Introduction

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

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

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.

Item #1 – Different types of grip design

Tensile testing grips include manual vise grips, pneumatic grips, wedge grips, hydraulic grips, rope and thread grips, webbing grips, pinching, 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