High-Velocity Impact of Composite Structures

Composite impact response

Composite structures respond to high-velocity impact differently from metallic structures. The anisotropic material behaviour, the layered construction and the multiple failure modes (fibre, matrix, interface) produce a complex damage pattern. The impact may cause fibre breakage, matrix cracking, delamination, local crushing or a combination of all four. The damage may be hidden — internal delamination or matrix cracking may not be visible on the surface, making inspection and residual strength assessment more challenging than for metallic structures.

Fibre failure

Fibre failure occurs when the local tensile or compressive stress in the fibre direction exceeds the fibre strength. Tensile fibre failure produces a fracture surface perpendicular to the fibre direction; compressive fibre failure produces microbuckling or kinking. Fibre failure is the most severe damage mode — it directly reduces the load-bearing capacity of the laminate. In high-velocity impact, fibre failure occurs locally at the impact point, where the contact stress is highest. The fibre failure pattern depends on the ply orientations — fibres in different directions fail at different locations and in different modes.

• Fibre & Matrix Failure Under Impact

Matrix cracking

Matrix cracking occurs when the local stress in the matrix exceeds the matrix strength. Matrix cracks typically form parallel to the fibres (intralaminar cracking) or perpendicular to the through-thickness direction (transverse cracking). Matrix cracking is less severe than fibre failure — it reduces the laminate stiffness and provides a path for delamination, but the fibres still carry the primary load. However, matrix cracking may be extensive — the impact may produce a network of matrix cracks throughout the laminate, significantly degrading the composite properties.

Delamination

Delamination is the separation of plies at the ply interface. It is driven by interlaminar stresses — the through-thickness tensile and shear stresses that arise from the impact loading. Delamination is the most insidious damage mode because it may be hidden — the surface plies may appear intact while the internal plies are separated. Delamination reduces the laminate stiffness, the compressive strength and the fatigue life. In high-velocity impact, delamination may be driven by the stress-wave reflection at ply interfaces and by the local contact deformation.

• Delamination Under High-Rate Impact

Local crushing

Local crushing is the compression failure of the composite material directly beneath the projectile. The material is crushed into a powder or a fragmented mass, absorbing energy through the fragmentation process. Local crushing is the primary energy absorption mechanism in some composite targets — the material sacrifice absorbs the projectile energy. The crushed region may be small (localised to the impact point) or extensive (spreading radially), depending on the impact energy and the composite architecture.

Perforation and residual strength

Perforation of a composite laminate produces a hole with damaged edges — broken fibres, cracked matrix, delaminated plies. The perforation hole reduces the load-bearing area, and the damage zone around the hole may extend several projectile diameters. The residual strength after perforation is significantly reduced — the damaged zone has degraded properties and may act as a stress concentrator. The residual strength assessment of composite structures after impact is particularly challenging because the hidden damage (delamination, matrix cracking) may be more extensive than the visible damage.

• Composite Residual Strength After Impact
• Composite Structures

Anisotropy and stacking sequence

Composite impact response is controlled by the directional stiffness and strength of the laminate. The same externally applied impact can produce very different damage in quasi-isotropic, cross-ply and highly directional laminates because load spreads preferentially along fibre directions. Ply thickness, stacking order and the position of off-axis plies affect both bending stiffness and interlaminar stress. Structural analysis should therefore use the actual laminate definition where available rather than an equivalent isotropic plate when damage morphology or residual strength is important.

Through-thickness resolution

Delamination and rear-face tensile failure are driven by through-thickness stress gradients that can be poorly represented by an overly simple shell model. Layered shell formulations can be effective for global response and intralaminar damage, but cohesive interfaces, solid layers or a local solid submodel may be needed when delamination growth or local crushing is a key output. The modelling approach should be chosen from the required result rather than from a blanket preference for shells or solids.

Progressive damage and energy accounting

Multiple damage mechanisms can operate simultaneously: matrix cracking, fibre failure, interface separation and local crushing. The model should track how stiffness and strength degrade after each mechanism initiates and should avoid releasing non-physical amounts of stored energy when damage variables change abruptly. Review internal energy, damage energy, contact work and kinetic energy through the event. A numerically stable simulation can still be non-physical if damage deletion removes load-carrying capacity too suddenly.

Test correlation

Composite correlation should use more than visible front-face damage. Useful measures include rear-face deformation, ultrasonic delamination area, damage shape at individual interfaces, residual compression strength and mass loss where applicable. Because internal damage is often larger than the visible surface mark, agreement based only on external photography can give false confidence. Sectioning or NDE data are particularly valuable for validating the through-thickness damage sequence.