Composite Design, Failure Modes & Testing: Delamination, ILSS & Fatigue

Composite structures fail by mechanisms completely different from metals — delamination, interlaminar shear failure, and fibre-matrix interface debonding are the dominant failure modes. Aerospace CFRP structures (Tata Advanced Systems, HAL composites division), wind turbine blades (Inox Wind, Suzlon — GFRP), and automotive CFRP bonnets must be designed against these failure modes. Understanding ILSS, fracture toughness, and fatigue crack growth in composites is essential for structural composite design.

• Identify and explain the six primary failure modes in fibre-reinforced composites.
• Calculate Interlaminar Shear Strength (ILSS) from Short Beam Shear test data.
• Distinguish Mode I (opening), Mode II (sliding shear), and mixed-mode delamination.
• Interpret S-N fatigue curves for CFRP vs. GFRP under cyclic loading.
• Identify ASTM D2344 (ILSS), ASTM D5528 (Mode I GIc), and ISO 14130 standards.

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3.1 Primary Failure Modes in Composites

3.2 Interlaminar Shear Strength (ILSS) — Short Beam Shear Test

ILSS is the most commonly measured composite interlaminar property. Measured by the Short Beam Shear (SBS) test per ASTM D2344 / ISO 14130:

$$ILSS = \frac{0.75 \times F_{max}}{b \times h}$$

Where: F_max = maximum force at failure (N), b = specimen width (mm), h = specimen thickness (mm).

Specimen geometry: Span-to-thickness ratio L/h = 4:1 (ASTM D2344) or 5:1 (ISO 14130) — minimises tensile/compressive contributions, maximises shear.

Typical ILSS values:

3.3 Delamination Fracture Toughness

Delamination is characterised by energy release rate G (J/m²):

Mode I (opening/peel): Tested by Double Cantilever Beam (DCB) per ASTM D5528.

$$G_{Ic} = \frac{3 P \delta}{2 b a}$$

Where: P = load, δ = displacement, b = width, a = crack length.

Mode II (shear sliding): Tested by End-Notched Flexure (ENF) per ISO 15114.

Typical G_Ic values:

• GFRP/Epoxy: 150–300 J/m²
• CFRP/Epoxy: 100–200 J/m²
• CFRP/PEEK (thermoplastic): 1000–2000 J/m² — 10× tougher than thermoset

3.4 Fatigue of Composites (S-N Curves)

Composites do not exhibit a clear fatigue endurance limit (unlike steel). CFRP shows superior fatigue performance vs. GFRP:

GFRP degrades significantly under cyclic loading due to matrix cracking and fibre debonding accumulation.

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Problem: A Short Beam Shear test on a CFRP/epoxy laminate gives: F_max = 1850 N, specimen width b = 10 mm, thickness h = 4 mm. Calculate ILSS.

$$ILSS = \frac{0.75 \times F_{max}}{b \times h} = \frac{0.75 \times 1850}{10 \times 4} = \frac{1387.5}{40} = \mathbf{34.7 MPa}$$

Interpretation: ILSS = 34.7 MPa — this is somewhat low for autoclave CFRP/epoxy (expected 65–100 MPa). This result suggests poor fibre-matrix interfacial bonding — possibly due to insufficient surface treatment on carbon fibre sizing, inadequate cure pressure, or void content. Further investigation with DCB delamination test and C-scan ultrasonic inspection recommended.

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Tata Advanced Systems Limited (TASL, Hyderabad) manufactures CFRP airframe components for Airbus, Boeing, and the Tejas LCA. Their composites quality lab routinely performs ILSS, DCB, and C-scan ultrasonic testing per ASTM D2344 and AMS specifications. BVID (barely visible impact damage) assessment uses ASTM D7136 to characterise post-impact compression strength retention.

Inox Wind Limited (Noida) manufactures 2–3 MW wind turbine blades in GFRP/epoxy at their Una (Himachal Pradesh) plant. Blade root attachment delamination is the critical design failure mode — IEC 61400-23 mandates fatigue testing of full-scale blades to 20-year service equivalent (10⁷–10⁸ cycles).

• ILSS: Interlaminar Shear Strength — measures resistance to ply delamination (ASTM D2344).
• G_Ic: Mode I fracture toughness — energy required to propagate delamination crack in opening mode.
• BVID: Barely Visible Impact Damage — sub-surface delamination from low-velocity impact; critical for aerospace CFRP.
• Matrix cracking: First failure event in composite laminates — transverse cracks in 90° plies.
• Delamination: Ply separation between laminae — catastrophic if unchecked.
• CFRP/PEEK: Thermoplastic composite with G_Ic 10× higher than thermoset CFRP — superior impact resistance.

1. ASTM D2344 — Short Beam Strength (ILSS) of polymer matrix composites
2. ISO 14130 — Fibre-reinforced composites — Determination of interlaminar shear strength
3. ASTM D5528 — Mode I Interlaminar Fracture Toughness (GIc) — DCB test
4. ISO 15114 — Mode II interlaminar fracture toughness (GIIc) — ENF test
5. ASTM D7136 — Measuring damage resistance from drop-weight impact on polymer matrix composites

1. An ILSS test gives F_max = 2200 N, b = 12 mm, h = 6 mm. Calculate ILSS. Is this consistent with GFRP/epoxy or CFRP/epoxy?
2. Explain why CFRP thermoplastic composites (PEEK matrix) have G_Ic values 10× higher than CFRP thermoset (epoxy matrix).
3. Define Mode I and Mode II delamination — which test method is used for each?

Q1. ILSS is measured by:

• C) Short Beam Shear test (ASTM D2344)

Q2. The ILSS formula is:

• B) ILSS = 0.75 × F_max / (b × h)

Q3. BVID stands for:

• B) Barely Visible Impact Damage — sub-surface delamination from low-velocity impact

Q4. Which composite matrix gives the highest G_Ic (delamination toughness)?

• A) Epoxy thermoset B) Polyester C) PEEK thermoplastic D) Vinyl ester

Q5. Which Indian company manufactures CFRP airframe components for Airbus and Boeing?

• A) Tata Advanced Systems Limited (TASL)