International Journal of Fracture最新文献

This paper investigates the fatigue behaviour of laser powder bed fusion (LPBF) processed Ti6Al4V samples under three build orientations. The post-heat treatment (PHT-1050 °C) was carried out. The microstructural characterization was performed using optical microscopy, X-ray diffraction SEM and EDS techniques. The tensile test and high cycle fatigue tests were performed. The PHT performed at 1050 °C exhibited Widmanstatten microstructure consisting of a higher volume fraction of elongated β and a small amount of α. PHT samples’ ductility was ~ 67%, 40% and 177% higher than the as-printed samples under horizontal, inclined and vertical orientations. Interestingly, the fatigue lives of PHT samples at higher stress levels were higher and nearly isotropic in all three build orientations than the as-printed samples due to enhanced ductility and fewer critical pores. Further strong correlation between PHT samples and ductility was established. Moreover, there was a marginal improvement in fatigue limit due to PHT at 1050 °C compared to as-printed samples.

A controlled fragmentation method, by deep laser melting, for a multi-purpose projectile (penetrator) is presented, using a full-sized projectile with 1100 mm long, 148 mm diameter, and 17.8 mm wall thickness. Effects on penetration and fragmentation performance, for various laser melting parameters, are explored through penetration and fragmentation field tests. It is shown that it is possible to attain an optimal local microstructure, in the melted regions, that ensures a pre-defined fragmentation pattern upon explosion without compromising on the penetration capabilities.

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.