Analysis of fatigue damage in composite laminates

The degradation of laminated composite materials under service loading occurs by means of a complex combination of damage modes and damage mechanisms. These damage events combine to produce a damage state which controls the state of stress and the state of strength of the degraded laminate. Although data have been generated dealing with various details of this process, an understanding or philosophy of the precise nature of damage development has not, heretofore, been developed. This paper makes such an attempt for the general case of damage development in laminated plates under cyclic (fatigue) loading. The philosophy is based on the discovery of a ‘characteristic damage state’ in such plates which forms independently of load history, and is determined only by the properties of the laminae, their orientations, and their stacking sequence. The detailed nature of this characteristic damage state, the nature of its formation, and its influence on strength, life and stiffness is discussed. The general need for further work in this, and related fields is also assessed.