British Standards compression testing

Note that a portable tester based on the same principle as the compression tackiness tester is defined by a British Standard for testing of soils16. It uses a split mould 38 mm in diameter and 76 mm long (note the 2 1 ratio), with the sample of soil extruded into it from a sampling tube. The load is applied manually by a rotary handle and a lead screw, through a calibrated spring. 

Flexometers or heat build-up fatigue apparatus operate in compression, shear or a combination of the two and various designs have been in use and standardised, particularly by ASTM, for many years. The test piece geometry and deformation cycle used are, inevitably, somewhat arbitrary and this perhaps contributed to it being much later before there was an international or British standard method. 

Semi-solid foods, such as soft butter and some cheeses, cannot be formed into samples capable of supporting their own weight. For such foods, compression testing takes the form of cone or die penetrometry, in which a cone, die, needle or sphere is made to penetrate the sample (held in a suitable container) either under constant load or at constant speed, and the penetration depth measured as a function of time. Standard methods for penetrometry of fats are published by the AOCS (AOCS Official Method Cc 16-60, Firestone, 1998) and the British Standards Institution (BS 684 Section 1.11 1976, BSI, 1976c). 

Breakdown by confined compression can be tested in a compression cell similar to that in compaction tests, except that the normal forces used are greater here. A British Standard method59 packs a sample of specified quantity and size grade into a steel cylinder, 150 mm nominal diameter a plunger is inserted into the open end of the cylinder and the whole compressed in a compression testing machine. The load is slowly increased up to 400 kN and the product is then sized. A similar test has been used in assessing friability of pellets and coal. 

BS2782, Part 9, Method 901 A, Methods of testing plastics Compression moulding test specimens of thermoplastic materials, British Standards Institute Publication. 1988. 

Lubrication of the test pieces is now standard practice in order to eliminate one obvious source of variation. The more uniform flattening of the test piece also eases measurement of thickness after release from compression. However, there remain specifications in which set is determined in the absence of lubricants. It has also become common practice with general-purpose rubbers to measure compression. set after just one day at 70°C, which for sulfur-vulcanized elastomers can be a. sensitive measure of the state of cure. Higher test temperatures are specified for special-purpose and speciality synthetic rubbers, but the one-day test has remained popular, not least as a cla.ssification criterion and grade requirement in such specifications as ASTM D2000 and the British Standard series of material specifications for individual rubber types. Tests seldom last more than seven days, and recovery is usually confined to the standard. 30 minutes after release, during which time the test piece cools to standard laboratory temperature if taken from an oven. The short-term nature of the test and the absence of isothermal conditions during recovery has been questioned by Birley and other workers [43]. 

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