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Stress concentration relief factor, which reflects the linear relationship between the stress concentration factors of composite joint and those of elastic isotropic joint, was proposed by Hart-Smith for the failure prediction of composite multi-bolt joints. It is deemed to be determined by experiments from either unloaded- or loaded-hole laminates and is suitable for investigations of bypass and bearing stress concentration factors. To examine this viewpoint, in this paper, the stress concentration relief factor is determined by tensile tests of open-, filled- and loaded-hole laminates failing in tensile mode, respectively. It can be found that the loaded-hole has obviously high-stress concentration level, while the filled-hole has slightly higher stress concentration than that of open-hole. Detailed 3D-FE analyses are performed to disclose the stress concentration mechanisms of three hole-laminates. Various stress concentration relief factors are combined to calculate bearing and bypass stress concentration factors, which provide the failure envelope for the strength prediction of composite joints. Moreover, both conventional and modified failure envelopes are discussed. Experimental results of two-, three- and four-bolt joints are provided to investigate the suitability of different failure envelopes. It follows that the stress concentration relief factor for bearing stress concentration factor needs to be determined by a loaded-hole laminate, while that for bypass stress concentration factor can be determined by each of open-, filled- and loaded-hole laminates.

A probabilistic model involving the randomness of basic design parameters was presented to predict the probabilistic strength of a typical composite double-lap single-bolt joint. In the probabilistic model, statistical models of the random parameters including geometrical dimensions, fiber orientation angles and material properties were constructed. In addition, a parametric failure prediction model employing a modified characteristic curve method was established. Static tensile tests of fifteen composite double-lap single-bolt joints made of T800 carbon/epoxy composites were carried out. The probabilistic failure load of the joint obtained from the proposed model is in good consistency with that from the experiments, which verifies the effectiveness and accuracy of the proposed model. Meanwhile, it shows that the proposed model is not sensitive to the probability distribution type of random variables, although the probability distribution type of random variables slightly influences the statistical parameters of the probabilistic failure load. Furthermore, a relationship between the failure probability and the tensile load is determined, and longitudinal compressive strength X C , ply thickness t ply and longitudinal elastic modulus E 11 of T800 unidirectional lamina are the key factors affecting the probabilistic failure load and reliability of the joint remarkably.