What does the torsion test primarily calculate?

The torsion test is designed to evaluate how materials react to twisting forces. Its primary focus is on measuring the shear modulus (G) of materials, which quantifies the material's response to shear stress. During the test, a material specimen is typically subjected to a twisting moment, allowing engineers to determine its ability to resist deformation under applied torque.

The shear modulus is particularly important in structural engineering as it describes the relationship between shear stress and shear strain, enabling engineers to predict how materials will behave under load conditions that involve torsion. This modulus is crucial for applications where materials are expected to undergo twisting or rotational forces, such as in shafts, beams, and various structural components.

In the context of the other options, while compressive strength, tensile strength, and bending stiffness are significant mechanical properties, they are not the primary focus of a torsion test. Compressive and tensile strengths pertain to axial loads, and bending stiffness relates to bending moments and deflections rather than torsional deformations. Thus, the correct understanding of the torsion test directs us to the shear modulus specifically, which is vital for characterizing material behavior under torsional loads.