Tensile Testing Machine: Definition, How It Works, Types, Components, and Examples
Learn more about these machines and the types of materials that are tested with it.
A tensile test is a destructive testing method carried out with the use of a tensile testing machine. A tensile strength tester works by holding a specimen in a pair of grips and then applying an increasing tensile load until the specimen breaks. If the original cross-section of the specimen is known as well as the rate of elongation (the rate of change in length), then the tensile strength can be calculated. Tensile testing can be performed using an electromechanical or hydraulic universal test machine. The machine consists of specimen grips to hold the test piece, a crosshead to move and apply loading, and a data acquisition system to record the test parameters. This article will further discuss tensile testing machines, how they work, the types and components, and provide examples.
A tensile testing machine is a device that measures the mechanical properties of a material. A tensile test machine uses either an electromechanical or hydraulic system to apply a tensile load to a test piece until failure. During the test, the tensile strength machine will record the rate of elongation, change in length, and the load applied which can be used to determine the material properties of the specimen.
A tensile tester works by gripping a specimen of a known cross-section and then pulling it apart at a predetermined speed until failure. The tensile test result can be interpreted to find the tensile strength, elongation, and stiffness. Many tensile testers can also perform compression. For a compressive test, the setup is the same but acts in reverse and uses different grips to hold the material.
Tensile test machines are used to determine the mechanical properties of a material including: yield strength, ultimate tensile strength, and ductility. The uses for tensile test machines are listed below:
- Material selection for engineering applications.
- Material quality assurance.
- Comparison of properties of new materials or processes.
- To identify material behavior under load.
It is important to use a tensile test machine to determine a material's properties and behaviors. Tensile tests must be carried out to determine mechanical properties so that the right materials can be selected for the application.
Tensile strength testing is important because it can be used to understand how a material acts. The tensile test is fundamental to understanding the basic mechanical properties of a material. Manufacturers must fully understand the mechanical properties of a material when selecting it for applications. For more information, see our guide on Tensile Strength.
It is important to use tensile testing for 3D printed materials to determine the material properties of the part. Since parts made from 3D printing have anisotropic characteristics, they require more extensive testing to be fully understood. For 3D printed materials, many tensile tests will be required to fully understand the material properties.
There are two main types of tensile testing machines as listed below:
A hydraulic tensile test machine is a device that uses a hydraulic system to apply a tensile load to a test specimen to determine its properties. Hydraulic tensile testers are based on a single or dual-acting piston that uses Pascal's law to apply a large force to the specimen. With a hydraulic servo system, an electronic servo valve is used to adjust the rate of hydraulic fluid movement. This adjustment controls the rate of loading. Hydraulic test machines are designed to test metals and concrete.
An electromechanical tensile tester is a device that uses a variable-speed motor to apply the required tensile load for a test. The machine works by using a variable-speed electric motor that connects to the 1–4 screws used to move the crosshead via a reduction gearbox. A microprocessor is used to control the variable-speed motor and adjust the speed of the crosshead. Electromechanical test machines are designed to test: polymers, elastomers, composites, and textiles.
A tensile test machine is a metal load frame that uses a pair of specimen grips to pull apart a test piece using a moving and a static crosshead. Figure 1 below illustrates the parts of a tensile test machine:
Tensile testing machine.
Image Credit: https://www.zwickroell.com/
The parts of a tensile test machine are:
- A load cell
- Specimen Grips
- Moving Crosshead
- Load Frame
- Data Acquisition System
In product design, tensile tests can be used to test the fundamental mechanical properties of a material to be used in a product. Whenever a new material is going to be used in a product, whether as part of a new or existing product, a tensile strength test needs to be carried out to ensure that the material is fit for purpose. However, sometimes the material will stay the same but the application will change. This will change the operating environment of the part which will also require new testing to certify that the part will work.
To perform a tensile test, an operator will load a specimen into the two specimen grips of the machine. After loading the specimen, the operator selects the correct rate of movement for the material type. The tester then pulls the specimen apart until failure. The extensometer will record the distance moved and plot the movement on a graph electronically. Once completed, the graphic results are interpreted to understand the properties and behavior of the material.
To prepare a material ready for testing, the material is shaped to resemble a “dumbbell” or “dog bone.” Preparing the material in this way gives the grips a pair of “shoulders” to hold onto to pull apart the specimen. Shaping the specimen in a “dumbbell” shape guarantees that the sample will fail in the narrow section. Figure 2 below shows how the specimen should be shaped:
Proper specimen shape for tensile testing.
Image Credit: Shutterstock.com/Sanphat Chaisong
The kinds of materials that can be tested are listed below:
Ceramics are non-metallic, inorganic solids that are usually brittle. Ceramics are tested using the same process as metals which use a “dog bone” shaped sample. However, ceramic tensile testing is hard and rare due to the brittleness of most ceramics. The brittleness of ceramics means it is hard to shape specimens, grip them, or align load stresses properly without breaking them before the test is conducted. While the most commonly used ceramic material, concrete, has a very low tensile strength in the range of 2-5 MPa, there are also advanced ceramics including alumina and zirconia which can both have a tensile strengths up to 1138 MPa and 551 MPa respectively.
Textile refers to any material that is fibrous. This includes yarns and woven fabrics. Materials can be woven, non-woven, or in films or sheets. Textiles can be tested wet or dry and use a variety of ASTM standards depending on the material and conditions used. These materials are tested to find their tear resistance. Textiles are tested before being incorporated into a composite such as a fiber-reinforced polymer or elastomer. Flax can experience a tensile strength of 280 MPa and nylon has a strength of 60–90 MPa.
Metals are materials that have metallic atomic bonding, conduct electricity, and heat well. Metals are the simplest materials to test as samples are easy to form into dog bone test specimens. These are commonly tested to understand their properties for use in structural components. Metals are tested for use in almost every industry including: automotive, aeronautical, marine, medical, and construction. The only challenge to testing metals is some high-strength steels will induce deflection in the testing machine causing inaccuracies. The tensile strength of copper is approximately 220 MPa whereas a titanium alloy can be 900 MPa.
Polymers are synthetic materials that are made up of a monomer chain. They have wide-ranging properties—some are extremely elastic and some are extremely brittle. Some brittle plastics do not experience a significant yield point at which the material changes from elastic deformation to plastic deformation. For that reason, a predetermined strain is used—that is under 0.5%—to determine the yield strength of these materials. Polymers are tested to understand their use in polymer composites. They range in tensile strength but are generally fairly weak. For example, ABS plastic has a tensile strength of 40 MPa.
Composites are two materials that when joined together create a material with more desirable properties. These usually refer to fiber-reinforced plastic. Composites can be tested in a number of conditions as either: a single filament, a strand of filaments, a unidirectional laminate, a multidirectional laminate, a pair of bolted laminates, or bonded laminates. Since composites are anisotropic they can be tested to determine their properties in different orientations. Composites are tested for use in applications including aerospace and high-performance automotive applications. Standard carbon fiber can benefit from an ultimate tensile strength of 600 MPa.
Elastomers have a very similar composition as polymers except they have fewer cross-linking bonds which makes them less stiff and more elastic. Elastomers are very sensitive to the chosen rate of elongation and the heat at which the test is carried out. They have extensive use as tires and latex products. Polyisoprene (hard rubber) has an ultimate tensile strength of around 39 MPa which is lower than most plastics.
No, tensile test machines are largely made out of aluminum and steel as these materials are strong and stiff which minimizes deflection during testing.
No, tensile testing machines cannot be used to determine material failure causes. Tensile testing machines can be used to simulate the failure of specimens under a tensile load. But this cannot determine the cause of an already failed part. Tensile testers could be used to aid a failure analysis to show how materials behave, but this will not definitively show what caused a material to fail.
Yes, biomaterials can be tested using tensile testing machines. For a medical device to be properly qualified for use, the biomaterial it is replacing must be understood fully. Bones, bone tissue, and ligaments are all biomaterials that are tested in a tensile test machine.
The benefit of using a tensile test machine is the ability to understand the properties and behaviors of a range of materials. Other benefits of using tensile testing include the ability to:
- Test the quality of a material.
- Identify material defects.
- Identify design defects.
- Find new materials to use for a product.
- Qualify a part or material for a new application.
The biggest limitation of tensile testing is that it is a destructive test method, meaning the part tested cannot be used. The other limitations of tensile testing include:
- There is a limitation on the size of the part to be tested (it cannot be too small or too big).
- It is time-consuming to prepare specimens to be tested.
- Requires training and retention of skilled staff.
- It is an expensive system to buy and maintain.
- Possibly requires specialist tooling to grip specimens.
Tensile testing machines are an accurate form of testing. Most modern tensile test machines are built to conform to either ISO 6892-1 or ASTM E8. To meet these standards, manufactured machines must have an accuracy of plus or minus 1% of the tested measurement. For example, if the test measured a 1-inch elongation, the true elongation would be between 0.99 inches and 1.01 inches.
A tensile testing machine can cost anywhere from $12,900–$150,000. The price will vary depending on factors such as: the force the machine is able to produce, the space it takes up, and the controller used. The force produced is the most relevant factor in relation to cost. For forces from 1–25 kN, a machine costs $12,900–$30,000. For forces of 50–150 kN, a machine costs $32,000–$57,000. Finally, for a machine that produces forces of 300–2,000 kN, it costs $77,000–$150,000.
This article presented tensile testing machines, explained what it is, and discussed its importance and different types of machines. To learn more about tensile testing machines, contact a Xometry representative.
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