Testing standard
ASTM D5961
Standard Test Method for Bearing Response of Polymer Matrix Composite Laminates
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D5961 measures the bearing response of a composite laminate — how much load a fastener can push against the side of a hole before the hole crushes. It gives bearing strength and the hole elongation that accompanies it, and it is the foundation of bolted joint design in composite structures.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ASTM
- Edition
- D5961/D5961M-23
- Material
- Composites & sandwich structures
- Runs on
- Series 7200 and Series 9000
What the test does
A composite coupon with a precisely machined hole is assembled into a bearing fixture, either double-shear — with the coupon sandwiched symmetrically between two plates — or single-shear, where two plates overlap as in a real lap joint. A fastener or pin of specified fit passes through the hole and is torqued to a controlled value with specified washers. The coupon is gripped and pulled at about 2 mm/min, so the fastener pushes against the side of the hole. Force is recorded against hole elongation, measured directly at the joint rather than from crosshead travel, until the hole has crushed or the coupon has failed.
What it measures, and why it matters
The method gives bearing strength — at a defined offset hole elongation and at ultimate — together with the load-deformation behaviour of the hole. Bolted and riveted joints are where composite structures are most often limited, because a fastener concentrates load into a small area of a material that cannot yield to redistribute it. Bearing allowables from this test size every bolted joint in a composite airframe, wind blade root or automotive structure. The offset figure usually governs, because a joint whose holes have elongated has already lost clamp-up and shed load onto its neighbours long before anything collapses.
Specimen and fastener
This is a joint test disguised as a coupon test. The fastener, its fit and its clamp-up are as much part of the specimen as the laminate.
- Configuration
- Single-shear or double-shearDouble-shear is symmetric and produces cleaner data; single-shear reproduces the bending a real lap joint puts into the fastener.
- Edge distance and width
- Set by the specification, as ratios of hole diameterToo small an edge distance changes the failure mode from bearing to shear-out.
- Hole fit
- Close, to the specified clearanceA loose hole lets the fastener move before it bears, which shows as a false toe on the load-deformation curve.
- Fastener torque
- Specified and recordedClamp-up raises apparent bearing strength substantially by suppressing delamination under the washer. A torqued and an untorqued result are different tests.
- Washers
- As specified
- Deformation measurement
- Hole elongation, measured directlyCrosshead travel includes fixture and coupon extension and will not do.
- Record the fastener condition after the test
- Bent, sheared or intactDakA bent fastener in a single-shear test tells you where the load actually went.
Bearing is only one of four possible failure modes. Net-tension, shear-out and cleavage are all geometry-driven, and a coupon that fails by any of them has measured its edge distance and width rather than the laminate's bearing strength.
Test speed
- Crosshead speed
- 2 mm/min nominal
- Offset bearing strength
- At a specified percentage of hole elongationCommonly 2 % of the hole diameter, because a bolted joint is usually limited by hole elongation rather than by ultimate collapse.
- Ultimate bearing strength
- At maximum force
- Report both
- Offset and ultimateDakDesign nearly always uses the offset figure; quoting only the ultimate overstates what the joint can be relied on for.
Calculations
σᵇʳ = P / (D × h)
- P
- force, N
- D
- hole diameter, mm
- h
- coupon thickness, mm
The projected bearing area — diameter times thickness — not the hole's circumference or any contact patch.
Bearing stress at a defined hole elongation, commonly 2 % of D
The design-relevant figure for most bolted joints.
εᵇʳ = δ / D
- δ
- hole elongation, mm
- D
- hole diameter, mm
How the test runs
- 01Cut coupons to the specified width and edge distance ratios.
- 02Drill and ream the hole to the specified clearance, backing the laminate.
- 03Inspect the hole for delamination before use.
- 04Measure coupon thickness, width, hole diameter and edge distance.
- 05Assemble the coupon into the single- or double-shear fixture.
- 06Fit the fastener with the specified washers and torque it to the specified value.
- 07Attach the transducer that will measure hole elongation.
- 08Grip the coupon and check alignment.
- 09Load at 2 mm/min, recording force against hole elongation.
- 10Record the force at the specified offset elongation and at maximum.
- 11Disassemble and classify the failure mode.
- 12Reject net-tension, shear-out and cleavage failures as non-bearing.
What the report has to contain
- Reference to ASTM D5961 and the edition, and the procedure used
- Material, lay-up and cure schedule
- Coupon dimensions, hole diameter, edge distance and width ratios
- Single-shear or double-shear configuration
- Fastener type, fit, washers and applied torque
- How hole elongation was measured
- Conditioning and test temperature
- Crosshead speed
- Offset and ultimate bearing strength for each coupon
- Failure mode for each coupon, classified
- Coupons rejected, with the reason
- 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 several tens, a crosshead holding 2 mm/min, and a displacement channel dedicated to hole elongation. That last requirement is the demanding one: hole elongation is on the order of a millimetre, while crosshead travel includes coupon, fixture, load cell and frame compliance that together dwarf it. A transducer arranged to read the elongation at the joint itself is essential. Grips must hold a coupon that may be thick and wide, and alignment must ensure the fastener is loaded squarely rather than being dragged sideways.
What goes wrong in practice
The most frequent error is accepting a non-bearing failure. Net-tension, shear-out and cleavage are all geometry-driven modes, and a coupon failing that way has measured its width or edge distance rather than the laminate's bearing strength — the result has to be rejected and the geometry revisited. The second is deriving hole elongation from crosshead travel, which buries a millimetre of real deformation inside several millimetres of compliance. The third is reporting only ultimate bearing strength when design uses the offset value. And torque that is not controlled between coupons introduces scatter that looks like material variability and is not.
Single-shear or double-shear
| Double-shear | Single-shear | |
|---|---|---|
| Load path | Symmetric, coupon between two plates | Asymmetric, two overlapping plates |
| Fastener bending | Minimal | Significant — as in a real lap joint |
| Data quality | Cleaner, less scatter | Noisier, more representative |
| Use | Material bearing allowable | Joint-level behaviour |
Double-shear characterises the laminate; single-shear characterises the joint. A single-shear result is usually lower because the fastener bends and loads the hole unevenly through the thickness, and substituting one for the other is not conservative in both directions.
Questions we are asked about this test
What is ASTM D5961?
It is the ASTM method for the bearing response of composite laminates. A pin or fastener passing through a hole in the coupon pushes against the side of that hole, and the method measures the bearing strength and the hole elongation that goes with it. It is the basis of bolted joint design allowables for composite structures.
Why is offset bearing strength usually more important than ultimate?
Because a bolted joint normally becomes unusable long before it collapses. As the hole elongates the joint loosens, load redistributes to neighbouring fasteners and clamp-up is lost. Design therefore limits hole elongation — commonly to 2 % of the hole diameter — and the bearing stress at that elongation is the figure the structure is sized on. Quoting only the ultimate strength overstates what the joint can be relied upon to do.
Does fastener torque change the result?
Substantially. Clamp-up presses the laminate together through the thickness and suppresses the delamination that otherwise develops under the fastener head, which raises apparent bearing strength by a considerable margin. A torqued result and a finger-tight result are effectively different tests, so the torque is specified, applied to a controlled value and recorded. Relying on a torqued allowable where the joint may relax in service is a known trap.
What is the difference between single-shear and double-shear?
In double-shear the coupon is sandwiched symmetrically between two plates, so the fastener is loaded evenly and barely bends — clean data, good for a material allowable. In single-shear two plates overlap as in a real lap joint, so the fastener bends and loads the hole unevenly through the thickness. Single-shear results are usually lower and more representative of an actual joint.
My coupons are splitting to the edge instead of crushing the hole. What is wrong?
The edge distance is too small, and the failure mode has changed from bearing to shear-out. Bearing, net-tension, shear-out and cleavage are four distinct modes, and which one occurs is governed by the width and edge-distance ratios rather than by the material. A coupon that fails by shear-out has measured its geometry, not the laminate's bearing strength, and must be rejected.
Why can't I use crosshead travel for hole elongation?
Because crosshead travel includes the extension of the coupon, the fixture, the load cell and the frame, and all of that is large compared with the millimetre or so of hole elongation being measured. The result would be dominated by compliance. Hole elongation has to be measured directly at the joint with an extensometer or transducer arranged for that purpose.
Does hole quality matter as much as in the open-hole test?
It matters, though not quite as decisively. In an open-hole tensile test the hole wall is the highest-stressed feature and any delamination there governs the result. In bearing, the fastener presses against the hole wall and the laminate is being crushed rather than pulled around a stress concentration, so it is somewhat more forgiving. Delamination from drilling still lowers bearing strength, and holes are still inspected before use.
Running ASTM D5961 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 5 to 40 kN depending on laminate thickness and fastener diameter | 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 to ASTM E83 Class B-1 where bearing deformation is reported, gauge length not applicable — hole elongation is the deformation measured | Certified to ASTM E83 and ISO 9513 Class 1 — non-contact video, clip-on and high-elongation |
| Gripping | Double-shear or single-shear bearing fixture with a close-fitting pin or fastener through the coupon hole | Our self-tightening serrated wedge grips, with V-jaws for round specimens, built to the specimen |
| Environment | Standard laboratory atmosphere, or conditioned and elevated-temperature where specified | 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.