Biaxial Film Testing using a Texture Analyser – Calculating Fundamental Parameters

The use of the Film Support Rig can provide a very useful test for monitoring the quality of irregular objects, such as the toughness of sliced meat. However, this test setup can also provide more fundamental stress-strain data if the sample has a known uniform thickness and homogeneous structure.

Sometimes the properties of a thin film are under investigation, but the film might be unsuitable for any other type of testing. An example would be a film of dried nail polish; depending on its properties, this can be too brittle to clamp in tensile grips, it is too thin to test under compression and it may not be large enough for bend testing. In cases such as this, the Film Support Rig is ideal, as the sample is clamped at the edges and put under “biaxial tension” by a spherical probe pushing into the centre. 

Contained Compression Testing using a Texture Analyser – Calculating Bulk Modulus

Bulk modulus is a defined as the relative change in a sample’s volume when a unit compressive or tensile stress acts uniformly over its surface. 

It is a measure of how resistant a substance is to pure compression. Its measurement is possible by the use of a Texture Analyser, providing care is taken over the sample setup.

The Measurement of Hysteresis using a Texture Analyser

Hysteresis in the context of deforming a material is the loss of energy in the form of heat when a sample is loaded then unloaded. 

This occurs due to internal friction, and so the effect is larger for a material such as rubber, which requires the movement of large molecular chains each time it is stretched or compressed. If a piece of rubber is deformed several times in the hands, this loss of energy can be felt directly as the sample heats up. 

As can be seen on the graph, the deformation of a viscoelastic material follows a different path on the load and unload cycles. The unload cycle shows that the material is slower to recover compared to the load cycle. This difference in recovery is known as hysteresis, which is due to energy dissipation by the generation of heat.

Compression Testing using a Texture Analyser – Calculating Fundamental Parameters

Compression testing involves a self-supporting sample being compressed under a flat probe large enough to cover the whole sample. 

The loading arm (attached to the probe) moves down at a constant speed to deform the sample, first deforming it elastically then plastically (if it is not completely brittle). If the force required to break the sample is within the limit of the load cell, fracture may occur. However, this is not always the case in compression as many samples are stronger (or tougher) in compression than in tension, especially ceramic samples.

Three Point Bend Testing using a Texture Analyser – Calculating Fundamental Parameters

When a customer is presented with an elongated or flat food sample, one of the first things they might do is snap it between their hands. 

During this process, they are subconsciously assessing whether the food product is brittle or ductile, stiff or compliant, and strong or weak. Consequently, bend testing is a technique used very frequently by Texture Analyser users, covering everything from the snap of tablets to the toughness of dog chews. 

The analysis of this type of testing is often limited to looking at a force peak and maybe the distance to fracture. It is a very useful test for monitoring quality of irregular objects. However, this test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample dimensions. “Flexure” and “bending” have the same meaning and are often used interchangeably.

Tensile Testing using a Texture Analyser – Calculating True Stress and True Strain

The calculation of parameters from the stress-strain graph of a tensile test has already been covered in a previous blog post. 

The calculations considered were those most often used when referring to stress and strain, and to give them their full name they would be called “engineering stress” and “engineering strain”. It is usually safe to assume that every time stress and strain are mentioned in the literature, this refers to the engineering values.
 
However, as the load on a sample increases, the cross-section over which the force is applied changes (it gets thinner). If the engineering stress is used (taking into account the initial area), the stress is underestimated. True stress solves this issue by using the instantaneous area over the course of application of load so that as the cross-section changes, the value of stress is calculated using the new cross-sectional area.
Additionally, whereas engineering strain is the amount that a material deforms per unit length, true strain is the natural log of the current length over the original length.

Tensile Testing using a Texture Analyser – Calculating Fundamental Parameters

Tensile testing involves a sample held in two grips a set distance apart. The loading arm (attached to the top grip) moves up at a constant speed to deform the sample, first deforming it elastically then plastically. If the force required to break the sample is within the limit of the load cell, fracture will occur.

It is a very useful test for monitoring quality of irregular objects, such as the toughness of pizza or the texture of fish. However, this test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample’s dimensions. “Dogbone” shaped specimens are often used in tension, with two wide sections tapering to a narrower central section.

Indentation Testing using a Texture Analyser – Calculating Fundamental Parameters

Although it has long been known that some materials are harder than others, indentation tests to find quantitative hardness values only came about in the 1800s. 

Once it was established as a valuable technique, hardness testing machines started to appear on the market early in the following century. Old fashioned indentation testing involved the application of a weighted probe onto a flat sample surface that was left for a set time period. The hardness of the sample was calculated from the area of the residual dent left in the sample. “Instrumented” (computer controlled) indentation testing has now been in use for many years, and involves the collection of force, displacement and time data, which is why the Texture Analyser is so well-suited to this type of measurement.