ADMET has many years of experience in the material testing industry and this has shown the significance of equipping test machines with the exact grips and fixtures.
Image Credit: Admet Inc.
A machine can only be exploited to its full capacity if the right fixtures or grips are used. With all the available options, selecting the correct accessories for a testing application can be more complicated than one thinks.
Grips and fixtures are mainly categorized by two key factors: the kind of testing that they will be used for and their capacity. Overloading the grips may damage not only the grips but also the machine frame and the load cell, so it is very important to avoid overloading. This article will focus on grips employed for tensile testing and guide customers through the steps involved in determining the appropriate tensile grips for their testing.
Specimen slippage from the grip faces may occur not only due the material characteristics of samples, but also due to several reasons associated with the grips selected:
Instead of slipping from the grips, certain specimens may break right at the grip faces. It is necessary to monitor the behavior of the sample during the tests and observe how the specimen breaks. The recorded breaking strength will not accurately represent the actual breaking strength if the break is at the line of the jaw faces. There are a few ways to handle specimen breakage:
What if the samples are too small or too large and have a non-standard shape that it is almost impossible to find the right grips? This might also be an issue if customers are interested in testing the end product, not just the material with which the end product is made. Very often, the solution is to have a custom grip engineered to suit the customers’ testing requirements or look into non-standard grips which are not always listed in product catalogs.
One example is a fixture that is used for holding needles. The ADMET’s Engineers have developed the Luer lock to adapter fixture, as shown in the picture below, for a quick solution.
Custom Luer Lock test fixture. Image Credit: Admet Inc.
Keeping those items in mind, users can avoid the most common issues associated with tensile grips by being conscious of five key factors in choosing the correct grips for their application.
Tensile testing grips include self-tightening grips, pinching, webbing grips, rope and thread grips, hydraulic grips, wedge grips, pneumatic grips and manual vise grips. The selection of the most suitable tensile grips to effectively secure the samples is essential in obtaining accurate measurements of tensile properties.
Vise grips are the most common tensile grips and ADMET offers these grips with one or two t-handles.
Figure 1. Manual Vise Grip diagram. Image Credit: Admet Inc.
Particular tensile grips may be limited to the opening width of the jaws or limited to a specific capacity due to their design specifications. For instance, manual vise grips can test samples up to 50 kN. Pneumatic grips can go up to 30 kN, but when the capacity increases, the opening width of the grip jaws decreases.
Wedge grips are often used to test higher capacity samples such as metals. They are also used with ASTM E8 metals testing and supplied with optional alignment tools in order to ensure axiality of the applied loads.
Figure 2. Specimen centering with wedge grips. Image Credit: Admet Inc.
Rope, thread, capstan and webbing grips come with custom designs to specifically test specific materials such as yarns, ribbons, wires and cords.
Figure 3. Webbing grip diagram. Image Credit: Admet Inc.
Figure 4. Rope tensile grips.Image Credit: Admet Inc.
Figure 5. Pneumatic capstan thread grips for tensile testing. Image Credit: Admet Inc.
Scissor grips provide a large opening width together with wide jaws, and hence they can be used with samples with non-standard shapes, such as plastic belts.
Figure 6. Scissor grips. Image Credit: Admet Inc.
O-ring tensile testing fixture, loop testing grips and hook grips are examples of non-standard solutions that can be a better choice for customers’ materials.
Figure 7. Hook grips. Image Credit: Admet Inc.
Figure 8. O-Ring tensile grips. Image Credit: Admet Inc.
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The same grip can be employed for testing several samples just by changing the grip face type. Without knowing the specifications of the sample material, it would be difficult to recommend a grip face. Certain ASTM standards will contain a recommended grip face, but most will leave it to the user to decide based on the dimensions and characteristics of the specimen material.
Grip Face DescriptionIn addition, ADMET provides a quick change system for all types of jaw surface mentioned above. Users who are interested in quickly changing from one jaw type to another can use the jaws with quick change carriers.
Figure 9. Quick change grip faces. Image Credit: Admet Inc.
Tensile grips are employed to achieve results on the tensile properties of customers’ samples such as yield, tensile modulus, elongation, peak load and tensile strength.
Different materials demonstrate different material characteristics. Before selecting grips, customers must be confident of the maximum force that will be needed to test their samples.
The end product and its internal component materials with varying features can be tested for tensile properties using a universal testing machine.
Common materials tested in tension include textiles, paper, metals, plastics and elastomers, composites, biomaterials and adhesives. At first, each material is prepared for testing in a particular geometry that is typically defined in the testing standards. Common specimen geometries include cylindrical or flat dog-bone geometries such as bars and tubes.
Sample preparation may be performed by die cutting or machining. ADMET provides dies conforming to particular testing standards. Sample preparation is essential and usually specific. One example is ASTM D638, which explains five different specimen dimensions that have to be selected based on the characteristics of the material that needs to be tested.
Figure 10. ASTM D638 die specifications. Image Credit: Admet Inc.
Very often, hard specimens such as composites, metals and certain plastics are difficult to hold properly as slippage and premature breakage can be common. Therefore, grips with high clamping forces, such as wedge grips and hydraulic grips are recommended.
On the contrary, elastomers and rubbers are more easily held in a range of grip designs such as wedge grips, pneumatic grips, manual vise grips or eccentric roller designs. The easy-to-use and most common grips are pneumatic or manual vise grips with rubber coated or serrated jaw faces.
In addition to recommending the test method, particular testing standards indicate a fixture design to be made specifically for testing in accordance with that standard. Examples of tensile fixtures that are designed to specific standards include ASTM D, ASTM D and ASTM C297.
Figure 11. ASTM C297 custom test fixture. Image Credit: Admet Inc.
The final item on the list is attaching the grips on the testing machine. Since grips are not linked to the electronics of customers’ universal testing machine, they can be used together with any make and model. Usually, ADMET tensile testing grips are mounted on Tinius Olsen, MTS and Instron frames using adapters.
Adapters can be provided with customers’ grips of choice of if customers would like to machine them in-house, the company’s Sales Engineers can provide drawings to match the precise dimensions required.
Figure 12. Male end adapter for mounting grips. Image Credit: Admet Inc.
To determine the best suited grips for tests, customers can let ADMET’s Sales Engineers know the maximum force capacities required to test their samples, shapes, dimensions, sample material and elongation properties. The Engineers will then discuss the options with the customers, and, if required, the team can even test their samples to select the right grips for their testing requirements.
This information has been sourced, reviewed and adapted from materials provided by Admet, Inc. - Materials Testing Equipment.
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