STRENGTH AND RELIABILITY OF FIBER-REINFORCED COMPOSITES - LOCALIZED LOAD-SHARING AND ASSOCIATED SIZE EFFECTS

The statistical aspects of the failure of large 3-D unidirectional fib er reinforced composites are studied numerically and analytically. A 3 -D lattice Green's function model is used to calculate the stress fiel d, damage evolution, and Failure in composites under ''Local Load Shar ing'' (LLS) conditions in which the stress from broken fibers is trans ferred predominantly to the nearby unbroken fibers. Failure by local a ccumulation of a critical amount of damage, and the associated decreas e in ultimate strength with increasing composite size, is explicitly d emonstrated. Weakest-link statistics are then employed to investigate size effects and reliability. An intrinsic ''link'' in LLS is found wh ich has the same Gaussian distribution function for strength as a bund le in Global Load Sharing (GLS) (no local stress concentrations) of th e same size. The size of the link is found to be comparable to the cri tical cluster of fiber damage observed in the simulations. Then, using known results for the GLS probability distribution function, analytic asymptotic results for the strength and reliability of large composit es in LLS are derived. The strength distribution shows excellent agree ment with the Monte Carlo simulation results for both the median stren gth and high reliability tail of the distribution. The implications of these results on the expected strength and reliability of moderate-si ze composites components is discussed, with applications to a Ti-MMC a nd a SiC/SiC CMC. Finally, the application of these results to modelin g of composite failure by the Finite Element Method is presented. (C) 1997 Elsevier Science Ltd.