Fengrui Liu

Influence of end distances on the failure of composite bolted joints

To explore the physical effect of end distances on mechanical behaviors of composite bolted joints, series of single-bolt composite joints designed with different end distances were tested. In conjunction with the experimental work, a numerical progressive damage method is introduced to trace the damage process from the onset and propagation up to ultimate failure of the joints. A group of material degradation factors is presented by a trial-and-error method to establish three-dimensional progressive damage models of the bolted joints. The progressive damage analyses show that the predicted load–displacement curves, failure loads, and failure patterns of bolted joints with different end distances are in good agreements with the related experimental outcomes. From the experimental and numerical results, it follows that the cleavage failure gradually switches to the bearing failure with the increasing ratio of E/D ranged from about 2 to 4. An economic and suitable ratio of E/D ≈ 3 is provided for the bolted joints made of X850 carbon/epoxy composites with balanced and symmetric layups [45/0/−45/0/90/0/45/0/−45/0]s. The understanding of the effect of E/D on the mechanical behaviors including the strength, stiffness, and failure patterns of single-bolt joints is strengthened.

Investigation on characteristic length testing methods for failure prediction of composite multi-bolt joints

Characteristic lengths (CLs) are key factors in the characteristic curve method (CCM), which is widely used in engineering to predict the failure of composite multi-bolt joints. They directly affect the accuracy of predicting the joint failure. To identify suitable CL testing methods, six schemes have been designed and tested, in which two tensile CL specimens (open- and filled-hole laminates) and three compressive CL specimens (semi-circular notch laminates, pin-loaded hole laminates and single-bolt joints) have been compiled. Additionally, 3D FE analyses on the stress concentration have been performed and a discussion of the unit failure index curves of different specimens has been carried out to explore their load-carrying mechanism and determine why they result in different CLs. Failure predictions of composite multi-bolt joints by using the CCM with different CL testing schemes have been compared and evaluated using related static tensile experimental results. Filled-hole laminates for the tensile CL and single-bolt joints for the compressive CL are recommended because they best approximate the actual structure and obtain accurate failure predictions for composite multi-bolt joints.