Crack initiation behavior in magnetoelectroelastic composite under in-plane deformation

Fretting fatigue crack propagation under out-of-phase loading conditions using extended maximum tangential stress criterion

A strain energy density-distance criterion was previously developed and used to correlate rising-load KC initiation data for notched and fatigue precracked specimens of hydrogen precharged Alloy X-750. This criterion, which was developed for hydrogen embrittlement (HE) cracking at 93°C, is used here to correlate static-load stress corrosion cracking (SCC) initiation times obtained at 360°C in deaerated water using smooth geometry, notched and fatigue precracked specimens. According to this initiation criterion, the onset of crack growth occurs when a critical strain energy density is attained at a microstructurally characteristic distance from notch and crack tips. In the previous HE study the characteristic distance for crack initiation was found to be one grain diameter, independent of notch radius, applied stress intensity factor and hydrogen level. For the current SCC study, initiation is shown to occur at a variable distance from notch and crack tips. The initiation site varies from very near the crack tip, for highly loaded sharp cracks, to a site that is one grain diameter from the notch, for lower loaded, blunt notches. The existence of hydrogen gradients, which are due to strain-induced hydrogen trapping in the strain fields of the notch and crack tips of the SCC test specimens, is argued to be responsible for variation in the crack initiation site. A variable crack initiation site for the SCC specimens is shown to be consistent with a fixed initiation site for the HE specimens by considering the source of the hydrogen, which is external to the specimen in the SCC study and internal to the specimen in the HE study.

Abstract

Underground coal mining activities are prone to cause movement and breakage in geological strata and also lead to mining subsidence and even ground fissures. Along the direction working panel advancing, ground fissures may occur in roof in front and/or behind working panel. However, the investigations of previous similarity tests in lab only emphasize on the region behind working panel. By improving strata material property in construction and mounting artificial pressure devices, two physical simulation tests were conducted and successfully investigated the simulated results. Then, the mechanical model of “cantilever beam and elastic foundation beam” was proposed to calculate the stress distribution and the crack initiation angle in overlying strata and it well explains the mechanisms of ground fissures generation and propagation. Results show that, the maximum internal force in roof always occurred in front of working panel. However, because the void space in gob due to excavation is large enough to cause the bend and rotation of roof strata, compare to the triaxially compressed region in front of working panel, the roof always broke off at some positions above gob since the stress concentration resulting from such bend and rotation of strata could easily reach the limit strength of strata rocks. Also, the length of cantilever beam changed dynamically as respect to the panel advancing and the breakage intervals. Thus, the breakage position where the internal force first reached the limit tensile strength is not fixed and there will be two different kinds of relative positions between the crack initiation point and the working panel. The crack initiation direction is always perpendicular to the internal force, and the crack propagation direction is affected by the initiation angle, overburden-separation degree and the position of the hydraulic shields. If there is no overburden-separation or less, the roofs will break off as a composite beam and the propagation direction will be roughly along the central line between the initial broken point and the support position. Otherwise, the roof strata will bend with the support shields moving forward, then the fracture angle will be close to the initiation angle and the fault surface will be stepped.

This paper presents a single‐domain boundary element method (SDBEM) for linear elastic fracture mechanics analysis in the 2D anisotropic bimaterial. In this formulation, the displacement integral equation is collocated on the uncracked boundary only, and the traction integral equation is collocated on one side of the crack surface only. The complete fundamental solution (Green′s function) for anisotropic bi‐materials was also derived and implemented into the boundary integral formulation so the discretization along the interface can be avoided except for the interfacial crack part. A special crack‐tip element was introduced to capture exactly the crack‐tip behavior. A computer program with the FORTRAN code has been developed to effectively calculate the stress intensity factors, crack initiation angle, and propagation path of an anisotropic bi‐material. This SDBEM program has been verified having a good accuracy with the previous researches. In addition, a rock of type (1)/(2) disk specimen with a central crack was made to conduct the Brazilian test under diametrical loading. The result shows that the numerical analysis can predict relatively well the direction of crack initiation and the path of crack propagation.

The biaxial low-cycle fatigue properties of anisotropic rolled steel plates were investigated. Biaxial low-cycle fatigue tests were conducted by utilizing the principle of anticlastic bending of flat rhombic plates. Crack initiation and propagation were examined with special reference to angles between the applied principal stress axes and the anisotropy axes of the material studied. The biaxial fatigue life of anisotropic specimens cut out in a given direction satisfied a linear relation on a log-log scale between an equivalent strain range and the number of cycles to failure. The direction of crack initiation under the uniaxial and biaxial stress conditions was coincident with that of the maximum shear stress. The fatigue life under the uniaxial stress condition was shorter than that determined under the biaxial stress condition for a given value of the equivalent strain range. The discrepancies were interpreted in terms of both normal stress component acting on the crack surface and the modes of crack propagation.

The effect of T-stress on the fracture of brittle rock under compression

Mixed mode I/II fracture investigation of Perspex based on the averaged strain energy density criterion

On crack initiation angle of mixed mode ductile fracture with continuum damage mechanics

Mixed mode I/II crack initiation from U-notches in Al 7075-T6 thin plates by large-scale yielding regime

Fatigue crack initiation around a hole subjected to biaxial fatigue loads with a phase difference was investigated. Axial and torsional biaxial fatigue loads with different phase differences and biaxiality of<br/> 1/ ??3 were applied to thin-walled tubular specimens. Five phase differences of 0, 45, 90, 145 and 180 degrees were selected. Directions of the fatigue crack initiation around the hole were found to approach to the circumferential direction of the specimen with increment of the phase difference for fatigue tests with phase differences less than 90˚. Whereas directions for tests with phase differences greater than 90˚ got away from the circumferential direction and those were symmetric to the directions for tests with phase difference less than 90˚ . Furthermore, it was shown that the fatigue initiation life decreased with increment of phase difference for fatigue tests with phase differences less than 90˚, but it increased for tests with phase difference greater than 90˚ . The crack initiation direction can be successfully explained by using the direction of the maximum tangential stress range obtained around the hole and at far-field.<br/>

Environmentally assisted crack initiation behavior is greatly affected by applied stress and environmental factors, such as water temperature, contained impurities and so on. On the other hand, crack initiation behavior also influences crack propagation. A typical example of this influence can be observed as the interference effects of multiple cracks, such as the coalescence of approaching crack tips or the arrest phenomena in the relaxation zone of an adjacent crack. To understand these effects of crack initiation on crack propagation behavior is very important to predict the lifetime of components, in which quite a few cracks tend to occur. This study aimed at revealing the crack initiation behavior and the influence of this behavior on propagation. At first, to evaluate the effect of applied stress on crack initiation behavior, sensitized stainless steel was subjected to a four-point bending test in a high temperature water environment at the constant potentials of ECP +50mV and ECP +150mV. Secondly, a crack initiation and growth simulation model was developed, in which the interference effect of multiple cracks is evaluated by the finite element method, based on the experimental results. Using this model, the relationship between crack initiation and propagation was studied. From the model, it was revealed that the increasing number of the cracks accelerates crack propagation and reduces life.

In this work, crack initiation from a notch tip in elements subjected to a mixed mode loading was studied. Two phenomenological criteria, based on a concept of Leguillon, were proposed and applied to predict crack initiation from a V-notch tip in a brittle or quasibrittle material. The first criterion, called in this paper the G- criterion, makes use of the critical strain energy release rate and the ultimate tensile stress as the main parameters of fracture. It requires calculations of strain energy release rate after an infinitesimal crack kinking. By means of some numerical effort, we established the indispensable relationships between the strain energy release rate after crack kinking and the remote mixed mode loads. These relationships enabled us to apply directly the proposed criterion to mixed-mode loaded notches. We also modified the strain energy density criterion proposed by Sih on the basis of Leguillon’s concept to form a criterion capable of predicting the crack initiation from a V-notch tip. This criterion, which we call the S- criterion, benefits from its simplicity in applications. Confrontation with experimental data shows that both the criteria enable reasonably accurate prediction of fracture in elements with V-notches subjected to mixed mode loading. Consequently, they can be effectively applied in practical engineering applications.

Investigations on the SCC initiation behavior of cold worked 316 L in high temperature oxygenated water at constant loads

On the application of the strain energy density theory in predicting crack initiation and angle of growth