Review of Environmentally Assisted Cracking

Abstract

Many efforts have been made in the past by several researchers to arrive at some unifying principles governing the embrittlement phenomena. An inescapable conclusion reached by all these efforts was that the behavior is very complex. Hence, recognizing the complexity of material/environment behavior, we focus our attention here only in extracting some similarities in the experimental trends to arrive at some generic principles of behavior. Crack nucleation and growth are examined under static load in the presence of internal and external environments. Stress concentration, either pre-existing or in-situ generated, appears to be a requirement for embrittlement. A chemical stress concentration factor is defined for a given material/environment system as the ratio of failure stress with and without the damaging chemical environment. All factors that affect the buildup of the required stress concentration, such as planarity of slip, stacking fault energy, etc., also affect the stress-corrosion behavior. The chemical stress concentration factor is coupled with the mechanical stress concentration factor. In addition, generic features for all systems appear to be (a) an existence of a threshold stress as a function of concentration of the damaging environment and flow properties of the material, and (b) an existence of a limiting threshold as a function of concentration, indicative of a damage saturation for that environment. Kinetics of crack growth also depends on concentration and the mode of crack growth. In general, environment appears to enhance crack tip ductility on one side by the reduction of energy for dislocation nucleation and glide, and to reduce cohesive energy for cleavage, on the other. These two opposing factors are coupled to provide environmentally induced crack nucleation and growth. The relative ratio of these two opposing factors depends on concentration and flow properties, thereby affecting limiting thresholds. The limiting concentration or saturation depends on the relative chemistry of environment and material. A dynamic dislocation model is suggested to account for crack growth.

Failure Diagram for Chemically Assisted Crack Growth

“…We show here only one example, since we have already provided many other examples in the companion article by us in this volume. [9] For the Al-6Mg alloy, the threshold stress r th % 30 MPa and fracture stress r f % 200 MPa, showing a significant reduction in the failure stress due to environment. The consistent behavior is also shown not only by smooth specimens but also by notched specimens.…”

“…The time depends on the relative aggressiveness of the environment, as indicated by many materials responses in different environments. [9] Even in the case of LME in metals or humidity effects in ceramic systems, once a crack is initiated, its propagation occurs very rapidly. With respect to the failure diagram, our attention is focused only on these limiting conditions, without getting into details of the kinetics of crack initiation/ growth at various stages, such as stages I through III.…”

“…While the detailed evaluation of the chemically assisted crack growth has been presented elsewhere, [9] we focus here on the representation of the material response in the form of a failure diagram, to facilitate its subsequent usage in the life prediction models. We approach this problem in the same way as we did in earlier articles where we combined the stages of nucleation and propagation of cracks under cyclic loads, using a modified Kitagawa failure diagram for fatigue.…”

Effect of Elastic-Plastic Stress in the Defect Tolerance Under Stress Corrosion Cracking

“…Moreover, observations that the severe embrittlement occurs near the freezing temperature of the liquid metal [28] [29] and observations of change in mechanism process at higher temperature [30] can be found in the literature.…”

“…Regarding the mechanism involved in the LME process, many authors use different mechanisms to explain the embrittlement caused by liquid metal and many references [28] [29] [30] [31] present a review between some available theories for the liquid metal embrittlement process.…”