Strength Predication for Load-Bearing Lugs of Three-Dimensional Braided Composites

Load response and failure modes of three-dimensional (3-d) four-directional braided composite lugs were studied analytically and experimentally. The objective of the study was to get information on the stiffness, strength and failure mode of the lug, as well as on the applicability of the analysis method used to predict lug load response and failure. The test lugs were manufactured with the RTM (Resin Transfer Molding) technique. The test specimens were loaded parallel to the lug centerline. Two types of specimens were tested to failure. Three of them were instrumented with 18 strain gages in each type of lug. There are three basic failure modes in braided composite joints: net-tension, shear-out, and bearing. Net-tension failure is associated with matrix and fiber tension failure due to stress concentrations. Shear-out and bearing failures result primarily from the shear and compression failures of fiber and matrix. The analyses were performed using finite element method. Shell elements were used. A steel pin was modeled to apply the loading. The loading was applied with a constant force distribution through the center of the pin. A contact was defined between the pin and the surrounding lug surface. The measured strains showed fairly good correlation with the analysis results. The strain response was almost linear. It can be concluded that with correct material properties the FE approach used in the analyses can provide a reasonable estimate for the load response and failure of 3-d braided composite lugs