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Tensile testing

For materials suppliers, the mechanical strength properties are an important measure of product quality, and tensile testing is often required for final certification.

Mechanical strength

Mechanical strength refers to the ability of a material or finished product to resist loads without failure. Failure in a material may occur due to breaking because of excessive force or because the specimen has stretched too far.

By using a tensile tester it is possible to characterise the tensile properties of a specimen by applying a force either in tension, compression, bending or shear.

The amount of force required to break a material and how far it extends before rupture are important properties to predict how the product will behave for its intended use.

For most materials you can determine these properties by plotting stress against strain (or force against elongation) on a graph and interrogating the data.

Many other useful material characteristics can be obtained from the graph and are commonly available as part of the calculations within the software of tensile testers.

Test speeds

It is important to make sure the correct test speeds (mm/ min) and loading / strain rate (N/s, %/t) are used.

The rate at which a test is performed can have a significant effect on material properties.

Plastics, polymers and metals are very sensitive to test speeds and loading rates, although aluminium exhibits little sensitivity. If a material is sensitive to strain rates it will generally show a higher tensile strength and lower elongation when tested at faster speeds.

This is the reason why it is important to establish an agreed testing rate and report it, so that results obtained across different batches of similar materials can be compared with a high degree of confidence.

International test standards (ISO, ASTM etc) are a good source as they often specify test speeds per material or specimen geometry. Selecting a tensile testing system which performs tests according to your methods, automatically calculates results, issues reports and seamlessly communicates with your IT network, can play a crucial part in testing your materials effectively

Selecting grips

International testing standards such as ISO, ASTM, DIN, JIS and BS EN specify the type of grip to be used but they all have the express purpose of holding the specimen without any slippage and minimizing the chance of a break within the jaw of the grip.

Considerations when selecting a suitable grip are:

specimen geometry
forces expected
clamping method
jaw type

For many applications it is the choice of the user to decide which is the most practical, cost effective and suitable for their material.

Specimen geometry and forces

The physical size of the specimen will often dictate the minimum and maximum opening of the grip, but consideration should also be given to the width of the grip face. Aim to use the centre section of the grip face with at least two-thirds of the jaw face maintaining contact with the specimen.

Destructive and non-destructive testing

Test methods may require the specimen to be pulled to a specific force and released (non-destructive). Alternatively, the standard may require the specimen to be stressed or strained until rupture (destructive).

When selecting grips it is good practice to estimate the highest tensile load expected and to add a safety margin (minimum 25% is recommended) to determine the minimum load rating for a grip.

Loadcell calibration classes

Loadcells are calibrated and given a classification. The class relates to the maximum permissible error found during calibration.

For ASTM E4, Class 1 allows +/- 1% for maximum error and repeatability. ISO 7500-1 allows for several classes, namely 0.5, 1, 2 and 3.

When a calibration is performed, measurements are taken down to the ‘lower limit of verification’, which is the lowest force reading required to achieve the appropriate class.

To establish the lower limit of verification you must multiply the display resolution—under ASTM standards for Class 1 you must multiply by 200, for ISO standards for Class 0.5 you must multiply by 400.

The loadcell will then be calibrated from this lowest point at various intervals across its range up to the loadcell capacity.

The loadcell is checked over three separate calibration runs and must achieve a value equal to or lower than the class for accuracy, repeatability, reversibility and zero.

Loadcell calibration

Calibration of loadcells, which are an intrinsic part of the measuring system, is important to perform on a regular basis to ensure they conform to their specification.

International testing standards covering the calibration of tensile testing machines have been harmonized.

The main standards used are:

ISO 7500-1: Calibration and verification of static uniaxial testing machines
ASTM E4: Standard Practices for Force Verification of Testing Machines
ISO 17025: General requirements for the competence of testing and calibration laboratories

In addition the equipment used to calibrate the tensile testers is covered by test standards:

ASTM E74: Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines.
ISO 376: Calibration of force-proving instruments used for the verification of uniaxial testing machines.

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Tensile testing tips and recommendations for accurate and repeatable strength testing

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Tensile testing

Mechanical strength

Test speeds

Typical points of interest when testing a material include:

Selecting grips

Specimen geometry and forces

Destructive and non-destructive testing

Clamping method

Self-tightening

Side-action

Loadcell calibration classes

Loadcell calibration