International Journal of Fracture最新文献

Construction of the Kitagawa–Takahashi (K–T) diagram requires inputs of two material properties, namely, endurance limit and threshold stress intensity range, ΔK_th. Both are sensitive to applied stress ratio. The effect of stress ratio on endurance limit is well known. Unfortunately, crack closure, associated with the nature of conventional testing practice obscures the effect of stress ratio on intrinsic, closure free ΔK_th that would apply to natural crack like defects and short cracks. This study was made possible by the development of a new test method to characterize closure free threshold conditions under controlled near-tip residual stress conditions that essentially determine near-tip stress ratio at threshold. A procedure is described to construct the K–T diagram, using ΔK_th values corrected for stress ratio and applicable to pre-existing defects and short cracks at notches that are unlikely to see closure. As a case study, a K–T diagram valid for different applied stress ratios is constructed for titanium alloy Ti-6Al-4V.

This research utilizes both single crystal and polycrystalline models to probe the fatigue crack propagation mechanism in pure silver via molecular dynamics (MD) simulations. A comprehensive validation approach at both micro and macro scales, incorporating transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and compact tension (CT) specimen fatigue testing, is developed to verify the reliability of simulation models and results. Simulation findings indicate that the initial crack orientation significantly influences crack propagation. As the crack advances within the crystal, two primary crack propagation mechanisms are discerned: (1) nano-voids appear at the crack tip, and the crack propagates by continuously aggregating with the nano-voids ahead; (2) the formation of Stair-rod dislocations and V-shape stacking faults due to dislocation reactions and slip band movements impedes crack propagation, accompanied by the dislocation reaction of Shockley partial dislocations ((tfrac{1}{6}) <112>) generating Hirth dislocations ((tfrac{1}{6}) <110>). The dislocation reaction is verified through the dislocation analysis of the crack tip area of the CT specimen after fatigue experiment by using TEM. In addition, the results of this study show that the angle between the direction of crack propagation and the grain boundary affects the fatigue crack propagation, e.g. when the angle is less than 60°, the crack rapidly propagates along the grain boundary. The orientation distribution function (ODF) results of EBSD can verify that the polycrystalline model containing 30 grains is a reliable model for the MD simulation of behavior of the crack tip of CT specimen. Lastly, the Paris law constants for pure silver are determined as m = 3.72 and lg C = − 10.77, providing a reference for the fatigue analysis and life prediction of silver components or silver soldering pots in engineering applications.

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.

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.