Delamination Under High-Rate Impact

Delamination in high-rate impact

Delamination — the separation of plies at their interface — is a critical damage mode in composite laminates under high-rate impact. Unlike quasi-static loading, where delamination grows slowly and may be arrested by through-thickness reinforcement, high-rate impact produces delamination that grows rapidly and may be extensive. The delamination may be driven by stress-wave reflection at ply interfaces, by local contact deformation, or by the global bending of the laminate.

Interlaminar stress

The interlaminar stress — the through-thickness normal and shear stress at the ply interface — is the driving force for delamination. Under impact, the interlaminar stress arises from three sources: the local contact pressure (compressive through-thickness stress beneath the projectile), the bending-induced through-thickness tension (on the rear side of the laminate), and the stress-wave reflection at ply interfaces (tensile through-thickness stress from impedance mismatch). The interlaminar stress may exceed the interlaminar strength, initiating delamination.

Wave propagation effects

Stress-wave propagation in composite laminates is complex because the wave speed depends on the ply orientation. A wave travelling through a 0-degree ply has a different speed than one through a 90-degree ply. At each ply interface, the impedance mismatch causes partial reflection and transmission. The reflected waves produce interlaminar tension that can drive delamination. The wave propagation effects are most significant in thick laminates with many ply interfaces — each interface is a potential delamination site.

• Wave Transmission Across Material Interfaces

Local contact and delamination growth

The local contact pressure beneath the projectile drives delamination in the impact zone. The contact pressure compresses the surface plies, and the through-thickness shear stress at the ply interfaces may exceed the interlaminar shear strength. The delamination grows radially from the impact point as the contact area grows. The delamination pattern typically follows the ply orientations — the delamination at each interface is elongated along the fibres of the adjacent plies, producing the characteristic "flower" or "peanut" pattern.

Hidden damage

Delamination is often hidden — the surface plies may appear intact while the internal plies are separated. This hidden damage is the primary challenge in composite impact assessment. Visual inspection may not detect internal delamination. Non-destructive examination — ultrasonic C-scan, thermography, or shearography — is needed to map the delamination extent. The hidden damage may significantly reduce the residual strength, particularly the compression-after-impact (CAI) strength, even when the visible damage is minor. The analysis must predict the hidden damage, and the inspection must confirm it.

Interface fracture representation

Delamination models commonly represent the ply interface using cohesive behaviour or a contact-based debond formulation. The interface requires normal and shear strength together with an energy-based propagation law. Mixed-mode loading is important because impact produces simultaneous opening and sliding. The parameters should come from appropriate interlaminar fracture data rather than being selected simply to reproduce one impact test. Where rate dependence is unknown, sensitivity studies can show whether reasonable variation changes the structural conclusion.

Mesh and interface discretisation

The predicted delamination area can be highly sensitive to in-plane element size and to the way cohesive interfaces are discretised. A mesh that is too coarse can delay initiation and force the crack front to advance in large jumps; an excessively fine mesh can add cost without improving the uncertainty in the material parameters. Establish a mesh level at which the principal delamination dimensions and absorbed interface energy are acceptably stable. Also check that the surrounding ply mesh can resolve the local bending and transverse shear that drive interface separation.

Interaction with intralaminar damage

Matrix cracking and fibre damage redistribute stress into neighbouring interfaces and can trigger or arrest delamination. Treating delamination in isolation may therefore miss the sequence observed in test. In a progressive model, review whether interface damage appears at physically plausible locations relative to matrix cracks and rear-face bending. The aim is not to reproduce every microscopic crack, but to capture the coupled mechanisms that control stiffness loss and residual strength.

Residual-strength significance

Delamination reduces the ability of adjacent plies to act together, lowers local bending stiffness and can promote sublaminate buckling under later compression. Its structural significance therefore depends on area, position through the thickness, proximity to free edges or holes and the post-impact load direction. Residual-strength assessment should use the actual or conservatively bounded delamination map where practical, rather than reducing the entire laminate stiffness uniformly.