MATERIAL FAILURE MECHANISMS AND DAMAGE MODELS

Failures in engineering systems occur due to specific causes, whether foreseen or not; many of those causes are related to specific procedures by the user. Failures are typically attributed (correctly or not) to: ignorance and/or indifference about user needs and desires, inattentive management, poor selection of materials or combinations of materials, inadequate design, inappropriate manufacturing and assembly processes, lack of adequate technology, improper treatment by users, and other poor control of quality. Failure is a complicated concept; four simple conceptual models for failure are: Stress-strength, damage-endurance, challenge-response, and tolerance-requirement. The specific failure mechanisms depend on material or structural defects, damage induced during manufacture and assembly, and on conditions during storage and field use. Conditions that affect the state of an item are broadly termed stresses (loads), eg, mechanical stress and strain, electrical current and voltage, temperature, humidity, chemical environment, and radiation. The effects of stresses are influenced by geometry, constitutive and damage properties of the materials, manufacturing parameters, and the application environment.