Our years in the material testing industry have shown the importance of equipping test machines with the right grips and fixtures because a machine can only be used to its full capacity if the right grips or fixtures are used. With all the options available, choosing the right accessories for your testing application can be more complicated than you think.
Grips and fixtures are primarily categorized by two important factors: the type of testing that they will be used for and their capacity. It is important to avoid overloading the grips as this may damage not only your grips but also the load cell and the machine frame. This post will focus on grips used for tensile testing and guide you through the steps in determining the right tensile grips for your testing.
Common issues experienced with grips
The three most common issues experienced with universal testing machine grips are
Specimen slipping from the grip faces
Specimen breaking at the grip faces
Not being able to firmly grip samples due to their non-standard shapes or dimensions
Specimen slippage may occur not only because of the material characteristics of your samples, but also due to various reasons related to the grips chosen:
The clamping force not high enough to grip the samples
To achieve higher clamping forces, we recommend switching to pneumatic grips. Almost every ADMET grip is offered with both the manual and the pneumatic clamping mechanism.
The surface area of the grip jaw faces might not be covering a large enough surface area to hold the samples
Most tensile testing standards recommend gripping at least 3/4th of the specimen surface area to ensure that the specimen is gripped tightly and is ready to undergo a tensile test. ADMET grip jaws come with various sizes to choose from.
The jaw surface might not be the correct type for your sample material
In order to select the most appropriate jaws, please discuss the specifics of your samples and testing application with our Sales Engineers.
Specimen breaking at the grip faces
Certain specimens may not slip from the grips but rather break right at the grip faces. It is important to observe the sample behavior during the tests and note how the specimen breaks. If the break is at the line of the jaw faces, the recorded breaking strength will not accurately represent the actual breaking strength. There are a few ways to deal with specimen breakage:
If manual vise grips are used, make sure to check that the manual clamping force on the specimen is not too high.
If pneumatic grips are used, then adjust the testing pressure to find the ideal clamping force that would be sufficient to avoid slippage yet that wouldn’t be excessive and result in specimen break at grip faces.
Another reason for specimen break at grips could be due to the serrations of serrated jaws damaging the specimen. In this case, we would recommend switching to another jaw type or going with a custom-made serrated jaw set.
If manual vise or pneumatic grip solutions do not work, we recommend utilizing grips with different designs such as webbing or eccentric roller grips. These grips are designed so that there is no contact between the sample and the square grip jaws.
Difficult or awkward sized/shaped samples
Custom Luer Lock test fixture
What if your samples are too large or too small and have a non-standard shape that it’s almost impossible to try to imagine finding the right grips? This could also be an issue if you’re interested in testing the end product, not just the material the end product is made of. Most often, the solution is looking into the non-standard grips that are not always listed in product catalogs or having a custom grip engineered for your testing needs.
One example is a fixture that is used to hold needles. A quick solution that our engineers have come up with is the Luer lock to adapter fixture, as shown in the picture to the right.
Five Things to Know Before Buying Tensile Testing Grips
With those items in mind, you can avoid the most common issues with tensile grips by being aware of five key factors in selecting the right grips for your application.
The most common tensile grips are vise grips. ADMET offers vise grips with one or two t-handles.
Figure 1 – Manual Vise Grip diagram
Figure 2 – Specimen centering with wedge grips
Certain tensile grips may be limited to a specific capacity or limited to the opening width of the jaws due to the specifications of their design. For example, manual vise grips can go up to 50kN. Pneumatic grips can test samples up to 30kN, but as the capacity increases, the opening width of the grip jaws decreases. Higher capacity samples such as metals are often tested with wedge grips. Wedge grips are often used with ASTM E8 metals testing and come with optional alignment tools to ensure axiality of the applied loads.
Rope, thread, capstan, and webbing grips come with custom designs to specifically test certain materials such as cords, wires, ribbons, and yarns.
Figure 3 – Webbing grip diagram
Figure 4 – Rope tensile grips
Figure 5 – Pneumatic capstan thread grips for tensile testing
Figure 6 – Scissor grips
Figure 7 – Hook grips
Scissor grips offer a large opening width with wide jaws and thus can be used with samples that have non-standard shapes, such as plastic belts.
Hook grips, loop testing grips, and o-ring tensile testing fixture are examples of non-standard solutions that can be a better option for your materials. Please refer to the What Kind of Tensile Grips Should I Buy? post here for more information.
Figure 8 – O-Ring tensile grips
Item #2 – Grip faces (Jaws)
The same grip can be used to test various samples simply by changing the grip face type. It is difficult to recommend a grip face without knowing the specifications of the sample material. Certain ASTM standards will include a recommended grip face but most will leave it up to the operator to decide based on the specimen material characteristics and dimensions.
Blank, smooth jaws are steel faced jaw surfaces without any coating. They are often used with film and foils.
Rubber jaws have rubber coatings and are recommended for biomaterials and paper specimen.
Serrated or pyramid jaws are nickel-plated jaws with pyramid surfaces. They are recommended for ASTM D412 rubber testing, ASTM D638 plastics testing, and ASTM E8 metals testing.
Diamond jaw surfaces resemble a sand paper-like surface due to their synthetic diamond coating.
Wave jaws are often used with flexible materials that are held in place between the 5mm waves.
V-jaws are used with round samples. Please make sure to check the diameter specification for each v-jaw before making your final decision. In addition, v-jaws may not always be the most appropriate for hollow tube samples. Tube samples may be tested with flat jaws if a pin is inserted through them.
Line contact jaws are made with a smooth, nickel-plated surface and a 3mm line in the middle of the jaw. Line jaws provide a better grip for plastic films and sheet materials.
ADMET also offers a quick change system for all the jaw surface types mentioned above. Users interested in a faster way to change from one jaw type to another can use the jaws with quick change carriers.
Figure 9 – Quick change grip faces
Item #3 – Grip capacity
Tensile grips are used to obtain results on the tensile properties of your samples such as tensile strength, peak load, elongation, tensile modulus, and yield.
Different materials show different material characteristics. Before selecting your grips, make sure you are confident of the maximum force that will be required to test your samples.
Hydraulic grips are the highest capacity grips and can go up to 3,000kN capacity.
Wedge grips also work with materials that require high capacities such as metals and composites and can go up to 500kN.
Each material is first prepared for testing in a specific geometry that is typically outlined in the testing standards. Common specimen geometries include flat dog-bone or cylindrical geometries such as tubes and bars.
Sample preparation may be done by machining or die cutting. ADMET offers dies conforming to specific testing standards. Sample preparation is important and usually specific. One example is ASTM D638, which describes five different specimen dimensions that need to be chosen based on the characteristics of the material to be tested.
Figure 10 – ASTM D638 die specifications
Quite often, hard specimens such as metals, composites, and certain plastics are difficult to hold properly as premature breakage and slippage can be common. As a result, grips with high clamping forces, such as hydraulic grips and wedge grips are recommended. Conversely, rubbers and elastomers are more easily held in a variety of grip designs such as manual vise grips, pneumatic grips, wedge grips, or eccentric roller designs. The most common and easy-to-use grips are manual vise or pneumatic grips with rubber coated or serrated jaw faces.
Figure 11 – ASTM C297 custom test fixture
In addition to prescribing the test method, certain testing standards specify a fixture design to be made specifically for testing in conformance with that standard. Examples of tensile fixtures designed per specific standards include ASTM C297, ASTM D1414, and ASTM D3039.
Item #5 – Adapting/mounting grips to testing machines