Composite Delamination Ultrasonic Inspection Case Study

This case study examines a composite laminate panel with suspected delamination after low-energy impact. The engineering question is not only whether ultrasonic testing detects an indication. It is whether the indication is sized, located, and dispositioned with enough evidence to release, repair, or reject the part.

Composite defects are difficult because damage can be barely visible at the surface while still reducing compression strength, bearing strength, fatigue life, moisture resistance, or inspectability. The worked disposition below connects inspection data to the load path and acceptance basis.

Case Context

A carbon-fiber reinforced polymer access panel is inspected after a maintenance tool drop. Visual inspection shows only a shallow surface mark. A tap test is ambiguous, so the quality engineer requests ultrasonic pulse-echo inspection.

The panel is in a compression-critical zone near a fastener row. The inspection team must decide whether the panel can remain in service, needs repair, or must be replaced.

Inspection Data

Quantity	Value
laminate thickness	4.8\ \text{mm}
detected indication area	2300\ \text{mm}^2
maximum indication length	68\ \text{mm}
maximum indication width	42\ \text{mm}
distance from fastener row	35\ \text{mm}
zone limit for equivalent delamination diameter	35\ \text{mm}
special disposition distance from fastener row	50\ \text{mm}
design compressive stress in service	150\ \text{MPa}
undamaged compression allowable	260\ \text{MPa}
damaged compression-after-impact allowable for this defect size band	175\ \text{MPa}
required minimum margin for release without repair	0.25

The ultrasonic procedure has been calibrated on a reference panel with known flat-bottom reflectors and embedded film inserts. The relevant reference indication is detected with a signal-to-noise ratio above the acceptance threshold.

Step 1: Convert Inspection Area to Equivalent Diameter

For disposition, reduce the irregular delamination map to an equivalent circular diameter:

where:

Therefore:

Engineering Comment

Equivalent diameter is a simplification. It does not capture shape, ply interface, distance to holes, fiber direction, or local buckling mode. It is useful as a screening metric only when the acceptance basis says it is valid for this geometry and defect type.

Step 2: Compare with Zone Acceptance

The acceptance limit in this zone is:

The measured equivalent diameter is:

Comparison:

The indication exceeds the zone limit.

The indication is also within:

of the fastener row because:

Engineering Comment

The location matters. A delamination near a fastener row can interact with bearing stress, bypass load, clamp load, moisture ingress, and local bending. A defect of the same size in a low-stress field may have a different disposition.

Step 3: Check Compression Margin

Use the damaged compression-after-impact allowable for the measured defect-size band:

The design compressive stress is:

Margin of safety:

The required minimum margin for release without repair is:

Comparison:

Engineering Comment

The damaged panel still has positive calculated margin, but it does not meet the release criterion. Positive margin is not automatically acceptable when the inspection basis, damage growth, environment, and uncertainty require a larger reserve.

Step 4: Check Inspection Uncertainty

Assume the ultrasonic sizing uncertainty for this procedure is:

Use a conservative upper-bound equivalent diameter:

This keeps the defect in the same damaged-allowable size band and remains well above the acceptance limit:

Engineering Comment

Uncertainty does not change the decision in this case. If the measured defect had been close to the limit, the engineer would need a clearer sizing method, repeated inspection, a more conservative disposition, or destructive evidence from a representative qualification panel.

Step 5: Decide Disposition

The evidence is:

Evidence	Result
ultrasonic indication detected	confirmed internal damage
equivalent delamination diameter	54.1\ \text{mm}
zone diameter limit	35\ \text{mm}
distance to fastener row	35\ \text{mm}, within special disposition zone
damaged compression margin	0.167, below required 0.25
sizing uncertainty	does not rescue acceptance

The engineering decision is:

Do not release the panel as-is. Repair or replace the panel, then verify the repaired condition with the qualified inspection method and updated structural disposition.

If a temporary concession is considered, it should be time-limited, load-limited, inspected again after use, and approved by the responsible structural authority.

Failure Mode Review

Failure mode	Cause	Effect	Initial rating
delamination left in compression-critical zone	impact damage accepted as cosmetic	local buckling, compression strength loss, moisture ingress, fatigue growth	S=8,\ O=3,\ D=5

Initial risk priority number:

After repair, qualified ultrasonic verification, and a documented inspection trigger:

Transferable Lessons

Composite damage disposition should not rely on surface appearance alone. Barely visible impact damage can be structurally important when it creates delamination, crushed core, bondline damage, or matrix cracking in a loaded zone.

Disposition Checklist

A strong disposition record includes:

1. inspection method and calibration reference;
2. defect map, depth or interface estimate, and sizing uncertainty;
3. location relative to fasteners, edges, ply drops, stiffeners, or bondlines;
4. structural stress state and relevant damaged allowable;
5. acceptance criterion or engineering authority for deviation;
6. repair or replacement instruction;
7. post-repair inspection and release evidence.

The practical lesson is that non-destructive testing is only useful when it is tied to a decision. A clear ultrasonic image is not enough. The engineer must know which defect size matters, where it is located, how reliable the detection is, and what action the evidence requires.