Time Dependent Probabilistic Failure Model for the Strength of Fiber Reinforced Plastics

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The fracture strength of GRP (Glassfiber Reinforced Plastics), which depends on the applied time of stress and the stress rate, was analyzed by using a time dependent probabilistic failure model. The fracture process was assumed to be a 1-step stochastic process and the transition probability of failure to be γS^δf(t) where S and t represent stress and time, respectively and γ and δare constants. The evaluation of the damage in the case when the stress varied was done by "the reduced time method" proposed in this paper. When f(t)=t^γ and S=at (γ, a; constants), the fracture stress S_c was calculated as follows. [numerical formula] where D_c is a constant. The constant a is stress rate and so the effect of stress rate on the fracture stress could be clearly presented. Static fatigue strength was also analyzed. The analytical results showed a good agreement with experimental results For glass plates and glassfiber composites, the constants γ and δ were found to be about -0.7 and 6.26, respectively. therefore, the traditional time independent probabilistic theory overestimates the effect of strain rate on the strength of materials.
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