Mixed-mode Failure Research Articles

Effect of artificial aging treatment on microstructure, mechanical properties and fracture behavior of 2017A alloy

The effect of artificial aging treatment on 2017A aluminum alloy microstructure, mechanical properties, and fracture behavior was investigated. The samples were taken from the as-received alloy and aged at 170 °C for 5, 10, 15, 20, and 30 hours. An optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), microhardness and tensile strength tests were used to characterize mechanical and microstructural properties. The microstructural analysis revealed that as the aging treatment duration is increased, the size and density of precipitates grow larger and more numerous. According to X-ray diffraction measurements, the microstructural evolution caused by aging treatments is primarily due to an increase in precipitation of the hardening phase -Al2Cu. According to the tensile test results, the yield stress increases with increasing aging duration. The fracture surface analysis of failed specimens subjected to tensile loading revealed that the aging treatment conditions had a significant impact on the morphology and mode of fracture: the mixed-mode (ductile-brittle) failure was established for intermediate aging durations (aging at 170 °C for 15 and 20 h), and the intergranular fracture was found to be more pronounced when the aging duration is prolonged due to the coalescence of fine precipitates at the grain boundary.

Experimental and Numerical Study of Fracture Behavior under Mixed-Mode of Al-Alloy AA3003 Not Welded and Welded by FSW Process

This study presents an experimental and computational analysis of fracture under mixed-mode conditions in aluminum alloy AA3003 using the Compact Tension Shear CTS specimen, both not welded and Friction stir welded. Mixed-mode fracture experiments were performed using the CTS specimen and an ARCAN loading device based on Richard's principle suitable for mixed-mode. The approach of linear elastic fracture mechanics allows for a better understanding of mixed-mode failure and the evaluation of the stress intensity parameters KI and KII. The variation of the stress intensity factor KI, KII is influenced by the pre-cracks length. The comparison between experimental results and the numerical results of simulation shows that there is a good agreement between these results.

Reflectance Transformation Imaging for the quantitative characterization of experimental fracture surfaces of bonded assemblies

Controlling failure modes is an important issue for the development of bonded assemblies of multi-material structures in industry. The analysis of the failure modes of bonded assemblies, and in particular the evaluation of the adhesive/cohesive failure rate, constitute a significant issue. Generally, it is considered in industrial applications that obtaining a cohesive fracture makes it possible to generate better predictions regarding the rupture of the adhesive joints, even if recent work has shown that a good prediction of the mechanical behavior can also be obtained when adhesive or mixed failure modes occur. Currently, these evaluations are carried out by experts by means of a visual analysis, either directly on the fracture surfaces, or sometimes from high-resolution microscopy. The approach presented in this paper aims to set out a methodology based on the Reflectance Transformation Imaging technique for the objective characterization of experimental fracture surfaces of bonded assemblies. We show how this imaging technique can be used to quantify objective surface features, and how these local descriptors can be used to estimate and map out the failure mode on the fracture surfaces.

A 2D generic multi-surface cohesive zone model for simulating FRP-to-concrete mixed-mode debonding failure

This paper proposes a 2D generic multi-surface cohesive zone model to simulate the debonding failure process of FRP-to-concrete joints within the framework of plasticity theory. The model combines a tension cut-off yield surface with a Coulomb friction yield surface, which can accurately predict the coupled mixed-mode debonding behaviour. Non-associated flow rule is adopted by the Coulomb friction yield formulation to reproduce the dilation effect, while the tension cut-off criterion is responsible for simulating the separation in the normal direction between two bonded substrates. Given the potential non-convergence problems caused by the presence of vertex region at the intersections between two yield surfaces, a remedy measure using the implicit backward Euler integration method incorporating Koiter’s rule and coupled independent state variables is applied. The proposed 2D interface model is versatile, and its accuracy has been validated by comparing the predicted results with their test counterparts from literature in different aspects, including the capacities of capturing mixed-mode failure, dilation behaviour, lateral compression effect, FRP deformation, and local bond stress-slip relationship. The model’s good compatibility with the constitutive relationship of CDP (concrete damaged plasticity) is also demonstrated.

Influence of accelerated corrosion on bi-material steel-CFRP double-lap joints bonded with thick adhesive

Bi-material steel-composite joints attract interest for marine applications. The marine environment imposes corrosion which is a prolonged process. This work presents a two-electrode electrochemical setup for accelerating free corrosion of the steel surfaces of bi-material joints. It is used to study the impact of accelerated corrosion on the mechanical performance of bi-material double lap specimens subjected to quasi-static tensile testing. Several test series have been conducted to evaluate the influence of overlap length and bond line thickness on shear strength and failure mode. Sixty specimens with a thick layer of methyl methacrylate adhesive have been fabricated and cured at room temperature for at least 24 h. Subsequently, 30 specimens were aged by subjecting them to electrochemical corrosion for 24 h. All specimens were tested for failure in quasi-static tensile loading while monitoring the strain fields in the joint area using digital image correlation. The measurements reveal a homogeneous shear strain field at the onset of loading, with a rapidly increasing shear strain concentration near the edges of the bond line preceding final failure. Both a decrease in the adhesive thickness and an increase in the overlap length increase the shear strength. Higher shear strength was observed for the electrochemically aged specimens than that of non-aged specimens. This is attributed to the faster residual curing of the adhesive during ageing because an increasing percentage of copper ions (released from the anode) accelerates the curing of the MMA adhesive. The electrochemically aged specimens showed mixed failure modes, i.e. cohesive failure and adhesive failure at the interface between steel and adhesive, and skin failure within the composite laminates.

Peridynamic analysis of materials interface fracture with thermal effect

A thermomechanical peridynamic framework is established for fracture analysis of dissimilar material interface coupled with residual thermal effect. In this frame, an extended peridynamic mixed-mode failure model considering thermal effect is proposed for interfacial fracture prediction, and the peridynamics-based methods for interface crack energy release rate and mode mixity computations are given. Meanwhile, a new peridynamic contact model is proposed for frictional contact modeling of crack surfaces. Then, three examples of the single edge-notched bimaterial (SENB), asymmetric bimaterial cantilever beams (ABCB) and four-point shearing (FPS) tests, are analyzed for the model verification and application. The elastic and interfacial fracture behaviors of these tests are predicted by the proposed peridynamic models, and compared to the analytical and the FEM solutions. The results show that the proposed peridynamic models can successfully predict the failure characteristics of materials interface, and capture the role of residual thermal effect on the interfacial fracture.

A coupled smeared crack-plasticity model in incremental sequentially linear analysis for mixed failure modes

In this paper, a coupled smeared crack-plasticity model is proposed in incremental sequentially linear analysis (ISLA) to simulate mixed failure modes of tension, shear and compression. ISLA provides a robust and extensible solution for quasi-brittle materials. The solution search path follows damage cycles sequentially with secant stiffness corresponding to local damage increments, which traces both damage history (explicit) and displacement history (implicit). The proposed rotating smeared crack model resolves the limitations of a purely rotating crack model in the case that the smallest and the largest principal stress value are almost the same or the situation that the principal directions interchange between the beginning and the end of a load step. ISLA with the proposed constitutive model were demonstrated by mixed flure modes of the in-plane tests with large overburdens of a double clamped short wall and a cantilever long wall. Stable post-peak results were computed for large displacements, and localized crack propagation was computed robustly and correctly.

Study of the fracture behaviour in hybrid fibers reinforced thermoplastic laminates: Influence of temperature and initial notch orientation

The present work was aimed at investigating the failure of quasi-isotropic fibers reinforced thermoplastic laminates solicited at different testing temperatures. Single-edge-notch bending (SENB) tests were conducted at room temperature (RT) and at a temperature higher than the glass transition temperature (Tg) to investigate the influence of failure mode (depending on initial notch orientation) as well as matrix ductility and toughness (depending on testing temperature) on: (1) damage mechanisms – (2) critical translaminar fracture toughness Gc and (3) G-R curves evolution. The mixed mode fracture toughness GI+II,c is dramatically lower than the mode I fracture toughness GIc, with −52% and −67% decreases, at RT and 150 °C respectively. In 45° notched specimens, most of the fracture energy (about 80%) stems from the mode I failure. Though they are not prominent on fracture, the failure mechanisms associated with mode II are instrumental in limiting the contribution of mode I on fracture behavior. Finally, though a temperature increase has very little influence on GIc, it significantly reduces the value of GI+II,c (−32%). This change primarily results from the formation of plastic-kink bands in compression that are promoted by both the ductility of the polyether ether ketone (PEEK) matrix at T > Tg and the mixed-mode failure.

Shear failure process of rectangular tunnel: Physical experimental test and numerical back-analysis

Shear failure is a typical failure mode in underground engineering and has resulted in a large number of instability issues. To study the shear failure mechanism, physical shearing tests for rectangular tunnel of sandstone, including single rectangular tunnel (SRT) and double rectangular tunnels (DRT) specimens, were carried out under three normal stress states. During the shearing tests, the internal failure process and the external cracks evolution of tunnel specimens were recorded by video camera, and the acoustic emission (AE) events of specimens were also monitored for exposing the mesoscopic failure characteristics. In addition, the discrete element method was also applied to analyze the corresponding shear experiment of rectangular-tunnel specimens. The tested results showed that the shear strength, shear modulus of the samples were positively correlated with the normal stress. For example, the shear strength, and shear modulus of SRT increased from 5.18 MPa to 8.70 MPa, and 1.02GPa to 1.88GPa, respectively when the normal stress increased from 2.0 MPa to 5.0 MPa. More importantly, the shear failure of rectangular tunnel model can be generally divided into four stages: (i) Shearing compression stage characterized by loading contacting and compressing; (ii) Shearing micro-crack propagation stage dominated by inner particles ejection of tunnel, tensile crack initiation and shear crack propagation of specimen; (iii) Shearing fracture stage dominated by the specimen’s crack propagation, coalescence, penetration, and corresponding rock ejection; (iv) Shearing friction stage dominated by shear damage of tunnel specimen. Corresponding back analysis by numerical simulation and scanning electron microscope results indicated that the shear failure of rectangular-tunnel specimen can be classified as tensile-shear mixed failure mode. The research results provide a new cures for shear prevention and support design of rectangular tunnels in underground engineering.

Fatigue Damage Diagnostics–Prognostics Framework for Remaining Life Estimation in Adhesive Joints

This work presents an integrated damage diagnostics–prognostics framework for remaining useful life (RUL) estimation in the adhesively bonded joints under fatigue loading. A matching pursuit algorithm is proposed as the diagnostics technique for estimating the damage extent followed by the fatigue damage growth (FDG) simulator as the predictive model for simulating fatigue degradation. The framework calibrates the FDG simulator by quantifying uncertainties in fatigue model parameters using the damage extent data. Bayesian inference via the Markov chain Monte Carlo method is used to quantify uncertainties and estimate the probabilistic RUL from the quantified uncertainties. The FDG simulator encompasses a physics-based fatigue damage degradation model with an artificial neural network-based hybrid machine-learning model for tracing the damage progression. In the diagnostic technique, ultrasonic guided waves are excited into the structure using a pair of piezoelectric wafers, and the damage extent is quantified by reconstructing the reflected signal from the bond region. The proposed diagnostic technique is verified using the ultrasonic signal obtained from the finite element simulations. The damage prognostics part of the integrated framework is verified by estimating RUL in a mixed-mode failure joint specimen using the experimental fatigue damage progression data. In addition, the integrated framework is then verified by estimating RUL in two adhesively bonded joints: a single lap joint and a tapered single lap joint using Gaussian noise added synthetic data and diagnostic damage extent data.

A combined phase-field and cohesive zone model approach for crack propagation in layered structures made of nonlinear rubber-like materials

The development of new high-tech products often requires the optimization of structural components having layer-based arrangements. The primary focus of the present study is the analysis of the failure mechanisms that may occur in thin layer-flexible substrate systems: crack propagation through the layers, delamination at interfaces, and mixed-mode mechanisms. The phase-field (PF) approach, formulated for hyperelastic materials, is herein exploited through a series of tests to prove its capabilities in simulating the mechanical response and the crack propagation mechanisms of these composite structures. While the PF approach already captures the competition between crack propagation and delamination at the interface for bilayer composites having properties mismatch, as in the case of metallic or ceramic layers on rubber-like substrates, the framework has been further enriched with a Cohesive Zone Model (CZM) approach to model imperfect interfaces. The benefit of the coupled method can be noticed, especially for mixed-mode failure patterns, where this combination of failure mechanisms is successfully captured.

Experimental study on crack overlaps between two blast-induced cracks in PMMA

Phenomena of crack overlaps are very common and essential in tunnel constructions by rock blasting. In this study, in conjunction with the high-speed camera, directional fracture controlled blasting experiments are performed on three sets of PMMA specimens using the optical caustics method. The lenticular fragments formed as two cracks approach each other are measured and compared, as well as other fracture parameters, such as crack velocity, mode I and II dynamic stress intensity factors and curvature of crack propagation path. It is found that both mode I stress intensity factor and crack velocity exhibit an abrupt rise upon crack interaction. Further, mode II stress intensity factors show more complicated variations in comparison with mode I, divided into three loading regimes, i.e. crack initiation, blast stress wave loading and crack overlap. And an appropriate ratio of crack offset distance/horizontal distance (h/s) between two boreholes for apparent mixed mode failure exists.

Interfacial Fracture Energy Between New Translucent Zirconias and a Resin Cement.

To determine the interfacial fracture energy (IFE) and stress distribution of Brazil-nut-shaped specimens made of translucent zirconia and resin cement. Three types of translucent zirconia were used: 3Y-TZP (high, Vita YZ HT), 4Y-TZP (super, Vita YZ ST), and 5Y-TZP (extra, Vita YZ XT). The adhesive surfaces were air abraded and 10-MDP-based resin cement was used. The cemented Brazil-nut-shaped specimens, with an elliptical defect in the center (as in real Brazil nuts), were thermally aged (5°C-55°C; 40,000 cycles). The IFE test was conducted with a piston to apply compression on the specimen, while the adhesive interface was positioned at four different angles (0, 10, 20, and 30 degrees) to measure the IFE during tensile, shear, and mixed failure modes. All adhesive interfaces were observed to determine failure patterns. The finite element analysis (FEA) was used to calculate tensile and shear stress distributions according to inclinations. Statistical analysis was conducted using the Kruskal-Wallis and Dunn's post-hoc tests (95%), as well as the Mann-Whitney test (95%) was applied to compare each group regarding the aging factor. According to Kruskal-Wallis and Dunn's post-hoc tests, there were no statistically significant differences between non-aged (p > 0.05) and aged materials (p > 0.05). However, there was a significant difference between aged and non-aged materials for all inclinations (p < 0.05) (Mann-Whitney test). According to the FEA, the compressive loading of Brazil-nut-shaped specimens at different angles showed a predominance of tensile stress at 0 degrees and shear stress at 30 degrees. The IFE under predominantly shear stresses is higher than when specimens are subjected only to tensile stresses, which allows the interpretation that failures in the oral environmental will probably occur preferentially under tensile stresses, because less energy is needed. All translucent zirconia bonded to resin cement has similar IFE, and thermal aging negatively affects these bonding interfaces.

A microstructure informed and mixed-mode cohesive zone approach to simulating hydrogen embrittlement

Hydrogen induced failure under uniaxial tension is simulated in a duplex stainless steel considering microstructural feature of the material. There are three key ingredients in the modelling approach: image processing and finite element representation of the experimentally observed microstructure, stress driven hydrogen diffusion and diffusion coupled cohesive zone modelling of fracture considering mixed failure mode. The microstructure used as basis for the modelling work is obtained from specimens cut in the transverse and longitudinal directions. It is found that the microstructure significantly influences hydrogen diffusion and fracture. The austenite phase is polygonal and randomly distributed in the transverse direction, where a larger effective hydrogen diffusion coefficient and a lower hydrogen fracture resistance is found, compared to the specimen in the longitudinal direction, where the austenite phase is slender and laminated. This indicates that the proper design and control of the austenite phase help improve hydrogen resistance of duplex stainless steel. The strength of the interface in the shear direction is found to dominate the fracture mode and initiation site, which reveals the importance of considering mixed failure mode and calibrating the hydrogen induced strength reduction in shear.

Bond degradation and EMI-based monitoring of CFRP to concrete interfaces exposed to wet-dry cycling

This study aims to examine the bond behaviour of CFRP to concrete interfaces that are exposed to wet-dry (WD) cycling and to explore the applicability of an electromechanical impedance (EMI)-based monitoring method for the bond via the use of piezoelectric transducers (PZTs). Single shear tests were performed on CFRP-concrete interface specimens subjected to a maximum of 360 WD cycles. WD cycling causes a shift in the debonding failure mode from cohesive failure within the concrete to a mixed mode of cohesive failure and adhesive failure. The bond capacity of the conditional specimens did not decrease significantly until 270 WD cycles, indicating that severe bond degradation occurred after 270 WD cycles. Moreover, a generalized method that yields representative evaluation results was applied based on the load response curve, and the bond slip relationship was proposed for the interfaces under WD cycling. In addition, an EMI method that is based on PZTs was employed to monitor the CFRP-concrete interface during exposure to WD cycling and single shear tests. It can be found that EMI monitoring can monitor WD cycling-induced bond degradation and interfacial debonding damage that occurs in single shear tests.

Comparative analysis of dynamic progressive failure of CFRPs using the scale-span and macroscopic modeling method in drilling

The focus of this paper is to explore the effects of different failure criteria, including Tsai-Wu, Hashin, Chang-Chang and micromechanics of failure criterion, on the dynamic progressive failure properties of carbon fiber reinforced polymers (CFRPs) in drilling. First, a unified finite element (FE) formulation on the dynamic intra-laminar damage and inter-laminar delamination of laminates was developed. Then, the intra-laminar damage model using four failure criteria were implemented by the explicit FE subroutine ABAQUS/VUMAT. Meanwhile, the delamination is simulated by the developed bilinear cohesive elements (CEs) model with mixed-mode failure criterion. Finally, numerical analysis and experiments were performed on drilling T700S-12K/YP-H26 CFRPs laminates to compare the predicted thrust force, torque and the visualized damage phenomenon. Results show that the prediction accuracy of the thrust force and torque is significantly improved. The actual damage behaviors which appear the tearing at the entry, pits of hole-wall’s surface, burrs and interlayer delamination at the exit are simulated when adopts the scale-span modeling method. In contrast, it is required to take more slightly simulation time under the same condition. This research provides fundamental support for appropriate selection and use of different failure criteria to achieve high-efficiency and high-fidelity simulation in the drilling of composites.

Study on the Fire Resistance of Concrete-filled Steel Tubular Column to Composite Beam Joints after Earthquake Damage

Background: Post-earthquake fire is one of the secondary disasters caused by the earthquake; many earthquakes around the world have brought about severe fires and led to enormous losses. It is of great significance to study the fire resistance of structures after earthquake to improve the level of urban disaster prevention and reduce the fire loss. Methods: A nonlinear finite element model of composite joints subjected to post-earthquake fires is established based on ABAQUS software. The state after the earthquake damage is imported into the thermal-stress analysis model through the restart request in ABAQUS, and considered as the initial state of the thermal-stress analysis model. Results: It is noticed that when the damage index is less than 0.517, the damage level shows significant effect on the fire resistance, but the influence is no longer significant when the damage index reaches 0.781. When only the column is exposed to fire, the column showed obvious local buckling and overall bending deformation. Meanwhile, when the joint area below the concrete slab is exposed to fire, the joint shows a mixed failure mode of the excessive vertical displacement of beam end and column instability. Conclusion: The fire resistance of the composite joint decreases gradually with the increase of seismic damage level. The fire condition has a significant effect on the fire resistance of the composite joints; with the increase of fire area, the fire resistance of composite joints with same damage degrees decreases gradually.

Mechanical behavior and failure mechanisms of CFRP and Titanium tubular adhesive lap joints at extreme temperatures

In this study, the mechanical behavior and failure mechanisms of thin-walled carbon fiber reinforced polymer (CFRP) and titanium adhesive tubular lap-joints (TLJs) at cold-temperature, room-temperature, and elevated-temperature are investigated. The prediction of TLJ’s performance at extreme temperatures is challenging due to their non-linear behavior and competing damage modes. To better understand their behavior, finite element analysis, static tensile tests, and design of experiments are employed to investigate the competing damage mechanisms within the TLJs, leading to a more detailed damage classification at extreme temperatures. Results show a mixed-mode failure region at elevated-temperature, where TLJs fail by either interlaminar shear or titanium net-section fracture. X-ray micro-computed tomography reveals that the initiation, propagation, and coalescence of delamination at the inserted end of the CFRP are created by shear stresses during deformation of the titanium. By utilizing numerical, experimental, and statistical techniques, this work aims to provide insights into the parameters and damage mechanisms affecting the performance of adhesively bonded thin-walled TLJs at extreme cold and hot temperature conditions.

Seismic behavior of 3-D beam to L-shaped concrete encased steel composite column joint: An experimental study

Experimental investigations were conducted on the behavior of three-dimensional (3-D) beam to L-shaped concrete encased steel (CES) composite column joint under seismic loading. A total of nine corner joints, including eight joint specimens with steel beam and one joint specimens with CES beam, were designed and tested. Test variables included the configuration of encased steel, axial compression ratio, loading angle and beam type. Failure patterns, hysteretic relations, deformation and strain distribution of these joints were recorded and analyzed. The seismic behavior was evaluated from peak load, stiffness and strength degradation, ductility and energy dissipation for these specimens. Test results indicated that the mixed failure mode acted on 3-D L-shaped CES column joint, including compression-shear failure in the joint core and bending failure at the beam-column connection. Seismic behavior of specimen with solid-web steel and T-shaped steel truss was much better than that of specimen with U-shaped steel truss. As the loading angle decreased from 45° to 0°, the peak load of joint specimen decreased continuously. Although reasonably increasing the axial compression ratio improved the bearing capacity and energy dissipation capacity of joint specimen, it also degraded the ductility and collapse resistance. Moreover, the joint with CES beam had better seismic behavior compared with joint with a steel beam. In this study, the ultimate drift ratio of all joint specimens exceeded the minimum limit of 1/50 in Chinese and American codes, indicating that 3-D L-shaped CES column joint had excellent collapse resistance.

Failure Behaviour of Carbon-Epoxy Composite in Crack Arrester and Crack Divider Mode

Failure behaviour of a carbon-epoxy composite is studied both in crack arrester (CA) and crack divider(CD) mode by conducting 3-point bend test of un-notched (UN) and single edge pre-crack notched (N) (notch depth varied from 0.5 to 3.3 mm) specimens at various cross head speeds(CHS) such as 0.2, 50, 100, 200 and 500 mm/min. Influence of notch depth (ND) and CHS on the conditional fracture toughness(CFT) is understood using ANOVA. In both the mode, while the CFT values varies linearly with CHS, its values are observed to be lower at short ND, higher at medium ND and again lower at high ND. In both the modes at high ND (~3.3mm) opening mode/fibre breaking (mode I) failure occurs because of high crack tip stress concentration (as analysed from ANSYS 15.0) which cause fibre failure at the tip and corresponding Mode I fracture toughness was observed to be~20 MPa.m0.5. The composite also shows similar flexural strength both in CA mode (641+48) and in CD mode (634+49 MPa). In CD mode N specimen predominantly fails by mode I. While in CD mode the delamination (mode II) is only observed in UN specimens, in CA mode UN or small N specimen shows mode II failure and medium notched specimen shows mixed mode failure. In CA mode, number of delaminated layer is higher for the specimen tested at high CHS. In CA mode during charpy test (strain rate 103/s), the composite shows numerous delamination and absorbed much higher energy(~40J/cm2) than that in CD (~10J/cm2) mode where mode I failure is predominant. During impact test, lowering of test temperature to -40oC has very little effect on the impact energy since it does not significantly affect the mode of failure.