Durability and Damage Tolerance

The durability and damage tolerance of an aerospace structure relate to the structures ability to resist the onset of damage and perform to required parameters with damage present throughout its remaining life time. This is a particular design criteria for composite structures that are exposed to a number of events during their lifetime. These can include in-service loading, environment and mishaps that can cause damage initiation and structure degradation. In addition, some form of damage may be present through manufacture or handling before the structure ever enters service.

The generally good fatigue and 'corrosion' resistance of composites aid in the durability and damage tolerance design of composites. However it is the inherent nature of composites that also present significant challenges to the designer to prevent matrix related damage initiation and growth. Most composite material resins are brittle and hence have limited resistance to damage. This damage manifests itself as matrix cracks and delaminations. These matrix failures can occur as a result of an impact event, some form of environmental degradation or out-of-plane fatigue loads.

MERL has pioneered the development of test methods, equipment and analysis approaches to predict delamination initiation and growth in composite structures. This includes the collaboration with other organisations internationally to develop new standards for interlaminar fracture testing. This involves developing methods for new material types such as fabrics, or new environmental conditions such as Mach 2+ temperatures.

To decrease the cost for generating these data, MERL have developed new test machines capable of testing multiple fracture specimens at one time.

MERL has worked on confidential client projects employing fracture mechanics to predict delamination initiation and growth in actual structures. This work often requires finite element analysis of the structure, to identify areas of high interlaminar strains, local modelling of delaminations, materials interlaminar fracture data generation followed by predictions and if possible validation to structural tests.