Service Life of Structures

Abstract

Assume that we have an engineered structure, such as an airframe, a bridge, a deep-sea platform, a high-rise building, or a nuclear containment facility et al., which is subjected to repetitive loads of varying stress amplitude or frequency. Each structure sustains repeated duty-cycles throughout its life during which the distribution of the loading spectrum remains unchanged or changes in a known way. Moreover, it is assumed that all load distributions imposed by usage can be classified into a finite number of categories and the frequency of occurrence of each category can be measured or is known. These empirical distributions are frequently incorporated into the the specifications for the design. The incremental damage to the structure, whether it is of metallic alloy or composite material, stems from strain imparted during the imposition of load. This may range from the rearrangement of dislocations within the crystaline lattice, the fracture of polymeric bonds in the material, the microscopic extension of a fatigue crack in some metallic sub-component, the abrasion or galling of some surface or any other form of deterioration for which there is no self-recovery or healing. The component strain, for a given geometry of structure and each type of load, is a complicated but determinable function which often is realized by performing computer-aided, finite-element analysis. Archetypical examples include an airframe component sustaining gust and maneuver loads during each ground-air-ground cycle or a bridge being subjected to the repetitive loads from its daily pattern of vehicular traffic, with seasonal damage due to winter weather and de-icing compounds.