Compression After Impact (CAI)

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Purpose of the Test

CAI tests characterize damage that can occur to airplanes or vehicles if hit by rocks or birds, or if damaged in an accident.

The measuring principle involves pre-damaging a test plate, which was tested previously using ultrasound to detect existing delaminations or imperfections, and then measuring the residual strength by means of a compression test.

This is why a compression after impact test consists of two parts: 

  • Pre-damage of a specimen using an instrumented drop weight tester
  • Compression test with a static testing machine to measure the residual strength 
cai-curve
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Pre-Damaging the Test Plates

  • The specimen is pre-damaged under set conditions in an instrumented HIT 230F drop weight tester.
  • Specimens are gripped in sections with the following dimensions: 76.2 x 127mm (ASTM, Boeing, SACMA, DIN), 75 x 125 mm (EN, Airbus) or 140 mm diameter (CRAG). Only Airbus AITM requires gripping within the section. For easier testing, the specimen is gripped outside of the drop weight tester and then moved into testing position.
  • The pre-damage process can be monitored and evaluated by the drop weight tester's instrumentation. The first damage peak on the force-time sequence also correlates to the Mode II fracture toughness of the laminate.

CAI Compression Test

The pre-damaged specimens are tested using a special compression test kit to determine residual strength. The compressive forces that arise are typically very high. A special compression test kit is used to load the test plates so they do not buckle. These test plates differ according to standard:

  • ASTM, Boeing, SACMA and DIN: All four sides are guided, but not gripped.
  • ISO, EN and Airbus standards: The upper and lower ends of the specimen are gripped. The sides are guided with linear contact.
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