Impact Test: Definition, Types, Benefits, and Standards
Impact tests measure the ability of a material to resist deformation in response to a sudden load. These tests are normally conducted according to test methods and standards published by ASTM International. Four commonly used types of impact tests include: Charpy, Izod, drop-weight, and dynamic tear tests. The benefits of conducting impact testing on materials include: enabling engineers and manufacturers to determine a material’s toughness, establishing quality control standards, optimizing designs, and picking appropriate materials for particular applications. While impact tests are very helpful in characterizing material behavior and conducting material selection, impact tests do not account for cyclical loads, external factors like temperature or humidity, and real-world applications where parts experience loads in all directions.
Overall, impact testing plays a vital role in science and industry, allowing engineers and researchers to make informed decisions regarding material selection, design, and safety. This article will review what impact testing is, and discuss its types, benefits, and the standards used to perform the tests and analyze the results.
An impact test is a technique used to determine a material’s ability to resist deformation when subjected to a sudden shock or impulse load. There are several different types of impact tests, but all entail striking a prepared test specimen with a weight. Different materials testing standards, such as ASTM E23, ASTM A370, and ASTM D256 govern the exact testing procedure and test specimen requirements for each type of impact test, and for different material groups (e.g., metals vs. plastics). While an impact test can help characterize a material’s ability to withstand sudden loads, the impact test does not account for real-world loading applications or cyclical loads. Still, the test enables the easy comparison of impact strengths between materials, so long as the materials were tested with the same standard.
An impact test works by striking a properly prepared and fixtures test specimen with a weight, either from the side or from above. For Charpy and IZOD impact tests, a pendulum with a weighted hammer is released from a specific height. The arc of its motion strikes the vertically oriented test specimen on its side. For drop-weight impact tests and dynamic tear tests, a weight is guided by rails and dropped directly onto a test specimen from above. For each type of impact test, a notch is cut into the test specimen, forcing the fracture of the specimen to occur at a repeatable location.
Impact tests are performed so that engineers, manufacturers, scientists, and researchers can characterize how a material behaves when subjected to a sudden load. This allows engineers and designers to optimize product designs for particular applications. It also helps manufacturers to establish quality control standards for their products and enables scientists and researchers to characterize a material’s toughness. Each of the items described facilitates the creation of products and structures that are safer for the end user.
Test specimen preparation is an important key to generating accurate impact test results. The test specimen for each type of impact testing requires a notch to be cut in the sample. However, the particular type of notch depends on the type of impact test to be conducted. Each impact test specimen has a V-shaped notch cut into it. Notches are cut into test specimens because it forces specimens to break if the force imposed by the impact is strong enough. As is the case with Charpy and Izod tests, the impact strength of a part can be determined since the energy absorbed by the test specimen can be related to the potential and kinetic energy of the pendulum. For drop-weight and dynamic tear tests, the breakage or deformation of the part is used as a pass-fail criterion.
Four types of impact testing are described in detail below, encompassing the pendulum, drop-weight, and dynamic tear methods:
The Charpy impact test, also known as the V-notch test, is a type of impact test where a weighted pendulum hammer is released from a specified height and strikes the part. A Charpy impact testing apparatus, a device with a pendulum with various locking points at specified heights and a fixture to hold the test specimen, is used to determine Charpy impact strength.
The standard test specimen is described in ASTM E23. It has a V-shaped notch cut into it. It is oriented so that it is simply supported on both ends horizontally and so the hammer strikes the face opposite the notched surface. The difference between the initial height of the pendulum and the height it reaches during the pendulum’s continued travel after striking the test specimen is used to calculate the energy absorbed by the material on impact. This is because it is assumed that all the potential energy before the pendulum is released is converted into kinetic energy when it strikes the specimen.
The Charpy impact test is most commonly used for ductile materials such as metals and thermoplastics. The test can be conducted at different temperatures and is often used to determine the ductile-to-brittle transition temperature of a material.
The Izod impact test is similar to the Charpy test in that a weighted pendulum hammer strikes a test specimen containing a V-shaped notch. An Izod impact testing apparatus — which is essentially identical to a Charpy impact testing machine — is used to determine Izod impact strength. The primary differences between the Izod and Charpy impact tests are the size of the test specimen, how it is restrained, and which side is struck by the pendulum hammer. The test specimen in the Izod test is fixed vertically on only one end, and the surface facing the notched surface is the surface struck by the hammer, in contrast to the Charpy test, for which the specimen is fixed at both ends and is struck on the side away from the notched surface. The test procedure for the Izod test is otherwise identical to the Charpy test. The test specimen is first prepared and mounted into the machine. Then, the pendulum is raised to a specified height and released to strike the upper tip of the specimen.
The Izod test, governed by ASTM D256, is most commonly used for thermoplastics. However, it can also be used for metals. Like the Charpy test, the Izod test is used to determine a material’s toughness and its ductile-to-brittle transition temperature.
The drop-weight impact test, also known as the Pellini test, uses a weight suspended over a simply supported horizontal test specimen and then dropped to produce the impact. A tube or rails guide the weight during its “free-fall” onto the specimen. Unlike Charpy and Izod tests, the height of the weight before and after it strikes the test specimen cannot be used to determine the energy absorbed by the test specimen. Instead, results only pass or fail since energy absorbed by the test specimen cannot be adequately determined. Fracture is not the only criterion for failure, deformation or the formation of a crack can also be considered a failure. The test specimen in a drop-weight impact test is notched, and the weight is dropped on the surface opposite that of the notch. Like the other impact tests previously described, the drop-weight impact test is frequently used to test for a material’s ductile-to-brittle transition temperature.
The dynamic tear test is similar to the drop-weight impact test. In the dynamic tear test, a notched test specimen is simply supported on both ends. A weight is dropped on the face opposite the notched side, and subjecting the test specimen to a bending impact load and 3-point bending. The primary difference between drop-weight impact testing and dynamic tear testing is that dynamic tear testing is often used for test specimens with a thickness less than 5/8” while drop-weight impact testing is for test specimens thicker than 5/8”.
The benefits of performing impact testing include:
- Useful for determining a material’s toughness and ability to resist deformation due to impact loads, as well as for determining a material’s ductile-to-brittle transition temperature. These properties can help engineers and designers pick appropriate materials for a particular application and design more effective and safe products for the end user.
- Serves as a way for manufacturers to maintain control of their products’ quality. The manufacturing of some parts that require a specific level of toughness, such as ship hull components or pressure vessels, benefits greatly from impact testing. Impact testing helps ensure that these components are made to the required specifications and are safe for use.
- Guides the design and optimization of components and structures. To obtain reliable data to optimize designs, complete parts must be used. Drop-weight tests are the preferred test method when it comes to design optimization. With drop-weight impact test data, engineers can make appropriate adjustments to material properties and design geometries to enhance product performance.
- Many different industries have stringent requirements for the materials that are used in their products. For instance, the automotive industry uses standards like ISO 8256 and ASTM D3763, as well as company internal standards, as criteria to establish impact strength requirements. Impact testing helps manufacturers maintain compliance with these standards and requirements.
Impact testing also has its downsides. Some challenges of impact testing are listed below:
- It is limited in its ability to characterize a material’s behavior in its actual service environment. Impact testing cannot account for real-world impacts that may vary in speed, impact force, angle of impact, chemical exposure, corrosion effects, and more. Therefore, impact testing should be used in conjunction with other material testing procedures that are carefully designed simulations of the real-world application to obtain a more comprehensive understanding of a material’s properties and behavior.
- Interpreting results from an impact test can be difficult since determining a material’s response to impact can be complex. For instance, results can be difficult to interpret for impact tests on brittle materials like ceramics and some composites. Expertise and an understanding of how a material behaves in dynamic conditions are essential to obtaining reliable impact test results.
- Results are highly sensitive to deviations from the established testing parameters. All impact test trials must be conducted in a consistent manner to obtain reliable impact testing data. Small changes in initial drop height, test specimen preparation, and test specimen orientation can drastically impact testing results. Test machine features like locking pins at fixed heights that allow tests to be conducted from the same height, specimens produced by CNC machining, and jigs and fixtures that help to orient the specimen the same way each time are all ways to ensure impact tests are conducted with the same parameters each time.
Some of the controlling standards establishing reliable impact testing methods are listed below:
- ASTM E23 / ISO 148-1: These standards govern impact testing for metals.
- ASTM D256 / ISO 180: These standards govern impact testing for plastics.
- ASTM A370 / ASTM E208: This standard governs impact testing for steel materials.
For more information, see our guide on Impact Strength.
All types of materials can be impact tested, including: metals, thermoplastics, thermosets, and composites. While certain types of impact testing are intended specifically for use with metals or plastics, any type of impact test can be used for any material.
Yes, Charpy is the most commonly used impact testing procedure due to its relatively easy test preparation, modestly priced test equipment, simple test procedure, and reliable data production.
Yes, there is a passing rate for impact testing but the particular “rate” often depends on a manufacturer’s specific requirements.
This article presented impact tests, explained them, and discussed the various types and benefits. To learn more about impact tests, contact a Xometry representative.
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