In-plane Damage Research Articles

Finite element modelling of interacting indentation, flexural, and delamination damage in lap joints of composites

Composite lap joints are essential for various applications, such as aircraft wings, piping networks, sporting equipment, and civil engineering works. Low-velocity impact on such joints is a common occurrence in real-life situations. The response of these joints under such impacts is quite complex. This involves multiple interacting damage modes that include delamination failure, ply failure (in-plane damage), and bond interface (joint) failure. These damages may lead to significant degradation of joint strength without apparent complete failure. Thus, it is very important to predict the response and behavior of lap joints under such impacts. The objective of this study is to demonstrate the methodology for the evaluation of damage parameters for built-in damage progressive models in ABAQUS. By introducing the concept of characteristic length to address mesh dependency, the method ensures accurate and reliable computation of damage parameters. Further, the proposed progressive damage model was improved to include the laminate (in-plane) damage, in addition to delamination and bond failure. Finally, the proposed methodology was validated by applying it to a real-world lap joint impact problem. The present study can be helpful in the initial design phase of composite structures, as it provides a consistent and easy-to-use methodology to evaluate damage parameters for practical impact simulation problems.

Lightning damage analysis of composite bolted joint structures based on thermal-electrical-structural simulation

To analyze the impact of connection behavior on lightning-induced ablation damage in composite bolted joints, this paper analyzes contact properties based on thermoelectric characteristics and develops a lightning ablation damage model for composite interference joint structures. A preliminary analysis of the electrothermal conduction process in the interference-fit composite joints clarifies the influence of connection behavior on lightning damage. The results indicate that a skin effect occurs near the fastener-to-CFRP interface, and both Joule heating and conductive heat significantly affect the temperature distribution in the composite bolted joint structure. A lightning current impact test was also conducted for validation and comparison. Quantitative comparisons show that the predicted in-plane damage aligns well with the experimental results, with an error of 7.37%, while the difference in thickness direction damage is more significant, with an error of 40.02%. Furthermore, the analysis of ablation damage characteristics reveals that the most severe damage occurs in the lower-middle region of the CFRP thickness due to the combined effects of interference damage and bolt preload, while damage in other areas is suppressed.

Out-of-plane behavior of unreinforced masonry walls with horizontal slip layers in RC frames

In some cases, the infilled walls may be subjected to in-plane（IP） earthquake and out-of-plane(OOP) earthquake action due to the uncertainty of earthquake action and infills’ position. As a new type of unreinforced masonry(URM) walls with horizontal slip layers, it can effectively reduce the in-plane damage of infilled walls of reinforced concrete frames under an earthquake. However，the infill wall out-of-plane damage has a greater security threat than in-plane damage. In this study, a numerical study about the difference of OOP behavior of URM walls with/without horizontal sliding layers are analyzed, mainly including failure mode, bearing capacity, displacement capacity and stiffness. The spacing of horizontal slip layers, length of annectent reinforcing bars and the friction efficient between slip layer and masonry unit are chosen to investigate the differences of OOP mechanical behaviors. The results show that the horizontal sliding layers reduce the integrity of URM walls in reinforced concrete frames, causing the constraint of surrounding frame on infilled wall and the arching effect inside infilled wall become weaker, and OOP bearing capacity to drop. It is found that i) the annectent reinforcing bars is conducive to reducing the out-of-plane failure of the wall; 2)the smaller the slip layer spacing of the wall is, the more serious the damage of the infill wall under the out-of-plane load is, and the smaller the out-of-plane bearing capacity and the out-of-plane stiffness of the wall are; iii) the influence of the friction coefficient between slip layers and masonry unit on OOP mechanical behaviors is small. Finally, Chinese building design code is also selected to estimate the OOP bearing capacity of URM walls with horizontal sliding layers. The value of λcr is recommended to be greater than 1.0, and the value of λP is greater than 3.3. The spacing of the slip layer should be greater than 1 / 5 of the wall height, and the length of the annectent reinforcing bars should not be less than 1 / 5 of the infilled wall length.

Novel application of ground penetrating radar for damage detection in thick FRP composites

This paper presents the first successful application of ground penetrating radar (GPR) to the inspection of thick (≥100 mm) fiber-reinforced composites. These thick composites are found in wind/tidal turbine blades and composite-hulled ships, where sufficient non-destructive testing (NDT) remains challenging. Polyester-glass specimens, ranging in thickness from 100 to 120 mm, were created with delamination-mimicking damage. Specimen thickness, damage depth location, antenna orientation and damage dryness were the test variables. Finite-difference time-domain simulations indicated the method’s feasibility, and experimental results confirmed these findings. GPR effectively detected and precisely located dry, in-plane damage, with increased detectability for water-filled damage due to the enhanced contrast of electrical properties that creates the damage response in the signal. This capability is particularly advantageous for marine composites, where extensive damage may lead to water ingress. In a comparison with an ultrasonic inspection, GPR proved superior for the thicker composites (≥100 mm). As the first successful application of GPR to composite structures, these findings significantly advance the field of NDT of these materials.

Fragility functions for masonry infill walls in RC frames with the in-plane and out-of-plane loading

This paper presents a comprehensive analysis of masonry infill walls under both in-plane and out-of-plane loading, offering limit state criteria and fragility functions. Four distinct damage states are defined to capture damage evolution under diverse loading conditions. The research establishes a thorough database documenting in-plane and out-of-plane drift levels corresponding to each damage state, derived from experimental testing on masonry-infilled frame specimens from existing literature. Fragility functions were developed based on in-plane or out-of-plane drifts for four distinct damage states. The study systematically evaluates the influences of masonry prism compressive strength, slenderness ratio, and aspect ratio. Notably, the impact of prior in-plane damage on out-of-plane vulnerabilities is investigated. The study introduces equations designed to predict changes in the central tendency and dispersion parameters of out-of-plane drift thresholds, accounting for variations in prior in-plane drift ratios. It’s crucial to highlight that the empirical fragility functions proposed in this study are exclusively based on experimental tests involving tight-fit single-leaf infill walls without openings or modifications.

Effect of in-plane damage on the out-of-plane strength of masonry walls with openings

Past earthquakes have shown that masonry walls are vulnerable to fail near the openings. Openings such as doors and windows are an essential part of the structure. The presence of an opening reduces the strength and stiffness of the masonry structures. It also affects the load-resistant mechanism of the masonry walls. The walls are subjected to simultaneous in-plane and out-of-plane forces during seismic events. Therefore, the analysis of the masonry walls either in in-plane or out-of-plane direction may not give a proper picture of the global behavior of the masonry structures. The masonry walls deform primarily in the in-plane direction if the walls are properly tied with each other, and the structure is constructed by following codal provisions. Under this situation, it is necessary to investigate the effect of in-plane damage on the out-of-plane strength of the masonry walls. Few researchers have studied the behavior of solid unreinforced masonry walls under combined loading. However, limited work can be found for masonry walls with openings. Therefore, the present study focuses on the effect of different types of opening layouts on the combined in-plane and out-of-plane behavior of masonry walls incorporating varying pre-compression. Further, in masonry buildings, walls are interconnected, making the masonry walls of various shapes. In this study, masonry walls of rectangular and I shapes have been considered to examine the effect of cross-walls under combined loading. Interaction curves have been developed for the walls subjected to the in-plane loading followed by an out-of-plane pressure. Based on the results, the maximum opening percentage in the masonry walls has been suggested, which is desirable in the case of masonry buildings constructed in high seismic zones.

Simulating lightning effects on carbon fiber composite shielded with carbon nanotube sheets using numerical methods

The demand for lightweight aircraft structures has shifted from traditional metals like aluminum to composite materials such as carbon fiber reinforced polymers (CFRP) to achieve weight reduction. However, this transition has led to decreased lightning strike protection efficiency due to the dielectric nature of CFRP. To address this problem, two CFRP samples—one unprotected and the other shielded with carbon nanotube (CNT) sheets—were subjected to artificial lightning strike testing. The research employed a coupled thermal-electrical finite element analysis method to investigate the lightning strike's impact and damage mechanisms on both samples. The numerical results closely aligned with published experimental data, validating the simulation. Unprotected CFRP sustained damage through the thickness direction up to 8 composite plies and in-plane direction over a length of 110 mm. In contrast, the sample protected with CNT sheets exhibited damage limited to the surface of the first 4 plies, with in-plane damage reduced to 24 mm. Notably, the damage area in the CFRP protected with CNT sheets showed a substantial 78.1 % reduction compared to the unprotected CFRP sample. This suggests that CNT can enhance the electrical conductivity of CFRP when incorporated between interlayers in both in-plane and thickness directions. The study enhances understanding of CFRP damage behavior and failure modes under lightning strike conditions, emphasizing CNT sheets as an improved and viable lightning strike protection system for aerospace applications, warranting further investigation.

Damage evolution and failure mechanism of masonry walls under in-plane cyclic loading

Masonry structures often exhibit poor seismic performance during earthquakes owing to their low material strength, structural integrity, and ductility. To address the mechanism of in-plane damage and collapse of masonry walls subjected to earthquake ground motion, three full-size brick walls, one with structural columns and two without structural columns, were designed and constructed in this experimental study to quantitatively investigate the effect of structural columns on the seismic performance of masonry walls. Quasi-static tests of masonry walls subjected to cyclic lateral loading were conducted using electrohydraulic servo actuators. The in-plane damage evolution and failure mode of the masonry walls under lateral loading were investigated. A numerical model with a novel plastic damage constitutive model of masonry was established. The accuracy of this model and the calculation results were verified by comparing them with the crack development and mechanical property curves obtained from the experiments. Based on the numerical model, a parametric analysis was conducted to investigate the influence of critical factors, such as mortar strength, vertical load, and height-to-width ratio, on the seismic performance of masonry structures. The test results showed that structural columns can effectively improve the performance of masonry structures. Properly installed structural columns can improve the peak load-bearing capacity, ductility, and energy dissipation of masonry structures. The masonry damage constitutive model used in this study could simulate the damage to masonry wall specimens well and reflect the hysteretic curves of the specimens within an acceptable range of accuracy. Thus, it provides a valuable reference for physical experiments. An appropriate mortar strength and vertical compressive stress can effectively enhance the load-bearing capacity of masonry structures within a certain range. For structural column walls. Walls with mortar strengths (average compressive strength) of 12.5 MPa, 10 MPa, 7.5 MPa, and 5 MPa showed an increase in peak bearing capacity of 26.2 %, 23.9 %, 18.5 %, and 9.4 %, respectively, compared to walls with mortar strengths (average compressive strength) of 2.5 Mpa. The aspect ratio had a significant impact on the shear load-bearing capacity of the masonry walls. An increase in the aspect ratio led to a transition from shear failure to flexural failure and a subsequent decrease in shear load-bearing capacity. For masonry walls with structural columns, increasing the diameter of the longitudinal steel in the structural columns can enhance the load-bearing capacity of the masonry wall, resulting in a significant improvement in its overall performance.

Effects of In-Plane Damage of infill Walls on Their Out-of-Plane Bearing Capacity Reduction Factors

Effects of In-Plane Damage of infill Walls on Their Out-of-Plane Bearing Capacity Reduction Factors

Finite element analysis of the effect of crystallinity on low-velocity impact performance of poly (aryl ether ketone) composites

A new copolymer poly (aryl ether ketone) (PAEK) resin was used as a matrix to prepare carbon fiber (CF) reinforced composites with different crystallinity by slow and rapid cooling. Finite element (FE) models were constructed according to micromechanics, and the progressive damage method based on the three-dimensional (3D) PUCK damage criterion was used to analyze the low-velocity impact damage mechanism of the CF/PAEK composites. The experimental results show that the composites with lower crystallinity have higher impact resistance and smaller impact damage area. In the FE simulated impact damage evolution process, the damage of the impact side is mainly compressive damage of the matrix and the fiber, while that of the impact back side is mainly tensile damage of the matrix. Moreover, due to their high interfacial bonding strength, the main damage modes of rapid-cooled composites with lower crystallinity are severe matrix tensile damage and slight fiber tensile damage in the impact back side. However, the slow-cooled composites show more delamination. In addition, for the damage evolution of compression after impact, the FE simulation shows that fiber compression, matrix tension, matrix compression and delamination expand along the transverse to the compression load. Furthermore, the in-plane damage expansion occurs earlier in the rapid-cooled composites, which also have graver interlaminar damage.

UNI-axial dynamic tests on a stone masonry subassembly before and after interventions

The present study deals with the seismic response of three-leaf stone masonry subassemblies before and after the application of intervention techniques. In detail, shaking table tests are performed on a scaled substructure in the Laboratory for Earthquake Engineering, at the National Technical University of Athens (NTUA). The substructure consists of two parallel walls. One wall is of rectangular section, whereas the other is provided with a portion of transverse wall at its mid-length. A flexible timber floor rests on the top of the walls. Sine-sweep and dynamic tests are performed perpendicular to the two walls, before and after interventions. The aim of this work is to experimentally study the response of three leaf masonry walls against in-plane and out-of-plane seismic actions, to investigate the effect of the transverse wall to the out-of-plane behavior of walls (and vice versa) and to assess the effect of a selected set of interventions on the behavior of the substructure. The results of this study confirm the vulnerability of three leaf masonry structures, evidenced by the premature detachment of masonry leaves. They prove the significant role of the transverse wall, in reducing the width of the separation cracks observed in the walls subjected to out-of-plane actions followed by an increase of its in-plane damage, as well as the positive effect (in terms of load and deformation capacity) of grouting the walls and enhancing the diaphragm action of the floor. Additionally, the experimental results of this study can serve as a validation tool for the calibration of numerical models and as input for further analytical research. In this paper, the calibration of a smeared crack model that is based on macro-modeling approach is presented, along with a comparison between experimental and numerical results for the specimen before interventions, based on the performed calibration.

Effect of in-plane damage on the out-of-plane mechanical performance of infill walls with recycled concrete blocks

Effect of in-plane damage on the out-of-plane mechanical performance of infill walls with recycled concrete blocks

Research on low-velocity impact characteristics of Z-pin reinforced quartz/phenolic laminates

The problem of in-plane damage caused by dynamic impact loading of composite laminates can be enhanced by implanting Z-pins. The effect of Z-pin implantation on the impact resistance of composite laminates is investigated experimentally, and the damage process is analyzed by establishing a low-velocity impact finite element model of Z-pin reinforced laminates. The test results show that the maximum impact contact force of Z-pin reinforced laminates can be increased by 8.23%, and the maximum contact force and absorbed energy of Z-pin reinforced samples both increase gradually with the increase of impact energy. The simulation results show that with the increase of impact energy, the expansion of impact damage is hindered by the Z-pin, and it reduces in-plane matrix damage. The expansion path of the interlaminar crack changes when it reaches the Z-pin enhancement area, and the shape of the interlaminar damage changes from circular to elliptical.

Experimental investigation on the in-plane and out-of-plane interaction of isolated infills in RC frames

This paper presents the findings of a series of bidirectional quasi-static tests conducted on isolated infilled frames to investigate the in-plane and out-of-plane interaction of the infills with different boundary constraints. The effects of prior in-plane damage on the out-of-plane behavior of isolated infill walls were assessed. It was found that the presence of existing cracks that developed during the in-plane loading stage had an effect on the out-of-plane cracking patterns, decreased the integrity of the isolated infills, and weakened the infill-frame boundary restraints. The behavior of isolated infill walls under combined in-plane and out-of-plane loading was also affected by the effectiveness of boundary constraints. The experimental results demonstrated that reliable boundary constraints were necessary for forming arching mechanisms and preventing rigid body motion.

Experimental investigations into delamination behavior of curved composite laminates reinforced by a pre-hole z-pinning (PHZ) method

In this paper, an experimental study is conducted on the resistance of delamination in curved carbon fiber/epoxy composite laminates reinforced by z-pins. The curvature of the specimens is designed according to that of the aero-engine fan blade roots. A cost-effective pre-hole z-pinning (PHZ) technique is employed to mitigate initial in-plane damage, and the impact of z-pin volume fraction and diameter on interlaminar fracture toughness and bridging behavior is determined through double cantilever beam (DCB) testing. The mode-mixity of z-pin is determined, and the fracture toughness of curved specimens is calculated based on Timoshenko curved beam theory. The results indicate that the mode-mixity varies depending on the location of z-pins. Z-pins are subjected to a mix of crack opening and crack sliding loads during the tests. The primary failure mode of z-pins is pull-out, with a minor portion of the pins experiencing fracture. Specimens with 0.8 vol% z-pins exhibit a fracture toughness that is approximately 567% higher than that of unpinned specimens. The delamination resistance capacity of z-pins is dependent on the mixed-mode ratio, due to the transition in delamination resistance mechanism and failure behavior of the z-pins.

Out-of-plane seismic performance of bed-joint reinforced Autoclaved Aerated Concrete (AAC) infill walls damaged under cyclic in-plane displacement reversals

The infill walls made of Autoclaved Aerated Concrete (AAC), which is a lightweight, fire resistant and energy efficient material, provide effective insulation solutions for building types of structures and becoming more and more popular in earthquake prone regions. Although the number of experimental tests examining the seismic response of clay brick infills is extensive, the amount of prior research on infill walls built of AAC blocks is rather limited. Past research revealed that the use of bed-joint reinforcement is one of the promising solutions to improve the global seismic response of masonry walls by enhancing strength and displacement capacity. In this study, the out-of-plane (OOP) seismic performance of AAC infill walls with flat-truss and innovative cord-type bed-joint reinforcement is experimentally evaluated. Also, consideration is given to the prior in-plane (IP) damage, which was found to degrade the seismic performance of infills in OOP direction. For this purpose, three IP and four OOP, in total, seven experimental tests were performed on four full-scale AAC infill wall specimens. The test parameters were selected in such a way as to make it possible to parametrically compare the OOP performance of AAC infills with flat-truss and cord-type bed-joint reinforcements with unreinforced AAC infill walls, together with the effect of prior IP damage on the OOP response of unreinforced AAC infill walls. It was found that the use of innovative cord-type bed-joint reinforcement improved the OOP strength to a similar extent to what was obtained from the truss-type reinforced specimen. In terms of ultimate displacement and energy dissipation capacity enhancement, the specimen with cord-type reinforcement performed better. In addition, the damages formed due to IP cyclic displacement reversals up to 0.005 drift ratio, which is defined as the drift limit for buildings with brittle infill walls in certain design codes, resulted in a significant reduction in the OOP strength and stiffness properties of AAC infills. The theoretical OOP strength calculations were found to provide unconservative strength values for the IP-damaged specimens.

Experimental and numerical investigation on the failure behavior of Bouligand laminates under off-axis open-hole tensile loading

The Bouligand structure is a fibrous laminate with uniaxial layers assembled periodically into a helicoidal pattern. Compared to other types of laminates, its resistance to in-plane damage still lacks clarification. The failure behavior of Bouligand laminates under off-axis open-hole tensile (OHT) loading was investigated compared to cross-ply and quasi-isotropic structures using experiment and numerical simulation. After verifying the progressive damage model based on Hashin criteria, the numerical predictions from various off-axis loadings were compared between these three structures. The results show that Bouligand laminates exhibit low but nearly stable OHT strength. In comparison, the cross-ply structure exhibits high OHT strength variation when loaded in different directions. With the change of off-axis angle, the OHT strength of the Bouligand structure changes by 10.1 % from 405.2 MPa to 364.1 MPa, while it changes by 69.1 % from 533.8 MPa to 165.1 MPa for the cross-ply structure. Besides, the Bouligand structure exhibits higher OHT strength when the off-axis angle exceeds 5°. It also exhibits a more stable OHT strength than the quasi-isotropic structure. It is shown that the failure of fiber and matrix is the primary factor affecting the OHT strength of the laminate, while the delamination has little effect on the OHT strength.

Continuum damage mechanics behavior of a carbon-fiber-reinforced epoxy composite fabricated by filament winding with different material and manufacturing conditions

The elastic, plastic, and in-plane damage evolutions of a carbon-fiber-reinforced epoxy composite fabricated via filament winding were studied based on continuum damage mechanics. The specific voids and resin-rich region distributions depended on the material and manufacturing conditions. The damage evolutions were expressed by a unified curve, indicating that the continuum damage mechanics approach gave a robust prediction of the damage evolution, although the critical damage at the ultimate failure depended on the material and manufacturing conditions. The damage coupling parameters were also affected by these conditions, which changed the damage evolution behavior under simultaneous shear and transverse damage development situations.

Coupled VEM–BEM Approach for Isotropic Damage Modelling in Composite Materials

Numerical prediction of composite damage behaviour at the microscopic level is still a challenging engineering issue for the analysis and design of modern materials. In this work, we document the application of a recently developed numerical technique based on the coupling between the virtual element method (VEM) and the boundary element method (BEM) within the framework of continuum damage mechanics (CDM) to model the in-plane damage evolution characteristics of composite materials. BEM is a widely adopted and efficient numerical technique that reduces the problem dimensionality due to its underlying formulation. It substantially simplifies the pre-processing stage and decreases the computational effort without affecting the solution’s accuracy. VEM is a recent generalization to general polygonal mesh elements of the finite element method that ensures noticeable simplification in the data preparation stage of the analysis, notably for computational micro-mechanics problems, whose analysis domain often features complex geometries. The numerical technique has been applied to artificial microstructures, representing the transverse section of composite material with stiffer circular-shaped inclusions embedded in a softer matrix. BEM is used to model the inclusions that are supposed to behave within the linear elastic range, while VEM is used to model the surrounding matrix material, developing nonlinear behaviours. Numerical results are reported and discussed to validate the proposed method.

Cyclic thermal shock behaviors of carbon fibers reinforced SiCN ceramics with bio-inspired Bouligand-like architectures

Ceramic matrix composites (CMCs) are commonly used for high temperature components in aircrafts. However, thermal shock, as a typical loading case, will cause high thermal stresses in CMCs resulting in brittle fracture failure, and material cracking caused by thermal shock can further reduce the effectiveness of thermal protection function. In the present paper, we propose a bionic hierarchical fiber preform design method to improve the thermal shock resistance of ceramics. The effect of architectures of fiber preforms of continuous carbon fiber-reinforced CMCs on the thermal shock resistance was investigated to understand its importance and the related mechanical mechanisms. Thermal shock (cycling) tests were performed with continuous carbon fibers reinforced SiCN ceramic matrix composites (Cf/SiCN) prepared by PIP. 3D micro-CT scan and three-point bending tests were also conducted to evaluated the resultant damage. The results showed that smaller internal damage and higher thermal shock resistance can be obtained in comparison to pure SiCN ceramics, and the underlying mechanism can be explained by the fact that smaller pitch angle can resist the through-thickness crack propagation via promoting diffused in-plane damage. The present study offers a possibility in developing biomimetic Cf/SiCN ceramics with excellent thermal shock behavior.