Estimating nominal strength of built-up CFRP laminates from standardized specimen tests

ASTM Standard D7290 defines a standard practice for determining the characteristic strength of polymeric composites for structural engineering applications. The characteristic strength is based on the estimate of the 5th percentile of strength modeled by a two-parameter Weibull distribution, in which the distribution parameters are determined from standardized methods of testing for tensile properties of the composite material, e.g., ASTM D638 or D3039. The situation frequently arises, particularly in field applications involving structural rehabilitation and retrofit, where the composite is applied in layers to achieve the required engineering strength properties. In these circumstances, the 5th percentile strength of the in situ composite laminate system may differ from the 5th percentile strength of the single-layer lamina that has been determined from standard laboratory testing. This technical note employs system reliability methods to determine the characteristic strength of an in situ composite system consisting of 2–25 laminae. It is found that using the 5th percentile strength determined from a standardized test of one lamina to estimate the comparable strength of an n-laminate composite system will lead to an unconservative estimate of the strength of the installed system.