The main failure mechanisms for maintenance diagnostics

Analysis of feedback from industrial equipment failure databases shows that, in the majority of cases, failures are due to the choice of materials at the design stage. During operation, equipment materials are subjected to mechanical and thermal stress, aggressive environments and aging phenomena. This article describes the main material failure mechanisms that lead to sudden failure, cracking before failure, loss of material or aging-induced changes in properties. Knowledge of these degradation and failure mechanisms is essential at the design stage, in order to select materials that will ensure optimum equipment reliability and service life. Knowledge of these mechanisms is also essential in diagnostics, to identify the causes of failures and select corrective actions, and to estimate and predict remaining useful life (RUL) and estimated time to failure (ETBF). The first section presents the issues and basic concepts involved in finding these failure modes for materials to ensure optimum equipment reliability, availability and safety. It provides a brief reminder of the terminology used to characterize failures in accordance with standards EN NF 13306, IEC 60050 and ISO13381-1. The second section presents the fundamental elements of materials science, essential for characterizing a material in terms of its chemical bonds and crystallographic structures. These properties make it possible to propose their classification and establish the relationships most commonly used in strength of materials between stresses, strains and displacements. The third section looks at failure mechanisms, distinguishing between rapid and progressive cracking. Particular attention is paid to the observation of facies, either visually or by microscopy, which enable specialists to identify the nature of the failure mode. The fourth section describes corrosion, defined according to ISO 8044 as "a physical-chemical interaction between a metal and its environment". The most frequently observed types of corrosion are briefly described. The fifth section is devoted to wear mechanisms linked to contact and friction, within the framework of the discipline known as tribology. It provides an inventory of the main modes of wear (abrasive, adhesive, gouging-grinding and fretting). Material ageing, defined by DIN 50035 as "all irreversible chemical and physical processes occurring in a material over time", is the subject of the sixth section. In particular, it describes aging processes in metallic, polymer and composite materials. The seventh section proposes a typology of diagnostic methods whose aim is to identify material failure mechanisms on the basis of their symptoms. After drawing distinctions between inductive and deductive methods, the principles of analytical or artificial intelligence-based approaches are briefly outlined, differentiating...