Torsion testing is a type of mechanical testing that evaluates the properties of materials or devices while under stress from angular displacement. Torsion testing can be split into two distinct categories: testing raw materials like metal wires or plastic tubing to determine properties such as shear strength and modulus, or functional testing of finished products subjected to torsion, such as screws, pharmaceutical bottles, and sheathed cables. The most common mechanical properties measured by torsion testing are:
- Modulus of elasticity in shear
- Yield shear strength
- Ultimate shear strength
- Modulus of rupture in shear
Why Perform a Torsion Test?
Many products and components are subjected to torsional forces during their operation. Torsion testing is necessary when engineers wish to change or update the materials used in these products. For example, the metal used in vehicle drive trains experiences a complex combined loading when in use, with torsion being the main component. An engineer trying to design a more fuel-efficient vehicle may need to change the material of the driveshaft in order to reduce vehicle weight. Torsional testing can help the engineer identify an appropriate material that will possess the required torsional strength while also contributing to the goal of lightweighting.
Many finished products are also subjected to torsional forces during their operation. Products such as biomedical tubing, switches, and fasteners are just a few devices subjected to torsional stresses in their everyday use. By testing their products in torsion, manufacturers are able to simulate real life service conditions, check product quality, verify designs, and ensure proper manufacturing techniques.
Types of Torsion Tests
Torsion tests can be performed by applying only a rotational motion or by applying both axial (tension or compression) and torsional forces. Types of torsion testing vary from product to product but can usually be classified as failure, proof, or product operation testing.
- Torsion Only: Applying only torsional loads to the test specimen.
- Axial-Torsion: Applying both axial (tension or compression) and torsional forces to the test specimen.
- Failure Testing: Twisting the product, component, or specimen until failure. Failure can be classified as either a physical break or a kink/defect in the specimen.
- Proof Testing: Applying a torsional load and holding this torque load for a fixed amount of time.
- Operational Testing: Testing complete assemblies or products such as bottle caps, switches, dial pens, or steering columns to verify that the product performs as expected under torsion loads.
Performing a Test and Calculating Results
Torsion testing can be performed on specialized torsion-only equipment such as Instron's MT series or, more commonly, on biaxial test equipment such as universal testing machines capable of applying both linear force and torque. Instron's Torsion Add-On is an advanced fixture which can be retrofitted to any 594X or 596X system to allow for biaxial testing of raw materials and products.
The torque vs rotation curve produced by testing raw materials in torsion is in many ways analogous to the force displacement curve captured during axial testing. Most materials exhibit a similar linear region, representing the shear modulus, followed by yield and ultimate failure.
Common ASTM and ISO Standards
ASTM F543 - Axial and Torsion Testing of Bone Screws
ASTM A938 - Torsion Testing of Metallic Wire
ISO 7800 - Torsion Testing of Metallic Wire
ISO 594 - Testing of Conical Luer Fittings
ISO 7206 - Endurance and Fatigue Testing Artificial Hip Implants