Environmental cracking, definition

Failure occurs when the component ceases to perform its required function. In the case of catastrophic failure, such as the rupture of a pipe or electrical breakdown of an insulator, this is obvious, but in many cases there is no such clear end of life. For example, is the end point when a small amount of environmental stress cracking has occurred, or when cracks have reached 5 mm in length Broadly, the definition of end point is that a property has reached a level at which safety, performance or market acceptance dictate that the component or product can no longer be used. 

Thermoplastic cracking develops under certain conditions of stress and environment, sometimes on a microscale. Because there are no fibrils to connect surfaces in the fracture plane (except possibly at the crack tip), cracks do not transmit stress across their plane. Cracks result from embrittlement, which is promoted by sustained elevated temperatures and ultraviolet, thermal, and chemical environments existing in the presence of stress or strain. There appears to be no practical definition that can sufficiently distinguish between environmental and other stress cracking, although the micromechanics of the two types of cracking may be quite different. 

Furniture Poiish 100 20 5 Satisfactory but environmental stress cracking may " occur 100 60 5 Limited resistance a higher level of absorbtion occurs " resulting in definite loss of physical properties... 

Thus, mechanism-informed predictions, together with an assessment of the uncertainty in the prediction associated with epistemic uncertainties in the system definition, may be made about the cracking susceptibility of components in a specific plant for proposed material, stress, or environmental modifications (Figure 18.31), and the cost effectiveness of these various mitigation actions may be assessed. It should be noted that these predicted crack depth/time loci are exactly those needed for the proactive life management decisions discussed in Section 18.1. 

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