Jaw Breaks and Specimen Slippage: How to Choose the Right Tensile Grips
Two problems ruin more tensile tests than anything else: the specimen slips in the grips, or it breaks right at the jaw face instead of in the gauge section. Both usually come down to the wrong grip type, the wrong jaw faces, or poor alignment. This guide explains why jaw breaks and slippage happen, how each type of tensile grip works, and how to match the grip to your material.
Why specimens break at the jaws
Stress concentration from the jaw faces: sharp serrations bite into the specimen and create notches right where the clamped and unclamped material meet.
Too much clamping force: crushing the specimen in the grip section weakens it before the test starts, especially with plastics, composites and fibers.
Misalignment: if the upper and lower grips are not on the same axis, the specimen sees bending on top of tension, and the highest stress lands at a jaw edge.
Specimen design: straight-sided specimens with no shoulders or tabs concentrate stress at the grip. Dogbones, tabs and end reinforcements move failure into the gauge section.
Most standards treat a jaw break as invalid or as a reason to retest, so fixing it is worth the effort.
Why specimens slip
Not enough clamping force for the load and specimen thickness.
Jaw faces too smooth for the material, or worn and filled with debris.
Specimens that thin under load, such as rubber and films, which loosen a fixed-gap grip as they stretch.
Slippery surfaces, such as coated fabrics, oily metals and low-friction plastics.
Tensile grip types and when to use them
Wedge grips
Wedge grips are self-tightening: as tensile load increases, the wedges slide down their tapered body and clamp harder. That makes them the standard choice for metals, rigid plastics and composites from a few kN up to several hundred kN. Interchangeable faces cover flat specimens and, with V-groove faces, round bars.
Pneumatic grips
Pneumatic grips close with air pressure, so clamping force is set by a regulator and stays constant no matter how the specimen behaves. They are fast to load and repeatable, which suits films, thin plastics, textiles, paper, wire and high-volume QC work. Foot pedal controls leave both hands free to position the specimen.
Hydraulic grips
Hydraulic wedge grips apply a set clamping pressure hydraulically before the test begins. They are used for high-capacity work such as rebar, bolts, high-strength steel and thick composites, where a mechanical wedge would need too much tightening or could slip at the start of the test.
Vise (screw) grips
Vise grips close with a screw or handwheel. They are simple, low cost and versatile for low to medium loads, and are often used for general tensile testing of plastics, textiles, webbing and wire. Because the gap is fixed after tightening, they can loosen on specimens that thin under load.
Roller (eccentric) grips
Roller grips use an eccentric roller that rotates and tightens as the specimen pulls. They are ideal for highly extensible materials such as rubber and elastomers tested to ASTM D412, where the specimen thins dramatically and a fixed-gap grip would let go.
Capstan, wrap and bollard grips
For rope, cord, yarn, webbing and strap, clamping directly causes breaks at the jaw. Wrap grips, rope grips and bollard grips wrap the specimen around a drum so friction carries most of the load before it reaches the clamp.
Choosing jaw faces
Serrated (pyramid or file-cut): metals and hard plastics. Coarser serrations for higher loads and thicker specimens.
Fine serrations: thin metals and rigid plastics where coarse teeth would cause jaw breaks.
Smooth or rubber-coated: films, foils, papers and fabrics that tear on serrations.
V-groove: round specimens, wire and bar.
Tabs and abrasive cloth: composites per ASTM D3039, where bonded tabs or emery cloth spread grip pressure.
Practical fixes for jaw breaks and slippage
Check alignment with an alignment specimen or by eye on a long rod; use self-aligning couplings where possible.
Lower clamping pressure on pneumatic or hydraulic grips until slipping just stops.
Change the face to a finer or rubber-coated profile for fragile specimens.
Use tabs, emery cloth or paper between the jaw face and specimen for composites and fibers.
Clamp the full grip length and center the specimen across the jaw width.
Clean faces regularly; debris fills serrations and causes slippage.
Grip selection FAQ
What is the best grip for plastics tensile testing to ASTM D638?
Wedge grips with fine serrated faces or pneumatic grips are the most common choices for ASTM D638. Pneumatic grips are faster and give constant clamping force, while wedge grips handle higher loads and stiffer materials.
What grips should I use for thin film to ASTM D882?
Pneumatic grips with smooth or rubber-coated faces are the usual choice for ASTM D882, because they clamp evenly without tearing the film.
Can one grip cover all my tests?
Rarely. A wedge or pneumatic grip with interchangeable faces covers a wide range, but rubber, rope and fibers usually need specialized grips. Browse all tensile grips or contact us with your materials and standards for a recommendation.
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