Why Impact Damage Reduces Compression Strength

What Is It?

The reduction in compression strength after impact is not caused by a single mechanism — it is the result of several interacting mechanisms. Delamination creates sublaminates that buckle locally. The buckling drives delamination growth. The delamination growth redistributes load. The matrix cracking reduces local stiffness. The fibre instability (compression failure) is triggered by the stress concentrations. Understanding how these mechanisms combine is essential for designing laminates with better CAI performance and for interpreting CAI test results.

Why It Matters

Understanding the mechanism of compression strength reduction is essential for two reasons. First, it guides laminate design — laminates that are more resistant to delamination, or that have better sublaminate stability, will have better CAI performance. Second, it guides the analysis — a CAI FEA must capture the governing mechanism (sublaminate buckling) to predict the residual strength. If the analysis does not capture the mechanism, it will not predict the correct strength, even if it predicts the correct damage state.

CAI strength reduction is caused by interacting mechanisms — delamination, buckling, growth, redistribution, cracking, fibre failure. Understanding the mechanism guides laminate design and analysis. A CAI FEA must capture sublaminate buckling to predict the correct strength.

Delamination

Delamination is the starting point for the compression strength reduction. The impact creates delamination at multiple interfaces. Under compression, the delamination separates the laminate into sublaminates. Each sublaminate is thinner than the parent laminate and has lower bending stiffness. The delamination is the enabler — without it, the compression strength reduction would be minimal. The delamination size and location determine the sublaminate geometry and therefore the buckling load.

• Delamination is the starting point — separates laminate into sublaminates
• Each sublaminate is thinner with lower bending stiffness
• Delamination size and location determine sublaminate geometry
• Without delamination, compression strength reduction would be minimal

Sublaminate Buckling

Sublaminate buckling is the primary mechanism. A delaminated sublaminate under compression buckles at a load lower than the intact laminate buckling load. The buckling load depends on the sublaminate thickness, the delamination size and the boundary conditions. A thinner sublaminate buckles at a lower load. A larger delamination allows more buckling. The buckling is local — it occurs at the delamination site, not globally. The local buckling changes the stress distribution and drives the next mechanism.

Sublaminate buckling load (simplified):

P_cr ∝ E × t_sub² / L²

where:
t_sub = sublaminate thickness
L = delamination length (buckling span)
E = sublaminate stiffness

Compared to intact laminate:
  P_cr,intact ∝ E × t_total² / L²

Ratio: P_cr,sub / P_cr,intact = (t_sub / t_total)²

For mid-plane delamination (t_sub = t/2):
  P_cr,sub / P_cr,intact = 1/4

The sublaminate buckles at 1/4 of the
intact buckling load — a 75% reduction.

Load Redistribution

When the sublaminate buckles, it can no longer carry the full compression load. The load redistributes to the intact parts of the laminate. The redistribution increases the stress in the intact material. If the redistributed stress exceeds the intact material strength, fibre failure occurs. The load redistribution is the link between the local buckling and the global failure — the local damage causes a global stress increase that drives the ultimate failure.

Buckled sublaminate cannot carry full load → load redistributes to intact material. Increased stress in intact material may exceed strength → fibre failure. Local buckling causes global stress increase → ultimate failure.

Matrix Cracking

Matrix cracking from the impact also contributes to the compression strength reduction. The matrix cracks reduce the transverse and shear stiffness of the damaged plies. The stiffness reduction changes the load distribution — more load goes to the undamaged plies. The matrix cracking also provides pathways for the delamination to grow — the crack tips are stress concentrators. While matrix cracking is less critical than delamination for CAI, it contributes to the overall stiffness and strength reduction.

• Matrix cracking reduces transverse and shear stiffness
• Stiffness reduction changes load distribution — more load on undamaged plies
• Matrix crack tips are stress concentrators — aid delamination growth
• Less critical than delamination but contributes to overall reduction

Fibre Instability

Fibre instability — fibre kinking or compression failure — is the final failure mechanism. The stress concentration at the delamination front, the bending of the buckled sublaminate, or the overall load redistribution can trigger fibre compression failure. Fibre compression failure is typically sudden and catastrophic — the fibres kink or crush, the load-carrying capacity is lost, and the structure collapses. The fibre instability is the ultimate failure event that defines the CAI strength.

• Fibre instability (kinking or crushing) is the final failure mechanism
• Triggered by stress concentration at delamination front or load redistribution
• Typically sudden and catastrophic — load-carrying capacity lost
• Defines the CAI strength — the load at which fibre failure occurs

Mechanism Interaction

The mechanisms interact in a cascade. Delamination enables sublaminate buckling. Buckling drives delamination growth. Growth increases the buckled area. More buckling increases load redistribution. Increased stress triggers fibre failure. The cascade means that the CAI strength is not simply the buckling load — it is the load at which the cascade reaches fibre failure. The cascade may be stable (gradual) or unstable (sudden), and the transition depends on the damage size, the laminate design and the load.

Mechanism cascade:

Delamination → Sublaminate buckling → Delamination growth →
More buckling → Load redistribution → Stress concentration →
Fibre instability → Collapse

CAI strength = load at which the cascade
reaches fibre failure.

The cascade may be stable (gradual) or
unstable (sudden).

Design Implications

Understanding the mechanisms has design implications. Laminates with tougher interfaces (higher G_Ic, G_IIc) are more resistant to delamination growth. Laminates with more 0-degree plies on the surface may have better sublaminate stability. Laminates with smaller delamination (from lower impact energy or better impact resistance) have higher CAI strength. Laminate design for CAI is a balance between impact resistance (minimising damage) and damage tolerance (maximising residual strength with damage).

• Tougher interfaces (higher G_c) → more resistant to delamination growth
• Surface ply design → better sublaminate stability
• Smaller delamination → higher CAI strength
• Design balance: impact resistance vs damage tolerance

Key Takeaways

• CAI strength reduction is caused by a cascade: delamination → buckling → growth → redistribution → fibre failure
• Sublaminate buckling is the primary mechanism — buckles at 1/4 of intact load for mid-plane delamination
• Load redistribution: buckled sublaminate cannot carry load → stress increases in intact material
• Matrix cracking contributes to stiffness reduction and aids delamination growth
• Fibre instability (kinking/crushing) is the final, catastrophic failure event