Environmental Effects on the Cracking of Engineering Materials
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RS BULLETIN/AUGUST1989
environmental cracking p h e n o m e n a , with losses occurring in a wide variety of service environments. Liquid métal embrittlement is of concern in nuclear power and other industries. Hydrogen embrittlement, first recognized as an embrittler of iron in 1873,1 causes cracking problems in applications ranging from welding to oil drilling. In ail, the list of situations in which environmentassisted cracking occurs is long and is likely to grow as materials are increasingly challenged by the severity of their service conditions. Primary Variables Affecting Environmental Cracking A tensile stress, necessary to drive a propagating crack, is common to ail forms of environment-assisted cracking. Such stresses may be static, residual, or dynamic. Cracking driven by a residual stress or a static stress is most common in service failures. Cracking driven by a cyclic stress can be enhanced by environmental interactions and is often considered a spécial case of SCC, LME, or HE. A useful représentation of the effect of stress on a material's résistance to an embrittling e n v i r o n m e n t is a plot of crack velocity (daldt) versus m o d e I stress intensity (K{). This relationship is shown in Figure 1 and is représentative of a large number of materials in many stress corrosion, liquid métal, and hydrogen environments. The curves exhibit three stages: the stage I région is characterized by a crack velocity which increases rapidly with small increases in
stress intensity; in the stage II région, crack velocity is relatively insensitive to stress intensity; and in the stage III région, generally the crack propagates mechanically and rapidly through the remaining material. The utility of representing data this way is that this is likely to be the séquence of cracking in service failures. However, extracting detailed mechanistic information from thèse data is difficult. Stress corrosion data also are presented in plots of applied stress versus time to failure. There exists a stress, called the threshold stress, below which cracking does not occur (during the time of testing). The threshold stress is generally greater than one half the yield stress and most often about 75% of the yield stress. Similarly, there exists a threshold stress intensity, labeled Kth in Figure 1. For materials forming passive films, a certain minimum stress may be required for r u p t u r e of the film and résultant exposure of bare métal to the damaging environment. The threshold stress is not a material property; it is sensitive to the SCC environment as well as to material m i c r o s t r u c t u r e . Mechanical treatment to provide residual compressive stresses near the surface is commonly used to combat SCC. For LME, fracture can also occur at stresses below the yield stress; however, it is generally accepted that some plastic d é f o r m a t i o n is r e q u i r e d for cracking. Additionally, intimate contact (wetting) of the solid substrate by the liquid métal is a prerequisite to cracking. As for SCC, the existence of a
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