Inspectioneering Journal

Fitness-for-Service Evaluation: What's In It For You?

Reliable Operations in an Imperfect World

By Ted L. Anderson at Quest Integrity Group. This article appears in the November/December 1995 issue of Inspectioneering Journal.

An inescapable law of nature is that engineering materials contain flaws. Perfect materials and welds simply do not exist in the real world. A newly fabricated pressure vessel, for example, contains numerous imperfections and will probably degrade further in service. The occurrence of flaws is of more than cosmetic importance, because cracks or other planar flaws can lead to brittle fracture, which in turn can result in injury, loss of life, and severe economic consequences. Although flaws are inevitable, we can minimize the risk of catastrophic failure by imposing limits on the size and number of flaws in a given structure.

Acceptable Versus Unacceptable Flaws

Intuition suggests that big cracks are worse than small cracks, in that the former are more likely to result in failure. But this qualitative statement begs the quantitative question: what is the maximum acceptable flaw size? In practice, flaw acceptance criteria are often based on arbitrary workmanship standards rather than the effect such flaws have on mechanical integrity. In some cases, flaw acceptance criteria are defined by the sensitivity of the nondestructive evaluation (NDE) technique and the skill of the inspector. That is, if your inspectors detect flaws, you are required to repair or replace the affected equipment. This "find-it-fix-it" mentality has undoubtedly led to a large number of unnecessary (and costly) repairs and replacements over the years.

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