Physical Property Measurement: Introduction to Fracture

What is Fracture? This is the first of a set of blog posts on the subject of fracture testing. Traditional information on fracture testing focusses on techniques for assessing engineering materials using standard methods and strict geometries. However, these may not be so useful for the typical user of a Texture Analyser.

The loosest definition of fracture is “a form of failure in which the material separates into two or more pieces due to an applied load”. Fracture strength is the stress at which a specimen fails or fractures. Fracture can be brittle, ductile or semi-ductile. This refers to the nature of deformation and will be covered in the next post.
Why is it important to perform fracture tests? 
Fracture testing is important in many sectors for many reasons. Here are a few examples of fracture testing likely to be carried out by the owner of a Texture Analyser.

Food Texture Analysis
The way that food breaks down in the mouth during chewing is an important indication of its texture. Foods can be classified as crispy, crumbly, hard, strong, chewy, etc. according to how easy they are to break down in the mouth. The mouth, in fact, acts as a very efficient mechanical testing machine sensing the fracture properties of the food and contributing in an important way to our oral appreciation of the texture. Understanding the fracture properties of foods allows us to manipulate its structure and hence produce a desirable texture. 

Fracture also occurs in natural conditions (bruising in fruit and vegetables), transport and storage, freezing and processing. An understanding of the fracture properties of such materials can reduce loss of products by mechanical damage.

Cosmetics, Personal Care and Pharmaceutical Products
These groups of products usually have some functional need for strength to prevent fracture. For example, a lipstick or eye pencil must not break off during use, a bandage must not split when being wound round a leg, and a split dose tablet must be easy enough to break between the fingers without being so weak that it breaks during transit. 

As with the food sector, it is not always the case that the stronger the sample, the better. Some investigation into acceptable ranges for breaking force is always necessary, as is the long term monitoring of changes to these properties.

Structural Materials and Packaging
Packaging, no matter what it contains, is generally designed to keep its contents safe from damage during transport and storage. Consequently, it has to be strong enough to stand up to external forces such as crushing or puncture. However, it is not always beneficial to have the greatest strength possible – for example, a peel off lid must have a seal adhesive of lower force than the strength of the lid itself so the packaging fails in the correct way. 

The same goes for perforations and heat sealed seams, for example on the top of a crisp packet. This means that the fracture properties of each component must be well characterised and tested under different conditions, ensuring they are repeatable in a large number of samples.

Methods for testing fracture properties of materials Fracture testing looks at the ability of a material containing a crack to resist fracture. If possible, it is best to carry out deformation in a controlled way in which the changes in the specimen geometry can be measured. 

In testing the fracture properties, we must bear in mind some basic material science principles. Energy has to be supplied to load the material so that it deforms physically. This comes in the form of applied force. The change in force reading as deformation increases can be measured by the Texture Analyser and plotted as a force/deformation or a force/time curve by Exponent software.

Fracture testing and analysis are very complicated fields, and the most reliable analysis involving the accurate determination of fracture toughness is carried out on linear elastic materials (which are few and far between) with known geometry and a measureable pre-crack so compromises must be made when testing materials of irregular shape or properties that may not be cracked before the test. 

However, these more empirical tests can still provide very useful information, imitating the forces on a sample that is bitten or on a package opened by tension across a sealed seam, for example. It can be beneficial to imitate the real life use of a sample as closely as possible in this case.

The best fracture tests are the simplest ones in which the specimen is deformed along its axis (uniaxial). This is usually in the form of a tensile test on specimens with known geometry and dimensions to a point beyond fracture. If test materials are not of a regular geometry or size then test specimens of reproducible dimensions can be cut from them. 

Sometimes specimens are of regular geometry but are not easy to grip in the jaws of the test machine. In such a case bending or cutting (wedge penetration) tests may be appropriate. Lastly, some products may be best tested as a whole – sweets, lipstick or tablets, for example. Bending or compression may be the most suitable in this case.

The group of tests that can be set up to open a crack in a sample in a controlled manner are called ‘crack opening tests’. These include tensile tests, wedge penetration and bending tests. Most crack-opening tests involve similar mechanisms of energy storage and release. They should produce similar results for the fracture properties. Different types of crack-opening tests are suitable for different products depending on their shape, composition and ease of clamping.

This series of blog posts will talk you through the different types of fracture, some of the main types of fracture test and finally, give you some useful hints for help with fracture testing.

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We can design and manufacture probes or fixtures for the TA.XTplus texture analyser that are bespoke to your sample and its specific measurement.

Once your measurement is performed, our expertise in its graphical interpretation is unparalleled. Not only can we develop the most suitable and accurate method for the testing of your sample, but we can also prepare analysis procedures that obtain the desired parameters from your curve and drop them into a spreadsheet or report designed around your requirements.

For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

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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