Fatigue crack growth predictions in Aramid reinforced aluminum laminates (ARALL)

Abstract

Aramid reinforced aluminum laminates (ARALL) consist of thin sheets of high-strength aluminium alloy that are laminated using a structural adhesive and high-strength aramid fibers. ARALL is extremely fatigue resistant because the aramid fibers remain intact if fatigue cracks occur in the metallic component. The crack growth rate in the aluminium sheets is reduced considerably due to crack bridging by the fibers. The mechanical properties of ARALL are discussed and some potential applications for ARALL are presented. The fatigue mechanisms in ARALL are explained and a model that is able to calculate the fatigue crack growth rates in ARALL for dif- ferent constant-amplitude fatigue loadings is presented. URING aircraft service, fatigue stresses occur in the material of the structure as a consequence of cyclic loadings. Fatigue stresses may cause the initiation and propa- gation of cracks in the structure. Consequently, a potential risk of a final catastrophic failure of the structure is present. The risk is kept low by inspections and repair of the aircraft. These aspects are especially relevant because fatigue stresses and cycle numbers in modern aircraft are high as a conse- quence of the need for weight savings in the structure. Weight savings are important for reasons of economy (fuel consumption) and aircraft flight performance. Improvements of aircraft designs become possible if stronger and lighter materials with a good fatigue resistance become available. Presently, the bulk of an aircraft construction consists of different aluminum alloys. Unfortunately, the strongest con- ventional aluminum alloys usually show only moderate fatigue performance; for fatigue-critical aircraft com- ponents, the designer must choose other, more fatigue- resistant alloys. The fatigue resistance of a metallic material can be classified by two properties: 1) the resistance against the initiation of fatigue cracks and 2) the resistance against the growth of fatigue cracks. The initiation of fatigue cracks often takes pilace at notches in the structure, because the stresses in the notch root are higher than in the rest of the structure. Due to the great number of notches in an aircraft structure and the scatter in the fatigue crack initiation behavior, it is impossible to prevent the initiation of cracks with enough certainty at an acceptable weight of the aircraft structure. Hence, the safety of an aircraft has to be obtained with a damage tolerance approach (fail-safe structure). Con- sequently, the fatigue crack growth properties of a structural material are of major importance. Cracks have to be found before they become dangerous, so materials showing slow fatigue crack growth rates are required. The hybrid material ARALL (aramid reinforced aluminum laminate) combines good strength properties and very slow fatigue crack growth rates; thus, it is especially suitable for use in fatigue-criti cal aircraft structures.