Tensile Testing with Texture Analysers: A Comprehensive Guide

Explore the fundamentals of tensile testing, including its applications and the common probes and fixtures used with a Texture Analyser.

What is a Tensile Test?

A tensile test involves pulling or stretching a sample to measure its extensibility/elongation and tensile strength, quantified by the force required to stretch the sample and the distance it can be stretched. This test can be conducted using a Materials Testing instrument or a Texture Analyzer. Both of these measure compressive and tensile forces to objectively determine compressive or tensile strength.

Tensile strength refers to the maximum force (or stress) a material can withstand before breaking, failing, or experiencing permanent deformation, also known as the yield point. Additional parameters measured during a tensile test include Young’s Modulus, Tensile Strain, and Elongation.

Why Perform a Tensile Test?

Many materials and products face tensile stresses that must be measured and controlled to ensure they maintain required functionality. Assessing tensile properties is crucial for determining a material’s suitability and safety. Additionally, for certain materials, the ability to endure tensile stresses impacts product durability and consumer safety, making it essential for engineers and quality managers to evaluate materials under tension. Without mechanical integrity, a material may fail, jeopardising safety and leading to consumer dissatisfaction.

Although many foods and pharmaceuticals are not typically subjected to tensile forces during production and consumption—since chewing involves compression—certain products like films, dough, gels, spaghetti, confectionery, and adhesives rely on tensile properties as a key characteristic.

Packaging materials across industries must be strong enough to withstand storage, handling, stacking, and transit conditions. Determining the tensile strength of these materials is critical to ensure they can endure the forces applied during use. The higher the tensile strength, the more robust the packaging, thus better protecting the enclosed products. Paperboard and plastics, among other packaging materials, can be tested to assess the force required to pull apart or damage the sample, providing valuable insights.

Types of Tests

Uniaxial Tension Test

The most straightforward tensile testing method involves elongated specimens, either rectangular or circular in cross-section, or reproducible test beams. The specimen is gripped at both ends and stretched until it breaks using Tensile Grips. This uniaxial tension test forms the basis of many ISO or ASTM Standard Test Methods for materials testing. However, new materials may require non-standard methods or specially prepared samples.

For instance, gripping food samples can be challenging to ensure they break within the sample, not at the jaws. Dumbbell-shaped test pieces, gripped at the wider ends, can help, ensuring the break occurs in the narrow center. Other methods include encasing the sample ends between two perspex pieces or freezing the ends before clamping to prevent cutting at the grip point.

Bi-iaxial Tension Test

A bi-axial tensile test is another approach, where the sample is stretched in all directions by forcing a ball probe through the center, useful for testing films and certain foods like tortillas to measure burst strength. Despite the challenges of clamping specimens for tensile tests, careful selection of clamping fixtures can mitigate these issues.

Tensile testing offers an advantage over compression tests as the initiation of fractures is easily observable from the outside of the sample. Conversely, in uniaxial compression, fractures often start inside the test piece.