Tips and tricks for successful tensile testing

Tensile testing is less commonly used than compression testing, because it is more difficult to grip the sample in such a manner that a tensile load can be applied.  

If the ends of a bar of uniform cross-section are clamped, compression of the ends causes stress concentrations to develop, which promote failure in the vicinity of the clamp.

This problem is usually overcome by making the sample wider in the vicinity of the clamp and ideally the sample will fail at its narrowest point. Therefore stress is calculated from the minimum cross-sectional area (see diagram A).


More unusual sample mounting techniques for tensile testing:
 
Cummings and Okos (1983) tested noodles using rapid-setting glue to secure them in a metal tube slightly larger in diameter than the noodles. The tubes were attached to a tensile testing machine via wires and loops which allowed the sample to align itself with the line of application of the force (see diagram B). 

Murase and Merva (1977) wrapped tomato skin samples in aluminium foil, clamped the ends, severed the aluminium foil to expose the sample, and then soaked the sample in solutions of known water potential while applying a tensile force (see diagram C).

Here at Stable Micro Systems we have used a similar sample supporting device – a business card – to enable the mounting of fragile samples such as a hair fibre. 

An aperture is cut into the mounting card and the strand affixed either side of this aperture via a hot glue bead. The mounting card can then be secured in tensile grips and just prior to testing the card cut either side of the sample so that it is then ‘free-standing’.

We have also encountered researchers – needing to grip food samples that are either moist or prone to slippage – who have used liquid nitrogen to dip the sample ends into before mounting as a means of providing a more solid grip face.

We also occasionally recommend the wrapping of the ends of the sample with cardboard before gripping, so that the tensile rig grip faces tighten against the cardboard rather than potentially damaging the surface of a softer sample.

Over the years, Stable Micro Systems have developed a wide range of tensile testing solutions, each addition being as a result of the requirement for a more ideal and successful way of mounting a specific sample. There are of course a couple of basic and popular tensile rigs that suit many requirements. 

TENSILE RIGS AND THEIR OPTIMUM APPLICATION

Chewing gum test

Tensile Grips are general purpose knurled jaw face grips suitable for holding samples which have a maximum thickness of 25mm and width of 37mm.         

Occasionally the upper grip is used alone – as seen in the video of tissue paper extraction below. 

Self-tightening Roller Grips are designed with spring-loaded cross-hatched rollers which provides the self-tightening mechanism by which samples up to 45mm wide and 10mm thick are gripped. These grips are ideal for measuring the tensile strength and tear characteristics of a wide range of materials which have a smooth surface finish or whose physical characteristics alter (e.g. due to thinning when stretched) when a tensile force is applied.
 
Mini Tensile Grips have smaller jaw faces with a maximum opening of 8mm and are used where testing stroke is limited – for example, when performing tensile tests using a TA.XTExpress Texture Analyser.
 
Pneumatic Grips are often a preferred means of holding a sample for tensile testing because the gripping pressure can be controlled precisely and because deformation of the specimen does not produce any change in the gripping pressure. This type of grip clamps the specimen by lever arms that are actuated by compressed-air cylinders built into the grip bodies. A constant force maintained on the specimen compensates for decreases in force resulting from creep of the specimen in the grip.  Another advantage of this design is the ability to optimise gripping force by adjusting the air pressure (up to a maximum of 10bar), which makes it possible to minimise specimen breaks at the grip faces.

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. Click here to view video...


RIGS FOR THE TENSILE TESTING OF SPECIFIC SAMPLES

The Spaghetti/Noodle Tensile Rig was developed to support soft noodle and spaghetti samples by locating them through slots in parallel friction rollers which ensure that the sample is not split or cut during the test. Samples are wound around the rollers two or three times to reduce any slippage and also to anchor the sample ends and thereby encourage the breakage along the exposed length of the sample.
 
For noodle samples that can be taken from sheeted dough, the Noodle/Pasta Loop Tensile Rig is recommended. By adaptation of the Spaghetti/Noodle Tensile Rig and use of the unique sample cutter, the rig has been designed to promote failure away from the sample mounting points, which is imperative to ensure good quality test data

The Capsule/Loop Tensile Rig has a similar approach to sample mounting. Empty capsule shells are mounted onto two separating rods.  Vertical movement of the upper rod is then applied until the capsule is split apart while the force and distance moved to do so is recorded.

When asked about how to measure the extensibility of molten cheese, Stable Micro Systems developed the Cheese Extensibility Rig, featuring a double-sided fork probe. The vessel and fork assembly is filled with a known weight of cheese and heated until the cheese is molten. The Texture Analyser pulls the fork through the molten cheese whilst the body of cheese is held by a sample retaining insert to resist lifting of the whole mass.

The Dough & Gluten Extensibility Rig was developed from the requirement to support a small (10g) section of dough whilst extending with a hook. The ends of the sample are held via a spring-loaded clamping plate.

When the need arose to successfully support a slice of pizza in order to measure its ‘tug’, the Pizza Tensile Rig was developed. This rig comprises two four-pronged attachments. The upper attachment fits directly to the loadcell and the lower to the base of the texture analyser. The rectangular shaped test sample is positioned on the attachments by impaling carefully on the prongs.

The Pizza Tensile Rig has also been used successfully in research carried out on Atlantic salmon at the University of St. Andrews Fish Muscle Research Group. It proved to be the most accurate predictor of post-mortem gaping (the appearance of tears or slits within the fillet on handling).

For the measurement of burst strength of thin, film-like samples the biaxial extension approach, as used in the Film Support Rig, may be more appropriate. In this situation, the sample is supported between plates which expose a circular section of the sample allowing a spherical probe to be pushed through to perform extension and elasticity measurements.

Similarly, the Pastry/Tortilla Burst Rig provides the means to support larger and thicker samples with a larger circular exposed region of the sample and the testing with a larger spherical probe. A smooth support ring prevents the breakage of samples at the inside edge of the support plates.

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.

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The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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