ASTM D7137

What the test does

A composite plate that has already been damaged under ASTM D7136 — struck by a hemispherical impactor at a controlled energy and characterised by ultrasonic scanning — is machined flat and square at its loaded ends and assembled into a fixture that restrains all four edges against buckling. Strain gauges are bonded back to back. The assembly is compressed between the machine platens at about 1.25 mm/min until the plate fails, which should happen through the damage zone. The residual compressive strength is calculated from the maximum force and the gross cross-section.

What it measures, and why it matters

The result is what the impact actually cost. Composites are damage-tolerant in tension and remarkably intolerant in compression, because impact damage is delamination and delamination is exactly what compression exploits: separated plies can no longer support one another against local buckling, so under compressive load they buckle individually and the damage spreads. In tension the plies are pulled straight and the same delamination matters far less. That asymmetry is why damage tolerance in composite structure is assessed in compression, and why this pairing of methods sits underneath every inspection interval and allowable damage limit in a composite airframe or blade. The figure that actually gets used is not the residual strength itself but the retention — what fraction of the undamaged strength survived — because that is what a designer applies as a knockdown factor.

Specimen and support fixture

The plate arrives already damaged. The fixture's job is to make sure the damage, and nothing else, decides the failure.

Specimen

A plate damaged under ASTM D7136The two methods are a pair. A plate damaged any other way is not this test.

Edge restraint

All four edges supported against bucklingA damaged plate buckles more readily than an intact one, so without restraint the test measures stability rather than residual strength.

End loading

Through machined flat, parallel endsOut-of-square ends crush before the damage does.

Strain gauges

Back to backThey reveal global buckling, which is the failure mode this test exists to exclude.

Damage state

Recorded from the D7136 scanResidual strength is meaningless without the damage area it corresponds to.

Check the fixture does not carry load

Before trusting a resultDakAn over-clamped edge restraint takes load itself and inflates the residual strength.

The result is only meaningful beside two other numbers: the impact energy that made the damage and the undamaged compressive strength of the same laminate. On its own a residual strength says nothing about damage tolerance.

Test speed

Crosshead speed

1.25 mm/min nominal

Valid failure

Through the damage zone

Watch the strain traces

Divergence means bucklingDak

Photograph the failure path

Before handlingDakWhether the failure ran through the delamination is the evidence the result is real.

Calculations

How the test runs

1. 01Take a plate damaged and characterised under ASTM D7136.
2. 02Confirm the damage area from the ultrasonic scan is recorded.
3. 03Machine the loaded ends flat, parallel and square.
4. 04Bond strain gauges back to back.
5. 05Assemble the plate into the support fixture with all four edges restrained.
6. 06Set the edge clamping so the fixture restrains without carrying load.
7. 07Place between the platens and check alignment.
8. 08Compress at 1.25 mm/min, recording force and both strain channels.
9. 09Watch for divergence between the traces, which indicates buckling.
10. 10Confirm the failure ran through the damage zone.
11. 11Report residual strength beside the impact energy and the undamaged strength.

What the report has to contain

• Reference to ASTM D7137 and the edition
• Material, lay-up and cure schedule
• Plate dimensions and thickness
• The impact energy and damage area from ASTM D7136
• Support fixture and edge clamping arrangement
• Conditioning and test temperature
• Crosshead speed
• Residual compressive strength
• Undamaged compressive strength of the same laminate, and the retention
• Failure location and mode
• Plates rejected for buckling or end crushing
• Mean, standard deviation and coefficient of variation

What the machine must be capable of

Force measurement to ASTM E4 at loads that commonly reach forty to two hundred kilonewtons, a crosshead holding 1.25 mm/min, platens flat and parallel enough that the fixture is loaded squarely, and two strain channels recorded alongside force. The fixture itself is substantial and the frame needs the daylight for it. The subtle requirement is the edge clamping: it must restrain the plate against buckling without carrying load itself, and an over-clamped fixture inflates the residual strength — which in a damage-tolerance programme is a dangerous direction to be wrong in. Checking that the fixture is restraining rather than sharing the load is worth doing deliberately rather than assuming it from the setup drawing.

What goes wrong in practice

Global buckling is the failure this method exists to prevent, and when it happens the number describes the plate's stability rather than its residual strength. End crushing is the second, and it points at the machining of the loaded ends. A failure that does not run through the damage zone is the third — all three are rejections rather than low results. Away from the bench, the most common reporting error is quoting a residual strength on its own: without the impact energy that caused the damage and the undamaged strength of the same laminate, the figure cannot be interpreted by anyone.

Compression after impact or open-hole compression

Both are notched compressive allowables and both are needed. The hole is a defect you design around because you put it there; the impact is one you must survive because you did not.

Questions we are asked about this test