The need for torsion testing
Many products and components are subjected to torsional forces during their intended application. With torsion testing, designers are able to understand real-life service conditions, improve product quality, validate design, and also verify actual production process. Products such as biomedical stents, implantable orthopedic devices, fasteners, and automotive steering/suspension components are examples of products that are subjected to torsional stresses in their typical service environment.
The types of torsion testing utilized vary depending on the information desired from the material or component. Examples include biomedical type torsional testing for bone, bone screws and plate assemblies. Whether it is for characterizing a torsion insertion test for bone screws, or performing basic failure tests, proof tests, or manufacturing process validation, some type of torsional testing method can be designed and performed to meet the needs of each application.
Analogies to tensile/compression testing
Torsion tests are typically performed to determine the torsional properties of a material or component. Analogous to the properties determined during a tensile/compression test, the "torque versus angle" diagram, for example, is similar to a "stress versus strain" curve. The properties determined include:
- Modulus of elasticity in shear
- Yield shear strength (torsional elastic limit)
- Ultimate shear strength
- Modulus of rupture in shear
- Ductility (along the maximum shear plane)
- Brittle failure (perpendicular to maximum tensile stress, 45 deg)
Types of torsion tests
Torsion tests can either apply to only a rotational motion or to the application of both axial (tension or compression) and torsional forces (moment). The types of tests include the following:
Torsion only: Applies torsional-only loads (torque or moment) to the specimen/device.
Axial-torsion: Applies both axial (tension or compression) load and torque.
Failure testing: Twisting the product, component, or specimen until failure.