Test Compression with Less Stress
August 31, 2010
To compress a specimen is to apply a force that attempts to reduce its volume by applying pressure. Compression tests are used to determine how a product or material reacts when it is compressed, squashed, crushed or flattened by measuring fundamental parameters that determine the specimen behavior under a compressive load. These include the elastic limit, which for “Hookean” materials is approximately equal to the proportional limit, and also known as yield point or yield strength, Young’s Modulus (these, although mostly associated with tensile testing, may have compressive analogs) and compressive strength.
Commercially speaking, a compression test can be undertaken as part of the design process, in the production environment or in the quality control laboratory, and can be used to determine functionality and suitability of components or materials such as springs, switches and spray nozzles; to evaluate consistency of manufacture, in seam seal tests for sachets or actuation of dispensers, for example; or as part of a thorough quality check or safety evaluation, for instance, by measuring the expression force of a syringe or in a tablet crush test.
A compression tester will normally measure two basic quantitative factors in a compression test: load (generally expressed in Newtons, pounds-force or kilograms-force) and deflection (inches or millimeters). These may be converted into more complex units such as stress (Mega Pascals or MPa) and strain (% elongation), depending upon how data is presented and calculated.
Types of Compression TestCommon types of compression test include: flexure/bend, spring and top-load/crush.
Flexure/bend test, also known as transverse testing, modulus of rupture testing, three-point bend and four-point bend testing, is a way of determining the behavior of samples when they are bent. While supported at each end, a compressive force is applied to, or across, the middle of the specimen to evaluate its performance. In performing a test of this type, parameters such as flexural stress, flexural strain, modulus of rupture and resilience are considered.
Spring test is a measure of the reciprocal force exerted by a spring at a specific deflection or length, or the deflection or spring length at a given force. Measurements such as spring rate can be determined.
A measurement of the resistance of a package or product to a compressive load, top-load/crush test often is used to determine how a product will cope with anticipated loads during storage and distribution.
Common TerminologyStress often is used when describing performance under compressive load. It is the load applied to a specimen, divided by its cross-sectional area and is expressed as units of pressure in newton per square millimeter (N/mm²) or MPa.
Strain is the proportional change in length of a specimen subjected to a compressive load expressed as a percentage. That is the instantaneous length of a specimen divided by its original length.
Materials Under CompressionCertain materials subjected to a compressive force show initially a linear relationship between stress and strain. This is the physical manifestation of Hooke’s Law, which states:
E = Stress (s) / Strain (e)
where E is known as Young’s Modulus for compression. This value represents how much the material will deform under applied compressive loading before plastic deformation occurs. A material’s ability to return to its original shape after deformation has occurred is referred to as its elasticity. Vulcanized rubber, for instance, is said to be very elastic, as it will revert back to its original shape after considerable compressive force has been applied.
Once a certain force or stress threshold has been achieved, permanent or plastic deformation will occur and is shown on graphs as the point where linear behavior stops. This threshold is known as the proportional limit and the force at which the material begins exhibiting this behavior is called the yield point or yield strength. A specimen will then exhibit one of two types of behavior; it will either continue to deform until it eventually breaks, or it will distort until flat. In either case a maximum stress or force will be evident, providing its ultimate compressive strength value.
Each of these parameters offers useful information relating to the physical characteristics of the material in question.
Some materials, such as a PET bottle, distort during a compression test and are measured by the degree of distortion, whereas other materials such as ceramics fracture, producing a definitive compressive strength value.
The Compression TestIn preparation for a compression test, a number of uniform test specimens should be selected to form a representative sample of performance. This will help to provide accurate comparative results.
Grip selection is crucial to ensure reliability of results and test accuracy. There are many specially designed grips to ensure consistent test performance, repeatability and minimal sample movement. Compression plates are one such example where variants are available for specific applications, a few of which are:
Specialized fixtures are available for applications involving flexure/bend tests and spring tests. Spring supports maintain vertical alignment of springs during compression testing, improving the level of safety for the operator and accuracy of results. Three and four-point bend jigs are engineered to support samples in the correct position for flexure testing.
When selecting grips, choose those that will enable an even load distribution across the sample.
Careful consideration also should be made before purchasing any test system to perform compression testing. For light applications, such as a mobile phone keypad test, where little deflection is expected, a machine with automatic system deflection compensation is required to provide suitable accuracy. A computer-controlled test system would be suitable in this instance.
How is a Compression Test Performed?A constant test speed should be employed when applying load to the specimen, and a motorized test system will guarantee a consistent rate of deflection. Crosshead speed should be slow enough to capture a reliable compression profile of the material, but also fast enough to complete testing within a reasonable amount of time.
For a specimen that is unlikely to break, the test method should specify at what point to take a measurement reading.
A typical test procedure to identify the characteristics between load and deformation of a specimen may be defined in as few as four steps:
1. Center the sample on the bottom platen of the testing machine.
2. Lower the compression platen until it touches the top of the sample.
3. Reset the systems readings to measure the distance between the bottom platen and the compression platen. Commence the compression test at a uniform speed to a predefined distance or until the specimen breaks.
4. Return to the start position.
Repeat tests should be performed to gain comparative results and ensure statistical significance of results.
ResultsAfter the test is complete, results will need to be viewed, stored, exported or analyzed depending on the application requirement. Good computer-controlled testing systems and most motorized systems will provide facilities for transfer of results to a PC, printer or data logger.
Compression tests can be used for a range of applications. As long as the test is performed correctly, it will yield a wealth of information to help in your quality control tasks. Q