Testing standard
Standard Test Method for Shear Strength of Plastics by Punch Tool
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D732 measures the shear strength of plastics by punching. A flat specimen is clamped by a bolt through its centre, seated over a die plate, and a 25.4 mm punch is driven down to shear the material around the clamped centre. The reported strength is peak force divided by the sheared surface — the punch circumference multiplied by the thickness.
A flat plastic specimen is clamped between a pair of hardened discs by a bolt through its central hole, seated over a die plate. A 25.4 mm punch is driven down onto the specimen by a compression platen, shearing the material around the clamped centre and pushing it clear of the sheet; the force needed to do it is recorded. Nothing is stretched and nothing is gripped — the material fails around the circumference of the punch, through the thickness of the sheet.
The reported quantity is a shear strength: peak force divided by the sheared surface, which is the punch circumference multiplied by the specimen thickness. It tells a manufacturer how much resistance a sheet or moulded plate offers to being cut through its thickness — the situation in punching and blanking, in fastener pull-through, and wherever a bolted joint bears a head or washer against plastic rather than steel.
The caveat travels with the number. Only relatively brittle grades shear cleanly; ductile grades draw down and tear ahead of the punch, so the peak force is a mixed-mode result and the calculated stress is a ranking figure, not a material constant. Where a quantitative in-plane shear value is wanted for design, this is not the method that supplies it.
τ = P_max / (π d t)
π d t is the sheared surface — the cylinder of material the punch pushes through. It is not a cross-sectional area in the tensile sense, which is why this figure does not compare with a tensile or flexural strength.
The punch is driven by a compression platen, so the load path is a straightforward direct compression arrangement. The punch, die plate and clamping discs are tooling made to the method's geometry.
SpecificationsCapacity is usually what decides whether a laboratory can run this method at all. The sheared surface grows with thickness: about 101 mm² at the 1.27 mm minimum, about 1013 mm² at the 12.7 mm maximum. With plastic shear strengths in the region of 20 to 70 MPa, that spans roughly 2 kN for thin, soft sheet up to 60 to 70 kN for thick, tough grades. A 50 kN frame is the sensible floor and 100 kN is wanted for thick nylon, acetal or rigid PVC plate.
The frame must load in compression, with a platen that is flat and travel that is square to it. Punch travel is fixed at 1.25 mm/min (0.05 in./min), the tolerance being 1.3 ± 0.3 mm/min (0.050 ± 0.010 in./min) measured with the machine running idle, so what the frame needs is stable low-speed control rather than range. The load cell must be verified to E4 across the full punch range, which spans thin sheet to thick plate. Punch displacement is not reported, so neither an extensometer nor a strain channel has any role here.
The punch shear tool does the work grips do elsewhere. Punch diameter, punch-to-die clearance and the sharpness of the cutting edge all sit inside the calculated stress, and the clamping bolt must hold the specimen flat against the die so the sheet cannot lift. A tool with a worn edge or an opened-up clearance goes on producing plausible numbers indefinitely.
Punch edge condition. A blunted or chipped punch crushes and drags rather than shearing, and the peak force rises. The failure still looks roughly right, so the drift is usually caught only when a control material starts reading high.
Ductile draw. Soft grades bulge and tear instead of shearing. The trace shows it — a rounded, extended peak rather than a sharp drop — and the result should be reported as what it is.
Clamping. If the bolt is under-tightened the specimen lifts, bends into the die opening and takes bending as well as shear, which lowers the apparent strength. Over-tightening a thin specimen dishes it around the hole instead.
Thickness measurement. The stress depends directly on measured thickness, so on a moulded plate that varies across its area, a single reading taken away from the punch line biases every result in the set.
| ASTM D732 punch | ASTM D5379 V-notch | ASTM D1002 lap shear | |
|---|---|---|---|
| Material | Plastic sheet or plate | Composite laminate | An adhesive bond |
| Loaded through | The thickness | In-plane or interlaminar | The glue line |
| Answers | Resistance to being punched through | Shear properties of the laminate | Comparative bond strength |
| Design allowable | No — an application proxy | Closer | No |
This is an application test rather than a material constant. It answers how a sheet resists being cut through its thickness — punching, blanking, fastener pull-through, a bolt head bearing on plastic — and those are the situations to use it for.
It is the ASTM test for shear strength of plastics by punch tool. A flat specimen is clamped by a bolt through its centre over a die plate, and a 25.4 mm punch is driven down to shear the material around the clamped area. The reported strength is the peak force divided by the sheared surface.
Because the punch pushes a cylindrical slug out of the sheet, and the material that resists is the wall of that cylinder — the punch circumference multiplied by the sheet thickness. It is not a cross-section in the tensile sense, which is exactly why a D732 figure should not be compared with a tensile or flexural strength.
Situations where a sheet is cut or bears through its thickness: punching and blanking operations, fastener pull-through, and any bolted joint where a head or washer bears against plastic rather than steel. It is an application proxy rather than a material constant, and those are the applications it proxies for.
Because a worn punch tears the material rather than shearing it cleanly, and tearing takes more force. The peak load rises for reasons that have nothing to do with the plastic, so the strength comes out high and the error grows steadily as the tooling wears. The appearance of the cut edge is the check.
It is in the calculation directly — the sheared surface is circumference times thickness — so an error in the thickness measurement propagates straight into the reported strength. Measuring at several points around where the punch will pass, rather than once at a convenient spot, is what keeps that honest on a sheet of variable gauge.
Not in the way tensile strength is. The measured value depends on the punch and die clearance, the sharpness of the punch and the specimen thickness, because failure involves bending and local crushing as well as shear. It is a repeatable engineering comparison rather than a fundamental constant, which is why the apparatus condition is part of the report.
Sheet thick enough to shear rather than simply bend out of the way, and thin enough that the punch can pass through cleanly. Very thin sheet tends to deform and tear rather than shear, giving a low result that describes the geometry; very thick sections need forces that take the fixture outside its intended range. Where the material is outside that band, an in-plane shear method is the better answer.
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 | The sheared surface is the punch circumference times the specimen thickness — π × 25.4 mm × t — so it grows from about 101 mm² at the 1.27 mm minimum thickness to about 1013 mm² at the 12.7 mm maximum. At plastic shear strengths of roughly 20–70 MPa that spans about 2 kN for thin, soft sheet up to 60–70 kN for thick, tough grades. This is the highest force demand in the shear group: a 50 kN frame is the sensible floor and 100 kN is wanted for thick nylon, acetal or rigid PVC plate. | 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 | Punch shear tool — clamping discs and die plate with a 25.4 mm punch, loaded through a compression platen | Our shear fixtures, built to the specimen |
| Environment | Conditioned in a controlled laboratory atmosphere before test, as for any plastics method | 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.
