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, flattened 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 known 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 tests provide objective quantitative data on the integrity and safety of materials, components and manufactured products thereby helping to assess whether items are suited to their application and will not fail during storage, handling, transportation or use. A manufacturer can choose to perform a compression tests to ascertain the quality of a batch of material, to measure compression properties during the design phase, in the investigation of material substitutions or to comply with international, industry or internal quality control standards.
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 Young’s modulus seldom applies to food materials and the term modulus of deformability is perhaps a better term. However, the concept of Young’s 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.
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, Young’s Modulus, work/load/deflection at rupture, maximum load, deflection at maximum load, stiffness, offset yield 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.
What does a Compression test result look like?
Axes units (N, g, kg, N/mm, N/mm2 etc.) can be chosen based on a standard method requirement, manufacturer’s material specification requirements, academic literature submission guidelines or operator choice.
What types of Compression Test are there?
Compression tests can be performed in a number of different ways depending upon the property that is required to measure.
Measure the Force to go to a chosen Distance – This is the most typical type of compression test. A distance is chosen to push down onto the sample and the force to achieve this distance is measured. This type of test is typical for establishing whether the application of a chosen force is large enough to cause failure or irreversible deformation in a product. The force can be applied for a chosen period of time if necessary. This type of test is useful to assess the affect of stacking height (and thereby additional weight/load) of materials on top of each other.
Measure the Distance to go to a chosen Force – This type of compression test is suitable for measuring the compactability of a sample, for example, when compressing granules to create tablets.
Measure the Force and Distance at which a material fails under compression – This type of test is typical for the assessment of strength of components where the point of failure/break point needs to be determined. The greater the force to break, the more resistant the product will be to compressive loading/failure and the greater the distance that the failure occurs during the test, the more flexible the product is providing the product with a degree of ‘give’ before ultimate failure occurs. A short distance to failure would be indicative of a brittle material that is able to withstand very little flexure or compressive load before irreversible damage or breakage/collapse is apparent.
Measure recovery/relaxation of a product by holding a Distance for a chosen time – This type of test is useful for recoverable materials like foams where the degree of ‘spring’ is an important characteristic. The force at a chosen deformation distance is compared with changing forces over a chosen period of time to assess whether the material relaxes or attempts to recover to its original uncompressed height.
Measure the expansion or disintegration of a product by holding a Force for a chosen time – Certain materials expand or disintegrate under particular circumstances. For example, bread dough expands during the proofing stage. On the other hand, once immersed in liquid, bath bombs disintegrate. A chosen small force can be applied to the surface of these materials so that the speed (change of distance with time) of expansion/disintegration and any plateau point can be measured.
When performing compression tests they can be setup in a fundamental, empirical or imitative way.
Fundamental Compression tests using a Texture Analyser
A large number of users use their Texture Analyser to calculate fundamental parameters by taking into account the geometry and dimensions of the sample as well as the specifics of the test set up along with the relevant engineering equations. This type of testing allows easier comparison of different samples and comparison with literature data, but does require more sophisticated analysis to convert forces to stress and distances to strain.
One of the most commonly used fundamental tests is the measurement of Young’s Modulus. Fundamental tests generally assume (1) small strains (1-3% maximum); (2) the material is continuous, isotropic (exhibiting the same physical properties in every direction), and homogeneous; and (3) the test piece is of uniform and regular shape. Whilst fundamental tests are popular in the materials world most textural tests made on foods fail to comply with the three assumptions listed above. Fundamental tests are not used to any great extent in the food industry as they do not correlate as well with sensory evaluation as do empirical tests but they do have a place in some research laboratories. Szczesniak (1963) aptly described the usefulness of fundamental tests as follows:
Since most foodstuffs do not have simple rheological properties that are independent of stress and strain conditions, and since rheological properties once measured and defined are not meaningful in a practical sense unless related to functional properties, fundamental tests serve the greatest value to the food technologist by providing bases for the development of more meaningful empirical tests.
Empirical Compression tests using a Texture Analyser
Texture Analysers are very often used for quality control of irregular products by the use of empirical measurements. The force required to crush a tablet or the distance a biscuit bends before it snaps are invaluable pieces of information to manufacturers looking to develop new products or ensuring products perform correctly during spot checks. This type of test is ideal for products that have an unusual geometry, but it is also useful for testing more complicated systems, such as the combination of a product and its packaging (a box of eggs, the force to open a ring pull can or a yoghurt in its pot) with the added benefit of time saved by allowing a product to be tested straight off the supply line.
Imitative Compression tests using a Texture Analyser
Imitative tests measure various properties under conditions similar to those to which the sample is subjected in practice, that is, the properties of the material during handling and the properties of the sample during consumption or use. This class may be considered as a type of empirical test because the tests are not fundamental tests. Examples of this kind of test are the TPA (Texture Profile Analysis) test that imitates the chewing action of the teeth, the dough extensibility rig and other dough-testing fixtures that imitate the handling and working (mixing time and mixing tolerance) of bread dough, the Volodkevich Bite Jaws (which are intended to simulate the incisor tooth) and many others.
If you’re not bound by any existing constraints and are looking for the most simple testing solution, sometimes the easiest approach is to set up a test that closely imitates the way the product is evaluated in real life. This type of test usually also makes data interpretation easier for you to understand.
Example Applications of Compression Testing:
Electronics: Keyboard actuation
Cosmetics: Powder compact compressibility
Personal care: Bath bomb disintegration speed
Pharmaceuticals: Granule compactability, tablet diametral failure
Medical Devices: Stent radial crush strength, intravaginal ring flexibility, syringe aspiration
Packaging: Box crush strength, blister pack removal force
Food: Marshmallow springiness, eggshell break force, breakfast cereal crispiness
Powders: Unconfined yield stress
Industrial: Upholstery foam firmness, golf ball hardness, spring characteristics, ceramic hardness
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.