International Journal of Fracture最新文献

This article presents a novel two-dimensional quadrilateral solid finite element model, enhanced by incompatible modes and embedded strong discontinuity for simulation of localized failure in quasi-brittle heterogeneous multi-phase materials. The focus of interest lies in the development of discontinuities and cracks induced by both tensile and compressive loads, considering mesoscale material constituents and very complex meshes. Multiple cracks are initiated within elements using local Gauss-point criteria for crack initiation. Rankine and Maximum shear stress criteria control the crack initiation, location, and orientation depending solely on the stress state within the finite element. The model identifies distinct clusters of cracked elements and merges them into continuous cracks. A tracking algorithm ensures crack continuity, eliminating spurious cracks ahead of the crack tip to prevent crack arrest and stress locking. This approach ensures the formation of various types of cracks within the constituents of composite materials and their spontaneous coalescence forming the final failure mechanisms. The constitutive model for the crack representation is the damage softening model, which accounts for opening and sliding behavior. The efficacy of the proposed model is demonstrated through numerical simulations of heterogeneous 3-phase and 4-phase composites subjected to both tensile and compressive load cases.

Accurate fatigue life prediction of polycrystalline materials is crucial for many engineering applications. In polycrystalline materials, a significant portion of life is spent in the crack nucleation phase at the microstructural scale. Hence, the total fatigue life shows high sensitivity to the local microstructure. To predict fatigue life accurately, the microstructure models of polycrystalline material i.e., titanium alloy are virtually generated with the help of the Voronoi tessellation technique. These models incorporate critical microstructural features such as grain size, grain shape, and the volume fraction of different phases within the material. To efficiently predict microstructure sensitive fatigue life, the smooth finite element method (SFEM) is coupled with continuum damage mechanics (CDM). The SFEM provides flexibility in the meshing of complex microstructure geometries as it alleviates the need to use only triangular and quadrilateral elements. Moreover, there is no need of isoparametric mapping and explicit form of shape function derivatives in SFEM, hence it requires less computation time. To obtain the fatigue life (in number of cycles), jump in cycles algorithm is implemented using SFEM-CDM. The numerical results of fatigue life data obtained from simulations are compared with experimental data, which reveals the validity of the present approach. This approach is useful to find out the scatter in fatigue life data of polycrystalline materials along with the source of scatter.

The fracture resistance of pinewood under mode I loading is investigated experimentally for different crack plane orientations and the crack propagation direction parallel to longitudinal cells. Experiments are conducted on double cantilever beams using a digital image correlation system to evaluate the crack tip opening displacement. The compliance based beam method is used to determine the energy release rate at various crack lengths. The decomposition of crack propagation into the pre-peak and post-peak propagations is proposed to find the fracture energy contributions from individual toughening mechanisms in pinewood. The cohesive strengths measured in the experiments are confirmed by comparison with the tensile strengths obtained from separate tests performed on pinewood. An analytical model for evaluating the fracture process zone is used to validate the experimental results. The difference between the fracture energy values in different crack propagation systems is explained by using X-ray microtomography images of the fracture surfaces.

Delayed hydride cracking (DHC) is a hydrogen embrittlement phenomenon that may potentially occur in Zircaloy-4 fuel claddings during dry storage conditions. An experimental procedure has been developed to measure the toughness of this material in the presence of DHC by allowing crack propagation through the thickness of a fuel cladding. Notched C-ring specimens, charged with 100 wppm of hydrogen, were used and pre-cracked by brittle fracture of a hydrided zone at the notch root at room temperature. The length of the pre-crack was measured on the fracture surface or cross-sections. Additionally, a finite element model was developed to determine the stress intensity factor as a function of the crack length for a given loading. Two types of tests were conducted independently to determine the fracture toughness with and without DHC, (K_{I_text {DHC}}) and (K_{I_text {C}}), respectively: (i) constant load tests at 150 (^{circ })C, 200 (^{circ })C, and 250 (^{circ })C; (ii) monotonic tests at 25 (^{circ })C, 200 (^{circ })C, and 250 (^{circ })C. The results indicate the following: (1) there is no temperature influence on the DHC toughness of Zircaloy-4 between 150 and 250 (^{circ })C ((K_{I_text {DHC}} in left[ 7.2;9.2right] ) MPa(sqrt{text {m}})), (2) within this temperature range, the fracture toughness of Zircaloy-4 is halved by DHC ((K_{I_text {C}} in left[ 16.9;19.7 right] ) MPa(sqrt{text {m}})), (3) the crack propagation rate decreases with decreasing temperature and (4) the time before crack propagation increases as the temperature and loading decrease.

In this investigation, the growth behavior of a crack in a nickel-based superalloy under a turbine standard load sequence was determined by experimental, analytical, and computational methods. In the first experimental approach, ASTM standard compact tension (CT) test specimens were fabricated and fatigue crack growth (FCG) tests were conducted in a universal test machine under cold-TURBISTAN, a turbine standard spectrum load sequence. In the second analytical method, after rain-flow cycle counting of the cold-TURBISTAN sequence, the crack growth was estimated for each counted cycle from the crack growth law. The accumulated crack extension for each block of loading was thus estimated to determine the FCG behavior. In the third computational approach, a CT specimen containing an initial crack was modeled and the FCG behavior was predicted under cold-TURBISTAN spectrum load sequence using FRANC3D. The FCG trend predicted by analytical and computational methods was almost similar to the observed experimental behavior. The predicted FCG life was conservative with a life ratio ranging from 0.9 to 0.95.

Damage and failure in quasi-brittle materials such as rocks, concrete, and ceramics, have a complex non-linear behavior due to their heterogeneous character and the development of a fracture process zone (FPZ), formed by micro-cracking around the tip of an induced or pre-existing flaw. A softening behavior is observed in the FPZ, and the linear elastic fracture mechanic (LEFM) cannot correctly reproduce the stress field ahead of the crack tip. The existence of the FPZ may be the intrinsic cause of the size effect. An appropriate modeling of this process zone is mandatory to reproduce accurately the failure propagation and consequently, the structural behavior. Different from most of the domain numerical techniques, the boundary element method (BEM) requires (besides the boundary division into elements) only the discretization of a small region where dissipative effects occur. Cells with embedded continuum strong discontinuity approach (CSDA), placed in the region where the crack is supposed to occur, are capable of capturing the transition of regimes in the failure zone. Numerical bifurcation analysis, based on the singularity of the localization tensor, is used to determine the end of the continuum regime. Weak and strong discontinuity regimes are associated with diffuse micro-cracks (strain discontinuity) and macro-crack (displacement discontinuity). A variable bandwidth model is used during the weak discontinuity regime to represent the advance of micro-cracks density and their coalescence. Continuum and discrete cohesive isotropic damage models are used to describe the softening behavior. Analysis of three-dimensional problems with single crack in standard and mixed fracture modes, using this transitional approach and the BEM cells is firstly presented in this work. Experimental reference results are used to attest the capability of the approach in reproducing the structural behavior during crack propagation. Some necessary advances required for its applications for general complex structural problems are pointed out.

The study of the mechanical action between a punch and a cracked substrate has some theoretical guidance for the material protection. So the coupling problem of a cracked semi-infinite harmonic substrate under the action of a rigid flat punch is studied. The mixed boundary value problem is transformed into the Riemann-Hilbert boundary value problem by applying the complex-variable method, and then converted into singular integral equation for a numerical solution. The stress intensity factors at the contact ends and crack tips and the Piola stresses of whole harmonic material can be expressed as complex functions. The results indicate that the stressed state of harmonic solid near the crack tip and contact ends have similar features as those in linear elastic solids. The crack causes an obvious impact on the stress distributions near the contact region. The study provides theoretical guidance for analyzing the damaged problems of some soft materials under small deformation.

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.