Tensile testing: Bend and break

Tensile strength depends on the type of material and its microstructure. Metals usually have a high tensile strength because their atoms are arranged in regular, dense lattice structures that form strong bonds. On the other hand, plastics usually have a lower tensile strength, but can be strengthened by various methods, such as adding fibres or changing their microstructure.

It is important to note that tensile strength is only one of many properties that can be determined during tensile testing. Depending on the method, statements can be made about the toughness of the material, among other things. In the case of screws, for example, the elongation and necking of the sample can be tested before failure.

The functional principle of tensile testing

Tensile testing is a standardised procedure in which samples with a defined cross-sectional area are usually stretched to breaking point under certain conditions. The data obtained from tensile testing is of crucial importance for material characterisation.

Screws are tested on the finished product. The screws are screwed into a threaded sleeve for this purpose. There is a washer under the screw head, the upper side of which has a defined angle. After inserting the fixture created this way into the tensile testing machine, the tensile force is applied to the screw via the washer and the sleeve. This tensile test under wedge loading provides information about the tensile strength, but also about the intactness of the underhead radius (screw head is bent over) due to the angle of the washer. As a rule, the failure of a hardened and tempered screw must occur in the freely loaded thread, as this is where the cross-section is at its smallest.

In order to determine other parameters such as elongation at fracture, necking and yield strength in addition to tensile strength, screws can be machined. This produces a test piece that complies with the standard.

There are numerous other options for subjecting items to tensile or compression testing. The ability to mount the components is particularly important here. Various tools and mounting devices are used for this purpose. Nuts, for example, are screwed onto a test mandrel and then loaded. In contrast to other products, however, they are not removed until they fracture. In fact, the nuts are subjected to a defined test force and must not fail during a certain period of time.

Tests at elevated temperatures can also be particularly interesting for plastic samples or bonded metal sheets. A temperature chamber integrated directly into the tensile testing machine is available for this purpose.