Three-dimensional layer-by-layer stress analysis of single-lap, countersunk composite joints with varying joining interface conditions

Due to manufacturing variations of composite parts, assembly gap and consequent shimming widely occur in the airframe skin-structure. To guarantee the safety and reliability, the aircraft manufacturers expect to fully understand the effect of assembly gap and shimming on the loading behavior of composite skin joints. One of the main concerns is the effect of interface gap and shimming on the elastic stress state at the hole due to skin joints involve significant stress concentration and composite parts present brittleness and limited stress relief ability. But this topic has not yet been reported so far. In this paper, the effect of joining interface condition (including both shimming and interface gap) on three-dimensional layer-by-layer stress field at the countersunk hole of single-lap countersunk composite joints was fully and systematically investigated by employing a three-dimensional FE model. Shimming conditions include material type and shim thickness, and interface gap conditions include gap span and gap thickness. Some previously unrevealed conclusions were concluded. This study could contribute to improving the reliability of composite airframe, and benefit both design and manufacture in aircraft industry.