Fatigue of metal-composite joints with penetrative reinforcement

An innovative, hybrid joining technology (HYPER) is investigated. Metal-composite single lap coupons, reinforced with additively manufactured metallic pins, are subjected to high-cycle fatigue. A backface strain technique is used to identify damage initiation/growth and an ultrasonic inspection method is also used to help characterise failure. Fatigue life curves are generated and it is found that there is a discrete change in failure mode when load severity is reduced. High severity testing results in pin fracture above the pin root - similar to that observed during a previous static test programme. However, for high-cycle fatigue, at low loads, pins are torn from the adherend and cracks propagate through the stock material below the pin microstructure. It is proposed that this results from a change to the pin load mixity as joint rotation increases. Coupon failure is controlled and repeatable (for a given load severity) due to the sacrificial role of the pins and there is no significant damage to the composite adherend. Hence, it is believed that HYPER joints not only have excellent mechanical strength, compared to benchmark tests with bonded joints, but also offer greater potential for in-service repair than conventional, mechanically fastened joints.