Fibre & Matrix Failure Under Impact
Fibre and matrix failure modes in composites under high-rate impact — fibre tension and compression, matrix damage, shear, progressive damage and strain-rate considerations.
Fibre tension and compression
Fibre failure occurs in both tension and compression. In tension, the fibre fractures when the local tensile stress exceeds the fibre tensile strength. The fracture surface is perpendicular to the fibre direction, with fibre pull-out in some composites. In compression, the fibre fails by microbuckling or kinking — the fibre buckles laterally, producing a kink band at an angle to the fibre direction. Fibre compression failure is typically initiated by a local imperfection or misalignment and propagates rapidly. In impact, both modes may occur — tension on the rear surface, compression on the front surface.
Matrix damage
Matrix damage includes matrix cracking (transverse and parallel to the fibres), matrix crushing and matrix yielding. Matrix cracking is the most common mode — the matrix cracks when the local stress exceeds the matrix strength. Transverse cracks form perpendicular to the ply plane, parallel to the fibres; they are driven by transverse tensile stress or by thermal residual stress. Matrix damage degrades the laminate properties: the stiffness is reduced (the matrix transfers load between fibres), and the compression strength is reduced (the matrix supports the fibres against buckling).
Shear failure
Shear failure in composites occurs at the ply level (intralaminar shear) and at the ply interface (interlaminar shear). Intralaminar shear failure is the shear fracture of the matrix between fibres; it produces a crack parallel to the fibre direction. Interlaminar shear failure is the shear fracture of the ply interface; it produces delamination. In high-velocity impact, shear failure may be concentrated around the projectile perimeter (similar to plugging in metals) — the material is sheared out ahead of the projectile. The shear failure mode depends on the strain rate, the temperature and the matrix ductility.
Progressive damage
Composite failure under impact is progressive — the damage accumulates over the loading duration. Fibre failure, matrix cracking and delamination initiate, grow and interact. Matrix cracking may initiate first (matrix is weaker), followed by delamination (at the cracked interfaces), followed by fibre failure (when the remaining fibres can no longer carry the load). The progressive damage must be modelled in the FEA using a damage model that tracks the damage state at each integration point — a simple "fail/no-fail" criterion is not adequate. Progressive damage models (e.g. Hashin, Puck, LaRC) track the damage modes separately and degrade the material properties as damage accumulates.
Strain-rate considerations
The strain-rate sensitivity of composite materials varies with the constituent properties. The matrix may show significant rate sensitivity (particularly for thermoplastic matrices); the fibres may show modest rate sensitivity (carbon and glass fibres). The composite strain-rate sensitivity is typically dominated by the matrix. The failure strains may change with strain rate — some composites become more ductile at high rates (matrix rate sensitivity delays cracking), others become less ductile (the fibre-matrix interface fails more readily). The material model must include rate-dependent failure criteria calibrated from high-rate test data.
Failure criteria and mode separation
Composite damage criteria should separate fibre-dominated and matrix-dominated modes because they have different physical consequences and different stiffness-degradation rules. A single scalar failure index can indicate that a ply is highly loaded but provides little guidance on whether the subsequent response should lose longitudinal stiffness, transverse stiffness, shear stiffness or interface capacity. For impact, this distinction matters because an overly aggressive degradation rule can cause premature loss of load spreading and artificially concentrate later damage.
Compression damage and kink-band sensitivity
Fibre compression failure is especially sensitive to initial fibre misalignment, matrix shear response and local constraint. Continuum models generally represent this behaviour in an averaged way rather than resolving individual fibres. The input strengths and degradation law should therefore be treated as calibrated laminate-level properties. If rear-face or local bending places plies into compression after initial damage, check that the model does not retain unrealistic longitudinal stiffness after a compressive failure criterion has been exceeded.
Rate dependence and temperature coupling
High-rate deformation can alter matrix yield and fracture behaviour, while local adiabatic heating may reduce matrix stiffness or strength. Whether this coupling needs to be represented explicitly depends on the material system and impact severity. At minimum, document the strain-rate range covered by the material data and compare it with the rates predicted in the critical plies. Extrapolation beyond the characterised range should be identified as model uncertainty rather than hidden inside a generic rate law.
Correlation hierarchy
Use a hierarchy of tests where possible: constituent or ply-level tests for material behaviour, laminate coupons for damage interaction, and representative components for structural load spreading. An impact model that is tuned only at component level can compensate for incorrect fibre, matrix and interface parameters. Separate correlation evidence for each mechanism makes the model more transferable to different laminate thicknesses and stacking sequences.