Local Surface Toughening – Improvement of Stress Resistance by using TPU

Kurzfassung

Bonded joints offer numerous advantages compared to bolted joints when connecting different materials. However, the disadvantages of an adhesive bond like fatigue cracks can quickly become apparent under increased load, a challenge that is particularly prevalent in aviation because of the thin substrate material and the limitation in joint design. To address this issue, critical stressed components in aviation utilize a hybrid approach, incorporating both rivets and adhesive for structural bonding. This strategy aims the benefits of structural adhesive bonding while maintaining the safety associated with traditional bolted joints. Referred to as a fail-safe design, this concept prioritizes damage tolerance and enhanced resilience against manufacturing defects, achieved through a secure double load path. Especially when joining fiber-reinforced composites, bolts weaken the adherends of the joint and only contribute to load transfer when the brittle adhesive fails [1]. This study investigates the impact of local surface toughening [2] on the bond strength of thermoset adhesive joints using additional thermoplastic polyurethane (TPU) as toughening material. The concept is designed to introduce a crack arrest inside a pure bonded structural joint. Structural adhesive Mode I loading conditions are tested through quasi-static Double Cantilever Beam (DCB) test. Single lap joint (SLJ) tests are carried out to determine the influence of the TPU strip on the joint strength of a typical bonded joint combined load condition (Mode I and Mode II). The results show a pronounced influence of the TPU on the joint strength and an increase of over 15% in fracture toughness, indicating a significant improvement in the robustness of the joint and its resistance to failure.