Which test involves applying a twisting force to an object until it permanently deforms?

The torsional test is a method specifically designed to evaluate an object's resistance to twisting forces. During this test, a gradual application of torsion is exerted on the specimen until it reaches the point of permanent deformation. This is essential in materials engineering, as it provides insights into the material's strength, ductility, and structural integrity under twisting loads, which are common in various applications such as shafts and beams.

The other options deal with different aspects of materials testing. The transverse test focuses on bending and flexural properties, measuring how materials respond to forces applied perpendicular to their length. X-ray testing is a non-destructive method used primarily to identify internal flaws and structural integrity without causing any harm to the tested object, while dye penetrant testing is a surface inspection technique that reveals flaws through the application of dye that penetrates cracks or porosities. These methods assess different material properties and are not designed to measure the effects of twisting forces.