What is a compression test?
A compression test is perhaps the most simple and popular test of instrumental texture measurement. In its simplest method, a sample is placed on a flat base/surface and a flat probe/platen is lowered onto the sample to a given force or distance (or the sample is compressed to a percentage of its original height). The sample is deformed and the extent of the deformation and/or the resistance offered by the sample is recorded. Simple compression tests are often termed uniaxial compression which means that the sample is compressed in one direction and is unrestrained in the other two. Either a low or high degree of compression can be chosen but a high degree of compression will usually cause the product to rupture, spread, fracture, or break into pieces.
Why perform a compression test?
When a sample is compressed, crushed or squashed you are able to determine how the product reacts by recording the way it fails, the behaviour of it under compressive load or the forces necessary to compress it. Early in the compression stage of a Hookean solid of uniform cross-sectional area there is a region of the test that is classed as small strain (before rupture). During this stage, the Young’s modulus of elasticity can be measured (which is the slope of this linear region of a stress-strain curve). Further compression will reveal the elastic limit (approximately equal to the proportional limit) and also know as yield point or yield strength. These fundamental parameters along with compressive strength and stiffness are properties that are widely used in engineering. Compression testing is used to guarantee the strength, quality or performance of components and the compressive properties of materials and finished products for a wide range of industries. Compression springs, keypads, solenoids, foams, plastics, rubber, syringes and tennis balls are typical examples of products where compressive properties would certainly need to be measured and controlled.
Since most foods are viscoelastic rather than elastic and are usually subjected to large compressions in testing, the strict definition of Youngs modulus seldom applies to food materials and the term modulus of deformability is perhaps a better term. However, the concept of Youngs modulus of elasticity is frequently used to express the stress-strain ratio of the food, at least under moderately light compressions and in the area of the force-compression curve that is reasonably linear.
Compression tests can be carried out on a wide variety of viscoelastic products that experience such a force in natural conditions. These may include fruit and vegetables, puffed cereals, cakes and biscuits, confectionery and pharmaceuticals.
Texture Profile Analysis
Texture profile analysis (TPA) is an objective method of sensory analysis pioneered in 1963 by Szczesniak who defined the textural parameters first used in this method of analysis. Later in 1978, Bourne adapted the Instron to perform TPA by compressing standard-sized samples of food twice.
The test consists of compressing a bite-size piece of food two times in a reciprocating motion that imitates the action of the jaw and extracting from the resulting force-time curve a number of textural parameters that correlate well with sensory evaluation of those parameters. See more information on this specific type of food compression test.
Properties that can be measured with a compression test
Compression tests are typically chosen to measure:
Compressibility, compactability, springiness, stress relaxation, creep compliance, crush strength, firmness, elastic recovery etc.
To understand how these properties are measured visit the Textural Properties page.
Typical probes/fixtures used for compression tests
The probe which comes into contact with the sample, exerting a force on it, usually takes the form of a simple cylindrical probe or a flat plate/platen. Compression testing involves the use of probes that should be of equal (throughout the duration of the test) or greater surface area than the sample. Testing using probes with diameters smaller than the sample are referred to as puncture/penetration tests.
Bulk Compression Tests
Some compression tests are performed in bulk where the sample is compressed in three dimensions, e.g. when using an Ottawa cell. This type of test is needed for multi-particle products which are irregular in shape and size from piece to piece – a chosen weight or number of pieces is tested ‘in bulk’. Usually a compressive test is the most reliable way of assessing their fracture behaviour.
Sample Preparation Tools
If specimens come in varying sizes and geometries such as fruit and vegetables, it is best to cut out a number of geometrically reproducible specimens from its flesh, e.g. cylinders using a cork borer. This way the variations in the mechanical properties due to size and shape differences are eliminated. A number of tools for repeatable sample preparation are available on the Accessories page.
To understand how these probes are designed and manufactured click here. Other probes/fixtures and accessories are available to accommodate many specialist needs, or can be designed and manufactured to a specific customer brief.
Items with codes prefixed 'HDP/' must be used with the HDP/90 Heavy Duty Platform.
Items tagged * are Community Registered Designs.