International Journal of Fracture最新文献

In this letter, we present interfacial fracture toughness data for a polymer-metal interface where tests were conducted at various test temperatures T and loading rates (dot{delta }). An adhesively bonded asymmetric double cantilever beam (ADCB) specimen was utilized to measure toughness. ADCB specimens were created by bonding a thinner, upper adherend to a thicker, lower adherend (both 6061 T6 aluminum) using a thin layer of epoxy adhesive, such that the crack propagated along the interface between the thinner adherend and the epoxy layer. The specimens were tested at T from 25 to 65 °C and (dot{delta }) from 0.002 to 0.2 mm/s. The measured interfacial toughness Γ increased as both T and (dot{delta }) increased. For an ADCB specimen loaded at a constant (dot{delta }), the energy release rate G increases as the crack length a increases. For this reason, we defined rate effects in terms of the rate of change in the energy release rate (dot{G}). Although not rigorously correct, a formal application of time–temperature superposition (TTS) analysis to the Γ data provided useful insights on the observed dependencies. In the TTS-shifted data, Γ decreased and then increased for monotonically increasing (dot{G}). Thus, the TTS analysis suggests that there is a minimum value of Γ. This minimum value could be used to define a lower bound in Γ when designing critical engineering applications that are subjected to T and (dot{delta }) excursions.

The single-peak overload test based on DIC technology was carried out in this study, and U71MnG steel was used to explore the influence of dislocation motion on crack propagation during overload. The changes in the shape and size of the plastic zone during the overload fatigue cycle are tracked and recorded, and the trends in the stress intensity factors of the Christopher–James–Patterson (CJP) and dislocation correction models are compared. The degree of influence of the dislocation motion on the variation in the stress intensity factors is evaluated, and the variation pattern of the plastic flow factor is derived (the amount of crack tip blunting, ρ). The results showed that the dislocation correction model increased the accuracy of the solution of the coefficient set, and the predicted size of the plastic zone of the correction model was more consistent with the experimental data. A better match with the crack tip was observed in the experimental plastic zone, the dislocation correction model error with the theoretical plastic zone fluctuates within 10%, whereas the CJP model can reach a maximum of 36.75%, demonstrating the insensitivity of the dislocation correction model. The plastic flow factor ρ follows the same pattern as that of the plastic zone area and stress intensity factor amplitude, ρ increases slowly with the increase of crack length before overload, ρ increases significantly after overload and then decreases sharply, and it recovers to be stable with the disappearance of the overload hysteresis effect of crack propagation.

This study compares the fatigue life of a 7475-T7351 aluminum alloy lower wall plate in an aircraft beam structure under alternating corrosion and fatigue conditions to universal fatigue life. It incorporates a corrosive environment and variable amplitude fatigue loads. The current study uses the “beach marking” technique and visual inspection to monitor crack propagation and evaluate the corrosive environment’s impact on fatigue life and damage tolerance. The experimental results indicate that during the fatigue crack initiation and penetration stages, the corrosion environment does not significantly impact the fatigue life of the beam structure because of the protection from uniform oxide films, epoxy primer, and sealants at joints. In the crack propagation stage, the corrosive environment speeds up crack growth compared to universal fatigue tests. Additionally, a “hysteresis effect” in alternating corrosion and fatigue tests shows the fatigue crack growth rate changing discontinuously, caused mainly by corrosion dissolving slip bands at the crack tip. Altogether, this study provides new insights into the influence of alternating corrosion and variable amplitude load on an aircraft beam structure’s fatigue life and damage tolerance.

In this study, AlSi10Mg samples were manufactured by laser-powder bed fusion process to explore the fracture toughness dependence on both built orientation and aging treatment. The experiments were performed on as-built and directly-aged (200(^{circ })C/4 h) conditions, with the latter revealed to be a valuable treatment for improving fracture toughness. A comprehensive investigation involving detailed microstructural analysis, grain-orientation mapping, and crack-tip strain measurements was conducted to investigate the mechanisms governing the material behavior. The results revealed that specimens subjected to direct aging display higher toughness, thereby enhancing the fracture resistance of AlSi10Mg. Moreover, a considerable variation in fracture toughness values was observed for the different printing orientations, indicating the existence of manufacturing-induced anisotropy. The findings highlight that this anisotropy mainly correlates with the distinctive microstructure induced by the additive manufacturing process. In particular, this study focuses on the different preferential crack paths dictated by the melt pool boundaries orientation respective the crack propagation direction. A substantial reduction in fracture toughness was observed when the crack propagates along the melt pool boundaries.

To improve the fatigue performance of laser-welded TC4 titanium alloy joints, ultrasonic rolling processing (USRP) is employed herein. Multiple passes of USRP (viz., one, three, and five) are conducted using an ultrasonic rolling device. Results reveal that USRP considerably improves the fatigue limit and life of the welded joints. At room temperature, the fatigue strength of the weldment increases by 2.04–4.58% and the corrosion fatigue life increases by 1.72–2.88 times. In addition, to reveal its strengthening mechanism, the effects of USRP on the surface morphology, microstructure, surface residual stress, and microhardness of the laser-welded TC4 titanium alloy joints are investigated. USRP leads to a shift in the crack initiation point to the subsurface and formation of a hardened layer with high residual stress on the surface via the application of considerable static pressure input and multiple passes. Consequently, fatigue striations become narrower and denser. Compared to the traditional weld surface treatment, USRP substantially improves the surface quality and fatigue performance of laser-welded TC4 titanium alloy joints.

In this expository Note, it is shown that the Griffith phase-field theory of fracture accounting for material strength originally introduced by Kumar, Francfort, and Lopez-Pamies (J Mech Phys Solids 112, 523–551, 2018) in the form of PDEs can be recast as a variational theory. In particular, the solution pair ((textbf{u},v)) defined by the PDEs for the displacement field (textbf{u}) and the phase field v is shown to correspond to the fields that minimize separately two different functionals, much like the solution pair ((textbf{u},v)) defined by the original phase-field theory of fracture without material strength implemented in terms of alternating minimization. The merits of formulating a complete theory of fracture nucleation and propagation via such a variational approach — in terms of the minimization of two different functionals — are discussed.

Phase-field models of brittle fracture are typically endowed with a decomposition of the elastic strain energy density in order to realistically describe fracture under multi-axial stress states. In this contribution, we identify the essential requirements for this decomposition to correctly describe both nucleation and propagation of cracks. Discussing the evolution of the elastic domains in the strain and stress spaces as damage evolves, we highlight the links between the nucleation and propagation conditions and the modulation of the elastic energy with the phase-field variable. In light of the identified requirements, we review some of the existing energy decompositions, showcasing their merits and limitations, and conclude that none of them is able to fulfil all requirements. As a partial remedy to this outcome, we propose a new energy decomposition, denoted as star-convex model, which involves a minimal modification of the volumetric-deviatoric decomposition. Predictions of the star-convex model are compared with those of the existing models with different numerical tests encompassing both nucleation and propagation.

Mesh size dependency caused by strain localization is an ongoing problem in numerical simulations using the finite element method. In order to solve this problem, the concept of including the characteristic element length for regularization is used in the literature. The estimation of the characteristic element length is not a straightforward task since normally the characteristic element length differs from one element to another in the simulation and depends not only on element geometry but also on fracture plane orientation and material orientation. In this paper, an innovative method is proposed to estimate the characteristic element length which works on the orthogonal projection of elements on the fracture plane. The method is implemented in Abaqus/Explicit finite element solver and is verified using simple and more complex load cases such as tensile specimens, open hole specimens, and low-velocity impact. A good correlation between the numerical and experimental results in all of the studied cases was achieved and the proposed method proved to be effective in reducing mesh sensitivity. The use of the volumetric method from the literature for the simulation of open-hole tensile specimens led to more than 25% increase in the estimation of specimen strength while similar values of strength for different element aspect ratios were achieved with the proposed method.

An energy-based analytical model is proposed here to investigate the mechanical behavior of adhesively bonded simple-lap and stepped-lap joints (SLJ) with carbon fiber-reinforced polymer (CFRP) adherends subjected to tensile loading. In this study, the CFRP uni-directional (UD) adherends of ([0]_{16}) and quasi-isotropic (QI) layup sequence of ([45/-45/0/90]_{2s}) are considered to be joined. The governing differential equations (GDEs) of equilibrium are derived for the adhesively bonded adherends in stepped lap joint configuration following an energy-based approach. Additionally, this model is reduced for GDEs of the simple-lap joint configuration. The finite difference scheme is employed to obtain the numerical solution of the proposed analytical model. The field distributions of strain and displacement over the specimen surfaces are captured in the experimental investigation using the full field technique of 2D digital image correlation (DIC). The analytical model generates the load–displacement curve, validated against experimental and finite element (FE) predictions. Additionally, a sensitivity analysis is conducted to assess the influence of the design parameters of the adhesive joint, including the thickness of the adhesive layer, length of overlap region, and elastic modulus. Finally, the analytical model prediction of the peak load for damage in adhesively bonded joints under shear loading is compared with experimental results. The developed analytical model provides an understanding of the mechanical behavior, including possible failure/critical locations of the adhesive joints from the design perspective.

The fatigue crack growth behavior of a submicron-grained WC–Co hardmetal is investigated by artificially introducing small flaws by means of sharp indentation. Similar fatigue testing is also conducted on notched specimens with long through-thickness cracks for comparison purposes. The use of controlled small indentations flaws is shown to be a valid and successful approach for studying and describing crack growth behavior under cyclic loading for the material under consideration. This statement is based on the similitude found in fatigue mechanics and mechanisms between both crack types. Regarding the former, accounting of the indentation-induced residual stresses is key to rationalize the experimental findings. Concerning the latter, inspection of crack-microstructure interaction as well as fracture surfaces permit to discern similar features and scenarios, at both meso- and micrometric length scales. Results from this research yield an immediate practical implication, as indentation techniques may then be proposed as an alternative testing route for investigating fatigue crack growth behavior of hardmetal grades where sharp indentation is capable to induce well-developed radial crack systems.