149 Service failure analysis

Despite our increasing knowledge of fracture and fracture behaviour, service failures will continue to occur. A proper analysis of the circumstanccs under which the event took place may yield valuable information for the prevention of future incidents. An inventory of the environmental circumstances, loads and stresses, is of paramount importance. The fracture surface may reveal sufficient evidence as to the nature of the defect that initiated crack growth and fracture. In the case where a fatigue crack induced the final failure, it is sometimes possible to establish a crack propagation curve by means of electron fractographical determination of striation spacing (chapter 2).

The size of the crack at the moment of final separation can usually be estimated: fatigue cracks and stress corrosion cracks have a different surface topography and consequently reflect the incident light in a different way than the final fracture surface. This implies that the crack that caused the failure is (sometimes sharply) delineated from the final fracture area. From knowledge of the fracture toughness of the material, and of the loading conditions following from the analysis, an estimate of the fracture load can be made. This provides information as to whether an exceptionally high load was involved. The fracture toughness of the material should preferably be determined from the remnants of the failed part.

On the basis of the failure analysis a judgement can be made of the likelihood of similar occurrences in other structures of the same kind. Measures can be taken to improve these structures, or the fracture mechanics analysis may provide information to prescribe safe inspection periods.

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