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.

Shear and torsion both have tensile components where bonds are uniaxially loaded in tension before they break.

Modes of structural fracture
Failures of the structures into which the material is orientated should not be confused with material failures. For instance, a cellular material under a compressive load can show failure where a zone of cells have collapsed, i.e. a compressive structural failure, but to achieve this the cell-walls have to be ruptured in localised tension, i.e. a tensile material failure.

Structural failures are the ones directly perceived in the texture analysis of a product or in the mouth during chewing food. They can be of the following modes:

Tensile: where the specimen is stretched until it breaks into two fragments. The fracture surface can be exposed. Here the bonds are uniaxially stretched until they break. Tensile tests are ideal for measuring the toughness of the specimen.

Compressive: here layers of the structure collapse at right angles to the direction of force as a result of bonds breaking in tension. In most cases a specimen under a compressive load fractures in shear at 45 degrees to the direction of force and the two fragments slide past each other. It is generally not possible to expose the fracture surface.

Shear: this is a result of two layers of the specimen fracturing and sliding past each other. In probe tests, for example, as a flat tip probe enters the specimen the material directly underneath will be compressed while a large zone around the probe is under shear. A conical probe largely removes the compressive component, giving mostly shear.

Torsion: where the material is twisted about its axis until fracture occurs. There is a large shear component but the bonds break in tension.

Bending: bending incorporates tensile failure in one side of the sample and compressive failure on the other. If the material has a higher tensile strength than compressive, the tensile region will be large, and vice versa.

Brittle and ductile fracture: In all modes of failure there are two extremes: brittle and ductile. Most fractures incorporate aspects of both extremes.

Brittle fracture: This is where as the specimen is deformed the stress increases as the strain increases. At fracture point the material catastrophically fails without prior warning. 

Products exhibiting this brittle fracture include boiled sweets, tablets, dried pasta and rice. Here cracking is sudden and unstable, i.e. once it starts it cannot be stopped and the material breaks into two or more fragments (shattering). The force reading suddenly falls to zero.

 Other dry products like puffed cereals, crisps, biscuits etc. may show a brittle fracture, but as they incorporate structural complexities such as air spaces which act as crack stoppers, they fracture not suddenly but in several smaller stages (crumbling). Here again the specimen will fail without prior warning with each small crack being sudden and accompanied with a drop in force reading, but in most cases it will stop soon until further energy is supplied. Crack propagation is therefore stable.

Ductile fracture: On the other extreme, when some products are loaded, they give plenty of warning before breaking. Dough, gels, meat and fresh pasta, for instance, yield to a varying degree when under tension, before they break. Materials that have long directionally orientated and compliant fibres or long chain molecules generally show this behaviour. Here, as the deformation is increased the force starts levelling off (or even decreasing) before the specimen fractures.

As most ‘real world’ products have a very complex structure and composition their mode of fracture can be a combination of any of the above and can lie within any of these extremes. Some products show a very stable crack propagation, i.e. when the crack starts it progresses quite slowly (fruit and vegetables, meat and fish). Instead of the force reading falling suddenly it gradually descends to zero. Further energy has to be supplied to propagate the crack. If the load is removed the crack stops. 

Many foods, especially under compressive loads, fail in a very complex manner so that clear failure modes are indistinguishable. This usually shows up as a three dimensional zone of damage (bruising).

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