fracture surface inclination angle, the more the crack tends to be perpendicular to the observation section; the smaller the fracture surface inclination angle, the closer the crack is to being parallel to the observation section.

[0038] When the image corresponds to a fiber fracture zone, the intersection line between the fracture surface boundary line and the observation section plane is identified in the local high-magnification image, and the tilt angle of the intersection line relative to the direction of the observation section plane is measured as the fracture surface dip angle at the corresponding position. It should be noted that the dip angle of the fracture surface of a fiber fracture zone reflects the spatial inclination of the fracture surface relative to the observation section. When the fracture surface intersects the observation section obliquely, it forms the angle between the trace left on the section and the plane of the section. The larger this angle, the steeper the fracture surface, and the fracture mainly extends in the direction perpendicular to the observation section; the smaller the angle, the gentler the fracture surface, and the fracture mainly extends in the direction parallel to the observation section. The observation section of wood is a two-dimensional plane obtained through polishing, while the fiber fracture zone is a fracture surface with a specific spatial orientation in three-dimensional space. The intersection of this fracture surface and the observation section is represented by the fracture boundary line in the image. Measuring the angle of this intersection line relative to a certain reference direction in the image plane yields the spatial dip angle of the fracture surface relative to the observation section.

[0039] When the image corresponds to the destructive feature region as the interlayer separation region, the intersection line between the plane where the interlayer separation region is located and the observation section plane is identified in the local high-magnification image. The tilt angle of the intersection line relative to the direction of the observation section plane is measured as the dip angle of the fracture surface at the corresponding position. It should be noted that the dip angle of the fracture surface in the delamination region reflects the spatial inclination of the delamination fracture surface relative to the observation section. It characterizes the attitude angle of the delamination fracture surface occurring along the wood ring interface or cell layer interface in three-dimensional space relative to the two-dimensional observation section. A larger dip angle indicates a steeper delamination fracture surface, almost perpendicular to the observation section. In this case, compressive loads mainly cause tensile or tearing along the thickness direction, and the delamination is primarily an opening-type failure. Conversely, a smaller dip angle indicates a gentler delamination fracture surface, almost parallel to the observation section. In this case, the load mainly causes shear slip along the interface direction, and the failure is primarily slip-type or peel-type.

[0040] Based on the above, all local high-magnification images are traversed to obtain the fracture surface dip angle corresponding to each image acquisition location.

[0041] Step 4: According to the next level of compression rate target value of the current compression rate target value, conduct the next level of axial compression test on the full-size wood sample. Repeat the above grinding, observation and calculation operations, record the local damage degree value of the next level of compression rate target value, and calculate the damage increment value of the local damage degree value relative to the previous level of compression rate target value. Based on the damage increment value, determine the changing trend of the failure mode of the full-size wood sample under each level of compression rate.

[0042] In this embodiment, the local damage level value of each local high-magnification image is calculated, and the specific steps are as follows: For the same region of damage features, extract the relative displacement and fracture surface dip angle corresponding to all image acquisition locations within the region; Calculate the average relative displacement at all sampling locations in the damaged feature area, and divide the average relative displacement by the extension length of the damaged feature area to obtain the average displacement per unit length. Calculate the average dip angle of the fracture surface at all sampling locations within the damage characteristic area, and obtain the tangent function value of this average dip angle; Multiplying the average displacement per unit length of the same damage characteristic region by the tangent function value of the average tilt angle yields the corresponding local damage degree value.

[0043] It should be noted that the local damage severity value describes the severity of local damage in a specific damage feature area on the observation section of a full-scale wood specimen under the current compression rate. It integrates two dimensions: deformation accumulation effect and damage geometry. The deformation accumulation effect is reflected by the average displacement per unit length. The average displacement per unit length is obtained by dividing the average relative displacement by the extension length of the damage feature area, eliminating the influence of size differences between different damage areas on the displacement, making the damage severity comparable between damage feature areas of different lengths. It reflects the average strength of normal separation or slippage of the material in the compression direction. The larger the displacement and the denser the distribution, the more severe the local plastic deformation or damage accumulation experienced at that location. The damage geometry is reflected by the tangent of the inclination angle. The larger the inclination angle, the more perpendicular the fracture surface is to the observation section, meaning that the opening damage component is more prominent, and the damage is more severe and unstable. The smaller the inclination angle, the more likely it is to be slip-type damage. Multiplying the two values ​​yields the local damage severity value, which comprehensively characterizes the substantial damage intensity that has occurred in the damage feature area during compression. The larger the local damage severity value, the more severe the damage.

[0044] The specific steps for determining the changing trend of the failure mode of full-size wood specimens at various compression rates based on the incremental failure value are as follows: After loading a full-scale wood sample to the preset next-level compression ratio target value and completing the acquisition of relative displacement and fracture surface inclination angle, calculate the local damage degree value of each damage characteristic region under the current compression ratio target value. Subtract the local damage degree value corresponding to the previous level of compression ratio target value from the local damage degree value corresponding to the current compression ratio target value to obtain the regional damage increment value corresponding to each level of compression loading. It should be noted that the damage increment value reflects the new damage intensity in the damage feature area between two adjacent compression ratios. When the damage increment value is greater than 0, it indicates that new substantial damage changes have occurred in the damage feature area, specifically including crack propagation, widening of the fracture zone, and intensified interlaminar separation. When the damage increment value is equal to 0, it indicates that no new macroscopic damage has occurred, and the full-size wood sample is relatively stable.

[0045] Record the proportion of the area of ​​all destruction feature regions with a destruction increment value greater than 0 to obtain the evolutionary activity level, and preset the evolutionary activity level threshold. When the evolutionary activity level is greater than the evolutionary activity level threshold, it is determined that the damage of the full-scale wood specimen is in the accelerated evolution stage, and the cumulative damage across the entire area is rapidly aggravated. When the evolutionary activity is greater than 0 and less than or equal to the evolutionary activity threshold, the full-scale wood sample is determined to be in the local evolution stage, with only local characteristic areas generating new cumulative damage and the overall destruction evolution being gradual. When the evolutionary activity level is equal to 0, it is determined that there is no new damage accumulation in each level of damage characteristic area, and the damage state of the full-scale wood sample has not been updated.

[0046] It should be noted that the damage increment value of a single region can only reflect the damage changes in a local area and cannot determine the overall damage situation. Therefore, evolutionary activity is introduced to reflect the overall damage trend. Evolutionary activity reflects the proportion of the area of ​​the damage characteristic region with substantial new damage on the full-scale timber observation section between two adjacent compression rates to the total area of ​​the total damage region. The higher the evolutionary activity, the larger the range of damage and the more global the damage tends to be. The lower the evolutionary activity, the more concentrated the damage is in a few weak areas and the overall observation section is relatively stable.

[0047] It should be noted that, based on the value of evolutionary activity, there are three typical evolutionary stages. When the evolutionary activity is equal to 0, there is no new damage in any area, and the specimen is stable. When the evolutionary activity is between 0 and the evolutionary activity threshold, it indicates that damage has occurred in a small number of areas, the overall damage is controllable, and gradual failure occurs. When the evolutionary activity is greater than the activity threshold, it indicates that most areas deteriorate simultaneously, and the full-scale wood specimen tends to become unstable. The evolutionary activity threshold is essentially the critical area percentage that distinguishes between locally controllable damage and global instability. The smaller the value of the evolutionary activity threshold, the more conservative the assessment, and lower area damage is considered as increased damage, resulting in higher early warning sensitivity, but lower wood material utilization. The larger the value, the more lenient the assessment, and the more fully the strength of the wood is utilized, but the safety redundancy is smaller. The evolutionary activity threshold can be set based on a baseline value derived from historical data, and then increased by a preset percentage. The specific evolutionary activity threshold is determined by adjusting the threshold according to the security and economic requirements of the actual application scenario.

[0048] The above formulas are all dimensionless calculations. The formulas are derived from software simulations based on a large amount of collected data to obtain the most recent real-world results. The preset parameters in the formulas are set by those skilled in the art according to the actual situation.

[0049] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented in software, the above embodiments can be implemented, in whole or in part, as a computer program product. Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution.

[0050] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment, depending on actual needs.

[0051] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A method for in-situ non-destructive microscopic characterization of the compression failure mode of full-scale wood, characterized in that, The specific steps include: Step 1: Preset several different compression ratio target values, and conduct multi-level axial compression tests on full-scale wood samples in order of increasing compression ratio target values. After each level of axial compression test, use sandpaper of different grit to refine the observation section of the full-scale wood sample step by step until the observation section reaches a smooth state without burrs or scratches. Step 2: Set the low magnification range and the high magnification range. Within the low magnification range, perform full-area scanning imaging on the observation section of the polished full-size wood sample to obtain microscopic images at each low magnification. Then, identify the damage feature areas in the microscopic images to form an initial damage feature distribution map. Step 3: Based on the initial damage feature distribution map, scan the marked damage feature area within a high magnification range to generate several local high-magnification images. Obtain the relative displacement and fracture surface dip angle on both sides of the crack from each local high-magnification image, and calculate the local damage degree value of each local high-magnification image. Step 4: According to the next level of compression rate target value of the current compression rate target value, conduct the next level of axial compression test on the full-size wood sample. Repeat the above sanding, observation and calculation operations, record the local damage degree value of the next level of compression rate target value, and calculate the damage increment value of the local damage degree value relative to the previous level of compression rate target value. Based on the damage increment value, determine the changing trend of the failure mode of the full-size wood sample under each level of compression rate. The damage feature regions are identified in the microscopic image to form an initial damage feature distribution map. The specific steps are as follows: Define the types of failure characteristic regions in the wood compression observation section, specifically including crack propagation paths, fiber fracture zones, and interlaminar separation regions; Based on the full-domain microscopic image corresponding to the lowest magnification within the low magnification range, the distribution range of crack propagation path, fiber fracture zone and interlaminar separation region is initially identified. Then, in the order of progressively increasing magnification, the full-domain microscopic images of each gradient magnification are switched sequentially to refine and iterate the various damage feature regions initially identified. The detailed features of crack propagation direction, fiber fracture zone morphology and interlaminar separation region outline at different magnifications are corrected. Finally, based on the full-domain microscopic image of the highest magnification within the low magnification range, the boundary calibration of various damage feature regions is completed. The location of each failure feature region is determined based on the crack propagation trajectory line, fiber fracture boundary line and interlayer separation region boundary curve, and an initial failure feature distribution map containing multiple failure feature region types is generated. The specific steps for calculating the degree of local destruction in each high-magnification image are as follows: For the same region of damage features, extract the relative displacement and fracture surface dip angle corresponding to all image acquisition locations within the region; Calculate the average relative displacement at all sampling locations in the damaged feature area, and divide the average relative displacement by the extension length of the damaged feature area to obtain the average displacement per unit length. Calculate the average dip angle of the fracture surface at all sampling locations within the damage characteristic area, and obtain the tangent function value of this average dip angle; Multiply the average displacement per unit length of the same damage characteristic region by the tangent function value of the average dip angle to obtain the corresponding local damage degree value; The specific steps for determining the changing trend of the failure mode of full-size wood specimens at various compression rates based on the incremental failure value are as follows: After loading a full-scale wood sample to the preset next-level compression ratio target value and completing the acquisition of relative displacement and fracture surface inclination angle, calculate the local damage degree value of each damage characteristic region under the current compression ratio target value. Subtract the local damage degree value corresponding to the previous level of compression ratio target value from the local damage degree value corresponding to the current compression ratio target value to obtain the regional damage increment value corresponding to each level of compression loading. Record the proportion of the area of ​​all destruction feature regions with a destruction increment value greater than 0 to obtain the evolutionary activity level, and preset the evolutionary activity level threshold. When the evolutionary activity level is greater than the evolutionary activity level threshold, it is determined that the damage of the full-scale wood specimen is in the accelerated evolution stage, and the cumulative damage across the entire area is rapidly aggravated. When the evolutionary activity is greater than 0 and less than or equal to the evolutionary activity threshold, the full-scale wood sample is determined to be in the local evolution stage, with only local characteristic areas generating new cumulative damage and the overall destruction evolution being gradual. When the evolutionary activity level is equal to 0, it is determined that there is no new damage accumulation in each level of damage characteristic area, and the damage state of the full-scale wood sample has not been updated.

2. The in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood according to claim 1, characterized in that: Full-field scanning imaging was performed on the observation section of the polished full-size wood sample within the low magnification range to obtain microscopic images at various low magnifications. The specific steps are as follows: A preset magnification threshold is used to divide the low magnification range into a high magnification range, and an equal gradient magnification is set in each of the two magnification ranges. The specific steps for low-magnification scanning of the whole field of view imaging within the low magnification range are as follows: Place the polished full-size wood sample on the stage of the stereomicroscope, and conduct observation and imaging from the lowest magnification level within the low magnification range; during observation, first complete the initial focusing and imaging of the upper left corner vertex area of ​​the observation section, and then control the stage to move horizontally step by step according to the preset scanning path, focusing and imaging field by field of view, and the horizontal step distance is the result of multiplying the horizontal width of the field of view by the preset scaling factor; after the stage moves to the right end of the observation section, complete the vertical step by multiplying the vertical height of the field of view by the preset scaling factor, and then scan horizontally in the opposite direction, repeating the cycle until the full coverage imaging of the entire observation section is completed; All gradient magnifications within the low magnification range are scanned and imaged using a low-magnification scanning method to generate full-domain microscopic images of the observation cross-section at each low magnification.

3. The in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood according to claim 2, characterized in that: The destructive feature areas of the marker are scanned within a high magnification range to generate several local high-magnification images. The specific steps are as follows: Extract the geometric center coordinates of each damage feature region in the initial damage feature distribution map, arrange the geometric center coordinates in the order of the distribution of each damage region, generate the planned movement path of the stage, align the geometric center of each damage feature region with the center of the microscope optical axis in sequence and complete the focusing, so as to realize the positioning of all target damage regions. After completing the area positioning and alignment, switch the microscope magnification to the high magnification range and retrieve several preset gradient magnifications within the high magnification range; at each high magnification, with the center of the microscope optical axis as the reference point, and combined with the exclusive extension direction of different damage feature areas, move the field of view segment by segment according to the preset step distance and acquire images to obtain several local high magnification images. Among them, the extension direction of the crack propagation path is the direction of the crack propagation trajectory line, the extension direction of the fiber fracture zone is the direction of the long axis extension of the fracture surface boundary line, and the extension direction of the interlayer separation region is the direction of the maximum span of the separation region boundary curve.

4. The in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood according to claim 2, characterized in that: The relative displacement between the two sides of the crack is obtained from each local high-magnification image. The specific steps are as follows: Displacement measurements were performed on each local high-magnification image, and the relative displacement was extracted using the corresponding measurement method based on the type of damage feature region corresponding to the image. When the image corresponds to the crack propagation path in the damaged area, the edge lines on both sides of the crack are identified in the local high-magnification image. Using the pre-calibrated crack propagation trajectory line as a reference, measurement lines are laid out perpendicular to the crack propagation trajectory line. The measurement lines intersect with the edge lines distributed on both sides of the crack at the left edge measurement point and the right edge measurement point. The vertical distance between the left edge measurement point and the right edge measurement point is measured as the relative displacement on both sides of the crack. When the image corresponds to the damaged feature region as a fiber fracture zone, the gray-scale boundary line between the fracture surface and the unfractured surface is identified in the local high-magnification image and used as the fracture surface boundary line. Measurement lines are laid out along the direction perpendicular to the fracture surface boundary line. These measurement lines intersect the fracture surface boundary line and the unfractured surface boundary line at the first boundary measurement point and the second boundary measurement point, respectively. The vertical distance between the first boundary measurement point and the second boundary measurement point is measured and used as the relative displacement of the fiber fracture zone. When the image corresponds to the destructive feature region as the interlayer separation region, the upper and lower boundary lines of the interlayer separation region are identified in the local high-magnification image. Measurement lines are laid out along the direction perpendicular to the upper and lower boundary lines. The intersection of the measurement line with the upper boundary line is the upper boundary measurement point. The intersection of the measurement line with the lower boundary line is the lower boundary measurement point. The vertical distance between the upper boundary measurement point and the lower boundary measurement point is measured, and this vertical distance is taken as the relative displacement of the interlayer separation region. Traverse all local high-magnification images to obtain the relative displacement of each damaged feature region.

5. The in-situ non-destructive microscopic characterization method for the compression failure mode of full-scale wood according to claim 2, characterized in that: The dip angle of the fracture surface is obtained based on each local high-magnification image. The specific steps are as follows: The dip angle of each local high-magnification image was measured one by one, and the dip angle of the fracture surface was extracted by a differentiated measurement method according to the different damage feature regions corresponding to the images. When the image corresponds to the crack propagation path in the area of ​​damage features, the intersection line between the crack propagation direction and the observation section plane is identified in the local high-magnification image, and the tilt angle of the intersection line relative to the direction of the observation section plane is measured as the fracture surface dip angle at the corresponding position. When the image corresponds to a fiber fracture zone, the intersection line between the fracture surface boundary line and the observation section plane is identified in the local high-magnification image, and the tilt angle of the intersection line relative to the direction of the observation section plane is measured as the fracture surface dip angle at the corresponding position. When the image corresponds to the destructive feature region as the interlayer separation region, the intersection line between the plane where the interlayer separation region is located and the observation section plane is identified in the local high-magnification image. The tilt angle of the intersection line relative to the direction of the observation section plane is measured as the dip angle of the fracture surface at the corresponding position. Iterate through all local high-magnification images and obtain the fracture surface dip angle corresponding to each image acquisition location.

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