Low Velocity Impact

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 variables associated with the impact event.

While the aerospace industry has an established procedure for handling impacts, they remain a key design limitation as all parts need to be designed as damage tolerant. This establishes inspection levels and limit loads.

MERL's high velocity servo-hydraulic machine can also simulate impacts up to 5m/s and +/- 5kN. This machine has also been configured to test impact conditions for composite panels that are under either tension or compression load, such as flight surfaces on take-off or landing.

To help protect composite structures, MERL investigated the use of protective coatings utilising thin layers of rubber and thin rubber/glass fabric coatings. Elastomer surface layers can be used to redistribute stresses that arise through in-service impact and, hence, reduce damage to the load bearing composite structure. Elastomer surface layers can also be configured to provide enhanced environment protection for the underlying composite structure. However, for larger energies a thin glass fibre layer over the rubber layer better distributed the impact load and allowed surface damage to be more readily identified.