If you run tensile tests on metal, plastic or composite specimens, the grip is the part that decides whether your result is valid data or wasted time. Wedge tensile grips are the most widely used fixture on a universal testing machine (UTM) — and the reason comes down to one word: self-tightening.
What Is a Wedge Tensile Grip?
A wedge tensile grip holds a specimen between two jaw faces seated in angled wedge bodies. As the crosshead pulls, the wedges are drawn deeper into their tapered housing, converting part of the tensile force into horizontal clamping force. The harder the pull, the tighter the grip — which is why high-load slippage is rare when the grip is correctly matched to the sample.
Main components: grip housing, wedge halves, interchangeable jaw inserts, and a manual lever or motorized drive.
How the Self-Tightening Principle Works
- Jaw faces contact the specimen surface.
- Tensile load pulls the wedge down along the taper.
- Taper geometry multiplies that load into clamping force.
- Clamping force rises with test force, up to the grip's rated capacity.
Because clamping scales with load, wedge grips suit high-strength materials — metal sheet, wire, rebar, FRP. The same principle is a risk for soft or fragile samples: it can crush them. That is when coated or soft jaw faces are required.
Jaw Insert Types
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Jaw insert
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Best for
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Note
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Serrated flat (pyramid)
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Flat dumbbell specimens, rigid plastics, metal sheet, laminates
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Standard; strongest anti-slip
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V-type
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Round bars, wire, cylindrical samples
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Select by diameter range
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Rubber-coated / soft
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Thin foil, coated sheet, delicate films
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Prevents crushing and surface damage
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With extensometer bracket
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Metals, FRP, composites
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For Young's modulus and Rp0.2 yield strength
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How to Choose — 4 Checks
- Capacity — match the grip rating to your load cell, not only to the expected breaking force.
- Specimen shape — flat goes to flat serrated jaws; round goes to V-jaws; thin or coated goes to soft jaws.
- Surface sensitivity — aggressive serration marks soft metals; use coated faces if surface damage affects results.
- Test environment — high/low temperature or humidity testing needs a compact body and chamber-compatible materials.
Common Issues
- Specimen slip: wrong jaw face, worn serration, or grip capacity too low.
- Specimen crush: serration too aggressive, or clamping force too high for the material.
- Off-axis break: misaligned wedges or worn wedge tracks — check alignment regularly.
FAQ
Q: Can one grip body handle different samples?
A: Yes. Flat serrated, V-type, rubber-coated and extensometer jaw inserts are interchangeable on the same body.
Q: Why does my specimen break inside the jaws?
A: Usually stress concentration from over-aggressive serration or excessive clamping. Switch to a softer jaw face.
Q: Do wedge grips need an extensometer?
A: Only for modulus and yield strength (Rp0.2). Choose the version with an extensometer bracket.
Wedge tensile grips are one of the core fixture types for universal testing machines. Browse our dual column tensile testing machine for compatible grip configurations, or contact us for a fixture recommendation based on your sample.
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