Static and Fatigue Strength of High Load Intensity Steel-Hybrid Composite Bonded Scarf Joints

This paper reports on analytical and experimental studies of the static and fatigue strength of high load intensity steel-hybrid composite bonded scarf joints. The hybrid composite consisted of T300/5208 at 0 deg and GY70/5208 at ±45 deg, with 0-deg direction being along the length direction of the joint. In sizing up scarf joints capable of carrying loads of 70 100 N/cm ≈ 40 000 lb per inch of width, properties of adherends, stiffness imbalance, coefficient of thermal expansion mismatch, and adhesive plasticity were taken into account. The joint test specimens were fabricated by cocuring at 450 K (350°F) the composite prepreg and the EA 951 Hysol adhesive. The scarf joints were tested in static tension, static compression, and tension fatigue. Test results showed the compressive strength of the scarf joint to be proportional to the scarf tip thickness, with joint strength increasing as the scarf tip thickness decreases. The fatigue strength was evaluated at 50, 60, and 70 percent of static ultimate strength using R = 0.05 and cyclic rate of 180 cpm. Also, the sensitivity of joint strength to bondline flaws was evaluated experimentally. For this purpose, a number of specimens containing debonds of various lengths and at various positions along the scarf length were fabricated and tested under static tension. The results from these tests were evaluated using fracture mechanics analysis. Limited data are presented on the repair of scarf joints that were tested to failure (and failed by microcracking and delamination) under tension fatigue. Preliminary experimental results are also presented on the response of a full-size [71.1-cm (28-in.)-wide] scarf joint in the skin of a box beam structural component simulating a foil structure.