Damage-tolerant composite structures: a review about the state of the art and future perspectives

In aerospace engineering, a structural design has to account for Damage Tolerance (DT). A flawed structure still has to sustain the design load until that flaw becomes discovered and is repaired. The corresponding inspection intervals are defined to detect a defect before it could become critical to the aircraft integrity. Thus, a damage-tolerant design ensures the airworthiness of aircraft under operative conditions. The DT analysis is well-established for metallic materials. The damage, in form of visible through-thickness cracks, grows stably and quantitatively predictable. Composite structures, which also have to account for DT, differ significantly in their damage behavior. Several interacting failure modes that are hardly visible and might grow abruptly, complicate a transfer of the standard DT procedures from metals. Instead, a conservative no-growth approach shall satisfy the safety demands. This no-growth design is approved by costly experimental qualification. The possible weight savings and cost benefit of composites structures decrease strongly. In the present investigation, we outline the current state of the art to design damage-tolerant composite structures. Furthermore, we identify the possibilities for better exploitation of the weight-saving potential. An improved determination procedure for a safe no-growth interval or a transition toward slow growth design could accomplish these advancements.