9 Steps on How To Do a Compression Test
Compression testing is a key part of manufacturing and quality control. A material’s compressive strength is a critical parameter for many applications, particularly in structural building components.
Compressive strength is important in many wider applications, from cardboard boxes to lipsticks, and these can be tested in different ways. Listed below are the steps on how to test an item’s compressive strength.
Select the material to be tested. Testing procedures depend on the material type and expected properties — you evaluate ABS plastic than carbon steel very differently, for example. Follow the procedures that best fit your material type and test article.
Most international standards prescribe a specific geometry for material test samples. These defined shapes and sizes generate standardized and relevant results even out of different testing rigs. Therefore, before testing, you need to form your material into the proper shape. Metals, for example, are usually machined for the task while concrete is cast in cubes.
Compression testing is usually accomplished using a universal testing machine. It consists of a table, a load frame, and upper and lower crossheads. It includes measuring instruments such as a load cell and extensometer. The “universal” moniker comes from its ability to evaluate multiple different material properties including tensile strength and puncture force
The size and type of machine will depend on the item you’re testing. Plastics require far less compressive force than metals or concrete. The machine’s fixtures (platen, load cell, and other accessories) need to be selected and fitted according to the sample’s material and shape.
Place the standardized material sample on the compression platen. You must align your sample properly to ensure that the force is applied evenly across its surface. Stress concentrations that are caused by the test setup rather than the sample will skew the results.
Input the test parameters into the universal testing machine’s software. These can be dictated by the testing standard or determined by experience. The parameters can include details such as: the maximum load to be applied, the platen’s speed of travel, and the maximum travel length.
You can now start the compression test. The upper platen will begin to move downward towards the lower one. This movement is slow and controlled. The operator should observe to make sure the sample is correctly aligned when the upper platen makes contact.
Remain and supervise the compression testing as it progresses. Take note of any problems that arise or if the part fails prematurely. Improper tests may need to be discarded or repeated, but detecting them often demands careful observation. Also, if anything goes wrong for any reason, you will be able to stop the test immediately.
The universal testing machine will output a stress vs strain plot. This plot depicts results such as the compressive strength — the highest stress experienced by the sample during the test. For more information, see our guide on What is Compressive Stress?
Other parameters can be read from the graph of stress vs strain, such as the modulus of elasticity and the yield stress (the stress at which plastic deformation begins). These measurements, together with the compressive strength, can be compared with international and internal standards. In this way, the data acts as a quality control method, ensuring that the samples under test meet specifications, and also alerting you to any potential failures or inadequate quality-control trends.
A compression test is a controlled and measured application of compressive force to a standardized sample of material. It is performed in order to determine properties of that material such as strength, elasticity, and ductility. The travel of a platen produces a compressive strain on the sample, while a load cell measures the stress applied. The machine measures the stress vs strain relationship, from which material properties can be determined. For more information, see our guide on What is a Compression Test?
Compression tests are meant to measure certain material properties. It reveals information such as the ultimate compressive strength, the modulus of elasticity, and yield stress (the stress at which the material begins to deform plastically).
Compression tests are used wherever an element’s strength under compression is a critical property. Some applications of compression tests are listed below:
Aerospace engineers need the results of compressive tests so they can be certain of their materials’ integrity under pressure. Whole components can also undergo compression testing to evaluate their failure points. Similarly, the compressive strength of structural components in vehicles is critical. This has a significant impact on passenger safety and the design of crumple zones meant to absorb impact.
Concrete samples regularly get tested for their compressive strength. That strength is critical to the safety and structural integrity of buildings, bridges, and other structures. Compressive strength tests on concrete are foundational to the quality control measures employed during planning and construction. Other building materials such as bricks also undergo compressive strength testing.
Compression testing is used in the cosmetics industry in a number of ways. Products such as lipstick are tested for the force they’ll absorb before deformation. The packaging for creams and lotions is tested for the squeezing force required to dispense them. Some powders such as eyeliners are also compacted before being sold, and the compacting force requires testing.
Many components used in the electrical and electronics industry need to reliably withstand compressive forces. These include not only interfaces such as LCD screens and keypads but also components that get crimped such as connectors and insulation.
Medical devices such as syringes and needles regularly experience compression. There are also devices that use compression to release pre-metered doses, as is the case with inhalers and pen injectors. Each of these items, therefore, needs to undergo compressive testing to ensure that they will function reliably each time.
A common application of compression testing is the crush test of a carton or container. This measurement is important as it illustrates the weight that can be safely stacked atop the container for storage or transport.
Products such as corrugated cardboard and other complex paper constructions are tested to establish their compressive strength. This is necessary to ensure that they are durable enough for their applications. The manufactured board is also tested for flexural strength.
Many elastomers and rubbers get employed to absorb compressive forces. Compressive strength is therefore a vital element of their engineering. Plastics run the gamut from cheap, weak options to high-end engineering materials. Compression testing helps engineers find plastics with the right balance between cost and capability.
Various sports equipment undergoes compression testing. This is particularly relevant for items such as balls (tennis, golf, etc) that need to bounce predictably and repeatedly.
Compression test equipment can be classified in a number of ways, but it’s generally organized based on the maximum force the machine is designed to deliver. Other aspects follow from this. Examples are listed below:
- Light Duty (<5 kN): This compression testing equipment is typically used on plastics, elastomers, and packaging materials. The test rig is bench mounted, uses electric servo motors to apply an accurate force, and typically has a single-column load frame.
- Medium Duty (approximate range of 5-100kN): This testing equipment is used for stiffer materials, harder plastics, and more robust packaging units. It is usually still bench mounted but has a dual-column load frame. Older models may still use hydraulics to apply force, but newer electric servo motors generally give the machine more precise control.
- Heavy Duty (>100 kN): Heavy-duty compression testing equipment is used for materials with high compressive strengths like metals and some ceramics. This equipment is floor mounted, being significantly larger and heavier than the lighter-duty machines. The load frame has two columns, and the force is usually applied by hydraulics to achieve forces of up to 2,000 kN.
Compression testing can illuminate multiple material properties. The most common are listed below:
- Compressive Strength: This is the maximum stress that the sample experienced during the compression test. It is taken from the stress vs strain graph.
- Modulus of Elasticity: Also referred to as Young’s modulus, this is a measure of the stiffness or rigidity of the material. It is the ratio between stress and strain — illustrated as the gradient of the graph of stress vs strain — in the elastic portion of its deformation. Young’s modulus can be calculated for compression as well as tension.
- Yield Stress: This is the stress at which the material begins to deform plastically (irreversibly) rather than elastically.
Compression tests come in three different flavors. The types are listed below:
- Top Load/Crush: This is the simplest type of compression test where the load is applied directly to the top of the sample. This demonstrates the material’s compressive strength. This type of test is also carried out on everything from concrete to packaging materials to measure their resistance to crushing.
- Flexure/Bend: This type of compression test uses two lower supports to hold the ends of the test article. A load from above is then applied between those supports, causing the part to bend. The deflection of the sample is measured according to the load applied, and this gives valuable information about the specimen’s stiffness and bending strength.
- Spring Testing: This type of compression testing is applied to springs. They’re compressed so the test equipment can measure various properties. The spring starts at rest, gets extended, and is then slowly compressed until all coils are touching. This type of compression testing demonstrates a solid height, maximum load, and spring rate of the spring.
Compression testing can be used in a number of beneficial ways. Some of its benefits are listed below:
- Can be used for quality control in manufacturing. Compression tests allow key parameters of the product to be tested across batches to ensure consistent quality.
- If carried out with the necessary quality controls and according to defined standards, the results are repeatable and accurate.
- Allow designs of components (such as packing cartons) to be optimized. Adjustments can be made to a design to reduce weight or save on material costs, while still ensuring that it meets the necessary specifications for compressive strength.
Compressive testing does have some limitations which can make the tests challenging. A few are listed below:
- If you fail to place the sample in the ideal spot on the platen, you may get imperfect measurements.
- Samples must be prepared to a specific geometry, which can be hard to do with some materials.
- The results of the compression testing can be affected by environmental factors such as temperature.
- Compression testing can be time-consuming.
Yes, compression tests are accurate if done properly. International standards help to guide testing methods so that everyone who follows them ends up with comparable results no matter where they’re done or which brand of testing machine they use. However, there are some factors that may decrease the accuracy of compression tests, such as: temperature variability, poor alignment on the platens, and worn or uncalibrated equipment. But if you are disciplined in your inspections and certifications, you can have confidence in your compression tests.
No, a compression test cannot reliably determine the tensile strength of a material. Compressive strengths are usually higher than tensile strengths. Most materials with high compressive strengths also have good tensile strengths. This is only a general statement, however, and can’t be depended upon for engineering purposes. Most materials behave differently under compression than they do under tension. Metallic atoms, for instance, rearrange themselves differently under tension than under compression, so one test cannot substitute for the other. For more information, see our guide on the Tensile Strength of a Material.
The difference between a compression test and a tension test is the direction in which the load is applied. For both tests, it acts perpendicular to the material’s surface. With compression testing, it is applied inward, towards the center of the sample to compress it while a tensile test applies it outward, stretching the material. Although some materials have similar strengths under both compression and tension, that is not always the case — many materials behave differently under one or the other.
This article presented how to do a compression step, explained each step, and discussed the various applications of these tests. To learn more about compression tests, contact a Xometry representative.
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