TJ-144
Heavy Duty Hydraulic Grips
Heavy duty hydraulic wedge grips hold a constant clamping force as the coupon thins slightly under load, which mechanical wedges do not always manage at composite failure loads.
SpecificationsTesting standard
ASTM D3518
Standard Test Method for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a ±45° Laminate
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D3518 obtains the in-plane shear response of a composite by pulling a ±45° laminate in simple tension. No shear fixture is involved — the lay-up does the work, because a ±45° laminate loaded along its axis puts each ply into shear. Biaxial strain measurement turns the tensile data into a shear stress-strain curve.
At a glance
- Test type
- Shear
- Published by
- ASTM
- Edition
- D3518/D3518M-18
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A straight-sided coupon is cut from a balanced, symmetric ±45° laminate and pulled in simple tension at about 2 mm/min — in ordinary wedge grips, with no shear fixture of any kind. Strain is measured in both the longitudinal and the transverse directions. The axial stress is halved to give the in-plane shear stress, which is an exact geometric consequence of loading a ±45° lay-up along its axis, and the shear strain is obtained as the difference between the two measured strains. From that pair comes a full shear stress-strain curve, and from its initial slope the shear modulus.
What it measures, and why it matters
In-plane shear modulus and shear strength are among the properties a laminate cannot be designed without. Every bolted joint, every ply drop, every place where load passes from one part of a structure into another puts the laminate into shear, and the matrix rather than the fibre carries it — which makes shear the property most sensitive to cure state, void content and moisture. What makes this method valuable out of proportion to its simplicity is that it needs no fixture at all, so a laboratory equipped only for tensile testing can produce shear data on the same day it produces tensile data.
Specimen
The lay-up IS the fixture. That is what makes this the cheapest shear test there is, and also what limits it.
- Lay-up
- ±45°, balanced and symmetricThe whole method depends on it. Any other lay-up in this geometry is not this test.
- Geometry
- A straight-sided tensile couponThe same coupon shape as ASTM D3039 — no notches, no fixture.
- Strain measurement
- Longitudinal AND transverseShear strain is the difference between the two. One gauge alone cannot produce the result.
- Tabs
- Usually unnecessaryA ±45° laminate is compliant and low in axial strength, so the grips rarely damage it.
- Conditioning
- As the specification requires
- Expect very large strains
- Well past what a metal coupon reachesDakChoose an extensometer or gauge rated for it, or the measurement stops before the material does.
Shear stress in this test is exactly half the applied axial stress. It is a geometric consequence of the ±45° lay-up, not an approximation — but forgetting the factor of two doubles every reported shear stress.
Test speed
- Crosshead speed
- 2 mm/min nominal
- Shear modulus
- From a defined strain range on the shear curve
- End of test
- Failure, or a strain limit the method setsMany ±45° laminates deform enormously without ever breaking cleanly, which is why a strain cut-off exists.
- Report which ending applied
- AlwaysDakA strength at rupture and a stress at a strain limit are different quantities.
Calculations
Shear stressτ₁₂
τ₁₂ = σₓ / 2
- σₓ
- applied axial stress, MPa
The factor of two comes from resolving the axial load onto planes at 45°. It is exact for this lay-up, and omitting it is the commonest error in the whole method.
Shear strainγ₁₂
γ₁₂ = εₓ − εᵧ
- εₓ
- longitudinal strain
- εᵧ
- transverse strain, negative
Subtracting a negative transverse strain adds it. This is why two gauges are mandatory.
Shear modulusG₁₂
G₁₂ = Δτ₁₂ / Δγ₁₂ over the defined range
How the test runs
- 01Lay up and cure a balanced, symmetric ±45° laminate.
- 02Cut straight-sided coupons; tab only if grip damage occurs.
- 03Measure the cross-section.
- 04Bond or fit strain measurement in BOTH directions.
- 05Condition to the specification.
- 06Grip and check alignment.
- 07Pull at 2 mm/min, recording force and both strain channels.
- 08Convert axial stress to shear stress by halving it.
- 09Compute shear strain as the difference of the two strains.
- 10Take the modulus over the defined range.
- 11Record whether the test ended at rupture or at the strain limit.
Grips and fixtures for this method
Self-tighteningTJ-15
Universal Parallel Wedge Grips
Universal parallel wedge grips where the coupon width and failure load sit within their range.
SpecificationsWhat the report has to contain
- Reference to ASTM D3518 and the edition
- Material, lay-up and cure schedule
- Coupon dimensions and cross-section
- How strain was measured in both directions
- Conditioning and test temperature
- Crosshead speed
- Shear stress-strain curve, shear modulus and shear strength
- Whether the test ended at rupture or a strain limit
- Failure mode where the coupon ruptured
- Mean, standard deviation and coefficient of variation
What the machine must be capable of
Force measurement to ASTM E4 across a range from a few kilonewtons to perhaps thirty, a crosshead holding 2 mm/min, and — the distinguishing requirement — two strain channels recorded simultaneously with force. Alignment matters as it does for any tensile coupon, since eccentricity introduces bending. The subtler requirement is stroke and strain capacity: a ±45° coupon can extend a great deal before it fails, and both the crosshead travel and the strain measurement have to accommodate that or the test ends for an instrumentation reason rather than a material one.
What goes wrong in practice
Forgetting to halve the axial stress is the commonest error and the hardest to catch, because a shear stress twice its true value is still an entirely plausible number. Fitting only one strain gauge is the commonest reason a test has to be repeated outright — there is no partial result. Beyond those, the honest limitation is ply rotation: at large strains the fibres turn away from ±45° toward the loading axis, so the geometry producing the shear state is itself changing, and a modulus taken early is more trustworthy than a strength taken late. Reporting a stress at a strain limit as though it were a rupture strength conflates two different quantities.
ASTM D3518 or ASTM D7078
| ASTM D3518 | ASTM D7078 | |
|---|---|---|
| Fixture | None — ordinary tensile grips | V-notched rail shear fixture |
| Lay-up | Must be ±45° | Any lay-up |
| Shear state | Approximate; plies rotate at large strain | Cleaner and more uniform |
| Cost and speed | Low — the cheapest shear test available | Higher; needs the fixture |
D3518 is excellent for screening and for material qualification where a ±45° laminate is representative. Where a design allowable is needed, or the lay-up is not ±45°, D7078 gives the cleaner shear state and is the better answer.
Questions we are asked about this test
What is ASTM D3518?
It is the ASTM method for in-plane shear response of a composite, obtained by pulling a ±45° laminate in simple tension. There is no shear fixture — the lay-up produces the shear state, because a ±45° laminate loaded along its axis puts each ply into shear.
Why is shear stress half the axial stress?
It is a geometric consequence of resolving the applied load onto planes at 45° to it. For this specific lay-up the relationship is exact, not an approximation. Forgetting the factor of two doubles every reported shear stress, and because the resulting number is still plausible it can survive a long way into a data set.
Why do I need strain gauges in two directions?
Because shear strain is the difference between the longitudinal and transverse strains, and the transverse one is negative. A single gauge cannot produce the result at all — not a less accurate result, but no result. This is the most common reason a D3518 test has to be repeated.
What is the catch, if it needs no fixture?
The shear state is approximate rather than pure, and it degrades as the test proceeds. At large strains the fibres rotate away from ±45° toward the loading axis, so the geometry that produced the shear state is progressively changing. That is fine for screening and for modulus, and it is why the method is less suited to producing a design allowable.
Why does the test often not break the coupon?
Because a ±45° laminate is remarkably ductile in this direction — the plies scissor rather than the fibres breaking, and the coupon can reach very large strains without a clean rupture. That is why the method sets a strain limit as an alternative ending, and why a stress at that limit and a strength at rupture are different quantities that must be labelled.
Do I need tabs?
Usually not. A ±45° laminate is compliant and comparatively low in axial strength, so wedge grips seldom damage it — which is another reason this test is quick and cheap. If failures do occur at the grips, tabs are the answer, but it is not the default here as it is for a unidirectional coupon.
When should I use D7078 instead?
When you need a design allowable, when the lay-up is not ±45°, or when the material reaches strains large enough that ply rotation makes the D3518 shear state questionable. D7078's V-notched rail shear fixture produces a cleaner and more uniform shear state, at the cost of the fixture and the preparation.
Running ASTM D3518 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 — a ±45° carbon coupon commonly fails between 5 and 30 kN | 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 | Ordinary tensile wedge grips — the shear state comes from the lay-up, not from a fixture | Our self-tightening serrated wedge grips, with V-jaws for round specimens, 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.