Composites are being widely used because of their high strength and stiffness, low density and high formability for creating complex shapes. An all-composite π joint is a structural connector known for its ability to reduce the weight and assembly cost while retaining a good load-carrying capability. Based on the material characteristics of unidirectional fiber composites used in composite π joints, a modified maximum stress failure criterion, which is able to assess damage onset, propagation and final failure, is presented for unidirectional fiber composites. The stiffness and strength of four types of composite π joints under tensile and bending loads are simulated by progressive damage models, involving a finite element analysis, failure criteria and a material degradation model. Numerical results from the application of this criterion in nonlinear element analysis show good agreement with experimental outcomes.

Integrated composite structures can significantly reduce the assembling cost and improve the performance of the aircraft. All-composite π joint, which is a connector of integrated composite structures, has good ability to reduce the weight and assembly cost while retaining good load-carrying capability. To disclose the mechanics behavior of the out-of-plane, complex all-composite joint, the progressive damage methods are investigated for its good ability to trace the damage onset, damage propagation, up to collapse of composite structures. A progressive damage model is established using a new modified maximum stress failure criterion and a material degradation model developed from Chang’s model. The material degradation model takes account of the matrix crack direction, which is predicted by the modified maximum stress-failure criterion. Meanwhile, five other progressive damage models are established and applied to static tensile analysis to study the mechanics behavior of the joint. To evaluate the prediction of the six progressive damage models in joint initial failure, damage propagation, joint strength and stiffness, the numerical results of the six progressive damage models are compared with the experiment results. The effects of failure criteria and material degradation model on the prediction results are discussed in detail. By the comparison, it can be concluded that the progressive damage model consisting of the modified maximum failure criteria and its corresponding material degradation model is considered as the best one for the analysis of the all-composite π joints, because its initial failure prediction, failure process and ultimate failure prediction agree well with the experiment result.

This article concentrates on the design of composite π joint, which is an effective connector for integrated composite structures in aerospace structures. A brief review of research on out-of-plane joints is first presented to current practice towards the design of such π joints. Progressive damage models are established to trace failure analysis from damage onset to ultimate collapse of joints, which highlight the failure mechanism and internal mechanical behaviours. Four kinds of failure criteria are used firstly, and after comparing the numerical prediction results with the experimental results, one failure criterion is chosen for the simulation of this joint. Influences of structural and geometric parameters such as structural configuration, overlaminate material, ply angle, fillet radius and upstanding/horizontal leg length of overlaminate on flexural resistance ability are studied to provide ‘optimal’ design schemes. Moment–displacement graphs from the mechanical experiments are presented and physical failure phenomena of the joint under bending load are depicted. The good agreement between the predictions and experimental results gives evidence of the efficiency of numerical analysis.

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.

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.