Weekend reading. The handbook covering impact damage is free on Kindle, with three other engineering books, until Tue 10 Nov 23:59 PT. Time remaining: ends Tue 10 Nov, 11:59 pm PT.
This look at impact testing was published during a free Kindle week for four engineering books in November 2026.
It seems an odd thing to do with an expensive panel. Engineers drop a weight on it, measure the damage, then crush it. Yet this simple sequence, called compression after impact, can determine how much strain a composite aircraft structure is allowed to carry.
The threat
Aircraft get knocked. Tools fall during maintenance, ground vehicles bump into doors, and runway debris flies up. On metal, such an event tends to leave a clear dent. On carbon fibre, the outer surface may show very little while internal plies separate. That hidden condition is known as delamination.
Under tension this matters less. Under compression, separated plies act like thin sheets with little support, so they can buckle and let the damage grow. Upper wing skins, which are compressed in flight, are an obvious concern.
How the test works
Standard test methods exist for this. ASTM D7136 covers impacting a flat laminate with a falling weight, and ASTM D7137 covers the compression test that follows. In outline:
- A coupon is held in a fixture and struck with a known energy.
- The damage is measured, often with ultrasonic scanning.
- The panel is loaded in compression until it fails.
- Its strength is compared with an undamaged specimen.
Repeating this at different energies shows how much strength is lost, and where the damage begins to be detectable by eye.
Why it drives design
Designers commonly assume that damage too small to find by visual inspection is already present, and the structure must still meet its requirements. Because of this, the result often sets a design strain limit below what undamaged material could achieve. That means a laminate may be limited by damage tolerance, not by its pristine strength.
Tougher resins, better stacking and thicker skins can all help, but each one comes with a cost or weight penalty. Engineers treat this as a trade-off, and how a laminate is stacked has a real influence on the result.
The four books
Practical Airframe Stress Analysis, Volume 2: Composite Structure
GD&T Professional Exam Guide
Practical Airframe Stress Analysis, Volume 1: Metallic Structure
Python for Aerospace Stress Engineers (Volume 3)
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Related gear
- Kindle Paperwhite on Amazon — Related gear
- Aero Structures and Composite Materials with Artificial Intelligence (book) — Sandeep's book
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