A method for testing damage effect of seawater coral reef sand concrete

Abstract

The present application belongs to the technical field of building materials, and relates to a seawater coral reef sand concrete damage effect test method. The present application determines the reference compressive strength through a uniaxial compression test, determines the driving parameters for formal impact by using a split Hopkinson pressure bar combined with pre-impact closed-loop calibration; based on the stress balance criterion, the effective waveform is screened to calculate the dynamic stress peak value, and the fragment dimension of the test piece is extracted; after normalization, the three are fused to calculate the damage effect index. The method effectively solves the problems of dynamic load mismatch, waveform distortion interference and single index difficulty in quantifying the irregular characteristics of crushing, realizes multi-dimensional comprehensive quantification of static reference, dynamic response and crushing form, and improves the repeatability of impact test data and the objective comparability of damage evaluation of different proportioning materials.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology and relates to a method for testing the damage effect of seawater coral reef sand concrete. Background Technology

[0002] Constrained by high transportation costs and supply difficulties over long distances, using seawater-mixed coral reef sand as a substitute for traditional concrete has become a key solution to the shortage of building materials on islands and reefs. Coral reef sand, due to its porous, brittle, sharp-edged, and low-strength nature, results in numerous micropores within the concrete, leading to a weak interface between the aggregate and the cement matrix. When such concrete structures are subjected to high strain rate loads such as explosions or high-speed impacts, their brittle characteristics and tendency to break are more pronounced than those of ordinary concrete, and their energy absorption and dissipation mechanisms are also more complex.

[0003] Given its unique failure mechanism, conventional static mechanical indicators or single dynamic parameters alone are no longer sufficient to meet the safety design requirements of island and reef protection projects in extreme environments. Therefore, it is urgent to conduct damage effect tests on this type of material.

[0004] In existing technologies, the testing of damage effects on seawater coral reef sand concrete largely relies on a two-bar high strain rate loading platform. However, this method has the following drawbacks in practical applications: existing methods typically set the power source driving parameters directly based on theoretical experience, ignoring the nonlinear effects of internal mechanical damping, elastic bar divergence effects, and differences in the static bearing capacity of the specimen itself on the actual loading amount. This results in the pulse load actually acting on the specimen surface often deviating from the preset target, leading to poor repeatability and cross-sectional comparability of the test data.

[0005] Traditional processes often directly extract the forward loading, reverse unloading, and penetration transmission pulses after acquiring the sensor's electrical signals for subsequent calculations. If the specimen experiences asymmetrical breakage or poor end-face contact during impact, it can easily introduce distorted waveforms and directly affect the reliability of the test results.

[0006] Existing assessment methods are mostly limited to describing physical quantities in a single dimension. For fragmentation groups generated after impact, traditional methods often employ simple mass sieving or macroscopic visual grading, which are insufficient to quantify the randomness of microcrack propagation and the irregularity of the fracture path. Furthermore, the damage resistance performance of specimens from different batches or with different mix proportions is difficult to objectively compare on the same scale, thus limiting the parameter optimization and engineering selection of this type of material in protective structure design. Summary of the Invention

[0007] In view of this, in order to solve the problems mentioned in the background art, a test method for the damage effect of seawater coral reef sand concrete is proposed.

[0008] The objective of this invention can be achieved through the following technical solution: a method for testing the damage effect of seawater coral reef sand concrete, comprising: conducting a uniaxial compression test on the test specimen, establishing a stress-strain curve, and determining the benchmark compressive strength based on the peak stress point of the curve.

[0009] Using a split Hopkinson pressure bar device, the target impact stress value was set according to the benchmark compressive strength and the impact bar parameters were configured. The opening of the air pressure valve was calibrated through a pre-impact test. After calibration, a formal impact test was conducted. The voltage signals of the strain gauges of the incident bar and the transmission bar were collected. After calibration and conversion, the incident wave, reflected wave and transmitted wave signals were extracted, and fragments of the specimen after the formal impact were collected.

[0010] Stress balance verification is performed based on incident wave, reflected wave and transmitted wave to obtain effective wave signal. Dynamic stress is calculated and dynamic stress peak value is extracted using effective wave signal. At the same time, the fractal dimension of the specimen fragments is obtained by morphological recognition.

[0011] Based on the baseline compressive strength, dynamic stress peak value, and fractal dimension, the damage effect index is obtained by normalization and calculation.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention introduces a pre-impact closed-loop calibration mechanism, sets the target impact stress value according to the benchmark compressive strength, adjusts the opening of the air pressure valve one by one and calculates the relative deviation between the incident wave equivalent stress and the target value until the deviation enters the allowable range. This method effectively solves the problem of dynamic load deviation caused by mechanical damping of equipment, rod dispersion effect and static difference of specimen, realizes the matching of actual impact stress and static strength of material, and improves the repeatability and lateral comparability of test data.

[0013] (2) This invention verifies the stress balance of the stress wave signal, calculates the absolute value of the difference between the superposition value of the incident wave and the reflected wave and the transmitted wave signal, and compares it with the preset deviation limit. Only waveforms that meet the balance condition are marked as valid wave signals. This method effectively solves the problem of distorted waveforms introduced by asymmetric breakage or poor end-face contact of the specimen, ensures that the dynamic stress peak calculation is based on real and reliable physical waveforms, and improves the reliability of the test conclusions.

[0014] (3) This invention obtains the fractal dimension by extracting the perimeter of the fragment outline and the projected area, and normalizes the benchmark compressive strength, dynamic stress peak value and fractal dimension respectively, taking the cube root of the three as the damage effect index. This method effectively solves the problem that traditional mass sieving methods are difficult to quantify the irregular characteristics of fragmentation, and realizes multi-dimensional comprehensive quantification of static strength, dynamic response and fragmentation morphology, providing a unified and comparable benchmark for the damage resistance performance of materials with different proportions. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a flowchart of a method for testing the damage effect of seawater coral reef sand concrete in this invention;

[0017] Figure 2 This is a flowchart illustrating the steps for obtaining the reference compressive strength in this invention.

[0018] Figure 3 This is a flowchart illustrating the specific steps of the pre-impact test in this invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0021] The following describes in detail, with reference to the accompanying drawings, a specific scheme for testing the damage effect of seawater coral reef sand concrete provided by the present invention.

[0022] Please see Figure 1 As shown, the implementation of the present invention includes S1 to S4: In order to solve the problem that the traditional single static strength index is difficult to accurately characterize the actual damage degree of seawater coral reef sand concrete under impact load due to the porous and brittle aggregate and large dispersion of mechanical properties, the present invention constructs a comprehensive testing mechanism that combines static benchmark strength, dynamic impact response and fractal characteristics of fracture morphology.

[0023] S1 is used to solve the problem of missing static reference. The stress-strain curve is established through uniaxial compression test and the peak stress point is used as the criterion to determine the reference compressive strength. This static reference serves as the reference zero point for subsequent dynamic loading. S2 is used to solve the problem of dynamic impact load mismatch and repeatability. The opening of the air pressure valve is calibrated through pre-impact test and relative deviation allowable range criterion to realize the control of dynamic impact load, ensuring that the actual stress matches the target value during formal impact. At the same time, incident wave, reflected wave and transmitted wave signals are collected and intercepted in time sequence to collect specimen fragments.

[0024] S3 addresses the issues of valid dynamic test data and quantification of fracture morphology. It filters valid wave signals using the absolute value criterion of stress balance difference, ensuring the validity of dynamic test data. Furthermore, it calculates the fractal dimension using the logarithmic fitting slope criterion of profile perimeter-projected area, achieving a joint characterization of dynamic strength and fracture characteristics. S4 addresses the incomparability of multi-source heterogeneous indices. It integrates three dimensions—minimum-maximum normalization and geometric mean—to form a single, comparable damage effect index. A higher index indicates more severe material damage under impact. Ultimately, this results in a quantitative assessment of the impact damage severity of coral reef sand concrete.

[0025] S1. Conduct a uniaxial compression test on the test piece to establish a stress-strain curve, and determine the reference compressive strength based on the peak stress point of the curve.

[0026] The aggregate in coral reef sand concrete is derived from marine organism remains, and is characterized by its porosity, high water absorption, and rough surface, resulting in a significantly higher dispersion in its mechanical properties compared to ordinary concrete. If conventional concrete specimen preparation processes are directly adopted, ignoring the pre-wetting of aggregates and mixing with seawater, the actual water-cement ratio will change due to water absorption by the aggregates during the molding process, causing the strength test results to deviate from the true values.

[0027] Therefore, by simulating the actual marine engineering environment, the collected coral reef sand aggregate was soaked in seawater. When its mass no longer increased, it was taken out and the surface water was wiped off with a damp cloth to make it saturated and surface dry. Seawater was used to mix it to eliminate the influence of aggregate water absorption on the hydration reaction of cementitious materials. The aggregate was then cast into a standard cylindrical specimen with a diameter to height ratio of 1:2.

[0028] Meanwhile, the flatness of the end face of the standard cylindrical specimen directly affects the uniformity of stress distribution during the compression test: if the end face is uneven, local stress concentration will lead to premature failure of the specimen, and the measured peak stress will be lower. Therefore, after demolding, a grinding machine is used to grind both ends of the specimen to control the elevation difference between the center point and the edge point of the end face of the specimen to not exceed one-thousandth of the specimen height.

[0029] Further, please refer to Figure 2 As shown, the steps for obtaining the benchmark compressive strength are as follows: S101, Place the qualified test piece at the center of the pressure plate of the compression testing device, apply an axial compressive load at a constant stress rate, and simultaneously collect load data and displacement data. Using a constant stress rate instead of displacement control is to maintain the quasi-static evolution of the initiation, propagation, and penetration of microcracks within the material, avoiding interference from the strength determination caused by the inertial effect of sudden changes in the loading rate.

[0030] S102. Divide the load data by the cross-sectional area of ​​the specimen to obtain the stress data, and divide the displacement data by the gauge length of the specimen to obtain the strain data.

[0031] S103. Construct a stress-strain curve showing the change of stress data with strain data. The peak stress point of this curve corresponds to the maximum stress that the specimen can withstand under uniaxial compression. At this point, the microcracks inside the material have propagated and penetrated, and the material loses its ability to continue bearing load. Based on this, the peak stress value is used as the benchmark compressive strength, representing the maximum axial compressive load that a standard specimen can resist per unit area under quasi-static loading conditions.

[0032] S2. Using a split Hopkinson pressure bar device, set the target impact stress value according to the benchmark compressive strength and configure the impact bar parameters. The opening of the air pressure valve is calibrated through a pre-impact test. After calibration, a formal impact test is conducted. The voltage signals of the strain gauges of the incident bar and the transmission bar are collected. After calibration and conversion, the incident wave, reflected wave and transmitted wave signals are extracted, and the fragments of the specimen after the formal impact are collected.

[0033] Given that the impact load amplitude of the split Hopkinson pressure bar device is directly related to the impact bar velocity, and that the impact bar velocity is driven by high-pressure gas, the opening degree of the pressure valve is a key parameter for controlling the velocity. If the pressure valve opening is set directly based on theoretical formulas, due to the nonlinearity of the gas drive system, the difference in the contact state between the pressure bar and the specimen, and the discreteness of the material itself, there is often a deviation between the actual impact stress acting on the specimen and the target value.

[0034] Therefore, by using a pre-impact closed-loop calibration mechanism to back-calculate the equivalent stress based on the measured incident wave amplitude and comparing it with the target value, the opening of the air pressure valve is adjusted successively until the deviation enters the allowable range, thereby ensuring that the impact stress borne by the specimen during the formal test matches the reference compressive strength.

[0035] In one specific embodiment, please refer to Figure 3 As shown, the specific steps of the pre-impact test are as follows: S201, the test piece is installed between the incident rod and the transmission rod of the split Hopkinson pressure bar device.

[0036] S202. Set the target impact stress value based on the reference compressive strength, taking 50% to 80% of the reference compressive strength as the initial value for pre-impact calibration. Select the material, length, and diameter of the impact rod based on the target impact stress value. The impact rod is made of the same material as the incident rod, with a length of 200mm to 400mm and a diameter consistent with the pressure rod.

[0037] It should be noted that the initial values ​​mentioned above are only used as calibration benchmarks. Because the wave impedance of the specimen is lower than that of the compression bar, the actual dynamic stress borne by the specimen needs to be determined through pre-impact closed-loop adjustment. The system corrects the opening of the pneumatic valve based on the relative deviation between the incident wave equivalent stress and the target value to compensate for energy attenuation during the impact process. After calibration, the peak dynamic stress during the actual impact is sufficient to induce brittle fracture, thereby ensuring the acquisition of fragments that meet the requirements for fractal dimension calculation.

[0038] S203. Adjust the opening of the air pressure valve to change the velocity of the impact rod and conduct a pre-impact test. After launching the impact rod, collect the voltage signal output by the strain gauge on the incident rod to obtain the voltage waveform as a function of time. The strain gauge is pre-attached to the middle of the incident rod and connected to the dynamic strain gauge via a Wheatstone bridge.

[0039] The voltage value at each sampling point in the voltage waveform is calibrated and converted into the corresponding strain value. All strain values ​​are arranged in chronological order to form the original strain waveform on the incident rod. The specific calibration and conversion method is as follows: multiply the voltage value by the bridge coefficient, and then divide by the product of the strain gauge sensitivity coefficient and the excitation voltage to obtain the strain value; where the bridge coefficient is determined according to the bridging method of the strain gauge, with a value of 2 for the half-bridge method and 4 for the full-bridge method.

[0040] S204. Extract the incident wave amplitude from the original strain waveform, i.e., the peak value of the first pulse. Convert the incident wave amplitude into the corresponding equivalent stress value. The conversion method is: multiply the incident wave amplitude by the elastic modulus of the compression bar, and then multiply by the ratio of the cross-sectional area of ​​the compression bar to the cross-sectional area of ​​the specimen. This equivalent stress value represents the nominal stress on the specimen end face, based on the stress amplitude carried by the incident wave, under the assumption that the specimen has not undergone significant deformation and the stress wave has not been reflected. It reflects the magnitude of the impact stress actually applied to the incident end of the specimen at the current pressure valve opening.

[0041] S205. Calculate the relative deviation between the equivalent stress value and the target impact stress value. If the absolute value of the relative deviation is less than or equal to 5%, the current air pressure valve opening is the final determined opening. If the relative deviation is greater than +5%, it indicates that the impact stress is too high, and the air pressure valve opening needs to be reduced. If the relative deviation is less than -5%, the opening needs to be increased. Each adjustment is 0.5 to 1 times the previous deviation, gradually approaching the target value. After adjustment, return to S203 and repeat the pre-impact test.

[0042] S206. If the number of adjustments reaches the preset maximum number, such as 5 times, and the relative deviation still does not enter the allowable deviation range, then stop the calibration and generate a calibration failure message.

[0043] Although the specimen did not break during the pre-impact process, microcracks may have formed internally, affecting subsequent test results. Therefore, after pre-impact calibration, a formal impact test was conducted using specimens prepared in the same batch that had not undergone any pre-impact. The air pressure valve opening was directly adopted from the value determined during the pre-impact test. The specific operation was as follows: The specimen, which had not undergone the pre-impact test, was installed between the incident rod and the transmission rod, and the air pressure valve opening determined during the pre-impact test was used as the air pressure setting value for the formal impact test. The impact rod was launched, and the voltage signals of the strain gauges on the incident rod and the transmission rod were simultaneously acquired. This air pressure setting value represents the optimal driving parameter that, under the current system conditions, allows the deviation between the target impact stress and the actual applied stress of the specimen to be controlled within the allowable range, ensuring the accuracy and repeatability of dynamic loading.

[0044] The raw strain waveform acquired during the formal impact test includes incident, reflected, and transmitted waves, as well as interference signals such as multiple reflections from the impact bar and reflections from the bar end boundary. To obtain an effective waveform, it is necessary to truncate the waveform using a physical time window based on the propagation time of the stress wave in the compression bar. The specific truncation method is as follows: Measure the distance from the center of the strain gauge on the incident bar to the impact end face of the incident bar. The wave velocity of the stress wave in the compression bar is approximately 5190 m / s for steel bars and approximately 5100 m / s for aluminum bars. Divide the above distance by the wave velocity to obtain the incident wave propagation time.

[0045] Centered on this propagation time, a preset duration, such as 100 μs, is extended forward and backward, defining the incident wave time window. Since the actual pulse width is typically 100 μs to 200 μs, extending by 100 μs centered on the propagation time can completely encompass the entire incident wave pulse. Waveform data within the incident wave time window is extracted from the original strain waveform of the incident rod and used as the incident wave signal. This incident wave signal characterizes the original compressive stress wave generated by the impact of the impact rod. A larger incident wave amplitude indicates a higher driving air pressure and higher applied impact energy; a smaller amplitude indicates a lower driving air pressure and lower impact energy.

[0046] Measure the distance from the center of the strain gauge on the incident rod to the contact surface between the incident rod and the specimen. Multiply this distance by 2 and divide by the wave velocity to obtain the propagation time of the reflected wave. Extend the propagation time of the reflected wave forward and backward by a preset time, and set it as the time window for the reflected wave.

[0047] Since the stress wave pulse has a certain width, extending it for a preset time ensures complete capture of the entire reflected wave pulse. Waveform data within the reflected wave time window is extracted from the original strain waveform of the incident rod and used as the reflected wave signal.

[0048] The reflected wave signal characterizes the tensile wave reflected from the incident wave at the end face of the specimen due to the difference in wave impedance. The larger the amplitude of the reflected wave, the greater the difference in wave impedance at the end face of the specimen, and the more significant the deformation and damage of the material during dynamic loading. The smaller the amplitude, the better the wave impedance matching and the less severe the damage.

[0049] The distance from the center of the strain gauge on the transmission rod to the contact surface between the transmission rod and the specimen is measured. This distance is divided by the wave velocity to obtain the transmission wave propagation time. A preset time window is defined, extending forward and backward from the transmission wave propagation time. Waveform data falling within this time window is extracted from the original strain waveform of the transmission rod as the transmission wave signal. This transmission wave signal characterizes the compression wave that penetrates the specimen and enters the transmission rod. A larger amplitude indicates a stronger stress wave penetrating the specimen, and a higher dynamic stress on the specimen; a smaller amplitude indicates a lower dynamic stress.

[0050] The preset duration should be greater than half the width of the stress wave pulse, typically ranging from 50 μs to 200 μs, depending on the length of the impact rod. For example, if the impact rod is 300 mm long and generates a stress wave pulse width of approximately 120 μs, then the preset duration can be set to 100 μs.

[0051] S3. Based on incident wave, reflected wave and transmitted wave, stress balance verification is performed to obtain effective wave signal. The effective wave signal is used to calculate dynamic stress and extract dynamic stress peak value. At the same time, the specimen fragments are identified by morphology to obtain fractal dimension.

[0052] The fundamental premise of the split Hopkinson bar test is that the specimen must be in stress equilibrium, meaning that the stress at the incident and transmitted ends should be approximately equal during dynamic loading. If the stress is unbalanced, it indicates the presence of inertial or wave propagation effects within the specimen, and the calculated dynamic stress will not accurately reflect the material's constitutive response. Furthermore, the irregularity of the impact fragments directly reflects the material's brittle fracture and energy dissipation characteristics, necessitating the introduction of geometric quantification indicators for characterization.

[0053] Therefore, stress balance screening is carried out by establishing a mechanism for comparing the absolute values ​​of amplitude differences between incident waves, reflected waves, and transmitted waves. Dynamic stress peaks are extracted based on the conversion of transmitted wave amplitude and cross-section. Fractal dimension is extracted by image morphology recognition and logarithmic fitting of fragments.

[0054] In one specific embodiment, the method for obtaining the effective wave signal is as follows: calculate the algebraic sum of the incident wave signal and the reflected wave signal at the same moment as the superposition value, and calculate the absolute value of the difference between the superposition value and the transmitted wave signal; the absolute value of the difference represents the degree of difference between the instantaneous stress at the incident end and the transmitted end of the specimen. The smaller the difference, the closer the stress at both ends is, and the more the equilibrium condition is satisfied.

[0055] The absolute value of the difference is compared with a preset deviation limit, which is typically 5% of the peak value of the transmitted wave. If the absolute value of the difference is less than or equal to the preset deviation limit, it indicates that the stress difference between the two ends of the specimen is within the allowable range, and the inertial effect is negligible. Therefore, the stress equilibrium condition is satisfied, and the incident wave signal, reflected wave signal, and transmitted wave signal that satisfy the stress equilibrium condition are marked as effective wave signals. This effective wave signal represents the effective physical waveform that truly reflects the dynamic mechanical behavior of the material after removing the interference of inertial effects.

[0056] Furthermore, in order to comprehensively quantify the damage degree of concrete under impact from two dimensions—macroscopic mechanical response and microscopic fragmentation morphology—the peak dynamic stress and fractal dimension were extracted.

[0057] It should be clarified that, given the established stress balance verified by incident and reflected waves, according to one-dimensional stress wave theory, the actual dynamic stress borne inside the specimen is physically equivalent to the stress output from the transmission end. At this point, the incident and reflected waves have completed data validity verification; the transmitted wave directly penetrates the specimen and propagates unidirectionally within the transmission rod, effectively avoiding the superposition interference of multiple reflections at the incident rod end, material dispersion effects, and high-frequency oscillations of the strain gauge, resulting in higher waveform purity and less phase lag. Therefore, this embodiment preferably uses the transmitted wave signal from the effective wave signal to calculate the peak dynamic stress.

[0058] In one specific embodiment, the method for obtaining the dynamic stress peak value is as follows: In the time series of the effective wave signal, the amplitude of the transmitted wave signal at each sampling point is extracted. This amplitude is multiplied by the elastic modulus of the compression bar material, and then multiplied by the ratio of the compression bar cross-sectional area to the specimen cross-sectional area to obtain the dynamic stress corresponding to that moment. The dynamic stress is calculated for all moments throughout the entire time range of the transmitted wave signal, and the maximum value is taken as the dynamic stress peak value. This peak value represents the maximum compressive stress level generated inside the specimen during impact loading. The larger the value, the more severe the stress response experienced by the specimen under dynamic load, and the more complete the internal microcrack propagation and energy dissipation.

[0059] The fractal dimension is obtained as follows: all specimen fragments collected after the formal impact test are laid flat on a white background, and images of the fragments are acquired using a high-resolution digital camera or scanner. Image processing software is used to perform binarization processing, identify the connected components of each fragment, and extract the contour boundary of each fragment.

[0060] For each fragment, calculate its projected area and perimeter. Treat the projected area and perimeter of each fragment as a pair of data points, and take the common logarithm for all data points. Using the logarithm of the projected area as the x-axis and the logarithm of the perimeter as the y-axis, perform a linear fit using the least squares method to obtain the slope of the fitted line. Take twice the slope as the fractal dimension.

[0061] The fractal dimension is a dimensionless number between 1 and 2: the closer the fractal dimension is to 1, the smoother the fragment outline, the more regular the shape, and the lower the degree of fragmentation. The closer the fractal dimension is to 2, the rougher the fragment boundaries, the more irregular the shape, and the higher the degree of fragmentation, meaning that more energy was absorbed during the impact.

[0062] S4. Based on the benchmark compressive strength, dynamic stress peak value, and fractal dimension, the damage effect index is obtained by normalization and calculation.

[0063] Because the dimensions and numerical magnitudes of the reference compressive strength, dynamic peak stress, and fractal dimension are different, and the reference compressive strength is inversely related to the degree of damage, while the dynamic peak stress and fractal dimension are positively related to the degree of damage, different normalization directions are adopted.

[0064] Specifically, the dynamic stress peak value and fractal dimension are subjected to forward minimum-maximum normalization, while the reference compressive strength is subjected to reverse minimum-maximum normalization. The minimum and maximum values ​​of the reference compressive strength can be obtained through statistical analysis of a large number of previous experiments; the minimum and maximum values ​​of the dynamic stress peak value are also set based on statistics; the minimum value of the fractal dimension is 1, and the maximum value is 2.

[0065] Then, the three normalized values ​​are multiplied together, and the cube root of the product is taken as the damage effect index. The closer the damage effect index is to 1, the lower the static reference strength of the specimen under impact load, the stronger the dynamic stress response, and the more complex the fragment morphology, that is, the more severe the overall damage to the material. The closer the index value is to 0, the higher the static strength of the material, the weaker the impact response, the slighter the fragmentation, and the better the impact resistance.

[0066] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.

[0067] Those skilled in the art will recognize that the algorithmic steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in 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. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this application.

[0068] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0069] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations 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. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0070] Finally, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for testing the damage effect of seawater coral reef sand concrete, characterized in that, include: A uniaxial compression test is performed on the test piece to establish a stress-strain curve, and the reference compressive strength is determined by the peak stress point of the curve. Using a split Hopkinson pressure bar device, the target impact stress value was set according to the benchmark compressive strength and the impact bar parameters were configured. The opening of the air pressure valve was calibrated through a pre-impact test. After calibration, a formal impact test was conducted. The voltage signals of the strain gauges of the incident bar and the transmission bar were collected. After calibration and conversion, the incident wave, reflected wave and transmitted wave signals were extracted, and fragments of the specimen after the formal impact were collected. Stress balance verification is performed based on incident wave, reflected wave and transmitted wave to obtain effective wave signal. Dynamic stress is calculated and dynamic stress peak value is extracted using effective wave signal. At the same time, the fractal dimension of the specimen fragments is obtained by morphological recognition. Based on the baseline compressive strength, dynamic stress peak value, and fractal dimension, the damage effect index is obtained by normalization and calculation.

2. The method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The method for obtaining the test piece is as follows: Coral reef sand aggregate was pre-wetted to a saturated surface-dry state, mixed with seawater, and cast into standard cylindrical specimens. The end face of the specimen is processed to ensure that the elevation difference between the center point and the edge point of the end face does not exceed one-thousandth of the specimen height.

3. The method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The method for obtaining the reference compressive strength is as follows: The test specimen is placed in the center of the pressure plate of the compression test device, and an axial compression load is applied at a constant stress rate, while load data and displacement data are collected simultaneously. The stress data is obtained by dividing the load data by the cross-sectional area of ​​the specimen, and the strain data is obtained by dividing the displacement data by the gauge length of the specimen. A stress-strain curve is constructed to show the change of stress data with strain data, and the stress value corresponding to the peak stress point of the curve is used as the reference compressive strength.

4. The method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The specific contents of the pre-impact test are as follows: The test specimen is installed between the incident rod and the transmission rod of the split Hopkinson pressure bar device; The target impact stress value is set according to the benchmark compressive strength, and the impact rod parameters are configured according to the target impact stress value; Adjust the opening of the air pressure valve, conduct a pre-impact test, collect the voltage signal output by the strain gauge on the incident rod, and obtain the original strain waveform on the incident rod after calibration and conversion; Extract the incident wave amplitude from the original strain waveform and convert the incident wave amplitude into the corresponding equivalent stress value; Calculate the relative deviation between the equivalent stress value and the target impact stress value. If the relative deviation exceeds the allowable deviation range, increase or decrease the opening of the air pressure valve according to the sign of the relative deviation and return to perform the pre-impact test. If the relative deviation enters the allowable deviation range, mark the current air pressure valve opening as the final determined air pressure valve opening. If the number of adjustments reaches the preset maximum and the relative deviation still does not fall within the allowable deviation range, a calibration failure message will be generated.

5. A method for testing the damage effect of seawater coral reef sand concrete as described in claim 4, characterized in that, After calibration, a formal impact test is conducted, including: The test piece that has not undergone pre-impact testing is installed between the incident rod and the transmission rod, and the air pressure valve opening determined by the pre-impact test is used as the air pressure setting value for the formal impact test.

6. The method for testing the damage effect of seawater coral reef sand concrete as described in claim 4, characterized in that, The method for obtaining the incident wave, reflected wave, and transmitted wave signals is as follows: Divide the distance from the center of the strain gauge to the impact end of the incident rod by the wave velocity of the stress wave in the incident rod to obtain the incident wave propagation time. Extend the incident wave propagation time forward and backward by a preset time, and set it as the incident wave time window. Extract the waveform data within the incident wave time window from the original strain waveform of the incident rod as the incident wave signal. Multiply the distance from the center of the strain gauge to the contact end of the incident rod and the specimen by 2, and then divide by the wave velocity of the stress wave in the incident rod to obtain the propagation time of the reflected wave. Extend the propagation time of the reflected wave forward and backward by a preset time, and set it as the reflection wave time window. Extract the waveform data within the reflection wave time window as the reflection wave signal. Divide the distance from the center of the strain gauge to the contact end of the transmission rod and the specimen by the wave velocity of the stress wave in the transmission rod to obtain the transmission wave propagation time. Extend the transmission wave propagation time forward and backward by a preset time, and set it as the transmission wave time window. Extract the waveform data within the transmission wave time window from the original strain waveform of the transmission rod as the transmission wave signal.

7. The method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The method for obtaining the effective wave signal is as follows: Calculate the algebraic sum of the incident wave signal and the reflected wave signal as the superposition value, and calculate the absolute value of the difference between the superposition value and the transmitted wave signal; The absolute value of the difference is compared with the preset deviation limit. If the absolute value of the difference is less than or equal to the preset deviation limit, the stress balance condition is determined to be met. The incident wave signal, reflected wave signal and transmitted wave signal that meet the stress balance condition are marked as effective wave signals.

8. The method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The method for obtaining the dynamic stress peak value is as follows: The transmitted wave signal is extracted from the effective wave signal, and its amplitude is multiplied by the elastic modulus of the compression bar material, and then multiplied by the ratio of the cross-sectional area of ​​the compression bar to the cross-sectional area of ​​the specimen to obtain the dynamic stress. The maximum value is extracted as the peak dynamic stress.

9. A method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The method for obtaining the fractal dimension is as follows: Identify the contour boundaries of specimen fragments and extract the contour perimeter and projected area; Take the logarithm of the perimeter of the contour and the projected area respectively, and perform linear fitting with the logarithm of the projected area as the x-axis and the logarithm of the perimeter of the contour as the y-axis to obtain the slope of the fitted line. Take twice the slope as the fractal dimension.

10. A method for testing the damage effect of seawater coral reef sand concrete as described in claim 1, characterized in that, The method for obtaining the damage effect index is as follows: The benchmark compressive strength is subjected to reverse minimum-maximum normalization, and the dynamic stress peak and fractal dimension are subjected to forward minimum-maximum normalization respectively. The cube root of the product of the normalized baseline compressive strength, peak dynamic stress, and fractal dimension is calculated as the damage effect index.

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