Fatigue spectra simplification approaches for accelerated testing in composite aircraft structures

The quest for light weight high stiffness structures that have excellent durability and corrosion resistance has led to the increasing use of composite materials in aircraft structures. Rigorous static and fatigue test programs are required to fully understand material behaviour, damage growth and fatigue characteristics of the composite structures to certify and prolong the usage of aircraft. Fatigue testing is a time consuming and costly process. Methods are in place to accelerate fatigue tests for metallic materials which in some cases have been translated across to the composites field. This paper reviews these methodologies and discusses their applicability and suitability for testing composite materials with the aim of improving composite fatigue test run times. Three approaches currently exist to simplify flight spectra, namely truncating portions of load cycles, applying load enhancement factors and merging cycles by using a turning point reduction technique. From the literature review it is clear that significant further work is required in developing turning point reduction techniques for composite structural fatigue testing applications. Based on the information gathered thus far, work in this area is currently underway covering aspects of theoretical modelling and fatigue test validation, starting from relatively simple load cases, such as specimens with a single delamination crack, right through to specimens subjected to impact damage under compressive fatigue loading.