Flat tensile specimens (often prepared in a dog-bone profile) are used when the starting material is sheet-based or thin, such as rolled metal sheets, stamped panels, polymer sheets, or composite laminates. These specimens are typically shaped by milling to create a narrow gauge section for controlled deformation and wider gripping shoulders that fit securely into testing fixtures. Flat specimens are the preferred choice when the material thickness is limited and when the structure of the material is better represented in sheet form.

Round tensile specimens (also referred to as cylindrical tensile bars) are used when the material is supplied as bar stock, forged blanks, machined components, or other solid sections. These specimens are usually prepared by turning on a lathe to create a uniform diameter gauge section and smooth transitions toward the shoulders. This geometry supports even axial loading, which helps reduce bending effects and provides a more uniform stress distribution across the sample.

The choice between flat and round specimens generally depends on the initial form of the material and the type of mechanical behavior you need to observe. Flat samples are selected for thin or layered materials, while round samples are selected for thicker or solid materials where the full cross-sectional response is important. Both formats can be produced to match ASTM, ISO, DIN, or internal testing standards.