Fractography, NDE, and fracture mechanics applications in failure analysis studies

Abstract

While identification of the precise mode of a failure can lead logically to the underlying cause, a thorough failure investigation requires much more than just the identification of a specific metallurgical mechanism, for example, fatigue, creep, stress corrosion cracking, etc. Failures involving fracture provide good illustrations of this concept. An initial step in characterizing fracture surfaces is often the identification of an origin or origins. However, the analysis should not stop there. If the origin is associated with a discontinuity, the manner in which it was formed must also be addressed. The stresses that would have existed at the origin must be determined and compared with material properties to determine whether or not a crack should have initiated and propagated during normal operation. Many critical components are inspected throughout their lives by nondestructive methods, for example, magnetic particle inspection or fluorescent penetrant inspection. The probability that a defect or crack will be detected generally increases with its size. When a crack progresses to failure, its nondetection at earlier inspections must also be understood. Careful study of the fracture surface combined with crack growth analysis based on fracture mechanics can provide an estimate of the crack length at the times of previous inspections. An important issue often overlooked in such studies is how processing of parts during manufacture or rework affects the probability of detection of such cracks. A recent example from the files of the authors is used to further illustrate and demonstrate the benefits of the comprehensive approach to failure investigations. The ultimate goal is to understand thoroughly the progression of the failure, to understand the root cause(s), and to design appropriate corrective action(s) to minimize recurrence.