A dynamic beam method for testing the Poisson's ratio of wood

Abstract

The invention discloses a dynamic beam method for testing the Poisson's ratio of wood based on beam theory. The length of a square-section cantilever wood beam and a cantilever wood composite beam specimen is l. A 0° strain gauge and a 90° strain gauge are pasted on the central lines of two opposite surfaces of the square-section cantilever wood beam specimen (b / h=1) and the cantilever wood composite beam specimen (1≤b / h<3). The 0° strain gauges and the 90° strain gauges on the two surfaces are symmetrically arranged. The 0° strain gauge is closely attached to the 90° strain gauge. The spacing between the center point of the 90° strain gauge and the fixed support edge of the square-section cantilever wood beam and the cantilever wood composite beam specimen is l / 3. The strain spectrum or the vibration wave curve of the first-order bending vibration of the square-section cantilever wood beam and the cantilever wood composite beam specimen is tested at 0° and 90°. The Poisson's ratio is calculated through the spectrum or the vibration wave curve. The invention is simple to operate and has high testing accuracy. A square-section cantilever wood beam specimen can test the Poisson's ratios of wood in two principal directions.

Description

Technical Field

[0001] The invention relates to a method for testing the Poisson's ratio of wood, in particular to a dynamic beam method for testing the Poisson's ratio of wood. Background Art

[0002] There are two methods for statically testing the Poisson's ratio of wood: the axial tension method and the four-point bending beam method. The axial tension method for testing the Poisson's ratio is based on the definition of the Poisson's ratio, that is, the absolute value of the ratio of the transverse strain to the longitudinal strain of the specimen under the action of axial tension is defined as the material Poisson's ratio. The axial tension method is a classic method for testing the Poisson's ratio of a material based on the definition of the Poisson's ratio. It is applicable to both isotropic and anisotropic materials. There are no other special requirements for the location of the patch except to avoid it near the chuck of the testing machine. It should be made clear that during axial tension, only longitudinal tensile stress exists in the specimen, and the transverse stress is zero.

[0003] The four-point bending beam method for static testing of the Poisson's ratio of wood uses the absolute value of the ratio of the transverse strain to the longitudinal strain at a point near the center of the pure bending section when the beam is subjected to symmetrical four-point bending loading to test the Poisson's ratio of the material. Since the upper and lower surfaces of the beam specimen in the pure bending section are subjected to equal reverse stresses (tensile stress or compressive stress) during four-point bending loading, this method can more comprehensively and truly display the mechanical behavior of wood than the axial tension method, which is the advantage of the four-point bending beam method. However, according to the static analysis of the four-point bending beam, there is no point where the transverse stress is equal to zero in the pure bending section of the four-point bending beam. Although the transverse stress is very small, due to the anisotropy of wood, its longitudinal elastic modulus is an order of magnitude larger than the transverse elastic modulus, which results in the absolute value of the ratio of the transverse strain at the center point of the beam surface to the longitudinal strain test value -ε y / ε x There is a certain error with the reference value of Poisson's ratio. The test results show that two sets of half-bridge measurements are required to reduce the dispersion of Poisson's ratio of wood in the four-point bending beam method.

[0004] In early studies, the Poisson's ratio of wood and OSB was tested by tensile tests. In recent years, a patch method with a transverse stress of zero for dynamic and static testing of the Poisson's ratio of wood has been proposed based on the plate theory. This method is suitable for testing the Poisson's ratio of wood and OSB boards with a length-to-width ratio of 4-6 and a width-to-thickness ratio of 6-13.67. The test results show that the patch method with a transverse stress of σ y = 0 position, the absolute value of the ratio of the transverse strain gauge test value to the longitudinal strain gauge test value can be used as the test value of Poisson's ratio. The test also verifies the dynamic and static transverse stress σ y =0 patch method determines the patch position is different, but can test the same Poisson's ratio results. According to the dynamic and static test of the board theory of wood and OSB board Poisson's ratio of the transverse stress equal to zero patch method (abbreviated as σ y=0 patch method), due to the width-to-thickness ratio of the board, a board specimen can only test the Poisson's ratio of one principal direction of wood or OSB board. In addition, σ y =0 patch method has the disadvantage that the patch position for testing Poisson's ratio is related to the main direction of the wood and the size of the specimen, which is troublesome and inconvenient to implement. It is necessary to find a simple and accurate method for dynamically testing the Poisson's ratio of wood and wood composite panels. Summary of the invention

[0005] The technical problem to be solved by the present invention based on beam theory is to provide a dynamic beam method for testing the Poisson's ratio of a plate in view of the deficiencies of the above-mentioned prior art. The dynamic beam method for testing the Poisson's ratio of a plate is simple to operate and has high testing accuracy. A square-section cantilever wood beam specimen can test the Poisson's ratio of the wood in two principal directions.

[0006] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0007] A dynamic beam method for testing the Poisson's ratio of wood, wherein the wood specimens are square-section cantilever wood beam specimens and cantilever wood composite beam specimens, the lengths of the square-section cantilever wood beam specimens and the cantilever wood composite beam specimens are both l, and the method is characterized in that:

[0008] A 0° strain gauge and a 90° strain gauge are attached to the center lines of two opposite surfaces along the length direction of the wood specimen, wherein the positions of the 0° strain gauge and the 90° strain gauge on the same surface are as follows: the angle between the 0° strain gauge and the center line of the wood specimen is 0°, the angle between the 90° strain gauge and the center line of the wood specimen is 90°, the line connecting the center point of the 90° strain gauge and the center point of the 0° strain gauge coincides with the center line of the wood specimen, the 0° strain gauge is symmetrically attached to the 90° strain gauge, and the distance between the center point of the 90° strain gauge and the fixed support edge of the wood specimen is 1 / 3;

[0009] Tap the center line of the wood specimen at a distance of 0.5l-0.6l from the fixed support edge to stimulate the free vibration of the wood specimen;

[0010] Two 0° strain gauges and two 90° strain gauges are connected by half-bridge method, and dual-channel signal acquisition and analysis display the 0° strain and 90° strain spectrum or the vibration wave curve of the first-order bending vibration;

[0011] Read the linear spectrum amplitude of 0° strain and the linear spectrum amplitude of 90° strain at the first-order bending frequency from the spectrum; or, read the peak-to-peak value of 0° strain waveform and the peak-to-peak value of 90° strain waveform from the vibration wave curve of the first-order bending vibration to obtain the test value of Poisson's ratio;

[0012] The frequency domain Poisson's ratio test value of the wood specimen is:

[0013]

[0014] The time domain Poisson's ratio test value of the wood specimen is:

[0015]

[0016] As a further improved technical solution of the present invention, when the upper and lower surfaces along the length direction of the square-section cantilever wooden beam are LT-direction tangential sections, the surface along the length direction of the square-section cantilever wooden beam and perpendicular to the LT-direction tangential section is the LR-direction radial section; when the upper and lower surfaces along the length direction of the square-section cantilever wooden beam are RT-direction transverse sections, the surface along the length direction of the square-section cantilever wooden beam and perpendicular to the RT-direction transverse section is the RL-direction radial section; when the upper and lower surfaces along the length direction of the square-section cantilever wooden beam are TR-direction transverse sections, the surface along the length direction of the square-section cantilever wooden beam and perpendicular to the TR-direction transverse section is the TL-direction tangential section; that is, if three types of specimens containing the main directions LT, RT and TR of wood are made, the Poisson's ratios of the six main directions LT, LR, RT, RL, TR and TL can be dynamically tested.

[0017] As a further improved technical solution of the present invention, the length-to-thickness ratio of the square cross-section cantilever wooden beam specimen is: l / h=8-20, the width-to-thickness ratio is: b / h=1-1.5, and the width h=15-20mm; the length-to-thickness ratio of the cantilever wooden composite material beam specimen is: l / h=8-20, and the width-to-thickness ratio is: 1≤b / h<3.

[0018] As a further improved technical solution of the present invention, the distance between the center point of the 90° strain gauge and the fixed support edge of the wood specimen is 1 / 3, that is, the 1 / 3 span patch method, and the patch positions of the 0° strain gauge and the 90° strain gauge are independent of the tree species and the main direction of the wood, as well as the longitudinal or transverse orientation of the wood composite board beam specimen.

[0019] The beneficial effects of the present invention are:

[0020] The dynamic beam method for testing the Poisson's ratio of wood proposed by the present invention: a square cross-section cantilever wood beam specimen with a length-to-thickness ratio of 8-20 is used; 0° and 90° strain gauges are pasted on the central line of the cantilever beam surface at a distance of 1 / 3 from its fixed support edge (square cross-section cantilever beam 1 / 3 span patch method, referred to as 1 / 3 span patch method); the Poisson's ratio of the main direction of the wood is tested by the 0° and 90° strain spectrum or vibration wave curve of the first-order bending vibration of the beam specimen. The advantages of the 1 / 3 span patch method for dynamically testing the Poisson's ratio of wood, in addition to the simple test operation and high test accuracy, are that a square cross-section cantilever wood beam specimen can test the Poisson's ratio of the two main directions of wood, that is, if three types of specimens containing the main directions of wood, LT, RT and TR, are made, the Poisson's ratios of the six main directions, LT, LR, RT, RL, TR and TL, can be dynamically tested. In this regard, the dynamic beam method for testing the Poisson's ratio of wood based on beam theory is superior to the test method based on plate theory.

[0021] The invention is simple to operate and has high testing accuracy. A square-section cantilever wood beam specimen can test the Poisson's ratio of two main directions of wood. That is, if three types of specimens containing wood main directions LT, RT and TR are made, the Poisson's ratio of six main directions, LT, LR, RT, RL, TR and TL, can be dynamically tested; for wood composite materials, such as OSB and MDF, the width-to-thickness ratio of the beam specimen applicable to the 1 / 3 span patch method for dynamic testing of the Poisson's ratio can be relaxed to 1≤b / h<3. This relaxed condition has practical significance for testing the Poisson's ratio of wood composite materials, because the thickness of the commonly used wood composite board is 15mm-20mm, which is rare and difficult to achieve with a square-section cantilever beam specimen. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the cantilever beam coordinate system.

[0023] Figure 2 -ε for radial cantilever beams such as spruce, Scots pine and beech y / ε x -x / l curve graph.

[0024] Figure 3 It is a cantilever beam in three main directions: chord, radial and transverse. y / ε x -x / l curve.

[0025] Figure 4 European red pine chord cantilever beam-ε y / ε x -x / l curve.

[0026] Figure 5 For European Scots pine transverse cantilever beam -ε y / ε x -x / l curve.

[0027] Figure 6 (a) is a schematic diagram of the LT main direction specimen of a square cross-section timber beam.

[0028] Figure 6 (b) is a schematic diagram of the RT main direction specimen of a square cross-section timber beam.

[0029] Figure 6 (c) is a schematic diagram of the TR main direction specimen of a square cross-section timber beam.

[0030] Figure 7 The experimental block diagram for testing the cantilever beam spectrum and waveform.

[0031] Figure 8 This is the chord-wise (LT) spectrum of larch specimen No. 1.

[0032] Fig. 9This is the chord-wise (LT) waveform diagram of larch specimen No. 1.

[0033] Fig.10 This is the radial (LR) spectrum of larch specimen No. 1.

[0034] Fig.11 This is the TR spectrum of Sitka spruce specimen No. 1.

[0035] Fig.12 This is the TR waveform diagram of Sitka spruce specimen No. 1.

[0036] Fig.13 This is the TL spectrum of Sitka spruce specimen No. 3.

[0037] Fig.14 For OSB longitudinal and transverse cantilever beams -ε y / ε x -x / l curve graph.

[0038] Fig.15 MDF cantilever beam-ε y / ε x -x / l curve graph. DETAILED DESCRIPTION

[0039] The specific embodiments of the present invention are further described below according to the accompanying drawings:

[0040] In order to simplify the method of testing the Poisson's ratio of wood and improve its testing accuracy, this embodiment proposes a new method for dynamically testing the Poisson's ratio of wood based on beam theory, namely, the dynamic beam method. First, for three main directions (LT direction, LR direction and RT direction) of square cross-section cantilever beam specimens of spruce, Scots pine and beech, whose length-to-thickness ratios are 8, 10, 12, 16 and 20, the ANSYS modal program block is used to calculate their first-order bending modal stress and strain, and determine the absolute value of the ratio of the transverse strain to the longitudinal strain - ε y / ε x The variation curve of x / l along the center line of the cantilever beam surface, i.e. -ε y / ε x -x / l curve; -ε of square cross-section cantilever beam specimen y / ε x -x / l curve, the characteristic of changing in a straight line in the range of x / l = 0.2-0.7 and -ε on it y / ε xThe value is equal to the corresponding reference value of the Poisson's ratio of the wood in the principal direction, and a 1 / 3 span patch method for testing the Poisson's ratio of wood is proposed (the strain gauge is pasted at 1 / 3 of the distance from the cantilever end of the specimen). The dynamic beam method for testing the Poisson's ratio of wood proposed in this embodiment: a square cross-section cantilever wood beam specimen with a length-to-thickness ratio of 8-20 is used; 0° and 90° strain gauges are pasted at 1 / 3 of the fixed support edge on the center line of the cantilever beam surface (1 / 3 span patch method for square cross-section cantilever beam, referred to as 1 / 3 span patch method); the Poisson's ratio of the wood in the principal direction is tested by the 0° and 90° strain spectrum or vibration wave curve of the first-order bending vibration of the beam specimen. The 1 / 3 span patch method was used to test the Poisson's ratio of larch in the LT and LR directions, Sitka spruce in the TR and TL directions, and the longitudinal and transverse directions of LVL, and the effectiveness of the 1 / 3 span patch method for testing the Poisson's ratio of wood was verified by the four-point bending beam method and the axial tension method. The advantage of the 1 / 3 span patch method for dynamic testing of the Poisson's ratio of wood, in addition to its simple test operation and high test accuracy, is that a square cross-section cantilever wood beam specimen can test the Poisson's ratio of wood in two principal directions, that is, if three types of specimens containing wood in principal directions LT, RT and TR are made, the Poisson's ratio of six principal directions, LT, LR, RT, RL, TR and TL, can be dynamically tested. In this regard, the dynamic beam method for testing the Poisson's ratio of wood based on beam theory is superior to the test method based on plate theory.

[0041] A square cross-section beam specimen with a width-to-thickness ratio b / h=1, for example, 20 mm in thickness, has been used in the four-point bending beam method to statically test the E, μ and G of wood. This embodiment intends to use a square cross-section cantilever beam specimen with a 1 / 3 span patch method to dynamically test the principal direction Poisson's ratio of wood (dynamic beam method).

[0042] First, ANSYS calculates the first-order bending modal stress and modal strain of square-section cantilever beam specimens in three principal directions of spruce, Scots pine and beech with different length-to-thickness ratios to determine the -ε y / ε x The distribution law of -ε y / ε x -x / l curve; -ε of square cross-section cantilever beam specimen y / ε x -x / l curve, in the range of x / l = 0.2-0.7, shows a flat straight line change characteristic and -ε y / ε x The value is equal to the reference value of the Poisson's ratio of wood. A 1 / 3 span patch method is proposed to test the Poisson's ratio of wood in the principal direction. That is, the strain gauge is pasted on the center line of the cantilever beam specimen surface and at a distance of 1 / 3 from the cantilever end, where l is the overhanging length of the cantilever beam.

[0043] For a square cross-section (b / h=1) wooden beam specimen, if one surface along the length of the wooden beam is a chordal surface, then the surface perpendicular to it is a radial surface; or if one surface is an RT surface, then the surface perpendicular to it is an RL surface; or if one surface is a TR surface, then the surface perpendicular to it is a TL surface. Therefore, the 1 / 3-span patch method for dynamically testing the Poisson's ratio of wood based on beam theory proposed in this embodiment can test the Poisson's ratio of wood in two principal directions from a beam specimen. That is, if three types of specimens containing principal directions LT, RT and TR are made, the six principal directions of LT, LR, RT, RL, TR and TL can be dynamically measured. In this regard, the 1 / 3-span patch method for testing the Poisson's ratio of wood proposed in this embodiment is superior to the σ method for testing the Poisson's ratio of wood based on plate theory. y =0 patch method.

[0044] The main points of this embodiment are as follows:

[0045] The dynamic beam method for dynamic testing of the Poisson's ratio of wood in the principal direction is based on: the square cross-section cantilever beam has a first-order bending vibration and its surface center line -ε y / ε x The -x / l curve shows a flat linear change in the range of x / l = 0.2-0.7; -ε on the flat line y / ε x The value is equal to the reference value of the Poisson's ratio of the corresponding wood in the principal direction; the 0° and 90° strain gauges are pasted on the central line of the surface of the square-section cantilever beam, and 1 / 3 away from the cantilever end (i.e. 1 / 3 span patch method); it is applicable to square-section cantilever beams with an aspect ratio of 8-20 and a square section width (thickness) of 15mm-20mm; the width-to-thickness ratio can be relaxed to 1.5; a square-section cantilever beam can test the Poisson's ratio of wood in two principal directions; it is applicable to testing the Poisson's ratio of wood in six principal directions, that is, making LT, RT, TR specimens to test the Poisson's ratio of wood in six principal directions μ LT , μ LR , μ RT , μ RL , μ TR and μ TL .

[0046] For wood composite panels, i.e. cantilever wood composite beam specimens, such as OSB and MDF cantilever beam specimens, the first-order bending vibration stress-strain analysis shows that the width-to-thickness ratio of beam specimens applicable to the 1 / 3 span patch method for dynamic testing of Poisson's ratio can be relaxed to 1≤b / h<3. This relaxed condition is of practical significance for testing the Poisson's ratio of wood composite panels, because the commonly used thickness of wood composite panels is 15mm-20mm, which is rare and difficult to achieve with square cross-section cantilever beam specimens.

[0047] The dynamic beam method for testing Poisson's ratio has the advantages of simple test operation and high test accuracy.

[0048] 1. Methods:

[0049] 1.1. First-order bending modal stress and strain of cantilever beam:

[0050] The cantilever beam coordinate system is Figure 1 As shown, the beam length is l, the beam width is b, and the beam thickness is h.

[0051] The ANSYS 19 modal program block solid-shell 3D Finite strain 190 element is used to calculate the first-order bending modal stress and strain of the cantilever beam specimen. The mesh is 40×8×4, and the input material parameters are from wood science.

[0052] 1.1.1. First-order bending modal stress and strain of a square-section cantilever beam:

[0053] For radial square cross-section cantilever beam specimens (200mm×20mm×20mm) of spruce, European pine and beech, ANSYS is used to calculate the transverse stress σ during the first-order bending vibration. y = 0 and the absolute value of the ratio of transverse strain to longitudinal strain along the center line of the beam surface ( Figure 2 );

[0054] from Figure 2 See: radial square-section cantilever beams of beech, pine and spruce on the center line of the surface -ε y / ε x As x / l changes, it changes in a straight line within the range of x / l = 0.2-0.7, and -ε on the straight line y / ε x The values ​​are equal to the respective radial Poisson's ratios of 0.45, 0.42, and 0.37; Figure 2 The x / l=0.156, 0.237 and 0.413 marked in the figure are the positions where the transverse stress of the radial square cross-section cantilever beam specimens of beech, Scots pine and spruce is equal to zero. y / ε x The values ​​are also very approximately equal to their respective Poisson's ratios.

[0055] The first-order bending mode of the square cantilever beam specimen (200mm×20mm×20mm) in the chord direction, radial direction and transverse direction of European red pine -ε y / ε x -x / l curve Figure 3 As shown. Figure 3 It can be seen that for the cantilever beam specimens with equal radial and transverse square sections of European red pine, in the range of x / l = 0.2-0.8 -ε y / ε xBoth show a straight line change, and their values ​​are equal to the radial Poisson's ratio of European Scots pine 0.42 and the transverse Poisson's ratio of European Scots pine 0.68 respectively; for the square cross-section cantilever beam specimen in the chord direction of European Scots pine, in the range of x / l = 0.3-0.7, -ε y / ε x It also changes in a straight line, and its value is equal to the chord-wise Poisson's ratio of European red pine, which is 0.57.

[0056] 1.1.2. The first-order bending modal stress and strain of cantilever timber beams when the width-to-thickness ratio is 1, 2 and 3:

[0057] For the European red pine chord and transverse cantilever beam specimens (specimen length l = 180mm, specimen thickness h = 10mm, specimen width-to-thickness ratio b / h equal to 1, 2 and 3 respectively), the first-order bending mode -ε calculated by ANSYS 19 y / ε x -x / l curves are as follows Figure 4 and Figure 5 shown.

[0058] like Figure 4 As shown: For the European red pine chord cantilever beam specimen 180mm×10mm×10mm (b / h=1), the transverse stress σ y =0 position x / l=0.265, -ε y / ε x In the range of x / l = 0.2-0.7, it changes in a straight line, and its value is equal to the chord Poisson's ratio of European red pine 0.57; for the chord cantilever beam specimen of European red pine 180mm×20mm×10mm (b / h = 2), the transverse stress σ y =0 position x / l=0.385, -ε y / ε x In the range of x / l = 0.3-0.5, it shows a slight upward trend, and its value is slightly equal to the chord Poisson's ratio of European red pine 0.57; for the chord cantilever beam specimen of European red pine 180mm×30mm×10mm (b / h = 3), the transverse stress σ y =0 position x / l=0.467, -ε y / ε x In the range of x / l = 0.4-0.6, it shows a rapid upward trend, and its value deviates greatly from the chord Poisson's ratio of European red pine of 0.57. Therefore, the -ε of the first-order bending mode of the European red pine chord cantilever beam specimen is y / ε x The variation law with x / l is related to the width-to-thickness ratio of the cantilever beam specimen. For the European red pine chord-directed cantilever beam specimen with a width-to-thickness ratio of b / h = 1, the y / ε xIt changes in a straight line only in the range of x / l=0.2-0.7, and its value is equal to the reference value of the chord-wise Poisson's ratio of European red pine.

[0059] The same analysis is applied to the radial cantilever beam specimen of European Scots pine. When the width-to-thickness ratio b / h = 1, 2, 3, -ε y / ε x The variation pattern with x / l is the same as that of the chord-wise cantilever beam specimen.

[0060] For the transverse cantilever beam specimens of European red pine, when the width-to-thickness ratio b / h = 1, 2, 3, in the range of x / l = 0.2-0.6, -ε y / ε x The value is equal to the transverse Poisson's ratio of European Scots pine, which is 0.68. Therefore, when the width-to-thickness ratio b / h of the cantilever beam specimen changes from 1 to 3, the transverse -ε of the wood y / ε x The variation of -x / l is different from that of -ε in the tangential and radial directions of wood. y / ε x -The changing pattern of x / l.

[0061] In summary, the -ε of the cantilever beam specimens in the three principal directions of the wood with a width-to-thickness ratio b / h = 1 on its central line y / ε x The value changes in a straight line in the range of x / l = 0.2-0.7, and its value is equal to the reference value of the Poisson's ratio in the main direction of wood. Therefore, when testing the Poisson's ratio of wood, if a square cross-section beam specimen is used, the error caused by the inaccurate position of the patch can be reduced, thereby improving the accuracy of the test Poisson's ratio.

[0062] Considering the -ε on the central line of the surface of the square-section timber cantilever beam specimen y / ε x The value changes with the x / l straight line in the range of x = 0.2l-0.7l, and its value is equal to the reference value of the Poisson's ratio in the main direction of the wood ( Figure 2-Figure 4 ), so the Poisson's ratio of wood can also be tested by pasting the strain gauge at a distance of 1 / 3 from the cantilever end, that is, the 1 / 3 span patch method. The 1 / 3 span patch method is suitable for dynamic testing of the Poisson's ratio of wood in three principal directions: chordal, radial and transverse.

[0063] Make as Figure 6 For the square cross-section main-direction timber beam specimens shown in (a), (b), and (c), the planes perpendicular to the main-direction planes of LT, RT, and TR timber correspond to the main-direction planes of LR, RL, and TL timber, respectively. Figure 6 The three types of square cross-section main-direction timber beam specimens shown in (a), (b) and (c) can dynamically test the six main-direction Poisson's ratios of timber, μLT, μLR, μRT, μRL, μTR and μTL.

[0064] 1.3. Definition of Poisson's ratio:

[0065] Definition of Poisson's ratio in frequency domain:

[0066] The ratio of the 90° strain linear spectrum amplitude to the 0° strain linear spectrum amplitude at the first-order bending frequency of the cantilever beam specimen spectrum is:

[0067]

[0068] Definition of Poisson's ratio in time domain:

[0069] The absolute value of the ratio of the peak-to-peak value of the 90° strain waveform to the peak-to-peak value of the 0° strain waveform of the first-order bending vibration of the cantilever beam specimen is:

[0070]

[0071] 2. Test:

[0072] 2.1. Test piece:

[0073] Three beam specimens were cut from the larch chordal slab, with the following dimensions and main directions: larch LT direction (chordal direction), dimensions: 320mm×20mm×20mm; larch LR direction (radial direction), dimensions: 320mm×20mm×20mm.

[0074] Three beam specimens were cut from Sitka spruce, with the following dimensions and main directions: Sitka spruce TR direction, dimensions: 300mm×30mm×20mm; Sitka spruce TL direction, dimensions: 300mm×20mm×30mm.

[0075] Three beam specimens were cut longitudinally and transversely from the whole LVL board, with dimensions of 380mm×15.7mm×15.7mm.

[0076] 2.1. Patch solution:

[0077] Larch beam chordal surface (LT surface), 0° strain gauge and 90° strain gauge are pasted on the center of the upper and lower surfaces (chordal surface) of the beam, a total of 4 strain gauges;

[0078] On the radial surface of the larch beam, the 0° strain gauge and the 90° strain gauge are pasted at the center of the upper and lower surfaces (radial surface) of the beam, with a total of 4 strain gauges;

[0079] On the TR surface of Sitka spruce beam, 0° strain gauge and 90° strain gauge adhesive sheets are pasted on the center of the upper and lower surfaces (TR surface) of the beam, with a total of 4 strain gauges;

[0080] Sitka spruce beam TL surface, 0° strain gauge and 90° strain gauge are attached to the center of the upper and lower surfaces (TL surface) of the beam, with a total of 4 strain gauges.

[0081] In the longitudinal and transverse directions of the LVL, 0° strain gauges and 90° strain gauges are respectively pasted on the upper and lower surfaces of the beam specimen and the center of the adjacent side surfaces, for a total of 8 strain gauges;

[0082] It provides the four-point bending beam method and axial tension method for testing Poisson's ratio, as well as the dynamic beam method in this paper for testing Poisson's ratio.

[0083] There are 72 strain gauges in total. The strain gauge specifications are BX-120, the strain grid is 5mm×3mm, and the strain gauges are glued with 502 glue.

[0084] The specific positions of the 0° strain gauge and the 90° strain gauge on a certain surface of the above wood are as follows: the angle between the 0° strain gauge and the center line of the square-section cantilever beam is 0°, the angle between the 90° strain gauge and the center line of the square-section cantilever beam is 90°, the line connecting the center point of the 90° strain gauge and the center point of the 0° strain gauge is parallel to the center line of the square-section cantilever beam, the 0° strain gauge is set close to the 90° strain gauge, and the distance between the center point of the 90° strain gauge and the fixed support edge of the square-section cantilever beam is l / 3.

[0085] 2.3 Experimental design:

[0086] First, the Poisson's ratio is tested statically using the four-point bending beam method and the half-bridge method using strain gauges on the upper and lower surfaces of the beam; then, the Poisson's ratio is tested using the 1 / 3 span patch method; and finally, the Poisson's ratio is tested statically using the axial tension method.

[0087] The four-point bending beam method and the axial tension method are used as experimental verifications of the effectiveness of the 1 / 3 span patch method for testing the Poisson's ratio of wood proposed in this embodiment.

[0088] For the specimens subjected to static Poisson's ratio testing using the four-point bending beam method, the depth of the clamped specimen was adjusted so that the strain gauges pasted using the four-point bending beam method met the requirements of the 1 / 3 span patch method, and the Poisson's ratio of wood was tested dynamically.

[0089] 2.4、Test block diagram:

[0090] The 0° strain gauges and 90° strain gauges on the upper and lower surfaces of the beam are respectively connected by half-bridge method and dual-channel acquisition to test the spectrum and vibration wave curve of the first-order bending vibration of the cantilever beam. The test block diagram is as follows Figure 7 shown.

[0091] 2.5. Spectrum and waveform:

[0092] from Figure 8 Reading: The first-order bending frequency of the larch specimen No. 1 in the chord direction is 265 Hz, the linear spectrum amplitude of the 0° strain gauge is 6.15 με, and the linear spectrum amplitude of the 90° strain gauge is 2.43 με. From the frequency domain (i.e., formula (4)), the test value of the Poisson's ratio in the chord direction of the larch specimen is 0.395.

[0093] from Fig. 9 Readout: 21.29ms-23.24ms, the peak-to-peak value of the 0° strain gauge waveform is 42.2με, the negative peak-to-peak value of the 90° strain gauge waveform is -17.05με, and the Poisson's ratio test value calculated according to formula (5) is 0.404;

[0094] 36.72ms-38.48ms, the peak-to-peak value of the 0° strain gauge waveform is 35.11με, and the negative peak-to-peak value of the 90° strain gauge waveform is -14.01με. The Poisson's ratio test value calculated according to formula (5) is 0.399; the positive peak value of the 0° gauge corresponds to the negative peak value of the 0° gauge.

[0095] from Fig.10 Read out: The radial first-order bending frequency of larch specimen No. 1 is 320 Hz, the linear spectrum amplitude of the 0° strain gauge is 8.18 με, and the linear spectrum amplitude of the 90° strain gauge is 2.49 με. From the frequency domain (i.e., formula (4)), the chord-wise Poisson's ratio test value of the larch specimen is 0.304;

[0096] from Fig.11 Reading: The first-order bending frequency is 80Hz, the linear spectrum amplitude of the 0° strain gauge is 42.69με, the linear spectrum amplitude of the 90° strain gauge is 15.19με, and the TR-direction Poisson's ratio test value of Sitka spruce is 0.356;

[0097] from Fig.12 Reading: 38.67ms-45.31ms, the peak-to-peak value of the 0° strain gauge waveform is 172.68με, the negative peak-to-peak value of the 90° strain gauge waveform is -63.57με, and the Poisson's ratio test value calculated according to formula (5) is 0.368; 76.56ms-82.81ms, the peak-to-peak value of the 0° strain gauge waveform is 142.76με, the negative peak-to-peak value of the 90° strain gauge waveform is -51.72με, and the Poisson's ratio test value calculated according to formula (5) is 0.362; the positive peak value of the 0° gauge corresponds to the negative peak value of the 0° gauge.

[0098] from Fig.13 Reading: The first-order bending frequency is 67.5Hz, the linear spectrum amplitude of the 0° strain gauge is 6.91με, the linear spectrum amplitude of the 90° strain gauge is 0.17με, and the TL Poisson's ratio test value of Sitka spruce is 0.025.

[0099] from Figures 8 to 13 It is found that the Poisson's ratio with approximately the same value can be determined from both the frequency domain and the time domain; the time domain vibration wave curve is attenuated, and the positive peak value of the 0° sheet strain corresponds to the negative peak value of the 90° sheet strain, which means that the phase difference between the transverse strain and the longitudinal strain is 180°, that is, opposite, which has obvious physical significance; in addition, the damping ratio of the material can also be determined from the attenuated vibration wave curve.

[0100] 2.5 Confirmatory test:

[0101] 2.5.1 Symmetrical loading four-point bending beam method test:

[0102] The four-point bending test bench is designed by ourselves, loaded with weights, and can measure E and μ.

[0103] Larch specimen: span is 300mm, symmetrical four-point bending loading: initial load is 24.99N (2.55kg weight), ΔP=24.99N, secondary loading.

[0104] Sitka spruce specimen: span 240 mm, symmetrical four-point bending loading: initial load 4.165 N (0.425 kg weight), ΔP = 4.165 N, secondary loading.

[0105] The four-point bending test is performed with two sets of half-bridges to measure the 0° strain gauge and the 90° strain gauge. That is, after one set of three consecutive measurements, the specimen is turned 180° around the center axis of the beam for another set of three consecutive measurements. The average of the two sets of E,μ test values ​​is taken as the E,μ test value of the specimen.

[0106] 2.5.2 Axial tensile test:

[0107] Four-channel measurement, one specimen simultaneously measures two principal directions E and μ, 0° strain gauge, 90° strain gauge each occupies one channel. In order to eliminate the bending strain caused by load misalignment during stretching, the 0° strain gauge and 90° strain gauge on the two surfaces are connected in series, and the 1 / 4 bridge tests the strain in the 0° and 90° directions.

[0108] Microcomputer controlled electronic universal testing machine, model UTM 4304, produced by Shenzhen Sansi, with a maximum test force of 30kN.

[0109] The loading scheme is:

[0110] Larch specimens: 500N-2500N-4500N; Sitka spruce specimens: 100N-200N-300N. The axial tensile test of each specimen was carried out three times, and the average value of E,μ in the last two tests was taken as the parameter test value of the specimen.

[0111] 3. Results and analysis:

[0112] The Poisson's ratio test values ​​of larch in the tangential direction (LT) and radial direction (LR) and Sitka spruce in the TR direction and TL direction tested by the dynamic beam method (this embodiment), the four-point bending beam method and the axial tension method are shown in Table 1 (the percentage in the brackets is the coefficient of variation, the same below).

[0113] Table 1 shows the Poisson's ratio of larch and Sitka spruce tested by 1 / 3 span patch method, four-point bending beam method and axial tension method.

[0114]

[0115] From Table 1, we can see that the Poisson's ratios of larch in the tangential direction (LT) and radial direction (LR) and Sitka spruce in the TR direction and TL direction tested by the dynamic beam method, the four-point bending beam method and the axial tension method are quite consistent.

[0116] The specimen used for the 1 / 3 span patch method can test both the Poisson's ratio in the principal direction of wood and the elastic modulus in the principal direction of wood. As long as the square-section wood beam specimen is freely suspended with a rubber rope (rigidity coefficient is less than 1N / cm) to test the first-order bending frequency, the elastic modulus is calculated according to the relationship between the elastic modulus of the Euler beam and the first-order bending frequency. After testing the elastic modulus E, the 1 / 3 span patch method can be used to dynamically test the Poisson's ratio in the principal direction of wood.

[0117] For the LVL beam of 380 mm × 20 mm × 20 mm, the elastic modulus and Poisson's ratio of the LVL tested by the dynamic beam method, four-point bending beam method and axial tension method are shown in Table 2.

[0118] Table 2 shows the elastic modulus and Poisson's ratio of LVL tested by the dynamic beam method, four-point bending beam method and axial tension method.

[0119]

[0120] x-LVL whole board longitudinal direction (along the length); y-LVL whole board transverse direction (along the width); z-LVL whole board thickness direction.

[0121] 4 Conclusion:

[0122] 4.1. The absolute value of the ratio of the transverse strain to the longitudinal strain during the first-order bending vibration of a square-section cantilever beam - ε y / ε x In the range of x / l = 0.2-0.7 on the center line of the beam surface, it changes in a straight line. y / ε x The value is equal to the reference value of Poisson's ratio;

[0123] 4.2. The dynamic beam method is applicable to the dynamic testing of Poisson's ratio in the six principal directions of wood, namely LT, LR, RT, RL, TR and TL, for wood beams with a length-to-thickness ratio of 8-20 or 8-12.

[0124] 4.3, the validity of the Poisson's ratios in the LT and LR directions of larch, the TR and TL directions of Sitka spruce, and the longitudinal and transverse directions of LVL in the dynamic test of the 1 / 3 span patch method was verified by the four-point bending beam method and the axial tension method;

[0125] 4.4. Dynamic beam method Since the patch positions for testing the Poisson's ratio of wood are all on the 1 / 3 span of the square cross-section cantilever beam, this method is used to test the Poisson's ratio of wood in the principal direction, which has the advantages of simple operation and high test accuracy.

[0126] Appendix: First-order bending modal strain analysis of a wood composite cantilever beam specimen (i.e., cantilever wood composite beam specimen).

[0127] For the OSB longitudinal, OSB transverse and MDF cantilever beam specimens (specimen length l = 180 mm, specimen thickness h = 10 mm, specimen width-to-thickness ratio b / h of 1, 2 and 3 respectively), the -ε of the first-order bending mode calculated by ANSYS 19 is y / ε x -x / l curves are as follows Fig.14 and Fig.15 As shown. Fig.14 , OSB longitudinal Poisson's ratio = 0.34, OSB transverse Poisson's ratio = 0.13. Fig.15 , MDF Poisson's ratio = 0.25.

[0128] from Fig.14 and Fig.15 See: For cantilever beam specimens with width-to-thickness ratios b / h = 1, 2, and 3 for OSB longitudinal, OSB transverse, and MDF, -ε y / ε x -x / l curve has the same variation behavior. For b / h = 1, x / l = 0.1-0.7 -ε y / ε x The ratio is flat and -ε within this range y / ε x The value is equal to the reference value of the respective Poisson's ratio; for b / h = 2, x / l = in the range of 0.2-0.6 -ε y / ε x The ratio is flat and -ε within this range y / ε x The value is equal to the reference value of the respective Poisson's ratio; for x / l = 3, x / l = in the range of 0.3-0.5 -ε y / ε x The ratio is flat and -ε within this range y / ε x is equal to the reference value of the respective Poisson's ratio.

[0129] OSB and MDF cantilever beam specimens with width-to-thickness ratio b / h = 1, 2 and 3 -ε y / ε x The variation of x / l is similar to that of the wood in the tangential or radial direction ( Figure 2 ) has a different change pattern, but is similar to the transverse -ε y / ε x The same applies to the variation of x / l.

[0130] In this embodiment, the 1 / 3 collapse patch method is used to test the Poisson's ratio. The patch position is independent of the tree species and the main direction of the wood, as well as the longitudinal or transverse orientation of the wood composite board.

[0131] The protection scope of the present invention includes but is not limited to the above embodiments. The protection scope of the present invention shall be based on the claims. Any replacement, deformation, and improvement of the technology that can be easily thought of by technicians in this field shall fall within the protection scope of the present invention.

Claims

1. A dynamic beam method for testing the Poisson's ratio of wood. The wood specimens are square-section cantilever wood beam specimens and cantilever wood composite beam specimens. The lengths of the square-section cantilever wood beam specimens and the cantilever wood composite beam specimens are both l , characterized in that: 0.0000 is pasted on the center lines of the two opposite surfaces along the length direction of the wood specimen. o Strain gauges and 90 o strain gauges, where 0 o Strain gauges and 90 o The position of the strain gauge is: 0 o The angle between the strain gauge and the center line of the wood specimen is 0 o , 90 o The angle between the strain gauge and the center line of the wood specimen is 90 o , 90 o The center point of the strain gauge and 0 o The line connecting the center points of the strain gauge coincides with the center line of the wood specimen. o The strain gauge is symmetrically attached to the o Strain gauge setup, 90 o The distance between the center point of the strain gauge and the fixed edge of the wood specimen is l / 3; The center line of the wood specimen should be struck at a distance of 0.5 mm from the fixed edge. l -0.6 l At, the wood specimen is excited to vibrate freely; Two of them are 0 o Strain gauge and two 90 o The strain gauges are all connected by half-bridge method, and dual-channel signal acquisition and analysis display 0 o Strain and 90 o Strain spectrum or vibration wave curve of first-order bending vibration; Read the first bending frequency at 0 from the spectrum o Strain linear spectrum amplitude and 90 o Strain linear spectrum amplitude; Alternatively, read 0 from the vibration wave curve of the first-order bending vibration o Strain waveform peak-to-peak and 90 o The peak-to-peak value of the strain waveform is used to obtain the test value of Poisson's ratio.

2. The dynamic beam method for testing the Poisson's ratio of wood according to claim 1, characterized in that: When the upper and lower surfaces along the length direction of the square-section cantilever beam are LT-direction tangent sections, the surface along the length direction of the square-section cantilever beam and perpendicular to the LT-direction tangent section is the LR-direction radial section; when the upper and lower surfaces along the length direction of the square-section cantilever beam are RT-direction transverse sections, the surface along the length direction of the square-section cantilever beam and perpendicular to the RT-direction transverse section is the RL-direction radial section; when the upper and lower surfaces along the length direction of the square-section cantilever beam are TR-direction transverse sections, the surface along the length direction of the square-section cantilever beam and perpendicular to the TR-direction transverse section is the TL-direction tangent section; that is, if three types of specimens containing the main directions LT, RT and TR of wood are made, the Poisson's ratios of the six main directions LT, LR, RT, RL, TR and TL can be dynamically tested.

3. The dynamic beam method for testing the Poisson's ratio of wood according to claim 1 or 2, characterized in that: The length-to-thickness ratio of the square cross-section cantilever beam specimen is: l / h =8-20, width-to-thickness ratio: b / h =1-1.5, width h =15-20mm; the length-to-thickness ratio of the cantilever wood composite beam specimen is: l / h =8-20, width-to-thickness ratio: 1≤ b / h <3.

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