Jaw Selection for Tensile Grips

Tensile grip jaw selection plays an important role for successful mechanical tests as if specimen slippage between the jaws occurs or if the specimen breaks or tears in the jaw area testing may need to be discarded. This post covers the available jaw surface types for tensile grips and also the operation of quick-change type jaws.

Introduction

Manual vise and pneumatic vise grips are the most standard grips used to clamp specimen in tensile testing applications. They can also be used in 180-degree peel testing, or in different test setups where one end of the specimen is held stationary inserted in a vise grip while the other end is pulled by another grip or fixture. On the other hand, wedge and hydraulic tensile grips are used in testing applications where higher capacity materials such as metals are tested.

Tensile Wedge and Eccentric Roller Grips and Jaws

See the post below for further information on tensile grip selection.

A variety of tensile testing grips and fixtures offered for different applications are supplied with a set of jaws, or inserts, that are in direct contact with the specimen. If the wrong jaw surface type is purchased, samples may slip from the grips even if the capacity of the grip matches the required testing capacity.

There should be adequate pressure between the gripping surfaces in order to:

• Clamp the specimen firmly prior to starting testing
• Prevent slippage during the progress of the test.

Graph below from ASTM E8 Metal Tensile Testing illustrates the behavior that can occur if the specimen slips in the grips and disturbs the extensometer as the force is applied.

Behavior that can occur if the specimen slips in the grips and disturbs the extensometer as the force is applied (ASTM E8)

Jaw Surfaces

The selection of grip jaw surface depends on the material type tested as well as its geometry. In addition, standardized test methods, such as ASTM or ISO standards, may recommend a specific type of grip jaw surface based on the specimen. Below is a list of standard jaw surfaces offered with tensile testing grips.

Plain (Blank) Jaws

Plain jaws have no coating and can be used with thin samples that would otherwise get damaged with coated jaws. They can also be chosen if further custom treatment needs to be done. Plain jaws can be lined with crocus cloth, emery cloth, or pressure sensitive tape.

Serrated (Pyramid) Jaws

Serrated jaws have serrations that help clamp the specimen. They are recommended for specimens such as metals and plastic. Serrated jaws are recommended for tensile testing using wedge grips if flat specimens are tested.

Rubber Jaws

Rubber jaws have a rubber coating for a softer clamping surface.

Note that the opening specifications of grips with rubber-coated jaws decreases by a few millimeters due to the thickness of the rubber coating on the jaw surface.

Diamond-coated Jaws

Diamond-coated jaws have a sand-paper like surface. Soft metals as well as paper and board specimens may be tested using diamond-coated jaws. Rope and thread grips that are also used to test yarn, filament, and narrow ribbons are often supplied with diamond-coated jaws.

Wave Jaws

Wave jaws are used with flexible specimen that require larger surface contact area a such as loose textiles and fabric.

Line-contact Jaws

Line-contact jaws come with a 3mm-radius single line mid-point of the jaws and deliver a better grip for sheet materials and plastic films.

V-Jaws

V-jaws are used with round samples. The round specimen sits on the tooth notch of the jaw. V-jaw specifications often include the range of specimen diameters that the v-jaw set can accommodate.

Note that not all round specimen require grips with v-jaws. Tubular hollow specimen may be tested with flat jaws by inserting snug-fitting metal plugs into the ends.

Quick-Change Mechanism

For customers testing a variety of specimen that require different jaw surface types, manually switching jaws can be time consuming. ADMET 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 quick-change jaws with quick change carriers. This mechanism is only available for manual and pneumatic vise grip jaws. Video below shows quick-change insert operation.

Different jaw surface types can be inserted into the quick-change carrier, allowing the operator to switch different jaws quickly instead of manually screwing in each jaw on the tensile grip.

Equipment

ADMET grips and fixtures are first categorized by the force capacity. See the list below for tensile grips with datasheets that specify available jaw surface types and dimensions.