ASTM D7136 Drop-Weight Impact Damage Testing of Composites

What the test does

A rectangular composite plate is clamped at its four corners over a support with a rectangular cut-out beneath it, leaving the centre free to flex. A guided mass carrying a 16 mm hemispherical impactor is released from a height calculated to deliver a specified energy. An instrumented tup records force and displacement through the strike, and a catch arrests the impactor on rebound so the plate is struck exactly once. The dent depth is then measured and the internal damage mapped by ultrasonic scanning.

What it measures, and why it matters

Strictly, its product is a damaged specimen rather than a number, and that is the point. Impact damage in a composite is mostly internal: a strike that leaves a dent an inspector would struggle to find can create a delamination many times larger inside the laminate, removing a substantial share of its compressive strength. That gap between what is visible and what has been lost — barely visible impact damage — is the central safety problem in composite structure. A metal panel struck by a dropped spanner dents visibly and keeps most of its strength; a composite panel struck the same way may look almost untouched and have lost a third of its compressive capability. Inspection intervals, allowable damage limits and repair thresholds all rest on that gap. This method makes that damage repeatably so that ASTM D7137 can measure what it cost, and the pair together is how damage tolerance is actually established.

Specimen and drop tower

This method's product is a damaged specimen. Everything is arranged so that the same damage can be made again tomorrow.

Plate

A rectangular laminate, clamped at four cornersClamped rather than gripped — the corners hold it down while the centre is free to respond.

Support

A rectangular cut-out beneath the plateIt defines the unsupported span and therefore how the plate flexes under the strike.

Impactor

Hemispherical, 16 mm diameterBlunt on purpose. A sharp tup would cut; this one delaminates, which is the damage aircraft structures actually get.

Energy

Scaled to thickness — commonly 6.7 J per mmScaling by thickness is what lets laminates of different thickness be compared at all.

Rebound catch

RequiredAn uncaught impactor strikes a second time and the specimen no longer represents one event.

Inspect ultrasonically before the next test

Map the damage areaDakBarely visible impact damage is mostly internal. What the eye sees understates what the laminate has lost.

The impactor MUST be caught after rebound. A second strike on an already-damaged plate produces a specimen that represents nothing, and it is easy to miss because the plate looks much the same.

Test speed

Energy, not speed

6.7 J per mm of specimen thickness — 33.5 J on the 5.0 mm laminate the method targetsNo rate of traverse is set anywhere in the method. Energy is the controlled quantity, and drop height follows from it.

Drop height and impact velocity

H = E / mg — 0.62 m for 33.5 J on the 5.5 kg impactor, arriving at about 3.5 m/sThe 5.5 ± 0.25 kg mass and the 16 ± 0.1 mm hemispherical tip are fixed by the method; the height and velocity are arithmetic from them.

Minimum drop

300 mm; below the energy that gives, use a 2.0 ± 0.25 kg impactor

Instrumented tup

Records force and displacement through the strikeThe force-time trace shows when delamination started, which the final damage cannot.

One strike per specimen

Always

Record the dent depth

After the plate has relaxedDakA composite recovers some indentation over minutes, so a depth read immediately overstates it.

What the machine must be capable of

This part of the pair needs a drop tower rather than a universal frame: a guided mass, an adjustable height, an instrumented tup, and — mandatory rather than optional — a catch that arrests the impactor on rebound. The universal testing machine's role comes afterwards, in the compression-after-impact test the damaged plate is passed to. Ultrasonic inspection equipment is required too, because the damage that matters cannot be measured any other way.

The method sets no rate of traverse and could not, since the strike is over in milliseconds. It fixes energy instead, and the height follows. The impactor is 5.5 ± 0.25 kg carrying a 16 ± 0.1 mm hemispherical tip, and the potential energy is 6.7 J per millimetre of specimen thickness. On the 5.0 mm laminate the method targets that is 33.5 J, so the drop height H = E / mg is 0.62 m and the tup arrives at √(2gH), about 3.5 m/s. There is a floor as well: the fall must be at least 300 mm, and where the wanted energy cannot be reached from that height with the standard mass, a 2.0 ± 0.25 kg impactor is used instead. Mass, height and energy are the three numbers a report has to carry. Crosshead speed is not one of them.

How the test runs

1. 01Cut rectangular plates from a balanced, symmetric laminate.
2. 02Measure thickness and compute the required energy.
3. 03Set drop mass and height to deliver it.
4. 04Clamp the plate at four corners over the cut-out support.
5. 05Confirm the rebound catch is armed.
6. 06Release the impactor and record the force-time trace.
7. 07Confirm only one strike occurred.
8. 08Let the plate relax, then measure the dent depth.
9. 09Ultrasonically scan the plate and record the damage area.
10. 10Photograph both faces.
11. 11Pass the plate to ASTM D7137 for the residual strength measurement.

Calculations

What the report has to contain

• Reference to ASTM D7136 and the edition
• Material, lay-up and cure schedule
• Plate dimensions and measured thickness
• Support cut-out dimensions and clamping arrangement
• Impactor diameter and mass
• Drop height and impact energy, and the energy per unit thickness
• Force-time trace and peak force
• Dent depth after relaxation
• Damage area from ultrasonic inspection
• Confirmation that only one strike occurred
• Photographs of both faces

What goes wrong in practice

A second, uncaught strike is the classic invalidation and the easiest to miss, because a plate hit twice looks much like a plate hit once. The damage from two impacts is not the damage from one at twice the energy, so the specimen then represents nothing. Reading dent depth inconsistently is another, since it is one of the few things an inspector can see in service. Assessing the damage visually rather than ultrasonically understates it profoundly — that understatement is precisely the hazard the method exists to characterise.