Resisting low velocity impact damage

Cracking, delamination and element debonding are the predominant damage characteristics associated with impact damage. Much development work has occurred to improve composites materials resistance to impact damage. These include developing new toughened resin systems, stitching, z-pinning, and textile material forms (with varying degrees of through the thickness reinforcement). Impact damage resistance is also very dependent on the geometry, support and loading of the structure. The geometric features include laminate stacking sequence, local thickness buildups at bonded elements, adhesive layer inserts, location of strengthening elements, and presence of mechanical fasteners.

The damage resistance and tolerance are strongly dependent on the constituent resin and fibre material properties and the material form. The high tensile strain-to-failure of glass and aramid fibres make them significantly more resistant to failure under impact loads than carbon. Laminates using fabric reinforcement have also shown better resistance to damage than laminates with unidirectional tape. The area under a resin's stress-strain curve indicates the material's energy absorption capability hence, toughened thermoplastic materials exhibit good energy absorption capabilities. In turn the materials interlaminar fracture toughness values are an indicator on a composite materials damage resistance or ability to resist delamination.

During an impact event on thin laminates, in the damage under contact region, the damage may consist of fibre and matrix damage. Beyond the contact region, the damage consists only of matrix damage. A dent is left under the contact region typically the diameter of the laminate thickness. The depth of the damage is less affected by the shape of the impactor. Conversely, for impact to thick laminates, damage is centralised under the impact zone and does not continue through the thickness. The depth and profile of the damage is not dependent on impactor shape.