Learn about the factors that affect torsion testing accuracy and reliability, such as sample preparation, material behavior, strain measurement, test standards, and test limitations.

Torsional testing can help the engineer identify an appropriate material that will possess the required torsional strength while also contributing to the goal of light weighting. Many finished products are also subjected to torsional forces during their operation.

Last updated on Aug 30, 2024

What are some challenges and limitations of torsion testing?

Torsion testing is a common method of evaluating the mechanical properties of materials, such as modulus of rigidity, shear stress, and shear strain. However, like any other testing technique, it also has some challenges and limitations that you should be aware of before conducting or interpreting the results. In this article, we will discuss some of the main factors that affect the accuracy and reliability of torsion testing, and how to overcome or minimize them.

1 Sample preparation

One of the first challenges of torsion testing is preparing the sample in a suitable shape and size for the test. The sample must have a uniform cross-section, preferably circular or square, and be free of any defects, cracks, or surface irregularities that could influence the torsional behavior. The sample must also be aligned properly with the axis of the torsion machine, and clamped securely to avoid slippage or bending during the test. Any deviation from these conditions could result in inaccurate or inconsistent measurements of the torque, angle of twist, and shear stress.

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Benyamin Piri

Corrosion Engineer, RBI and RCFA Specialist
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Torsional testing can help the engineer identify an appropriate material that will possess the required torsional strength while also contributing to the goal of light weighting. Many finished products are also subjected to torsional forces during their operation.

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Rahul Verma

| Materials Associate at Infinita Lab Inc.| Ex-Corrosion Intel | Co-founder & COO at GreeNext Materials | IIT B | PSCL | IIT P | UCER | UP Sainik School |
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Torsion testing, used to evaluate the shear strength and ductility of materials. One significant challenge is ensuring uniform torque application along the entire length of the specimen, which can be difficult with heterogeneous, leading to inconsistent results. The geometric constraints of the sample, such as length and diameter, also limit the test, as they must be precisely calibrated to avoid premature failure or buckling. Moreover, accurately measuring the angle of twist and the corresponding shear strain can be complex, requiring high-resolution equipment and sophisticated data analysis techniques. Another limitation is the difficulty in applying torsion tests to brittle materials, which may fracture before yielding.

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2 Material behavior

Another challenge of torsion testing is accounting for the complex and nonlinear behavior of some materials under torsional loading. For example, some materials may exhibit elastic-plastic behavior, where the shear stress-strain curve is not linear beyond the elastic limit, or anisotropic behavior, where the properties vary depending on the direction of loading. These factors affect the calculation of the modulus of rigidity, which is based on the assumption of a linear elastic relationship between shear stress and shear strain. Therefore, you need to select the appropriate material model and test parameters to analyze the data correctly.

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3 Strain measurement

A third challenge of torsion testing is measuring the strain accurately and precisely. Unlike tensile or compressive testing, where the strain can be measured directly by using an extensometer or a strain gauge, torsion testing requires measuring the angle of twist of the sample, and then converting it to shear strain by using the geometry and dimensions of the sample. However, this method has some limitations, such as the difficulty of attaching a device to measure the angle of twist, the sensitivity of the measurement to the alignment and clamping errors, and the uncertainty of the measurement due to friction, backlash, or hysteresis in the torsion machine.

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Varun Pushpa

PhD student | Metallurgist at South Dakota Mines | Material Science | Process - Friction stir processing/welding, Heat treatment | Material Characterization | SEM/FIB/XRD/Micro XCT |
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Accurate measurement of angular deformation (twist) and shear strain in torsion testing can be challenging. Sophisticated instrumentation and careful calibration are required to obtain precise strain measurements.

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4 Test standards

A fourth challenge of torsion testing is following the appropriate test standards and procedures for different materials and applications. There are various standards and specifications that define the sample preparation, test setup, test speed, data acquisition, and data analysis methods for torsion testing of metals, polymers, composites, and other materials. Some examples are ASTM E143, ASTM D1043, ASTM D5448, and ISO 6721. You need to consult and comply with these standards to ensure the validity and comparability of your test results.

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Varun Pushpa

PhD student | Metallurgist at South Dakota Mines | Material Science | Process - Friction stir processing/welding, Heat treatment | Material Characterization | SEM/FIB/XRD/Micro XCT |
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Torsion induces a complex stress state in the material, involving both shear and normal stresses. Interpreting the results and extracting specific material properties can be more challenging compared to simpler tension or compression tests. Torsion testing may lead to non-uniform stress distribution across the specimen. This non-uniformity can be particularly pronounced at the ends of the specimen, leading to stress concentration effects that may not represent real-world applications accurately. Torsion testing is sensitive to temperature variations, and the results can be influenced by changes in temperature. Controlling and monitoring the testing environment becomes crucial to obtaining accurate and repeatable results.

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5 Test limitations

A final challenge of torsion testing is recognizing its limitations and scope of applicability. Torsion testing is useful for measuring the modulus of rigidity and the shear strength of materials, but it cannot measure other properties, such as tensile or compressive strength, fracture toughness, or fatigue resistance. Moreover, torsion testing is not suitable for materials that have a low modulus of rigidity or a high ductility, because they may undergo excessive twisting or necking before reaching failure. Furthermore, torsion testing is not representative of the actual loading conditions that some materials may experience in service, such as combined or cyclic loading, temperature or environmental effects, or stress concentrations.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are some challenges and limitations of torsion testing?

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1 Sample preparation

2 Material behavior

3 Strain measurement

4 Test standards

5 Test limitations

6 Here’s what else to consider

Materials Testing

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