Fatigue indicators for metal structures: from aircraft to bridges

The papers focus on a fatigue monitoring method for two types of engineering structures: (a) aircraft and (b) steel bridges. However, the potential application of this new approach extends beyond these examples. It is demonstrated that, despite advancements in stress-strain analysis, improvements in fatigue life prediction methods, and progress in contemporary non-destructive inspection techniques, unexpected failures of metal structures still occur. This paper outlines the evolution of Fatigue Indicators for Metal Structures, progressing from conceptual development to a family of indicators capable of monitoring fatigue damage in key structural components. These include aircraft parts made of aluminum alloys and load-bearing elements of steel bridges. All the indicators discussed share a unifying concept: the metal surface subjected to cyclic loading reflects accumulated fatigue damage. The primary parameter used to assess this damage is the intensity of the surface deformation relief (extrusion/intrusion patterns), measured using a computer-aided optical method. The conceptual design of Fatigue Indicators for both uniaxial and biaxial fatigue is explored. The required sensitivity level of the indicators is achieved through the redistribution of the strains by geometric optimization, which is carried out using Finite Element Analysis.