ASTM D7137 PDF  - Standard Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plates

ASTM D7137/D7137M is the standard test method for compressive residual strength properties of damaged polymer matrix composite plates, better known as the compression after impact (CAI) test. A laminate plate is first damaged, usually by a drop-weight impact to ASTM D7136, and then loaded in compression in a stabilising fixture until it fails. The result shows how much compressive strength a carbon or glass fibre laminate keeps after barely visible impact damage, which is a key damage-tolerance number in aerospace, automotive, wind and sporting goods design.

Test Specimen

• Flat rectangular plate 150 × 100 mm (6 × 4 in); the 150 mm length is the loading direction

• Recommended thickness about 4 to 6 mm (0.16 to 0.24 in), commonly a quasi-isotropic layup

• Damaged before testing, typically by drop-weight impact per ASTM D7136 (or quasi-static indentation per D6264)

• Record dent depth and damage size (for example by ultrasonic C-scan) before the compression test

• Ends machined flat and parallel so the load is shared evenly

Fixture and Machine

• CAI support fixture with adjustable side plates and knife-edge supports that stop the plate buckling, while top and bottom plates restrain the loaded ends

• Load applied end-on through flat, parallel compression platens on a universal testing machine

• Load cell capacity typically 250–300 kN for thick laminates

• Strain gauges back-to-back on both faces (optional but recommended) to check for bending and global buckling

Test Procedure

• Measure width and thickness of the plate away from the damage

• Install the plate in the fixture and set the side supports snug but not clamped

• Load in compression at a constant crosshead rate, commonly 1.25 mm/min (0.05 in/min)

• Record force (and strains) to maximum load

• Record the failure mode and location: a valid failure runs through the damage zone

CAI Strength Formula and Worked Example

Compressive residual strength: FCAI = Pmax / (w × h), where Pmax is the maximum force, w the specimen width and h the thickness.

Example: a 100 mm wide, 5.0 mm thick carbon/epoxy plate impacted at 30 J fails at 120 kN. FCAI = 120,000 / (100 × 5.0) = 240 MPa. If an undamaged plate of the same laminate reaches 400 MPa, the impacted plate keeps 240 / 400 = 60 % of its compressive strength.

Valid and Invalid Failures

• Valid: compression failure through or near the impact damage

• Questionable: failure away from the damage, at the ends, or under the side supports

• Invalid: global buckling (shown by diverging back-to-back strain gauges) or end crushing

Related Standards

• ASTM D7136 – drop-weight impact damage of composite plates

• ASTM D6264 – quasi-static indentation damage resistance

• ASTM D6484 – open-hole compressive strength of laminates

• ASTM D6641 – combined loading compression of composites

• ASTM D7766 – practice for damage resistance testing of sandwich constructions

ASTM D7137 FAQ

What is a CAI test?

Compression after impact: a composite plate is impacted and then tested in compression to measure how much strength it keeps after damage.

What size specimen does D7137 use?

A flat plate 150 × 100 mm (6 × 4 in), typically 4 to 6 mm thick.

How is the plate damaged?

Usually by a drop-weight impact to ASTM D7136, or by quasi-static indentation to ASTM D6264, before the compression test.

Why does the fixture have side supports?

A thin plate would buckle under compression. Knife-edge side supports prevent buckling so the plate fails in compression through the damage. Confirm the rate and dimensions against your copy of ASTM D7137.