Failure Modes Of Specimens Research Articles

Influence of curing conditions on the interface bonding properties of CFRP plate–steel

To understand the influence of different curing conditions on the interface bonding performance of CFRP-reinforced steel plates, tensile tests were conducted on epoxy resin dog bone specimens and CFRP-reinforced steel plate double lap specimens. The study explored the tensile performance of dog bone specimens under different curing temperature/time conditions and the mechanical properties of double lap specimens under different curing pressures and curing temperature/time conditions. The results indicate that increasing the curing temperature can shorten the curing time of dog bone specimens and also improve their elastic modulus and tensile strength. Elevating the curing pressure, curing temperature, and extending the curing time cause a transition in the failure mode of the double lap specimens from surface fiber stripping of the CFRP plate to a mixed failure dominated by surface fiber stripping and adhesive–steel interface debonding, and the bearing capacity, interface ultimate slip, and peak shear stress of the specimens significantly increase, with the shear stress concentration zone gradually expanding from the loading end towards the free end, simultaneously, the ductility of the specimen increased, and the failure process was delayed. The ultimate load, interface peak shear stress, and ultimate slip of the specimens cured at 90 °C are significantly higher compared to the control group cured at 20 °C and 0.01 MPa. However, with increasing curing time, the rate of growth shows a decreasing trend.

Pull-out test of hybrid connection in steel-UHPC composite slab

This paper presents an experimental study on tensile behavior of steel and ultra-high performance concrete (UHPC) composite structure connected by different connections including epoxy resin adhesive, headed stud, and hybrid of the former two. Thirty-two pull-out tests are conducted by taking the connection type, specimen size and steel fiber content of UHPC as the main parameters. The influences of these main parameters on the failure mode, load-separation behavior, and ultimate resistance of connection are researched. The failure modes of specimens with hybrid connection are UHPC interface debonding and UHPC cone failure, which is identical with the epoxy resin adhesive connection and headed stud connection, respectively. The tensile resistance ratio of headed stud connection to epoxy resin adhesive connection has a significant influence both on the separation performance and tensile resistance of hybrid connection. An empirical equation for predicting the tensile resistance of hybrid connection was proposed by taking the tensile resistance ratio of headed stud to epoxy resin adhesive. The ratio of predicted to test result was 0.973with standard deviations of 0.070. The pull-out tests and analysis of the hybrid connection in steel-UHPC composite structure were still needed to study to get it improved.

Fracturing behavior and mechanism of flawed disc specimens under compressive loading using geometry‐constraint‐based nonordinary state‐based peridynamics

AbstractA geometry‐constraint‐based nonordinary state‐based peridynamic (GC‐NOSBPD) model is developed to investigate the fracture characteristics of flawed brittle disc specimens under compressive loading as well as its fracturing mechanism. The bond‐associated horizon (BAH) size is directly defined based on the kinematic constraint (KC). This proposed model can well mitigate the numerical oscillation. The failure criteria based on the bond‐associated stress state are utilized to simulate the fracture of materials. Two stress effects can offer an insight into the fracturing mechanism of flawed specimens. The failure modes of flawed specimens are controlled by the tensile failure, and the crack growth trajectories acquired by the present numerical model are equivalent to those obtained by the experimental observations. The effects of size and crack inclination angles on the fracture toughness of brittle materials are assessed. The GC‐NOSBPD model is competent for evaluating the fracture damage of flawed brittle materials and understanding its failure mechanism.

Axial compressive behavior of reinforced hollow circular high strength concrete filled steel tubular short columns

This study aims to investigate the mechanical behavior of reinforced hollow circular high-strength concrete-filled steel tubular (RHCFST) short columns under axial compression. The axial compression test of 6 RHCFST short columns was carried out, and the bearing capacity, ductility and failure modes of the columns were analyzed. The results revealed that the failure modes of RHCFST short columns primarily included mixed failure, shear failure, and crushing failure. Comparatively, the failure modes of specimens with ordinary reinforcement exhibited a shift towards mixed failure, significant improvement in ductility, and a decrease in the increase trend of ultimate bearing capacity with the growth of steel tube thickness. Based on the test results, finite element models were developed using ABAQUS software for parameter analysis. The results demonstrated a positive correlation between the short column’s bearing capacity and parameters such as steel strength, sandwich concrete strength, steel tube thickness, and ordinary reinforcement diameter. Additionally, the initial stiffness of the short column increased with the increase of steel tube thickness. Considering the mechanical properties and engineering applicability of the member, the recommended design of steel tube yield strength, sandwich concrete strength, steel tube thickness and ordinary reinforcement diameter were given. Furthermore, the axial bearing capacity calculation formulae for RHCFST short columns were proposed. The calculated values were found to closely match both experimental and simulated values, with errors falling within 10 %. Consequently, the formulae can offer a reliable prediction of the member’s axial bearing capacity.

Experimental investigation on bond-slip performance and damage evolution mechanism of deformed reinforcing bar embedded in steel polyvinyl-alcohol hybrid fiber high performance concrete after high temperature

Hybrid Fiber High Performance Concrete (HFHPC) utilizes the synergistic effects of macro and micro fibers at different stages, which can effectively improve its mechanical properties, fracture toughness, ductility, and impact resistance after high temperature. The bonding between steel bars and hybrid concrete is a fundamental prerequisite for ensuring that these two materials with completely different mechanical properties work together. In order to study on bond-slip performance and damage evolution mechanism of deformed reinforcing bar embedded in steel-polyvinyl-alcohol (S-PVA) hybrid fiber high performance concrete (HFHPC) after high temperature, a total of 75 specimens were designed and manufactured by selecting volume fraction of steel fiber and PVA fiber and dosing of slag powder as the orthogonal test factors, and the central pull-out test were completed after exposure to room temperature, 200℃, 400℃, 600℃, and 800℃. The experimental results indicate that the bonding failure mode of HFHPC specimens all showed steel bar pull-out failure after high temperature, and the bond stress-slip curve can be divided into 4 stages: elastic rise stage, nonlinear rise stage, plastic degradation stage, and residual stage. As the temperature increases, the bonding strength of HFHPC specimens gradually decreases and the peak slip gradually increases. The addition of hybrid fibers can significantly improve the bonding ductility and energy dissipation capacity of deformed reinforcing bar embedded in S-PVA HFHPC after high temperature. When the temperature is below 200℃, the bonding strength damage rate of S-PVA HFHPC specimens is relatively slow; when the temperature exceeds 200℃, the bonding strength damage rate of S-PVA HFHPC specimens increases.

Characterization of steel T-stub compressing timber parallel to the grain component

Although there are several steel-to-timber joint types, there is still a lack of studies that more precisely characterize the components in the interaction between steel and timber in compression. Thus, this work aims to characterize the steel T-stub compressing the glued laminated timber (GLT) parallel to the grain component, focusing on formulating analytical expressions for design resistance, stiffness coefficient and deformation capacity. In order to validate the analytical models and increase the reliability of the component method, an experimental campaign was conducted through compression tests on 90 specimens composed of steel T-stubs with five different flange thicknesses under full and partial compression. It was identified that although GLT has lower compressive strength and lower stiffness than steel, the main failure mode in specimens studied was T-stub flange yielding. The proposed formulations were consistent with the experimental results and it can be used to predict the structural behavior of this component. The application of the component analytical variable elastic approach to a semi-rigid connection showed an increase in the accuracy for predicting its moment resistance.

Evolution of LWAC–OC interfacial properties during sulfate dry–wet cycles: Bonding strength and micro–properties

The interface properties between lightweight aggregate concrete (LWAC) and ordinary concrete (OC) have drawn significant research interest. However, its evolution during sulfate dry–wet cycles has yet to be elucidated. Therefore, the LWAC–OC composite specimens were subjected to 15 cycles of dry and wet cycles test, using 5 wt% sodium sulfate solution. The dry–wet cycling method consisted of complete immersion for 7 days, followed by natural drying for 8 days. Moreover, tests for splitting tensile, direct shear, scanning electron microscopy and microhardness were conducted on the LWAC–OC interfaces at varying numbers of dry and wet cycles. The results showed that the failure mode of specimens changed from mixed cohesive failure to interfacial bonding failure during sulfate dry–wet cycles. Direct shear and splitting tensile strength slightly increased and then markedly decreased as the number of dry–wet cycles increased. The ratio of direct shear strength to splitting tensile strength showed a distinct negative linear relationship with the number of sulfate dry–wet cycles. Moreover, when the number of dry–wet cycles increased, the porosity of interfacial transition zone first decreased and then increased. The mean value of the interfacial micro–hardness first increased and then decreased. Interfacial micro–properties correlated well with bonding strength throughout the process.

Experimental studying the effect of transverse reinforcement passing through perfobond holes on the structural behavior of perfobond shear connection

The push-out test program was carried out on three groups of nine small specimens to investigate the effect of the transverse reinforcement passing through the perfobond holes on the structural behavior of perfobond shear connector. The number of transverse reinforcements passing through the perfobond holes was changed in each group. One of the groups had no transverse reinforcement, and the other groups had one and two transverse reinforcements passing through the perfobond holes. The quantities observed in this program were the load capacity, the relative slip between the hot-rolled steel and the concrete slab, and the failure mode of specimens. The results showed that the transverse reinforcement passing through the perfobond holes enhanced the load capacity and significantly reduced the relative slip between the hot-rolled steel and the concrete slab.

Post-Fire Mechanical Properties of Half-Grouted Sleeve Connectors with Grouting Defects

Half-grouted sleeve connectors are a primary method for connecting rebar in prefabricated concrete structures. However, due to limitations in the construction environment, all kinds of grouting defects are inevitable, especially grouting voids. Additionally, fire disasters, among the most common types of disasters, significantly threaten the structural performance and safety of these prefabricated structures. Therefore, it is imperative to determine the mechanical properties of half-grouted sleeve connectors with grouting voids after high temperatures. This study designed and prepared 48 groups of half-grouted sleeve specimens with different grouting voids and defect locations. These specimens were heated to the specified temperature (25 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C), followed by unidirectional tensile testing after natural cooling. The experimental results showed that rebar fracture failure and rebar pulled-out failure were the failure modes of specimens. With the increase in temperature, bearing capacity, safety factor and ductility coefficient of specimens all decreased. When the temperature was lower than 400 °C, the specimen with void length less than twice the diameter of the rebar (i.e., 2d) had sufficient connection performance. For specimens with the same total void lengths, the bearing capacity of discrete voids is lower than concentrate voids at the same temperature. The load-displacement curve, safety coefficient, ductility coefficient and grey correlation degree of half-grouted sleeve specimens with grouting voids at different temperatures are analyzed and discussed, and the bond stress slip constitutive model is given. Grouting defects have greater influence on specimens after grey correlation analysis. Findings from this study provide valuable references for the safety performance evaluation of prefabricated structures with half-grouted sleeve connectors after exposure to fire.

Investigation on axial compression performance of self-compacting fly ash concrete filled square steel tube column: Tests and numerical simulation

Self-compacting concrete has the advantages of high fluidity, high uniform density, and low porosity. To promote the engineering application of self-compacting concrete, this article studies the axial compression test of ten square steel tube self-compacting concrete columns. The effects of design parameters such as different grades of concrete strength and internal structures of circular steel tubes, square steel tubes, and H-shaped steel on the mechanical properties of self-compacting fly ash concrete filled square steel tube columns are studied. The failure mode, characteristic load and characteristic displacement, ductility, and energy dissipation capacity of each specimen are analyzed. On the basis of the test, the finite element analysis model of steel tube self-compacting concrete column is established, and the test results and simulation results are compared and analyzed. The results show that: the failure mode of the square steel tube specimen is the multilayer middle and upper ring buckling failure. Internal structural measures of square steel tube and H-shaped steel can improve the bearing capacity, ductility, and energy dissipation capacity of the specimen. It is not recommended to use the specimen with an outer square and an inner circle design. The results of the finite element simulation have a high degree of matching with the test results, which intuitively reflects the changes in force of each component.

Study on Bonding Behavior between High Toughness Resin Concrete with Steel Wire Mesh and Concrete

This paper investigates the interfacial bonding behavior between high toughness resin concrete with steel wire mesh (HTRCS) and concrete. A total of five sets of fifteen double shear specimens were tested for parameters including concrete strength and material properties of HTRCS composites. The test results showed that the failure mode of DS1 specimens was partial debonding and fracture, and the rest of the specimens were the fracture of HTRCS. The concrete strength and reinforcement ratios of HTRCS composites were positively correlated with interfacial adhesion properties. When the concrete strength was increased from C30 to C40 and C50, the ultimate load increased by 43.4% and 43.2%, respectively. The ultimate load capacity increased by 32.1%, with the reinforcement ratio of HTRCS composites increasing from 1.05% to 1.83%. Moreover, the bonding slip model and the bearing capacity formula for the interface between HTRCS composites and concrete were proposed, and the calculation values were in good agreement with the test values, with an average value of 0.978.

Mechanical properties of recycled aggregate concrete reinforced with steel fibers under triaxial loading

The use of steel fiber reinforced recycled concrete (SFRAC) can help achieve resourceful reuse of construction waste and environmental sustainability. However, SFRAC structural members may experience complex stress states. Therefore, triaxial compression tests were conducted on 168 cylindrical SFRAC specimens to investigate their mechanical properties. The failure modes, stress-strain behavior, strength, and deformation of SFRAC under triaxial compression were observed. The effects of lateral confining pressure, steel fiber (SF) content, and recycled coarse aggregate (RA) replacement rate on these properties were analyzed. The internal micro-morphology of SFRAC was analyzed using SEM to better understand its failure mechanism under compression. The result shows that an increase in lateral confining pressure alters the failure mode of SFRAC specimens, resulting in a significant increase in peak strength and ductility. The enhancement of the elastic modulus by steel fibers decreases as the lateral confining pressure increases, suggesting a coupling effect between the two parameters. The inclusion of steel fibers in SFRAC did not have a significant impact on its failure mode or compressive strength. And yet, it did increase the ductility and toughness of the material, with an optimal steel fiber content of 1 % (by volume). On the other hand, the addition of recycled coarse aggregate resulted in a significant reduction in the concrete's compressive strength, by almost 17 %. Finally, several common material failure criteria for concrete were used to evaluate the impact of RA substitution rate and SF content on the strength of SFRAC. The Willam-Warnke failure criterion showed a higher strength. These findings provide a theoretical basis for future engineering applications of SFRAC.

Behavior and design of demountable and replaceable RC columns with bolted flange connections under eccentric compression

This study developed an innovative bolted flange connection for reinforced concrete (RC) columns, through which the RC columns could be removed and replaced in the event of a change in structural function or seismic damage. The detailed construction of the demountable and replaceable reinforced concrete columns with flange connection (DRCC-FC) is introduced firstly. Then the installation method and replacement requirement of DRCC-FC is presented, and the design formula for flange connection is also given. This study conducted experiments on nine DRCC specimens and one cast-in-place specimen under eccentric compression. The failure modes, loading capacity, deformability as well as strain on rebars and flange plates were investigated. Furthermore, finite element models of DRCCs were established and a parametric analysis was conducted. The effect of flange thickness, flange steel strength and length of connectors on mechanical properties was discussed. Results demonstrated that the failure modes of DRCC specimens were consistent with that of cast-in-place specimen, both of which exhibited yielding of rebars on the tensile side and crushing of concrete on the compressive side. The flange of the specimen with a low thickness appeared plasticity, and the other specimens did not show a significant deformation on the flange plates and bolts, which was favorable for the removal and replacement of the column components. The loading capacity of the DRCC specimens was slightly lower than that of the cast-in-place one, but the deformability was superior. Flange thickness is the main factor affecting the loading capacity and rotational stiffness of the flange connection. Finally, the formula for loading capacity of DRCCs related to flange thickness was presented. Comparison with the finite element and test results indicated that the calculation error of the formula is within 12%.

The macroscopic and microscopic fatigue failure mechanisms of high-temperature thermally-damaged granite under cyclic impact loading

The rock surrounding wells for developing geothermal energy are in an extreme environment with high temperature and long-term dynamic disturbance. To study the influences of different temperatures and dynamic disturbances on the damage, mechanical properties, and fatigue failure mechanism of rocks, a series of microscopic thermal damage analysis and dynamic cyclic impact tests were conducted on granite specimens treated at 25 ∼ 900 °C. Results show that after being thermally treated to different extents, granite specimens are weakened in terms of the dynamic strength and deformation resistance under cyclic impact loading. As the temperature rises, the number of impacts bearable by specimens before failure decreases accordingly. Under the joint action of temperature and strain rate, 450 °C is found to be the threshold temperature for the significant reduction of dynamic strength of the rock. Under action of temperature and strain rate, the proportion of dissipated energy gradually reduces with rising temperature. Moreover, the dynamic failure mode of rock specimens at high temperatures gradually changes from splitting failure to crushing failure, the fractal dimension gradually increases, and the cumulative dissipated energy density decreases. Finally, the deterioration degree of elastic modulus was adopted to describe the progressive damage evolution of thermally-treated granite under cyclic impact loading. Furthermore, the fracture morphologies of rock specimens after failure were analyzed from the microscopic perspective to reveal the macroscopic and microscopic fatigue failure mechanisms of the rock under the coupling of temperature and strain rate.

Dynamic mechanical properties and failure mechanisms of rock-like materials containing main and auxiliary holes with various arrangement inclinations and spacing ratios

As a typical rock defect, circular holes significantly influence rock masses, diversifying their failure modes and ultimately weakening their bearing capacities. To study the effects of hole arrangement inclination and spacing ratio on the mechanical properties and failure mechanisms of rock-like specimens, dynamic uniaxial compressive tests were conducted by employing a modified split Hopkinson bar apparatus and digital image correlation. The results reveal a V-shaped trend in the peak stress, Young’s modulus, and absorbed energy proportion as the hole arrangement inclination increases from 0° to 90°. However, the peak strain is insensitive to changes in the hole arrangement inclination and spacing ratio. The hole spacing ratio has no significant effect on the Young’s modulus, but it causes a gradual reduction in the proportion of energy absorbed in the specimens. Moreover, shear and tensile cracks emanating from the main hole dominate the failure mode of specimens with main and auxiliary holes, as demonstrated by the resultant displacement and vertical strain contours. Rock bridges experience shear failure as the hole arrangement inclination increases from 15° to 75°, whereas the rock bridges of specimens with various hole spacing ratios tend to fail owing to tensile cracks. The interaction between main and auxiliary holes with an arrangement inclination of 0° is the weakest, suggesting that tunnel groups should be arranged perpendicularly to the direction of dynamic loading.

Experimental Study on the Seismic Performance of Insulated Single-Sided Composite Shear Walls under Different Shear Spans and Axial Compression Ratios

The new insulated single-sided composite shear wall (NISCSW) composition involves setting a precast wall panel on one side and an insulation panel on the other side, with a middle cavity for casting concrete. To investigate the seismic performance of NISCSW under different shear spans and axial compression ratios, eight specimens are made, including six composite and two cast-in-place walls. The shear span ratio is controlled at 1.2 and 1.9, and the axial compression ratio is controlled at 0.1, 0.3, and 0.4. The specimens are subjected to quasistatic tests to analyze failure modes, hysteresis characteristics, stiffness degradation, displacement ductility, and energy dissipation capacity and to compare the seismic performance of the composite and cast-in-place walls. Results show that for each composite specimen, under the same axial compression ratio, the large shear span ratio specimen has a lower ultimate bearing capacity and faster stiffness degradation but better ductility and postyield energy dissipation capacity. Under the same shear span ratio, the high axial compression ratio specimen had a higher ultimate bearing capacity, slightly worse ductility, and similar stiffness degradation and energy dissipation capacity compared to other specimens. Compared with the cast-in-place specimen with the same axial compression ratio, the composite specimen failure mode and hysteresis characteristics are similar, and the ductility and energy dissipation capacity are comparable to the cast-in-place shear wall specimen, indicating that NISCSW has similar seismic performance to the cast-in-place shear wall under conditions of a large shear span ratio and high axial compression ratio. Based on the test results, the program ABAQUS is used to simulate the specimens. Compared with the test results, the simulated specimen failure mode is consistent with the test results, and the hysteresis and skeleton curves are consistent with the test curve, indicating that the model is correct, reliable, and can be verified with test results.

Bond performance of recycled coarse aggregate concrete with rebar under freeze–thaw environment: A review

Abstract In this article, a review focusing on the frost resistance of recycled coarse aggregate concrete (RAC) and the bond performance of RAC with rebar under freeze–thaw environment is carried out. The results show that there are still some controversies about the advantages and disadvantages of the frost resistance of RAC, and many influencing factors have not been considered. The mass of the RAC pullout specimens shows a trend of first increasing and then decreasing after freeze‒thaw cycles. The failure modes of the RAC pullout specimens after freeze‒thaw cycles include pullout failure, pullout-splitting failure, splitting failure, and rebar yielding failure. The bond performance of RAC with rebar shows a degradation trend after freeze‒thaw cycles, mainly in the decrease in bond strength and the increase in bond slip. The bond performance of RAC specimens is inferior to that of natural aggregate concrete specimens after freeze‒thaw cycles. The bond strength prediction models and the bond–slip constitutive relation prediction models of RAC with rebar after freeze‒thaw cycles have been summarized to ensure that engineers can better understand their applicability. The bond stress distribution between the RAC and rebar in the anchoring area is not uniform and constantly changes with the number of freeze‒thaw cycles. The degradation mechanism of the bond performance of RAC with rebar after freeze‒thaw cycles is analyzed.

Study on the characteristics of granite rock impact crack based on grain-based model

The type and heterogeneity of minerals control the initiation, aggregation, and expansion of rock blasting cracks and significantly affect the rock failure process. Granite was used to investigate the law of rock cracking under impact loading at the scale of mineral particles, and a two-dimensional grain-based model (GBM) was developed using PFC based on CT) scanning and laboratory mechanical tests. The GBM’s reliability was validated using laboratory uniaxial compression tests and numerical simulations. The specimen failure modes and evolution of microcracks during rock impact were analyzed, as well as the impact cracks’ characteristics under various impact velocities. The findings show that the GBM can simulate the microfracture behavior of several types of mineral fractures during rock impact. The evolution of granite cracks under impact loading can be divided into three stages: rapid growth, decreased growth, and gradual stabilization. The impact failure cracks of the samples were primarily tensile and intragranular. Granite’s tensile and shear fractures and intergranular and intergranular cracks of granite exhibit various trends with varying impact velocities. The GBM is viable for researching the dynamics of crystalline rocks and is a powerful tool for exploring the dynamic characteristics of rocks at the mineral particle level.

Interaction and initiation mechanism of non-parallel multiple cracks by strain gauge method

The maximum stress intensity factors, KI,kmax(θ) and KII,kmax(θ), of three non-parallel oblique cracks were calculated to reveal the interaction mechanism of multiple cracks. The multiple crack initiation criterion was adopted to predict the non-parallel multiple crack initiation parameters. Uniaxial compression tests were performed on rock specimens (red sandstone) with three non-parallel oblique cracks, where the strain near the crack tips was monitored by the strain gauge measurement method to determine the crack initiation sequence. The results showed that the change of crack relative inclination angle had little effect on the fracture trajectory. Multiple crack tips were seldom initiated at the same time. The failure modes of red sandstone specimens with three non-parallel oblique cracks under uniaxial compression are divided into two types: wing crack penetration and wing crack–secondary crack penetration. All of the fracture mechanisms are Mode I (tensile) initiation. The predicted results coincided well with the test results, verifying the validity of the theoretical results.

Damage characteristics of sandstone subjected to freeze–thaw cycles under different stress paths

The damage characteristics and failure modes of rock specimens subjected to different freeze–thaw (FT) cycles are vital to rock engineering in cold regions. Hence, in this study, the physical and mechanical parameters of sandstone specimens under different FT cycles were measured, and their deterioration laws were analyzed. The damage features of the rocks in different FT states under loading along various stress paths, including the main-frequency variation and crack propagation path evolution, were statistically analyzed. Two damage variables, one related to the acoustic emission (AE) and the other related to the rock strength, were combined to reveal the AE characteristics and damage evolution laws in the different damage states. The results showed that FT cycling led to a decrease in the peak stress, an increase in the number of microcracks, an increase in the AE activity, a shift in the main-frequency distribution toward the low-frequency range, and a shift in the damage mode from shear damage to a mixed damage dominated by tensile damage. The degree of strength deterioration in the rock specimens was affected by FT cycling under the different stress paths in the following influence order: conventional uniaxial loading > equal-amplitude cyclic loading> multistage cyclic loading; cyclic loading was more likely to induce tensile rupture than conventional uniaxial loading. The AE characteristic parameters were consistent with the stress–strain curve variations, which could reflect the damage evolution process of the rock specimens. The main frequencies showed a band-like evolution trend with the loading process and could be divided into three bands: a low-frequency interval (0–50 kHz), a medium-frequency interval (50–150 kHz), and a high-frequency interval (150–300 kHz); the main frequencies were concentrated in the low-frequency interval. With the increase in the number of FT cycles, the main frequency in the high-frequency interval and the number of corresponding microcracks gradually decreased.