Durability of polymer matrix composites: Viscoelastic effect on static and fatigue loading

The structural applications of polymer matrix composites (PMC) demand lifetimes of 15, 25 and 50 years. However, the mechanical properties of these composites have a time dependent nature, i.e. strength and stiffness are time-dependent due to the hereditary nature (viscoelasticity) of polymers. In this context lifetime models for viscoelastic materials, i.e. energy-based criteria and fracture mechanics extended to viscoelastic media, are revised. These models are applied to predict the lifetime of composite materials under special cases of constant load (creep rupture) and constant stress rate to failure. It is verified that these lifetime theories predict similar relationship between creep failure and constant stress rate failure strength. Alternative approaches based on Strength Evolution Integral [Reifsnider KL, Stinchcomb WW. A critical element model of the residual strength and life of fatigue–loaded composite coupons. In: Hahn HT, editor. Composite materials: fatigue and fracture (ASTM STP 907). Philadelphia (PA): American Society for Testing and Materials; 1986. p. 298–313; Reifsnider KK, Case SC, Duthoi J. The mechanics of composite strength evolution. Compos Sci Technol 2000; 60:2539–46; Reifsnider KK, Case SC. Damage tolerance and durability in material systems. Wiley-Interscience; 2002] and on Linear Damage Accumulation (LCD) law confirm these results. In addition the LCD law was found to be generally unsatisfactory except for the special case of constant stress rate to failure. Accordingly this result validates the accelerated methodology proposed by [Miyano Y, McMurray M, Enyama J, Nakada M. Loading rate and temperature dependence on flexural fatigue behavior of a satin woven CFRP laminate. J Compos Mater 1994;28(13):1250–60; Miyano Y, Nakada M, McMurray MK, Muki R. Prediction of flexural fatigue strength of CRFP composites under arbitrary frequency, stress ratio and temperature. J Compos Mater 1997;31(6):619–38; Miyano Y, Nakada M, Kudoh H, Muki R. Prediction of tensile fatigue life for unidirectional CFRP. J Compos Mater 2000;34(7):538–50; Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of GFRP laminates for marine use. Compos: Part B 2004;35:497–502; Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of FRP laminates for marine use. J Compos Mater 2005;39(1):5–20], which is based on LCD law, to characterize long-term creep failure of polymer composites based on the constant stress rate failure strength curves.Finally a new formulation is proposed, based on Strength Evolution Integral, to predict of fatigue failure load for an arbitrary load ratio.