Tension Test Methods
Tension methods pull or stretch the test sample, to measure the elasticity and the ultimate strength of the product. This is a less common, but valuable procedure for testing food itself, but has relevance in the simulation of production processes and the handling of the final product. As such, the texture analysis system set up for a tension method is often used in determining packaging recommendations.
The texture analyser may pull the sample apart by moving the crosshead upwards, or the fixtures may enable tension in the product, even when using the system in compression, by punching through in a slow, controlled manner.
Products such a stick chewing gum, restructured deli meats and cheeses, can be successfully tested for their elasticity with tensile testing by pulling apart. The burst strength of thin products (e.g. tortillas) are tested for by extensibility - slowly stretching the structure with a round probe (until tear and penetration). Product stickiness, adhesion, can measured by the tensile adhesive force exerted as a probe moves back upwards after initially compressing the sample. Dough stickiness is tested with a bakery-specific fixture, and is useful for texture analysis relating to influencers of the kneading process (by hand or machine).
In most cases it is quite difficult, if not impossible, to consistently grip a food sample during a tension test without causing a false failure point. This may be overcome by forming the samples into a dog bone shape so that the cross sectional area in the center of the sample, is smaller than that in the region being gripped. This ensures that the deformation and failure are consistent from sample to sample and accurately representative of the material - not influenced by the test system.
FTC offers several tensile grip designs to accommodate test samples in elasticity, strength and extensibility applications.