Testing standard
ASTM D7136
Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D7136 creates a controlled, repeatable impact damage in a composite plate by dropping a guided hemispherical mass onto it. It is not a strength test — its output is a damaged plate and a record of the strike. What that damage costs the laminate is measured afterwards by ASTM D7137.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ASTM
- Edition
- D7136/D7136M-25
- Runs on
- Series 7200 and Series 9000
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 — commonly 6.7 joules per millimetre of laminate thickness. An instrumented tup records force and displacement through the strike. A catch mechanism arrests the impactor on rebound so the plate is struck exactly once. The plate is then allowed to relax, its dent depth measured, and its 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 knowing how large that gap is for a given laminate. 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
- Set by drop height and mass
- 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.
Calculations
E = m × g × h
- m
- drop mass, kg
- g
- 9.81 m/s²
- h
- drop height, m
Set by choosing mass and height together. The same energy from a heavier mass at lower height is not identical in effect, which is why both are reported.
Energy per unit thickness, commonly 6.7 J/mm
Scaling by thickness is what makes laminates of different thickness comparable.
Measured by ultrasonic C-scan of the impacted plate
The visible dent is a small fraction of it. This is why the damage is called barely visible.
How the test runs
- 01Cut rectangular plates from a balanced, symmetric laminate.
- 02Measure thickness and compute the required energy.
- 03Set drop mass and height to deliver it.
- 04Clamp the plate at four corners over the cut-out support.
- 05Confirm the rebound catch is armed.
- 06Release the impactor and record the force-time trace.
- 07Confirm only one strike occurred.
- 08Let the plate relax, then measure the dent depth.
- 09Ultrasonically scan the plate and record the damage area.
- 10Photograph both faces.
- 11Pass the plate to ASTM D7137 for the residual strength measurement.
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 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 recording force and displacement through a strike lasting milliseconds, 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 also required, because the damage that matters cannot be measured any other way.
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 immediately rather than after the plate has relaxed overstates it, sometimes considerably, and dent depth 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.
ASTM D7136 or ISO 6603
| ASTM D7136 | ISO 6603 | |
|---|---|---|
| Purpose | Create controlled damage for a later test | Characterise puncture behaviour |
| Impactor | 16 mm hemispherical | Hemispherical striker |
| Primary output | A damaged plate, plus the strike record | Force-deflection and puncture energy |
| Usually followed by | ASTM D7137 compression after impact | Nothing — it is the whole test |
D7136 exists to make a specimen for D7137. ISO 6603 is a complete test in itself. Treating a D7136 result as a strength figure misunderstands what the method is for.
Questions we are asked about this test
What is ASTM D7136?
It is the ASTM method for creating a controlled drop-weight impact damage in a composite plate. A guided 16 mm hemispherical impactor is dropped onto a clamped plate at an energy scaled to its thickness. The output is a damaged specimen and a record of the strike, not a strength figure.
Why is it not a strength test?
Because the number that matters comes afterwards. Impact damage in a composite is mostly internal delamination that costs the laminate a great deal of compressive strength while leaving the surface almost unmarked. D7136 makes that damage repeatably; ASTM D7137 then compresses the plate to find out what it cost. Quoting a D7136 energy as though it were a material property misses the point of running it.
Why a blunt hemispherical impactor?
Because it delaminates rather than cuts, and delamination is the damage real structures actually receive. A dropped tool, a hailstone or a knock during maintenance spreads its energy over a blunt contact; the laminate absorbs it by separating between plies rather than by being penetrated. A sharp tup would produce a hole, which is a different and less dangerous kind of damage because you can see it.
What is barely visible impact damage?
It is the reason this pair of tests exists. A strike can leave a dent a millimetre deep that an inspector would struggle to find, while creating a delamination many times larger inside the laminate that removes a substantial share of its compressive strength. The eye sees almost nothing; an ultrasonic scan sees the real extent. That gap is the whole safety problem in composite structures.
Why must the impactor be caught after rebound?
Because a second strike on an already-damaged plate produces a specimen that represents no defined event at all. The damage from two impacts is not the damage from one at twice the energy, and it is easy to miss because the plate looks much the same either way. The rebound catch is mandatory equipment, not a refinement.
Why scale the energy to thickness?
Because a thicker laminate absorbs more energy before it delaminates, so a fixed joule figure would damage a thin plate severely and a thick one hardly at all. Normalising to energy per millimetre of thickness — commonly 6.7 J/mm — is what allows laminates of different thickness to be compared on equal terms.
Why measure the dent after the plate relaxes?
Because a composite recovers part of its indentation over the following minutes as the matrix relaxes. A depth read immediately after the strike overstates the permanent dent, sometimes considerably. Since dent depth is one of the few things an inspector can actually see in service, measuring it consistently matters.
Running ASTM D7136 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Not a force test — the controlled quantity is impact ENERGY, commonly 6.7 J per mm of thickness | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | A drop tower with a hemispherical 16 mm impactor, over a rectangular cut-out support with the plate clamped at four corners | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.