•Mode I fatigue delamination tests are conducted and mode II R-curve behaviour of fatigue delamination has been observed.•A fatigue delamination model is proposed to characterize the combined effects ...of fiber bridging and stress ratio on the mode II fatigue delamination growth behaviour.•All fatigue data collapse into one single master curve in the log–log plot if using the proposed model.•The model has been further validated by extra public fatigue data of other material type and interfaces.
Shear-dominated delamination is among one of the most common damages types. Deep understanding of the mode II fatigue delamination growth (FDG) behaviour is critically important for the damage tolerance design of composite laminates. Researchers have paid much attention to mode II FDG behaviour in unidirectional laminates and its stress ratio effect. However, the FDG behaviour in the widely used multidirectional laminates with effects of fiber bridging is still lack of studies. This work investigates FDG behaviour of multidirectional laminates under three kinds of stress ratio by using end-notch flexure set-up. Corrected beam theory with effective crack length is applied for calculating strain energy release rate (SERR), which can avoid visual observation for the delamination length. The R-curve behaviour of fatigue delamination has been observed. In addition, a FDG model has been proposed to characterize the combined effects of fiber bridging and stress ratio on the mode II FDG behaviour. It is found that all fatigue data collapse into one single master curve. The proposed model is also validated by data from public literature results.
This paper aims to discuss the effect mode II type tests have on the cohesive law of bonded joints. For this reason, an objective inverse method has been developed to extract the cohesive law for ...experimental load-displacement data using an analytical procedure to represent the fracture process zone of mode II tests. The method is implemented on the more popular and standardized mode II tests: End-Notched Flexure and End-Load Split. The analysis of the experimental data shows that the cohesive law is independent of the test type. In addition, the obtained fracture toughness calculated by integrating the cohesive law shows good agreement with the J-integral method.
Knowledge of the fracture properties of composite materials is fundamental to predict the impact behaviour of the lightweight structures currently used in the automotive industry. Although there is ...substantial research on mode I fracture behaviour of composites, limited information exists on mode II behaviour. This work aims to fulfil this gap, presenting experimental data regarding the influence of temperature and strain rate on the fracture energy in mode II, GIIC, of composite plates. Significant influence of the strain rate and temperature on GIIC of the composite materials was found. Higher strain rates led to a decrease of GIIC up to 17%. An increase of temperature corresponded to an increase of GIIC up to 15% and a decrease of temperature originated a decrease of GIIC up to 7%.
This article investigates the shear behaviour of Recycled Aggregate Concrete (RAC) Z push-off specimens. Fifteen specimens with different replacement levels of recycled concrete aggregate (RCA = 0 %, ...25 %, 50 %, 75 % and 100 %) were tested. It is shown that a 100 % RCA replacement level reduces shear strength by 17.3 %. The shear behaviour of the specimens was further analysed using nonlinear finite element analysis (FEA). The results show that the shear strength results from the FEA and Digital Image Correlation measurements agree well (within 5 %) with the experimental results. This study proposes a new semi-empirical equation to calculate the shear strength of specimens with different RCA replacement levels. The new equation adopts a fracture mechanics approach, and it explicitly considers the shear slip deformation and crack opening. Compared to existing models, the new equation fits better the experimental data in this study, as well as test results from an extensive database obtained from the literature.
Existing standards for delamination tests on composite materials typically employ one-dimensional (1D) beam specimens. However, such specimens may not represent real delamination scenarios in ...composite structures, where cracks tend to propagate in two dimensions. To address this limitation and compare the delamination behavior under both 1D and two-dimensional (2D) Mode-II fracture conditions, a numerical investigation was carried out based on previous experiments. A novel cohesive zone model, considering both microcracking and fiber bridging within the fracture process zone, was developed using a semi-experimental approach and incorporated in three-dimensional finite element simulations. According to the numerical results, the investigated laminates exhibited similar maximum strain energy release rates in both 1D and 2D delamination; however, different traction-separation responses were obtained. Practical methods of locating the tip of an embedded crack were proposed based on curvature and strain measurements on the laminate surface, demonstrating potential for application in structures with irregular crack shapes.
This article introduces a new method developed for characterizing the mode-II dominant interfacial fracture in a layered system composed of materials with significantly different mechanical ...properties. The proposed method consists of 3 parts. First, using digital image correlation (DIC) technique to acquire the full field strain in layered composites and then calculating the variation of elastic strain energy in multiple loading steps. Second, using DIC to acquire the relative displacement between layers and then calculating the possible strain energy release rates in multiple loading steps based on a presumptive bi-linear cohesive zone model (CZM) with unknown parameters. Third, developing an algorithm to find the optimal solutions for the presumptive bi-linear CZM by equilibrating the elastic strain energy and fracture energy with the minimum error in multiple loading steps. For verification, the proposed method was applied to characterize the mode II dominated interface fracture of a bi-material and bi-layer system under tension. The feasibility and accuracy of the proposed method are experimentally demonstrated.
•Mode II fatigue crack growth with biaxial compression was studied in bearing steels.•Normal compression hindered O2 access into the closed crack & favored adhesive wear.•Friction-corrected crack ...growth kinetics were obtained using DIC and FE simulations.•The apparent crack face friction coefficient rose, due to an oxidation gradient.•Many aborted branches quasi-orthogonal to the main crack reduced its driving force.•Both normal and parallel compressions favored mode II growth rather than branching.
Mode II fatigue crack growth under reversed shear and static biaxial compression was investigated in two bearing steels. Many aborted branches, quasi-orthogonal to the main crack, were observed along the crack face. The compressive stress parallel to the main crack hindered the growth of these branches and favored coplanar mode II crack growth. The crack face sliding displacement profiles measured by DIC were used to derive ΔKII,eff, at the main crack tip, using elastic–plastic FE simulations with crack face friction, by an inverse method. Friction-corrected crack growth kinetics were obtained for mode II crack growth in both steels.
•The effects of wetting and drying cycles on rock fracture toughness and crack propagation were investigated using centrally cracked Brazilian disk specimens.•The degree of degradation of KIIC is ...always larger than that of KIC.•The ratio of KIIC to KIC decreases with increasing number of wetting–drying cycles.
The effect of cyclic wetting and drying on the fracture toughness and crack propagation of rock has been studied using centrally cracked Brazilian disk specimens. The results show that the failure characteristics of sandstone change from brittle to ductile at higher wetting–drying cycles. With increase in the wetting–drying cycles both KIIC and KIC decrease, furthermore, the degradation degree of KIIC is always larger than that of KIC. The ratio of KIIC to KIC decreases with increase in the number of wetting–drying cycles. The experimental results and theoretical values of the fracture criterion agree very well with each other.
•The Eccentric End Notched Flexure (EENF) test is analyzed.•A compliance based method to obtain the mode II cohesive law in the EENF test is presented.•Based on the equivalent crack length approach ...including bond line thickness effect.•An extrapolation procedure is proposed to predict the mode II cohesive law.•The advantages and the suitability of the data reduction scheme are numerically confirmed.•The validity of the extrapolation procedure is experimentally confirmed.
A novel extrapolation procedure to predict the mode II cohesive laws of adhesive joints is presented. At first, a recently proposed compliance based experimental method to extract mode II Cohesive Laws is extended to the eccentric end-notched flexure test EENF and generalized including the effect of the bond line thickness and to this end, improved expressions for the compliance, J-Integral and shear displacement at the crack tip are derived.
Assuming that every effect associated to the damage is included in the equivalent crack length, new expressions related to the Compliance (C0), J- Integral (J0) and crack tip shear displacement (Δ0) are defined and invariant relations between J0-Δ0 and Δ0- C0 are elicited for a given material system and test configuration.
Finally, an extrapolation procedure is presented, based on the J0-Δ0 and Δ0- C0 calibrated curves, which enables to estimate the cohesive laws for a wide range of adhesive to adherend ratio of a given material system by processing only the load –displacement curve.
Z-pin through-thickness reinforcement is used to improve the impact resistance of composite structures; however, the effect of loading rate on Z-pin behaviour is not well understood. The dynamic ...response of Z-pins in mode I and II delamination of quasi-isotropic IM7/8552 laminates was characterized experimentally in this work. Z-pinned samples were loaded at both quasi-static and dynamic rates, up to a separation velocity of 12m/s. The efficiency of Z-pins in mode I delamination decreased with loading rate, which was mainly due to the change in the pin misalignment, the failure surface morphology and to inertia. The Z-pins failed at small displacements in the mode II loading experiments, resulting in much lower energy dissipation in comparison with the mode I case. The total energy dissipation decreased with increasing loading rate, while enhanced interfacial friction due to failed pins may be largely responsible for the higher energy dissipation in quasi-static experiments.
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