Practical considerations for successful tensile testing

Tensile testing is used less frequently than compression testing in food and biomaterial analysis, largely because it is more challenging to grip specimens in a way that applies a clean tensile load. Poor gripping leads to stress concentrations, premature failure at the grips and data that are difficult to interpret.

This article outlines practical approaches to specimen design, grip selection and sample-specific fixtures, with particular reference to testing on a TA.XTplusC Texture Analyser. The focus is on obtaining failures within the gauge section and reproducible measurements of tensile strength, extensibility and related parameters.

Specimen design and failure location

If a bar of uniform cross-section is clamped directly at both ends and loaded in tension, local compression and deformation at the grips generate stress concentrations. These promote failure near the clamp rather than in the central region of interest.

To overcome this, tensile specimens are usually designed:

• Wider at the clamped ends to increase load-bearing area and reduce local stress.
• Narrower in the gauge section so that the maximum stress and strain occur away from the grips.

In practice, stress is calculated using the minimum cross-sectional area in the gauge region:

• Stress, σ = F / A
• Strain, ε = ΔL / L₀

where F is the applied force, A is the minimum cross-sectional area, ΔL is the extension and L₀ is the original gauge length.

Correct specimen geometry is a prerequisite for reliable tensile data, irrespective of the grip type or fixture used.

General-purpose tensile grips and their applications

Stable Micro Systems have developed a range of tensile fixtures to address different sample geometries and surface properties. Selecting an appropriate grip is critical to avoid slippage, local damage and grip-induced failures.

Tensile Grips

General-purpose Tensile Grips use knurled jaw faces to clamp samples up to 25 mm thick and 37 mm wide. They are suitable for:

• Strips of semi-rigid materials
• Simple dog-bone specimens
• Conventional film or sheet materials where a firm mechanical grip is acceptable

In some cases, the upper grip is used alone for hanging-type tests or where the lower support is defined by a different fixture - as seen in this video of tensile testing options. 

Self-tightening Roller Grips

Self-tightening Roller Grips employ spring-loaded, cross-hatched rollers. As tensile force increases, the grip tightens automatically on the specimen. These grips are particularly useful for:

• Smooth or low-friction surfaces
• Materials that thin appreciably when stretched, such as some elastomers or thin films

The self-tightening mechanism helps maintain contact pressure as the specimen necks, reducing slippage and improving repeatability of tensile strength and tear measurements.

Mini Tensile Grips

Mini Tensile Grips provide smaller jaw faces with a maximum opening of 8 mm. They are advantageous where:

• The available stroke is limited, for example on a TA.XTExpressC  Texture Analyser
• Small or delicate specimens require proportionally smaller clamping areas

They allow tensile testing in more compact test spaces while preserving well-defined grip conditions.

Pneumatic Grips

Pneumatic Grips use compressed air to actuate lever arms that clamp the specimen. Key advantages are:

• Precisely controllable gripping pressure, adjustable via air pressure up to a maximum of 10 bar
• Constant clamping force that is maintained even if the specimen creeps or deforms at the grip interface
• Reduced incidence of grip-face failures, as gripping force can be optimised to prevent both slippage and damage

By decoupling clamping force from crosshead motion, pneumatic grips provide highly reproducible boundary conditions, which is particularly important in comparative studies or quality control protocols.

Articulated Tensile Grips

Articulated Tensile Grips are small lightweight grips suitable for gripping of thin materials whilst providing a good degree of rotational flexibility which may be caused by product distortion during a tensile test and makes loading of difficult samples easier. 

Sample-specific tensile rigs

Certain food and biomaterial systems require specialised fixtures to support the sample appropriately and to promote failure in the region of interest.

Spaghetti / Noodle Tensile Rig

The Spaghetti/Noodle Tensile Rig supports soft noodle and spaghetti samples using parallel friction rollers. Samples are wound around the rollers two or three times in order to:

• Prevent cutting or splitting at the grips
• Minimise slippage under load
• Encourage failure along the exposed central length

This configuration enables measurement of tensile strength and extensibility in slender, fragile strands.

Noodle/Pasta Loop Tensile Rig

For noodle or pasta samples cut from sheeted dough, the Noodle/Pasta Loop Tensile Rig is preferred. It combines:

• A unique sample cutter to form consistent loops
• A mounting geometry that promotes failure away from the clamping points

This is essential for obtaining high-quality tensile data where failure in the gauge section reflects material properties rather than fixture artefacts.

Capsule/Loop Tensile Rig

The Capsule/Loop Tensile Rig uses two separating rods to hold empty capsule shells. Vertical separation is applied while Force and Extension /displacement are recorded until the capsule splits. The resulting data can be used to assess shell integrity, closure strength and the effects of formulation or processing on capsule performance.

Cheese Extensibility Rig

The Cheese Extensibility Rig was developed to quantify the extensibility of molten cheese. It incorporates:

• A double-sided fork probe mounted to the Texture Analyser
• A vessel and sample-retaining insert that holds a known mass of cheese while preventing bulk lifting

The cheese is heated until molten, then the fork is drawn through the cheese. The force–distance curve characterises extensibility, stringiness and melt behaviour, parameters highly relevant to pizza and processed cheese applications.

Dough & Gluten Extensibility Rig

The Dough & Gluten Extensibility Rig supports a small dough sample, typically around 10 g. The ends of the specimen are secured using a spring-loaded clamping plate while a hook extends the sample. This configuration allows:

• Measurement of dough and gluten network strength
• Assessment of formulation effects on extensibility and resistance to extension

It is particularly useful for laboratory-scale rheological characterisation of bread and bakery doughs.

Pizza Tensile Rig

The Pizza Tensile Rig comprises two four-pronged attachments, one connected to the load cell and one to the base. A rectangular section of pizza is impaled on both sets of prongs. As the crosshead moves, the rig measures:

• The “tug” or separation force required to pull the slice apart
• The contribution of cheese, toppings and crust to the overall structural integrity

The same rig has been used in research on Atlantic salmon at the University of St Andrews Fish Muscle Research Group, providing a sensitive predictor of post-mortem gaping. The occurrence of tears or slits in the fillet under load correlates with fillet quality and handling robustness.

Film Support Rig

For thin, film-like samples, a biaxial extension approach may be more representative than simple uniaxial tension. The Film Support Rig supports the specimen between plates, exposing a circular area. A spherical probe is then pushed through the film to measure:

• Burst strength
• Extension at rupture
• Elastic recovery

This method is particularly relevant to edible films, packaging films and other membrane-like materials.

Tortilla/Pastry Burst Rig

The Tortilla/Pastry Burst Rig extends the same biaxial principle to larger and thicker samples. It includes:

• A larger exposed circular region
• A larger spherical probe
• A smooth support ring to reduce edge-induced breakage

This enables measurement of burst strength and extensibility of products such as tortillas, flatbreads and pastry sheets under conditions more akin to real handling or filling.

Unconventional mounting techniques for difficult samples

Specialised or fragile samples sometimes require non-standard mounting strategies to achieve reliable tensile loading conditions.

Adhesive mounting in tubes

Cummings and Okos (1983) tested noodles by securing them with rapid-setting glue inside metal tubes slightly larger than the noodle diameter. The tubes were attached to the tensile tester via wires and loops that allowed the sample to self-align with the loading axis. This approach:

• Improves alignment
• Reduces bending stresses
• Minimises damage at the clamping interface

Tomato skin in foil supports

Murase and Merva (1977) wrapped tomato skin samples in aluminium foil, clamped the foil ends, then cut the foil to expose the central gauge region. The specimens were soaked in solutions of known water potential while tensile tests were performed, enabling:

• Measurement of static elastic modulus
• Investigation of the effect of water potential on epidermal mechanical properties

Card-mounted fragile fibres

At Stable Micro Systems, a similar approach has been used for fragile fibres such as hair. An aperture is cut into a mounting card and the fibre is secured across it using beads of hot glue on either side of the aperture. The card is clamped in the tensile grips and, immediately before testing, the card is cut either side of the sample so that the fibre becomes free-standing between the grips. This method:

• Protects the fibre during mounting
• Facilitates accurate alignment
• Reduces the likelihood of introducing damage at the clamping points

Low-temperature gripping and soft-interface wraps

For moist or highly slippery food samples, some researchers have:

• Briefly dipped the sample ends in liquid nitrogen prior to mounting to create a temporarily rigid gripping section.
• Wrapped the sample ends in cardboard before clamping so that the grip faces engage with the cardboard rather than directly deforming or tearing the soft sample.

Both techniques aim to improve grip without compromising the integrity of the gauge section.

Practical guidelines for reliable tensile data

Across all these configurations, a few general principles improve tensile test quality:

• Design specimens to fail in the gauge section, not at the grips.
• Select grips that match the surface and mechanical properties of the sample.
• Control gauge length, test speed or strain rate, and environmental conditions (temperature, relative humidity).
• Record and interpret full force-distance or stress-strain curves rather than relying on a single peak force value.
• Inspect failure mode after testing to confirm that it is consistent with the intended measurement.

By combining appropriate fixturing with rigorous control of test parameters, tensile testing on a Texture Analyser becomes a powerful tool for quantifying strength, extensibility and structural integrity in a wide range of food and biomaterials.

References: 

CUMMINGS, D. A. & OKOS, M. R. (1983). Viscoelastic behaviour of extruded durum semolina as a function of temperature and moisture content. Transactions of ASAE, 26: 1883-1893.

MURASE, H. & MERVA, G. E. (1977).  Static elastic modulus of tomato epidermis as affected by water potential. Transactions of ASAE, 20: 594-597.