Testing standard
ASTM D7078
Standard Test Method for Shear Properties of Composite Materials by V-Notched Rail Shear Method
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D7078 measures the shear properties of a composite using a V-notched rail shear fixture. The coupon is clamped between rail pairs which are pulled in opposite directions, forcing shear across a notched gauge section. It is the method ASTM named as the replacement for D4255 when that was withdrawn in 2025.
At a glance
- Test type
- Shear
- Published by
- ASTM
- Edition
- D7078/D7078M-20
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A rectangular composite coupon is machined with symmetric V-notches on opposite edges, leaving a reduced section between the notch roots. The coupon is clamped between two pairs of rails, bolted to a specified torque, and the two halves of the fixture are drawn apart at about 2 mm/min. Because the load is introduced on either side of the notched section in opposing directions, that section is placed in shear. Strain gauges bonded at ±45° to the loading axis, back to back on the two faces, record the shear strain and simultaneously reveal any out-of-plane bending.
What it measures, and why it matters
The result is a shear stress-strain curve, from which shear modulus and shear strength are read. Shear is the property that governs load transfer everywhere in a composite structure — into and out of bolted joints, across ply drops and thickness changes, around cut-outs, through bonded overlaps. It is also carried largely by the matrix rather than the fibre, which makes it the property most sensitive to cure state, void content and moisture absorption, and therefore the one that most often reveals a processing problem before anything else does.
Specimen and fixture
The notches concentrate shear into a defined section, and the rails put it there. Both have to be right or the coupon fails somewhere the calculation does not describe.
- Notches
- Symmetric V-notches, machined to the specified angle and root radiusThey set where the shear concentrates. A rough or asymmetric notch root starts the failure early and in the wrong place.
- Coupon
- A rectangular plate, larger than a beam-shear specimenMore material to clamp is what lets this fixture transfer higher loads than the Iosipescu geometry.
- Clamping bolt torque
- Specified and recordedLoad is transferred by friction. Too little and the coupon slips; too much and it is crushed at the rails.
- Strain gauges
- ±45° to the loading axis, back to backBack to back detects out-of-plane bending, which this geometry is prone to.
- Lay-up
- Any — this is the method's advantage over D3518
- Deburr the notch roots
- Before gaugingDakA machining burr at the root is a crack starter in exactly the highest-stressed place on the coupon.
A coupon that fails outside the notched section, or that shows significant out-of-plane bending on the back-to-back gauges, has not produced a shear strength. Both are rejections rather than low results.
Test speed
- Crosshead speed
- 2 mm/min nominal
- Shear modulus
- Over the defined strain range
- Check the gauges agree
- Through the runDakDiverging back-to-back traces mean the coupon is bending and the shear figure is contaminated.
- Re-check bolt torque between specimens
- Faces bed inDak
Calculations
τ = P / (w × h)
- P
- applied force, N
- w
- distance between notch roots, mm
- h
- coupon thickness, mm
The width is the notch-root separation, not the coupon width. Using the full width understates the stress substantially.
γ = |ε₊₄₅| + |ε₋₄₅|
- ε₊₄₅, ε₋₄₅
- strains from the two gauges at ±45°
G = Δτ / Δγ over the defined range
How the test runs
- 01Machine the coupon and cut symmetric V-notches to the specified angle and root radius.
- 02Deburr the notch roots.
- 03Measure the notch-root separation and the thickness.
- 04Bond ±45° strain gauges back to back across the notched section.
- 05Condition to the specification.
- 06Clamp the coupon between the rail pairs.
- 07Torque the bolts evenly, in stages, to the specified value.
- 08Fit the fixture to the frame and check alignment.
- 09Load at 2 mm/min, recording force and both strain channels.
- 10Watch the two traces for divergence, which indicates bending.
- 11Reject any coupon failing outside the notched section.
- 12Re-torque and re-check between specimens.
What the report has to contain
- Reference to ASTM D7078 and the edition
- Material, lay-up and cure schedule
- Coupon dimensions, notch angle and root radius
- Measured notch-root separation
- Clamping bolt torque
- Strain gauge orientation and placement
- Conditioning and test temperature
- Crosshead speed
- Shear modulus and shear strength
- Failure location and mode
- Coupons rejected, with the reason
- Mean, standard deviation and coefficient of variation
What the machine must be capable of
Force measurement to ASTM E4 over a range from around ten kilonewtons to sixty, a crosshead holding 2 mm/min, and at least two strain channels recorded with force. The fixture is bolted to the load train, so alignment must be good enough that the two halves separate along the load axis without cocking — any tilt introduces the out-of-plane bending the back-to-back gauges exist to detect. Because load reaches the coupon through clamping friction rather than a mechanical key, the frame also needs to accommodate a fixture of some mass and the bolt torque has to be applied and re-checked with a calibrated wrench.
What goes wrong in practice
Using the full coupon width rather than the notch-root separation in the stress calculation is the commonest arithmetic error, and it understates the shear stress by a wide margin while producing an entirely believable figure. Failures outside the notched section are rejections rather than low results, and they usually point at notch machining or at torque. Out-of-plane bending is the quiet one: it contaminates the shear figures without any obvious signature in the force trace, and it is visible only in the two strain channels diverging. Torque drifting between specimens as the clamped faces bed in produces scatter that reads as material variability.
ASTM D7078 or ASTM D5379
| ASTM D7078 V-notched rail | ASTM D5379 V-notched beam | |
|---|---|---|
| Load introduction | Clamping friction through rails | Four-point loading of a beam |
| Coupon | Larger rectangular plate | Small notched beam |
| Load capacity | Higher — more area to clamp | Lower |
| Best for | Thicker laminates and higher-strength lay-ups | Thin coupons, small material quantities |
Both give a genuine shear state and both are current. D7078's larger clamped area lets it carry higher loads, which is why it suits thicker and stronger laminates where the beam geometry runs out of capacity.
Questions we are asked about this test
What is ASTM D7078?
It is the ASTM V-notched rail shear method for composites. A rectangular coupon with symmetric V-notches is clamped between two pairs of rails, which are then pulled in opposite directions so that shear is concentrated across the notched section. Shear modulus and shear strength come from the force and a pair of ±45° strain gauges.
Did this replace ASTM D4255?
Yes. D4255/D4255M, the older rail shear method, was withdrawn in 2025, and ASTM named D7078/D7078M as its replacement in the withdrawal work item. If a specification still calls for D4255, the practical course is to run D7078, report it as such, and flag the withdrawal to whoever wrote the specification.
Why is the width in the calculation the notch-root separation?
Because that is the section actually carrying the shear. The notches exist precisely to concentrate the stress into that reduced width, so using the full coupon width would divide the load by an area that is not resisting it, understating the shear stress substantially. It is the most common arithmetic error in the method.
Why do the strain gauges go at ±45°?
Because shear at the gauge location appears as tension along one 45° diagonal and compression along the other. Gauges aligned to those diagonals read the shear directly, and adding their magnitudes gives the shear strain. A gauge aligned with the loading axis would read almost nothing useful.
Why back-to-back rather than a single pair?
To detect out-of-plane bending, which this geometry is prone to because the load is introduced on the outer faces of a comparatively thin plate. If the two faces disagree, the coupon is bending and the shear figures are contaminated. Traces that diverge as the test proceeds are the warning to watch for.
What does bolt torque have to do with the result?
Everything, because load is transferred into the coupon by clamping friction rather than by a mechanical key. Too little torque and the coupon slips through the rails; too much and it is crushed where the rails bite. That makes torque a controlled test variable to be specified, applied evenly in stages, recorded, and re-checked between specimens as the faces bed in.
When would I choose D5379 instead?
When the coupon is thin, when material is scarce, or when the expected failure load is comfortably inside the beam fixture's range. D5379's V-notched beam uses a much smaller specimen. D7078's larger clamped area is what lets it reach higher loads, so it earns its place on thicker laminates and stronger lay-ups where the beam geometry runs out of capacity.
Running ASTM D7078 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 | Moderate to high — commonly 10 to 60 kN through the fixture | 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 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | V-notched rail shear fixture: the coupon is clamped between two pairs of rails which load it in opposing directions across a notched gauge section | Our shear fixtures, 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.