Cyclic Damage Evolution in a PMC Laminate

Using damage mechanics, cyclic damage evolution has been described and evaluated in a non-crimped glass epoxy fabric composite. A fundamental fatigue study has been carried out by progressively monitoring the fatigue damage modulus and crack density throughout the life of an [0,+45,90,-45]2 (antisymmetric) laminate cycled at a stress ratio R (minimum stress/maximum stress) of 0.1. Development of damage can be separated into two main stages. Initially, damage increases very quickly during the first 10% of life (Stage I). Afterwhich, it increases more slowly at a relatively constant rate to failure (Stage II). The changes in the fatigue modulus and crack density both show the same behaviour. A large amount of damage in the form of transverse matrix cracks develops during the first cycle. These then remain constant throughout life. By contrast, the number of shear matrix cracks increase continually. The crack density is cycle, not stress dependent. This behaviour is reflected by changes in the fatigue modulus. Using damage mechanics, a representative equation has been applied to express the progressive evolution of damage. The significance of which is that the amount of fatigue damage at the end of Stage I for any stress level can be used to predict fatigue life and the stress-life diagram for the laminate.