Physical Property Measurement: Useful hints for conducting fracture experiments

For reliable and reproducible results from fracture testing, some basic rules must be taken into consideration. 

Biological materials are notoriously variable in their morphology and structure. Variation can occur not only at any or all of the hierarchical levels from the molecular, through to material and structural level, but also within the entire test specimen and even between specimens. 

Physical Property Measurement: Fracture Testing by Cutting

Wedge penetration, a type of crack opening test that is particularly suitable to testing fruit and vegetables, cheese, stiff gels and cooked meats, involves a wedge driven into a block of material. 

The dimensions of the test specimen affect the results and so it is important to use reproducible rectangular blocks. This test is similar to a tensile test but instead of the two halves of the specimen being pulled, they are pushed out by the penetrating wedge. The two halves bend outward storing strain energy. 

At the point of fracture this energy is fed to the tip of the wedge where stress concentration is highest and a free running crack starts and propagates ahead of the wedge.

Physical Property Measurement: Fracture Testing in Bending

This is a versatile test and can be applied to many types of products that are available in, or can be cut or shaped into, elongated test beams. 

The most conventional method is a three-point bend test in which the specimen is supported horizontally at either end like a bridge and a probe moving downwards bends it in the centre. As the specimen bends it stores up strain energy, which is dissipated in cracking at the point of fracture.

Physical Property Measurement: Fracture Testing by Compression

Compression tests can be carried out on a wide variety of products that experience such a force in natural conditions. 

These may include fruit and vegetables, puffed cereals, cakes and biscuits, confectionery and pharmaceuticals. 

Normally, as these products may be oddly shaped, a compressive test is the most reliable way of assessing their fracture behaviour.

Physical Property Measurement: Fracture Testing in Tension

Many foods and pharmaceuticals are not normally subject to tensile forces in their manufacture and consumption. There are, however, exceptions, e.g. dough, gels, spaghetti and adhesives. 

These tests can also be performed on materials from which elongated test specimens can be cut and gripped within the clamps of the Texture Analyser to be stretched. This could include fruit, vegetables and meat. In any case this type of test yields reliable results for tensile modulus, yield stress and strain, strength and strain to fracture, and, if the crack area can be measured, the fracture toughness of the material.

Physical Property Measurement: Types of Fracture

Fracture is simply crack propagation. Force has to be exerted to initiate a crack, then energy has to be supplied to propagate it. This is the energy that goes into breaking the bonds within the material in order to generate new surfaces. 

There are three ways a crack can propagate within a material. All structural failures are the result of material failure in one of these three modes.

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.