Impact Damage & Damage Tolerance in Composites
Why composite impact damage can remain internally significant despite limited visible surface evidence.
What Is It?
Impact damage in composites is damage caused by a foreign object striking the composite surface. Unlike metallic structures, where impact damage (dents, scratches) is generally visible and proportional to the impact energy, composites can sustain significant internal damage — delamination, matrix cracking, fibre breakage — with very little visible surface evidence. This is one of the most important and challenging aspects of composite structural engineering: the damage that cannot be seen may be the damage that reduces structural capability the most.
Why It Matters
Impact damage is one of the primary threats to composite structural integrity. A composite panel that is fully adequate in its undamaged state may have significantly reduced compression capability after an impact that is barely visible on the surface. This is the basis of the barely visible impact damage (BVID) concept and the compression-after-impact (CAI) assessment. Understanding how impact damage forms, how it affects residual capability and how it is detected is essential for designing damage-tolerant composite structures.
Limited external damage does not necessarily mean limited internal damage. A composite can sustain significant internal delamination and matrix cracking from an impact that shows only a small surface dent or mark.
Why Composites Sustain Internal Damage with Limited Surface Evidence
When a composite is impacted, the impact energy is absorbed through several mechanisms: matrix cracking, delamination, fibre breakage and friction. In a thin composite panel, the damage often forms in a characteristic pattern — a cone of delamination spreading from the impact point through the thickness, with matrix cracks in the plies. The surface ply may remain largely intact, showing only a small dent or surface mark, while the internal plies have extensive delamination. This is because the impact energy dissipates internally through the thickness before reaching the back surface. In metallic structures, the ductile material deforms plastically and absorbs energy through yielding — the damage is visible as a dent. In composites, the brittle matrix and fibres absorb energy through fracture — the damage is internal.
Types of Impact Damage
| Damage Type | Location | Effect on Structural Capability |
|---|---|---|
| Matrix cracking | In plies beneath impact point; transverse and shear cracks | Reduces local stiffness; precursor to delamination |
| Delamination | Between plies; conical damage pattern through thickness | Reduces compression strength; reduces bending stiffness; grows under fatigue |
| Fibre breakage | At impact point in primary load-direction plies | Direct reduction in load-carrying capability; most severe damage type |
| Core crushing (sandwich) | Core beneath impact point in sandwich panel | Reduces shear transfer; may cause face-sheet debonding |
| Back-face damage | On opposite side from impact; fibre breakage and splitting | Can be more visible than front-face damage; indicates through-thickness damage |
Impact Energy
The severity of impact damage depends on the impact energy, the impactor geometry, the panel thickness, the laminate architecture and the boundary conditions. Higher energy impacts produce more damage. Sharp impactors concentrate energy and produce more local damage. Blunt impactors spread energy and may produce more delamination. Thicker panels resist impact damage better than thin panels. The relationship between impact energy and damage size is not linear — there is a threshold energy below which damage is minimal and above which damage grows rapidly.
BVID — Barely Visible Impact Damage
BVID is the concept that an impact can produce significant internal damage with only minimal visible surface evidence. The threshold of visibility depends on the panel thickness, surface finish, inspection method and access. BVID is the critical condition for damage-tolerant design — the structure must be able to tolerate damage at least up to the level that is reliably detectable by the specified inspection method. If the structure cannot tolerate BVID, the inspection method must be improved (e.g. using ultrasonic instead of visual) or the structure must be redesigned.
CAI — Compression After Impact
Compression after impact (CAI) is a standard test and assessment concept for composite damage tolerance. A panel is impacted at a specified energy, then loaded in compression to measure the residual compression capability. The CAI strength is compared to the undamaged compression strength to quantify the damage tolerance. CAI is a practical measure of how much compression capability is retained after a representative impact. It is not a universal property — it depends on the material, laminate, impact energy, panel geometry and boundary conditions. CAI testing provides data for damage tolerance assessment but should not be used without understanding its specific conditions.
COMPOSITE CHECK: Has impact damage tolerance been assessed? A composite structure that has not been assessed for CAI or residual strength after impact may not be damage-tolerant. The inspection programme and the structural capability must be consistent.
Residual Compression Capability
The residual compression capability after impact is typically the governing damage tolerance condition for composite structures. Delamination from impact reduces the compression strength because the delaminated plies can buckle locally at a lower stress than the intact laminate. The compression load that the damaged panel can carry is therefore lower than the undamaged panel — sometimes significantly lower. The residual compression capability must remain above the required load (typically limit load) with appropriate margin until the damage is detected and repaired.
Inspection
Impact damage detection depends on the inspection method. Visual inspection may detect large dents but will miss BVID. Tap testing can detect near-surface delamination but is subjective and limited in depth. Ultrasonic inspection (pulse-echo or through-transmission) can detect internal delamination and is the standard method for impact damage assessment. The inspection method must be capable of detecting damage at least at the BVID level. If the damage tolerance analysis assumes a certain detectable damage size, the inspection method must be able to reliably detect damage at that size.
Why Composite Damage Tolerance Differs from Metallic
Metallic damage tolerance is based on crack growth — a crack grows under fatigue and must be detected before it reaches a critical size. Composite damage tolerance is different. Impact damage is typically present from the moment of impact — it does not necessarily grow under fatigue in the same way. The concern is not crack growth but residual strength — the damaged structure must retain sufficient capability until the damage is detected and repaired. This is a fundamentally different assessment from metallic crack-growth-based damage tolerance, and the methods are not interchangeable.
| Aspect | Metallic Damage Tolerance | Composite Damage Tolerance |
|---|---|---|
| Primary concern | Crack growth under fatigue | Residual strength after impact damage |
| Damage initiation | Fatigue crack from stress concentration | Impact damage from foreign object |
| Damage growth | Predictable crack growth (da/dN) | May or may not grow; delamination can grow under fatigue |
| Critical condition | Crack reaches critical size | Residual strength drops below required load |
| Inspection target | Detect crack before critical | Detect impact damage before residual strength inadequate |
Key Takeaways
- Composites can sustain significant internal damage with limited visible surface evidence (BVID)
- Impact damage includes matrix cracking, delamination, fibre breakage and core crushing
- CAI — compression after impact — is the standard measure of residual capability after damage
- Delamination from impact reduces compression strength through local ply buckling
- Composite damage tolerance is based on residual strength, not crack growth — it differs fundamentally from metallic damage tolerance