Testing standard
ASTM D953
Standard Test Method for Pin-Bearing Strength of Plastics
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D953 determines the pin-bearing strength of rigid plastics: a flat coupon with a hole is loaded through a pin fitted in that hole, and the bearing stress is read against the deformation of the hole. Procedures A and B cover thermoplastics in tension and in compression loading; Procedure C covers reinforced thermosets and reports an ultimate strength. The current edition is D953-19, retitled from Bearing Strength of Plastics with the -18 edition.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ASTM
- Edition
- D953-19
- Material
- Plastics, polymers & films
- Runs on
- Series 7200 and Series 9000
What the test does
A flat plastic coupon with a hole near one end is loaded through a pin or bolt fitted in that hole. The pin bears against the wall of the hole and the load is carried edgewise into the sheet, exactly as it would be in a bolted or riveted joint. The method offers three procedures. Procedures A and B apply to thermoplastics, loaded in tension and in compression respectively, and both plot pin-bearing stress against the deformation of the hole. Procedure C applies to reinforced thermosets and reports the ultimate pin-bearing strength at which the specimen fails.
What it measures, and why it matters
Bearing stress — load divided by the projected bearing area of the hole — and, for the thermoplastic procedures, how far the hole has deformed while carrying it. That second quantity is the point of the test. A bolted joint in a plastic sheet rarely fails by snapping; it fails by the hole slowly ovalising until the fastener is loose, the joint rattles and the load path changes. Reporting a strength alone would miss that entirely, which is why the thermoplastic procedures record the stress at which the hole has deformed by a defined fraction of its diameter rather than the stress at rupture. For reinforced thermosets, where the hole wall crushes rather than creeps, the maximum stress is the meaningful figure, and that is what Procedure C reports. The method is written for specifying sheet products joined by rivets, bolts or similar fastenings, and more generally wherever a sheet carries edgewise load through a pin passing perpendicularly through it.
Specimen and the hole that is the specimen
The coupon is ordinary. The hole is the thing being measured, and how it was made decides the answer.
- Material
- Rigid plastics in sheet form, moulded form, or reinforced thermoset form
- Geometry
- A flat coupon with a single fastener hole, dimensions fixed by the standard
- Thickness
- Tied to the diameter of the fastener hole usedMore than one pin size is defined, and each needs its own fixture — a laboratory running both keeps two.
- The hole
- Drilled or reamed square to the surface, free of tear-out on either facePracticeAn oversized, out-of-round or smeared hole sets the whole bearing curve low, and nothing later in the test recovers it.
- Reinforced material
- Drilled with backing to prevent exit-side delaminationPracticeDelaminated material around the hole carries no bearing load, so the effective thickness is less than the coupon measures.
- Conditioning
- Pretreatment, temperature and humidity as the standard defines them
Every defect in the hole reduces the result and none of them are visible in the load trace. Drill quality is the single largest source of scatter in this test.
Loading and rate
- Load path
- Through a pin or bolt bearing on the wall of the holeThe sheet carries the load edgewise, exactly as a bolted or riveted joint does.
- Procedures A and B
- Thermoplastics, in tension loading and in compression loading respectivelyBoth plot pin-bearing stress against the deformation of the hole rather than reporting a single strength.
- Procedure C
- Reinforced thermosets, reporting ultimate pin-bearing strength
- Speed
- As specified by the procedure in use
- Measure hole deformation locally, not from the crosshead
- DakCrosshead travel carries fixture take-up and coupon stretch. On a test whose whole point is how far one hole has ovalised, that flatters the material.
Calculations
σb = P / (D × t)
- P
- load carried through the pin, N
- D
- diameter of the hole, mm
- t
- thickness of the coupon, mm
D × t is the projected bearing area — the area the pin presses against, not the area of the hole.
The bearing stress at a defined deformation of the hole
Procedures A and B report a stress at a deformation criterion rather than at rupture, because a bolted joint in a thermoplastic fails by the hole ovalising, not by the coupon breaking.
The maximum bearing stress reached before failure
Procedure C reports an ultimate value, because the hole wall in a reinforced thermoset crushes rather than creeping.
How the test runs
- 01Choose the procedure: A or B for a thermoplastic, C for a reinforced thermoset.
- 02Machine coupons to the specified geometry, with the thickness matched to the pin size.
- 03Drill or ream the hole square to the surface, with backing where the material can delaminate.
- 04Measure the actual hole diameter and coupon thickness and use them in the area.
- 05Condition to the specified pretreatment, temperature and humidity.
- 06Fit the coupon into the fixture for the chosen loading direction, with the pin seated square.
- 07Load at the specified rate, recording force and the deformation of the hole.
- 08For the thermoplastic procedures, plot bearing stress against hole deformation throughout.
- 09Read the stress at the deformation criterion, or the maximum stress for Procedure C.
- 10Inspect the hole afterwards for ovalisation, crushing or cracking, and record what is seen.
What the report has to contain
- Reference to ASTM D953 and the edition
- Which procedure was used, and therefore whether the result is a deformation-based or an ultimate value
- Full identification of the material and its form
- Coupon thickness and the measured hole diameter
- Pin diameter and fixture used
- Pretreatment, conditioning atmosphere and duration
- Rate of loading
- The bearing stress against hole deformation curve, or the ultimate bearing strength
- The condition of the hole after test
- Number of specimens and the mean and scatter
What the machine must be capable of
A universal frame with force indication verified to ASTM E4, and a fixture that loads through the pin without letting the coupon rotate or lift. Forces are modest by frame standards — the bearing area is small — so the load cell should be chosen for the specimen rather than the frame, or the early part of the stress-deformation curve is lost in the noise. The measurement that matters most is the deformation of the hole, and it has to be taken so that machine compliance, fixture take-up and coupon stretch do not contaminate it; crosshead travel alone flatters the material. The standard also names pretreatment, temperature and humidity as defined conditions, so conditioning capability is part of the requirement rather than an optional refinement.
What goes wrong in practice
Reading the deformation from crosshead travel, which adds the whole load train to what should be a local measurement of one hole. A hole that is oversized, out of round or drilled with a worn bit, which sets the whole bearing curve low. Confusing the procedures: a thermoplastic result read as an ultimate strength, or a thermoset judged against a deformation criterion. And citing the standard by its former title, Bearing Strength of Plastics, without noticing that the current document is the pin-bearing method with three named procedures.
Bearing tests on plastics and composites
| ASTM D953 | ASTM D5961 | ASTM D5766 | |
|---|---|---|---|
| Material | Rigid plastics, sheet, moulded or reinforced thermoset | Polymer matrix composite laminates | Polymer matrix composite laminates |
| Hole | Loaded by a pin | Loaded by a fastener | Open, carrying no fastener |
| Reports | Bearing stress against hole deformation, or ultimate stress | Bearing response for bolted-joint design | Tensile strength with a stress concentration |
| Used for | Specifying sheet products that are bolted or riveted | Aerospace bolted-joint design data | Notch sensitivity of a laminate |
D5766 and D5961 answer two halves of one question — what the hole costs you, and what the fastener bearing in it costs you. D953 is the plastics-sheet equivalent and is not interchangeable with either: the specimen, the procedures and the reported quantity all differ.
Questions we are asked about this test
What is ASTM D953?
It is the ASTM method for the pin-bearing strength of rigid plastics. A flat coupon with a hole is loaded through a pin fitted in that hole, so the sheet carries the load edgewise as it would in a bolted or riveted joint, and bearing stress is reported against the deformation of the hole. The current edition is D953-19.
Why was the title changed?
Every edition up to and including D953-10 was published as Standard Test Method for Bearing Strength of Plastics. The -18 edition retitled it Pin-Bearing Strength of Plastics, which is more precise about what is loaded and what is measured. A drawing or specification citing the older wording is citing this same method under its former name.
What is the difference between Procedures A, B and C?
Procedures A and B both apply to thermoplastics and both plot pin-bearing stress against the deformation of the hole; A loads in tension and B loads in compression. Procedure C applies to reinforced thermosets and reports the ultimate pin-bearing strength at which the specimen fails. Choosing the wrong one produces a number that cannot be compared with anything.
Why report deformation rather than strength for thermoplastics?
Because that is how the joint actually fails. A bolted joint in a plastic sheet rarely fails by the coupon snapping; the hole slowly ovalises under load until the fastener is loose, the joint rattles and the load path changes. A strength at rupture would say nothing about that, so the thermoplastic procedures record the stress at which the hole has deformed by a defined fraction of its diameter.
What does the standard cover it for?
It is written for specifying thermoplastic and thermosetting products in sheet form where rivets, bolts or similar fastenings join members or sections, and more generally wherever a sheet has to carry edgewise loads applied through pins or rods of circular cross section passing perpendicularly through it.
How much force does the test need?
Not much by frame standards, because the bearing area is small — a hole diameter multiplied by a coupon thickness. That makes load-cell choice more important than frame capacity: a cell sized for the machine rather than for the specimen loses the early part of the stress-deformation curve in its own noise, and the early part is where the deformation criterion sits.
Why does the hole quality matter so much?
Because the hole is the specimen. An oversized or out-of-round hole starts with a loose pin and a false zero. A blunt drill smears a thermoplastic and leaves a melted edge with different properties from the bulk. Drilling a reinforced laminate without backing delaminates the exit side, and delaminated material carries no bearing load, so the effective thickness is less than the coupon measures.
Can it be used on composite laminates?
The fixture is sometimes used that way, but the method for polymer matrix composites is ASTM D5961, which is what aerospace bolted-joint design data is generated to. D953 was written for rigid plastics in sheet, moulded and reinforced thermoset form, and a result from it should be quoted as a D953 result rather than offered against a specification that names D5961.
Running ASTM D953 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 | Modest. The bearing area is one hole diameter multiplied by one coupon thickness, so the load cell has to be chosen for the specimen rather than the frame or the early part of the stress-deformation curve is lost in its own noise. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | Force verification to Practices E4, which the method references | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Gripping | Pin-bearing fixture loading the coupon through a pin fitted in the hole; separate tension-loading and compression-loading arrangements | Our a fixture built for this method, built to the specimen |
| Environment | Defined pretreatment, temperature and humidity; Practice D618 conditioning is referenced | 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.