Tear Strength: Definition, Relation to 3D Printing and More

What Is Tear Strength?

Tear strength is a material’s ability to resist failure perpendicular to the stress being applied. This is usually tested by measuring the force required to begin a tear, while that force is applied to an unrestrained area close to a clamp retaining the material edge.

In 3D printing, tear strength helps define structural strength advantages of particular methods of 3D print construction. This is particularly relevant to anisotropic construction methods such as FDM printing. Anisotropic materials have different properties depending on the mode and orientation of the material. This can be interpreted as a “grain” similar to wood, where forces along the grain (i.e., aligned with the build plane and the primary filament direction) are much better resisted than those which pull filaments apart from each other.

Tear strength values are measured in newtons per mm or pounds force per unit thickness (lbf/in or N/mm) of the material sample. Although, results from different test setups are not directly comparable because of methodological differences. Test rigs restrain a part of the sample edge in a fixed plane and clamp a “free” edge close to the restraint. This allows a load to be applied perpendicular to the sample to induce a tear. Some tests require samples to have a smooth edge, others use a specified V-cut nick as an initiator, and others use a specified straight cut for this purpose. The measured value is the force required to initiate tearing only. 

This article will discuss tear strength definition, its relation to 3D printing, its formula, and more.

Measuring Tear Strength and 3D Printing

Measuring tear strength relates to 3D printing in that understanding the tear resistance properties of the various 3D print technologies and the orientation of the part helps make prototypes that are useful for active force tests rather than simply for fit and shape. The selection of process, material type, and build orientation can be critical factors in determining the suitability of a 3D-printed part for the intended purpose.

Parts that must flex or resist flexing, suffer repeat cycles of loading, and undergo complex load scenarios must be made by a process that delivers the required performance. Where flexibility is required, a basic SLA part will tend to fracture immediately. An FDM-printed part, on the other hand, that has been built such that the filament direction lies along the line of bending (rather than across it or through it) will perform better. For more information, see our guide to 3D Printing.

How to Calculate Tear Strength

The formula for tear strength is:

Tear strength = F/t

Whether the tear is pre-initiated with a V nick or I cut, or the edge is smooth and undamaged, a force will be measured that initiates the tear. That force (in N or lbf) is then divided by the sample thickness (in mm or inches) to provide a standardized measure of force/unit thickness.

Tear strength is measured in newtons per mm or pound-force per inch  (N/mm or lbf/inch).

Testing Tear Strength: A How-To Guide

Testing tear strength is generally performed in a tensile test rig. A sample is clamped to the upper and lower jaws of the test machine. It is oriented such that the stress is applied as a tearing motion. The most common format uses the aptly named trouser test. The “legs” or a trouser shape are clamped in a plane and pulled as if worn during a splits posture. Failure occurs at the point where the “legs” of the trousers meet. This stretches or deforms the material edge into a displaced line (when loaded to less than the tear point). Force is then increased in steps until tearing commences at the stress concentrator.

Three forms of edge preparation are made: a V nick or I nick to act as initiator, or the edge is smooth and forms an undamaged curve where the “legs” of the sample meet. These tests are not necessarily comparable with each other. The initiator is likely to result in a much lower tear strength than from otherwise identical samples. The stress (force) concentration occurs at much lower strain (extension/displacement) levels.

Tear strength testing is therefore considered a qualitative test to demonstrate failure mode rather than a value comparative test to allow precise comparisons of various materials. It can be used for qualitative comparison of the force resilience of identically shaped/tested material samples. However, quantitative measures in test samples are likely to inform only qualitatively about failure risks in real-world conditions, where load applications are rarely as simplified/ideal as those in a test laboratory.

ASTM International Standards For Measuring the Tear Strength of Different Materials

ASTM D264 is the US test standard for tear resilience, and it specifies five sample types: A (crescent, razor nicked), B (Winkelmann), C (Graves), T (Trouser tear)/ASTM D470, and CP (Constrained path trouser tear). For test specimen types A, B, or C, the measured value of tear strength is simply force (to initiate tear) divided by sample thickness. For specimen types T or CP, the measured value is the average or median force applied at the curve divided by the specimen thickness.

ISO 34-1 is directly comparable in principle but differs in a number of key details that make the two test standards very hard to directly compare, even for identical materials.

Tear Strength of Common Materials

1. Tear Strength of Fabric

Fabrics are materials produced by weaving together materials such as wool, nylon, and cotton. Cotton fabrics vary widely in tear strength depending on the base material and the force applied.

2. Tear Strength of Rubber

The values for various rubber materials are: natural rubber (23.95 +/-1.85 kN/m), nitrile rubber (9.14 +/-1.54 kN/m), styrene-butadiene rubber (4.88 +/-0.47 kN/m), and EPDM rubber (7.27 +/-0.86 kN/m).

3. Tear Strength of Plastic

The tear strength values for plastics vary, depending on the elongation orientation, polymer property variations, and the wide availability of polymer types. Results are usually only of interest in film materials that will be subject to manufacture or use stress. Polymer films also tend to be tested using ASTM D1922, the Elmendorf tear strength test (results in grams). For example, a modal HDPE has an Elmendorf tear strength of 120g MD (machine direction) and 24g TD (transverse direction). LDPE, on the other hand, has an Elmendorf tear strength of 320g MD and 170g TD.

What Are the Types of Materials With High Tear Resistance?

In fabrics, synthetic fibers have higher tear resistance. Kevlar® and nylon are good examples of extreme tear resilience in flexible fibers. Parachute materials are made of fine woven nylon due to the severe consequences of catastrophic failure. Military body armor and motorcycle armor are generally kevlar®, in which the combination of low elasticity and huge tensile resistance make for more rigid but tough fabrics.

In elastomers, the highest tear resilience comes from natural rubbers and compounds containing them. This is a result of the very high elongation at the break of natural (vulcanized) rubber. However, the balance of properties favors synthetic rubbers in many applications, which are generally stiffer and more durable.

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1. Kevlar® is a trademark of E. I. du Pont de Nemours and Company

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