Long-Term Predictions Relating to Environment Sensitive Cracking
- PDF / 792,152 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 54 Downloads / 138 Views
LONG-TERM PREDICTIONS RELATING TO ENVIRONMENT SENSITIVE CRACKING. R. N. PARKINS University of Newcastle upon Tyne, Newcastle upon Tyne, NEI 7RU, England. ABSTRACT
Consideration is given to the environmental requirements of environment sensitive cracking (ESC) of ferritic steels, involving hydrogen-induced or dissolution-related mechanisms of cracking. Pitting, with associated local pH changes, may result in hydrogeninduced cracking of simple ferritic steels, as has been observed with high pressure gas pipelines in contact with ground waters, and may constitute a greater potential failure mechanism for waste containers than dissolution-related cracking with its requirement of relatively concentrated solutions. However, the stochastic nature of pit initiation, together with the distributions of crack nucleation and growth rates, suggest that a probabilistic, as opposed to a purely deterministic, approach will need to be applied to life prediction estimates for waste containers. INTRODUCTION
It is obvious that the great majority of instances of service failures of engineeering structures by ESC are not anticipated at the design stage, which serves to indicate the difficulty of long term predictions relating to this mode of failure. In some cases, service failure results from the relief of fabrication stresses not being carried out, or being performed incorrectly, while in other instances the structure is designed on the basis of it being statically loaded, whereas in operation cyclic loads are applied, so that either the static or cyclic threshold stresses, respectively, for cracking are exceeded. However, in most cases of unexpected service failure it is probably the potency of the environment that causes the initiation and growth of cracks which is not identified at the design stage. That difficulty may be further exacerbated because the environment that results in cracking is locally generated, possibly within a pit or other geometrical discontinuity, from the bulk environment, which itself does not directly induce cracking in the given material. Indeed in many instances of service failure analysis, identification of the environment that resulted in cracking often presents the greatest difficulty in arriving at a convincing explanation of the cause of the failure. Until some two or three decades ago that did not appear to be a problem, because failure was ascribed to the presence of those species known to cause such failure in given materials, i.e. hydroxides or nitrates in the case of ferritic steels, chlorides in the cases of stainless steels or aluminium alloys, ammonia in the case of brasses, etc.. However, more recent service experience and laboratory studies have shown that the lists of environments capable of promoting ESC in given alloys is much more extensive than the, so-called, classical solutions of 30 years or more ago. The relatively recent observations of cracking of materials as diverse as type 304 stainless steel, Inconel 600, ferritic steels and even a-brass in pure water serves to show the dang
Data Loading...
