Crack closure

About: Crack closure is a research topic. Over the lifetime, 28157 publications have been published within this topic receiving 588158 citations.

Effects of crack tip stress states and hydride-matrix interaction stresses on delayed hydride cracking

Abstract: A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature. For test temperatures approached from below, factors such as the hydridematrix accommodation energies, the stress state at the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward and inside the plastic zone boundary with increasing temperature. At lowK I values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition forK IH . For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above parameters become less evident.

Failure of brittle polymers by slow crack growth

Abstract: The mechanical properties of a silica particle-filled epoxy resin composite system have been investigated in air as a function of volume fraction of particles for volume fractions ranging from 0 to 0.52. The Young's modulus and the compressive yield stress both increase as the volume fraction of silica particles is increased and various models of particle strengthening have been used to explain this behaviour. Slow crack growth in the various particulate composites has been studied using a fracture mechanics approach. The variation of crack velocity (V) with stress intensity factor (KI) has been measured for each of the compositions investigated. In each case, a unique relationship between V and KI has been found with KI increasing with volume fraction of particles at a given value of V. The failure mechanisms and the variation of other fracture mechanics parameters, for example, crack opening displacement and plastic zone size with increasing particle volume fraction have been discussed.

Method for Laboratory Determination of J c

Abstract: Possible methods for evaluating J for a three-point bend specimen are reviewed and the effects of slow crack growth, experimental limit load, and lateral constraint noted. A procedure, compatible with linear elastic fracture mechanics (LEFM) and crack opening displacement (COD) testing, is suggested for finding J c .

Fatigue of cold-work tool steels: Effect of heat treatment and carbide morphology on fatigue crack formation, life, and fracture surface observations

Abstract: The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M7C3 carbides. These M7C3 carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.