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Compression after impact (CAI)

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 

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) gripped. 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.

Compression After Impact (CAI) Test to ASTM D7137

During compression after impact (CAI) tests, a pre-damaged composite specimen is tested using a special compression test fixture up to 600 kN. The CAI test is performed with a Z600E materials testing machine in a temperature range of -60 °C to +350 °C.

CAI compression test kit (compression after impact)

  • Applications: Determination of the residual compressive strength after impact loading (CAI) to:
    • Boeing BSS 7260c, ASTM D7137, DIN 65561, ISO 18352, prEN 6038
    • Airbus AITM 1-0010 issue 3
  • Function description: The compression test kit is used to determine the residual compressive strength following impact loading. The compressive force is applied to the specimen in-plane. Depending on the version, the specimen is either clamped or simply supported at the upper and lower ends.
  • Advantages:
    • Easy changing of the specimen
    • The fixtures can be used over a wide temperature-range from -70 to 300 °C
    • Increased lateral stiffness through support plates
    • Interchangeable carbide insert on the upper and lower contact surfaces

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