How do I evaluate durability, damage resistance & tolerance?

The durability and damage tolerance of a composite material 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. Composite structures 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 location of damage initiation sites may be difficult to identify and thorough inspections are required.

The generally good fatigue and corrosion resistance of composites aid in durability and damage tolerance design. However, it is the inherent nature of composites that also presents 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.

The ability of a composite material to resist certain potential damage inducing events is termed damage resistance. It differs from durability in that durability addresses the prevention of damage under normal operating conditions.

Material or structural design must incorporate a consideration of damage resistance, durability and damage tolerance as they are interrelated. Improving one of these aspects, may not necessarily improve the others. An important part of a structural development program is to determine the damage that the structure is capable of carrying at the various required load levels. A damage tolerance design approach involves the use of inspection techniques along with structural design concepts to protect safety. An extensive residual strength database addressing the full range of potential damage events (including, impact damage, holes, manufacturing flaws) is required to ascertain allowable damage limits. This procedure will allow the critical damage threshold to be identified that will allow quantitative inspection procedures and periods to be set.

One of the limiting factors to the use of laminated composites is their internal damage state, particularly after an impact event. Impact events may occur from many sources including tool drop and collision with vehicles. The internal damage is a complex pattern of cracks, delaminations and fibre failure. These patterns are heavily dependent on the nature of the impact event.

From Mil-Hdbk-17 the method to evaluate damage tolerance requires the following steps:

1. Derive the entire residual static strength versus impact energy curve from analysis supported by test.
2. Determine accidental impact threats in terms of energy versus probability curves.
3. Calculate, within each scheduled inspection interval, the probability to have such accidental damages on the structure.
4. Determine load (or stress, or strain) occurrences versus probability curves.
5. Check that the scheduled inspection program will make damage detection highly probable before the probability target is exceeded.