Tensile Damage Research Articles

Finite Element Analysis of the Effect for Different Thicknesses and Stitching Densities under the Low-Velocity Impact of Stitched Composite Laminates.

In this study, a progressive damage model was developed for the mechanical response and damage evolution of carbon fiber stitched composite laminates under low-velocity impact (LVI). The three-dimensional Hashin and Hou failure criteria were used to identify fiber and matrix damage. The cohesive zone model was adopted to simulate the delamination damage, combined with the linear degradation discounting of the equivalent displacement method to characterize the stiffness degradation of the material, and the corresponding user material subroutine VUMAT was coded. The finite element analysis of the LVI of stitched composite laminates under different energies was finished in Abaqus/Explicit. Furthermore, the simulation predictions matched well with the results of the experimental tests. Based on this, composite laminates' mechanical response and damage forms with different thicknesses and stitch densities were analyzed. The findings show that the main damages of composite laminates were matrix tensile damage and delamination. The stitching process could improve the impact tolerance of composite laminates, inhibiting delamination and reducing the area of the delamination damage. The higher the density of the stitching, the more noticeable its inhibition would be. The thickness of the laminate also had a more significant effect on the damage to the laminate. Thin plates were more prone to matrix tensile damage due to their lower flexural rigidity, whereas thick plates were more susceptible to delamination because of their higher flexural rigidity.

Mechanical properties and damage characteristics of concrete under different atmospheric pressures

The aim of this study was to investigate the mechanical properties and damage characteristics of concrete under different atmospheric pressures (50 kPa, 80 kPa, and 101 kPa). The failure modes, compressive strength, dynamic elastic modulus, stress–strain curve, air pore structure, and acoustic emission (AE) parameters of concrete specimens were evaluated under the different atmospheric pressures. The results showed that as the atmospheric pressure decreased, the concrete damage gradually transitioned from tensile damage to shear damage, the compressive strength and dynamic elastic modulus decreased, and the peak strain increased. The pore structure of the hardened concrete changed, the number of air pores and air content decreased, and the average pore size and pore spacing coefficient increased considerably. Additionally, an analysis of the AE parameters showed that the development pattern of the concrete ringing counts and energy were mostly the same under different atmospheric pressures. The decrease in atmospheric pressure was accompanied by a corresponding gradual decrease in the AE rate of change parameters and cumulative AE parameters, indicating that the concrete crack expansion rate slowed down while the total cracking activity decreased. Moreover, the inverted V-shape of the b-value curve was not obvious, and the peak b-value decreased with decreasing atmospheric pressure. Finally, a combination of the Weibull distribution theory, Lognormal distribution theory, and strain equivalent hypothesis was used to establish a concrete uniaxial compression constitutive model considering atmospheric pressure. The model can reflect the evolution law of the mechanical properties of concrete under uniaxial compression under different atmospheric pressures.

Mechanism of surrounding rock failure in impact stress and in-situ stress in circular tunnel

With depth increase of many mines, the damage of surrounding rock by high ground stress cannot be ignored under the blasting excavation method. In view of the strong disturbance of surrounding rock caused by dynamic excavation and unloading under high initial rock stress conditions, based on the elastic unloading theory, the analytical solution of dynamic excavation and unloading stress is given using the residue theorem and Laplace inverse transform. The stress field distribution under the coupling effect of blasting load and initial rock stress is described. By making a rock model and implementing biaxial loading using a drop hammer to simulate the impact load, radial cracks generated by the impact stress wave and circumferential cracks generated by unloading during excavation were captured, and the stress change curve during excavation was monitored. The monitoring curve and theoretical curve have consistency in trend. Under the coupling effect of blasting load and initial stress, surrounding rock will generate tensile stress and broken, and the unloading effect of initial stress occurs after the blasting load, which causes tensile damage to the surrounding rock. The stress field curve plotted by the theory explains well the crack propagation caused by the coupling effect of blasting load and initial stress.

Numerical simulation method for quantitative assessment of fully-grouted sleeve connection damage state and performance under thermal coupling

Post-earthquake secondary fire is the major hazard prefabricated concrete (PC) structures face and is currently a hot research topic. Fully-grouted sleeve connection (FGSC) is the main connection method for PC structures, whose failure can result in serious consequences. However, the research on FGSC with tensile damage at elevated temperatures is limited and only at the experimental stage. To enrich the related studies, we proposed a 2D model that considers material damage in good agreement with the corresponding experimental results. 15 finite element models are established with two key parameters, tensile displacement, and temperature, to further investigate the mechanical performance of FGSC under force-thermal coupling. The temperature and damage distribution of FGSC are presented, in addition to the critical temperatures that cause the force state to change. The study results show that the damage pattern of FGSC mainly depends on the tensile phase, and the temperature-induced force can create non-negligible damage to the material. Based on this, the trends of axial and radial forces of FGSC have been summarized as well as their new prediction equations whose coefficients of determination are 99.36 % and 99.83 % in the elevated temperature stage. The numerical simulation method and conclusions obtained are the essential basis for the subsequent study of the response of PC structures under post-earthquake secondary fires.

An experimental and numerical study on the hybrid effect of basalt fiber and polypropylene fiber on the impact toughness of fiber reinforced concrete

As the civil aviation industry advances, higher requirements are put forward for the performance of airport cement pavement. The frequent takeoffs and landing of aircraft will cause continuous and large impact load on airport pavement, and combined with long-term natural factors, there will be various structural diseases. These diseases not only affect the service life of pavement, navigation safety. To enhance the impact resistance and durability of concrete, the incorporation of fiber has proven an effective approach, and basalt fibers (BF) and polypropylene fibers (PF) are selected as reinforcement materials in this study. Firstly, the effects of single BF and hybrid BF and PF on the mechanical properties of concrete were explored through compressive and flexural tests considering fiber volume contents (0.1%, 0.15, 0.2%) and lengths (6 mm, 9 mm, 12 mm). Then, the drop hammer impact test method, recommended by ACI committee 544, was refined to evaluate the impact resistance toughness of concrete. Finally, the numerical simulation of the impact test was established to explain the concrete impact damage mechanism. The results show that under less than 0.15% volume content, the single BF can effectively enhance the compressive strength, flexural strength, and impact resistance of concrete. Compared with the single BF, the combination of 12 mm BF and 6 mm PF yields favorable results, encompassing improved mechanical properties and impact resistance, thus exhibiting a positive hybrid effect. The increase in PF length (9 mm) leads to a reduction in compressive strength but an improvement in flexural strength and impact resistance. The simulation of the impact test is constructed and the accuracy is confirmed by comparing the impact times and failure patterns of the specimens. The tensile damage of plain concrete is much greater than the compressive damage, and the cracks will spread rapidly from the upper surface to the lower surface. Compared with plain concrete, the crack width of hybrid fiber reinforced concrete is larger, but the damage degree is more uniform. Fiber can absorb a large amount of impact energy, reduce the emergence of micro-cracks, and improve the impact toughness and toughness coefficient of concrete.

Low‐velocity multiple impact damage characteristics and numerical simulation of carbon fiber/epoxy composite laminates

AbstractLow‐velocity impact damage is one of the main defects of composite laminates, which seriously affects the bearing capacity and service life of composite laminates. Multiple impact experiments of carbon fiber/epoxy composite laminates were carried out under different impact energies. The finite‐element model was established according to the experimental conditions. The impact damage mechanism of laminated plates was studied by means of simulated damage cloud image, water immersion ultrasonic C‐scan, and computed tomography. The results showed that the matrix damage occurred mainly during the whole impact process, and the tensile damage of the matrix was dominant. As the number of impacts increased, the impact resistance of composite laminates decreased and the damaged area increased, with the second impact having the greatest influence on the degree of damage to the laminates; multiple impacts with low energy were easy to cause delamination at the top of the laminated plate. With the increase in impact energy, the damage at the bottom of the laminated plate was more serious, and the propagation mode of delamination damage in laminates changed from top‐bottom to bottom‐up.Highlights Establishing a multiple impact finite‐element model for composites. Accurately predicting the impact mechanical response of composites. Revealing multiple impact damage evolution of composites by simulation.

In situ damage propagation and fracture in notched cross-ply SiC/SiC composites: Experiment and numerical modeling

In this paper, the damage progression and fracture behavior in notched cross-ply SiC/SiC composites were investigated using the in situ digital image correlation (DIC) and acoustic emission (AE) techniques. Under tensile loading, the mechanical properties of composite’s elastic modulus, proportional limit stress (PLS), ultimate tensile strength (UTS), and fracture strain were obtained for non-notched, single-hole, double-holes, and four-holes samples. Based on the analysis of composite’s tangent modulus, the nonlinear tensile stress-strain curves were divided into three main domains, relating with internal multiple damage mechanisms observed using the in situ DIC and AE. Effects of the number and location of the circular holes on the full-filed composite’s damage and strain evolution, AE energy and ring-down count were investigated. After tensile fracture, the composite’s macro fracture morphology and micro damage mechanisms were observed under the scanning electronic microscopy (SEM). Numerical modeling of tensile damage and fracture process of different notched samples were conducted using the extend finite element method (XFEM). Relationships between the notch types, tensile mechanical properties, macro strain evolution, and micro damage mechanisms were established.

A modified HJC model for geological materials subjected to blasting loadings

In this paper, a modified HJC model is proposed to address the problem that the HJC model cannot simulate tensile damage cracks. The modified HJC model modifies the strength surface, strain rate effect and damage model of the original HJC model, respectively. The modified model is embedded in LS-DYNA software for single and multiple finite element validation. The effects of different parameters on the modified HJC model stress–strain curves are analyzed in the single finite element validation. In multiple finite element verification, uniaxial and triaxial compression experiments, Brazilian disc splitting experiments, SHPB dynamic compression experiments, and SHPB dynamic splitting experiments are performed. The results of single and multiple finite element numerical validation illustrate that the modified HJC model can well simulate the pre-peak and post-peak nonlinear mechanical behaviors of geological materials such as rocks and concrete. Compared with the HJC model, the damage pattern of the modified HJC model is closer to the real damage pattern. For the strain rate effect, the modified HJC model can reflect the real compressive strain rate effect and improve the sensitivity of tensile strength to strain rate. The results of the numerical simulation of the blasting funnel illustrate that: the modified HJC model can simulate explosive tensile cracking well and can explain the formation mechanism of blasting funnel due to the distinction between tensile and compressive meridian surfaces and the addition of tensile damage. The modified HJC model is more applicable to the numerical simulation of engineering blasting.

Mechanical and morphological investigation of laminated composite polymers depending on increasing vibration cycle

Composite polymers are widely used in vibrating environments such as the aerospace, automotive, marine, and sports industries. Accordingly, composite materials are exposed to vibration at various periods. In this study, the effect of the vibration cycle on the mechanical properties of composite plates was investigated. The plates were vibrated at different revolutions. The tensile strength of these plates after vibration was examined, and these effects were compared with each other. As the vibration cycle increased, it was observed that tensile damage gradually occurred at different angles, and scanning electron microscopy (SEM) analysis revealed that the fibers were damaged at different angles. Accordingly, it was observed that the increased vibration cycle caused an angular fracture in the composite plates and decreased the tensile strength from 9.7 to 7.9 kN by nearly 23% as well as the elongation from 3.4 to 2.76 mm.

Different effects of interfacial properties on the tensile and compressive damage mechanisms of 3D woven composites: Multiscale damage model and numerical comparative study

A multiscale damage model is proposed based on a micro-mesoscale bridging model and a multiscale kinking model, which can predict the damage development processes of 3D woven composites (3DWCs) under different loading conditions using only few material parameters. At the microscale, the various mesoscopic damage modes of yarns, including tensile fiber breakage, compressive fiber kinking and transverse inter-fiber crack, can be simply divided into matrix- and fiber-dominated damage. Using this multiscale damage model, a numerical comparative study is conducted to investigate the effect of interfacial properties on the mechanical performances and damage mechanism of 3DWCs. The numerical results indicate that the interfacial properties exhibit different effects on the tensile and compressive strengths and damage mechanisms of 3DWCs. The interaction of different damage modes controls the overall trend of the tensile strength of 3DWCs. There is an optimum point for the tensile strength of 3DWCs at different interfacial fracture toughness. Beyond the optimum point, the tensile strength of 3DWCs is negatively correlated with interfacial properties. Unlike the tensile loading case, the better the interfacial properties, the later fiber kinking occurs, resulting in higher compressive strength of 3DWCs.

Constitutive modeling hot tensile behavior of Ti6Al4V alloy by considering phase transformation and damage mechanisms

The hot tensile and fracture behavior, as well as damage mechanisms of Ti6Al4V alloy are investigated by the hot tension tests and microstructure observation experiments. For comprehensively characterizing the hot tensile behaviors, a developed dislocation density-based constitutive model was established by coupling the flow softening, phase transformation and damage mechanisms. The results show that the flow stresses under the relatively high strain rates (0.01–1 s−1) easily peak at low strains and then decrease with the further straining. However, under the low strain rate (0.001 s−1), the flow stress tends to a continuously increasing trend until fractured. After hot deformation, the fraction of β phase greatly increases compared to that under the initial state. Meanwhile, β-phase fraction increases with the strain rate decreasing or the deformation temperature increasing. The great input thermal energy by the high temperature and the long deformation period under low strain rates promote the α-β phase transformation. Furthermore, the necking diffusion caused by dislocation motion enhances the homogeneous deformation under low strain rate or high temperature. The cavitations preferentially nucleate at the α/β phase interface due to the inconsistent strain. The comparison of the measured and calculated results confirms the excellent prediction ability of the established model in reproducing the hot tensile behavior, β-phase fraction and damage evolution under different processing parameters.

A new analytical method for stability analysis of rock blocks with basal erosion in sub-horizontal strata by considering the eccentricity effect

Abstract. The basal cavity of a rock block formed due to differential weathering is an important predisposing factor for rockfall in hard–soft interbedded rocks, which induces an eccentricity situation at the base of the rock block. Rock block falling due to the non-uniform distribution with the failure modes of toppling or sliding is defined as biased rockfall in this study. Taking into account the non-uniform stress distribution due to the eccentricity effect, a new analytical method is proposed for three-dimensional stability analysis of biased rockfall. The development of non-uniform stress distribution stress calculated by this analytical method was verified by numerical simulation. The biased rockfall progresses from partial damage of the soft underlying layer, caused by non-uniform distributed stress, to toppling and sliding of overhanging hard rock block due to overall unbalanced force. Therefore, a set of factors of safety (Fos's) against partial damage (compressive and tensile damage of the soft underlying layer) and overall failure (toppling and sliding of the hard rock block) are used to determine the rockfall susceptibility level. The analytical method is applied and validated using biased rockfalls on the northeastern edge of the Sichuan Basin in southwest China, where a significant number of rockfalls consisting of overhanging thick sandstone and underlying mudstone occur. The evolution process of biased rockfalls is divided into four stages, initial state, basal cavity formation, partially unstable and failure. The proposed method is validated by calculating Fos's of the typical unstable rock blocks in the study area. As the cavity continues to grow, the continuous retreat of mudstone causes stress redistribution between the hard and soft rock layers. This results in damage to the underlying soft rock layer due to the development of the non-uniform distribution, ultimately leading to the failure of the hard rock block. The critical retreat ratio is determined to be 0.33, which is used to classify the low and moderate rockfall susceptibility in the eastern Sichuan Basin. The proposed analytical method provides insights into the evolution of biased rockfall and a means for early identification and susceptibility assessment of rockfall.

A three-dimensional numerical simulation method for seismic nonlinear response of underground structure in liquefiable site and analysis of structural damage

Considering the effect of dynamic interaction between liquefiable soil and underground structure, simulating the nonlinear seismic response of underground structures through simple, convenient, and reliable 3D numerical model is still a great challenge at present. A three-dimensional numerical simulation method for dynamic response of underground structure in liquefiable site is proposed based on the centrifuge shaking table test designed for a subway station buried in liquefiable interlayer site carried out in the previous period. The numerical model takes into account the characteristics of sand prone to large deformation after liquefaction and the nonlinear characteristics of the contact between saturated soil and structure. The typical numerical calculation results are compared with the experimental results to verify the correctness of the numerical model, and the extended analysis of the structural damage is carried out, which visually shows the damage distribution of structure under different functional states after earthquake. The analysis of structural damage shows that the junction position of middle column and longitudinal beam is more prone to tensile damage under the horizontal earthquake action; while after considering the horizontal-vertical seismic effect, the middle column will produce additional compressive damage due to the vertical inertia force, and the end of middle column should be paid attention to in seismic design.

Strain based damage indicators for finite element analysis of reinforced concrete structures

Numerical analysis conducted using a finite element model provides a quantitative and detailed evaluation of damage in concrete. As a result, the performance verification based on the local damage obtained numerically is considered an effective method for achieving a rational design of RC structures. Stresses and strains computed from numerical analysis are suited to indicators for representing local damage in concrete instead of sectional forces calculated from design equations; however, the performance evaluation using stresses and strains lack versatility owing to their dependency on constitutive equations and mesh discretization in numerical models. This study presents two types of strain-based damage indicators for performance verification in the design of RC structures using nonlinear finite element analysis. The second strain invariant of the deviatoric strain tensor is used to assess tensile damage, such as the opening of flexural cracks and the development of shear cracking in concrete, whereas the normalized accumulated strain energy is used to evaluate compressive damage, i.e., concrete crushing. These scalar indicators have high general usefulness, with calculation as non-local values by a weighted average, to reduce their mesh dependency. Furthermore, numerical analyses were performed to evaluate the damage condition and failure modes of RC beams, and the proposed damage indicators helped accurately understand flexure and shear failure. Lastly, we propose threshold values of the damage indicators for the performance verification of safety, represented by the load-carrying capacity in the design of RC structures.

Characterizing fatigue damage evolution in asphalt mixtures using acoustic emission and Gaussian mixture model analysis

The identification and investigation of fatigue crack evolution and damage modes in asphalt mixtures are crucial for understanding the corresponding mechanisms and characteristics of fatigue failure. In this study, the four-point bending fatigue tests were performed on the asphalt mixture specimens with precast joints using the acoustic emission (AE) technique to analyze the damage evolution. The failure modes of the asphalt mixtures were identified using the Gaussian mixture model (GMM). The results revealed that the fatigue damage of the asphalt mixtures can be categorized into four stages: Stage I (void compaction), Stage II (microcrack initiation and stable propagation), Stage III (crack aggregation and unstable propagation), and Stage IV (complete fracture). This division was defined based on the inflection points of the axial displacement curves, the cumulative number of AE events, and the cumulative ringing counts. The evolution of the AE b value was found to be correlated with the crack propagation. The rapid decline in the b value in Stage III was considered a harbinger of the eventual complete fracture in the asphalt mixtures. The GMM clustering analysis indicated that the fatigue damage of the asphalt mixtures primarily involved tensile damage (90%), and shear damage made a small contribution of 10%. Existing AE studies have focused on the quantitative evaluation of AE parameters, while research on clustering identification algorithms for these parameters is scarce. The analytical methods and findings of this study enable an effective identification of fatigue damage in asphalt mixtures at the laboratory scale and offer insights into the fatigue damage mechanisms of asphalt mixtures.

Tensile strength and damage of open-cell ceramic foams under cylindrical splitting test

Tensile strength and damage of open-cell ceramic foams under cylindrical splitting test

The Effect of Asynchronous Grouting Pressure Distribution on Ultra-Large-Diameter Shield Tunnel Segmental Response

The complex distribution of synchronous grouting pressure results in excessive tunnel deformation and various structural diseases, especially for ultra-large-diameter shield tunnels. In this study, to reduce the risk of tunnel failure, a three-dimensional refined finite element model was established for the Wuhan Lianghu highway tunnel project, taking into account the non-uniform distribution of synchronous grouting pressure. This study focuses on investigating the development patterns of internal forces, deformations, and damages in segment structures under varying grouting pressure ratios. The results indicate that the primary failure mode of a segment is tensile failure occurring at the outer edge of the arch. Moreover, an increased ratio of grouting pressure between the arch bottom and top leads to a higher positive bending moment value and greater tensile damage at the arch waist. The tunnel ring gradually exhibits distinct “horizontal duck egg” shape deformation. When the grouting pressure ratio is 2.8, there is a risk of tensile cracking at the outer edge of the arch waist. At this time, the segment convergence deformation is 39.71 mm, and the overall floating amount reaches 43.12 mm. This research offers engineering reference for the prediction of internal forces and deformations in ultra-large-diameter shield tunnels during grouting construction, thereby facilitating their application in the development of resilient cities.

Investigating the Impact of Pores on Rock Damage during Thermal Spalling Drilling

Numerous microcracks and pores in geological rock formations cause early flaws. High temperatures increase these fractures and pores, thermally damaging reservoir rocks and changing the rock failure mechanism. However, research on pores' high-temperature thermal spalling and fragmentation effects on heterogeneous rocks is sparse. This study built a finite element numerical model of heterogeneous granite rock thermal damage with pores based on rock thermal fracture theory and the Voronoi method and explored the mechanism under varied pore settings. The research's findings indicate that the application of high temperatures to local heterogeneous porous rocks results in a higher proportion of tensile damage. The proportion of shear damage and tensile damage constantly varies due to the changing position and shape of the pores. The rock's porosity has the effect of decreasing temperature in the direction of heat transfer while increasing the extent of temperature transfer along the pore parallel to the heating surface. The potential degree of damage increases as the density of pores increases, the distances between them decrease, and the pore lengths increase. The thermal damage resulting from heating in the vicinity of the pore is primarily localized in the area between the pore and the heated surface. This effect becomes more significant as the distance between them decreases. The findings of this study can serve as a theoretical framework for understanding the impact of rock pores on rock thermal fracturing and fragmentation in the thermal spalling-assisted development of deep oil and gas resources.

The effect of interface on the strength characteristics and fracture mechanism of the coral reef limestone-concrete combination specimen

Biological and biochemical geneses bring complex structures to coral reef limestone (CRL), leading to pronounced anisotropy and heterogeneity. The stress field concentration induced by pore structure introduces a considerable dispersion in mechanical parameters and reduce the strength of CRL significantly. The CRL-C specimens were made using coral reef limestone to conduct uniaxial compression tests with loading rate of 0.002 mm/s, revealing the controlling mechanism of interface inclination angle for the strength of CRL-C specimens. The experiment results shows that the peak strength, peak strain, and modulus of elasticity of the combination all tend to lessen and then grow with increasing the interfacial inclination angle. The achieved results indicate that the tensile damage occurs in specimens with 0°, 15°, and 90° inclination angles, whereas the tension-shear stress damage takes place for specimens with 30° inclination angle. In turn, the shear damage occurs along the interface for specimens with 45°, 60°, and 75° inclination angles. The strength of combination specimens underwent the shear failure is noticeably smaller than that of the combination specimens damaged across the interface. Additionally, the cement mortar at the interface has a remarkable reinforcing effect on the interface strength, and the CRL-C interface possesses the highest cohesion and a lower internal friction angle compared to terrestrial rocks. Simultaneously, the parameters of the CRL-C specimens in the single-interface model are appropriately determined based on the interface strength. The inclination range of the combination specimen damaged along the interface is 35.58°<α < 85.83°. The predicted strength parameters based on the Coulomb model and the single-interface model provide a solid reference for the engineering design of CRL-C.

Electromechanical behavior and damage index system of 3D carbon fiber angle-interlock woven composites with FEA and data processing tools

The relationship between structural damage and electrical resistance change of carbon fiber reinforced composites is important to structural health monitoring. Here, we investigated the electromechanical behavior and damage index system for identification of various damage types in 3D angle-interlock woven composites under tensile loading in the warp direction. The mechanism of electrical resistance change was explored by combining current injected from both ends (Electrical current is in the plane of the laminate and goes through the entire cross section of the specimen) and current injected diagonally (Direction of electrical current between the in-plane direction and the through-thickness direction). A finite element model was established to analyze the electric potential distribution and tensile damage evolution process of composites. We found current injected from both ends can only detect with yarn damage and current injected diagonally can detect with matrix cracks, interface cracking and yarn damage. Based on the electrical resistance data of current injected diagonally, we used principal component analysis and K-means clustering methods to establish a damage index system to reflect the damage accumulation degree of different damage modes. The finite element analyses verified the rationality of the damage index system.