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5 Things to Know when Selecting Grips

For the full blog post, click here: http://www.admet.com/5-things-know-buying-tensile-testing-grips/

  1. Item #1 – Different types of grip design

Tensile testing grips include manual vise gripspneumatic gripswedge gripshydraulic gripsrope and thread gripswebbing gripspinching, and self-tightening grips. Choosing the most appropriate tensile grips to effectively secure your samples is critical in getting accurate measurements of tensile properties.

The most common tensile grips are vise grips. ADMET offers vise grips with one or two t-handles.

 

manual vise grip for tensile testing

Figure 1 – Manual Vise Grip diagram

wedge grip tensile testing

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.

 

webbing grip for tensile testing

Figure 3 – Webbing grip diagram

rope grips for tensile testing

Figure 4 – Rope tensile grips

pneumatic thread grips for tensile testing

Figure 5 – Pneumatic capstan thread grips for tensile testing

scissor grips for tensile testing

Figure 6 – Scissor grips

hook grips for tensile testing

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.

o-ring tensile testing grips

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. 

ADADMET 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.

quick change grip faces for tensile testing

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.
  • Manual vise grips are versatile tensile grips and can go up to 50kN.
  • Pneumatic grips are used with rubber and plastic samples as they can only go up to 30kN.
  • Pinching grips are used with low-capacity samples such as electronic components and biomaterials. The maximum force that pinching grips can go up to is 2kN.
  • Webbing grip capacities range from 5kN to 200kN. As the capacity of the webbing grip increases, the roll diameter and the accommodated sample width increase as well.
  • Rope grips can be used up to 50kN.
  • Thread grips, which are often used with wires, cords, yarns, and fishnets, are offered in capacities from 200N to 5kN.

Item #4 – Specimen material and geometry and their impact

The end product and, more often, its internal component materials with varying characteristics, can be tested for tensile properties with a universal testing machine. Common materials tested in tension include adhesivesbiomaterialscompositesplastics and elastomersmetalspaper, and textiles.

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.

ASTM D638 tensile testing die specifications

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.

astm c297 custom tensile testing fixture

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 C297ASTM D1414, and ASTM D3039.

Item #5 – Adapting/mounting grips to testing machinesmale end adapter for tensile testing grips

Figure 12 – Male end adapter for mounting grips

The final item on the list is attaching the grips on your testing machine. Since grips are not connected to the electronics of your universal testing machine, they can be used with any make and model. ADMET tensile testing grips are often mounted on Instron, MTS, and Tinius Olsen frames using adapters.

Adapters can be supplied with your grips of choice or if you would like to machine them in-house, our Sales Engineers can send drawings to match the exact dimensions required.

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