EFFECTS OF PHYSICAL AGING ON LONG-TERM CREEP OF POLYMERS AND POLYMER MATRIX COMPOSITES

For many polymeric materials in use below the glass transition tempera ture, the long term viscoelastic behavior is greatly affected by physi cal aging. To use polymer matrix composites as critical structural com ponents in existing and novel technological applications, this long te rm behavior of the material system must be understood. Towards that en d, this study applied the concepts governing the mechanics of physical aging in a consistent manner to the study of laminated composite syst ems. Even in fiber dominated lay-ups, the effects of physical aging ar e found to be important in the long term behavior of the composite. Th is paper first lays out, in a self-consistent manner, the basic concep ts describing physical aging of polymers. Several aspects of physical aging which have not been previously documented are also explored in t his study, namely the effects of aging into effective equilibrium and a relationship to the time-temperature shift factor, The physical agin g theory is then extended to develop the long term compliance/modulus of a single lamina with varying fiber orientation. The latter is then built into classical lamination theory to predict long time response o f general laminated composites. Comparison to experimental data is exc ellent. In the investigation of fiber oriented lamina and laminates, i t is illustrated that the long term response can be counter-intuitive, stressing the need for consistent modeling efforts to make long term predictions of laminates to be used in structural situations.