Tensile Testers / Universal Tensile Testing Machine

What’s A Tensile Test?

A tensile tests is the pulling apart of material by a universal testing machine to see how it reacts, and the pulling is continual until it’s permanent deformation (Poisson’s ratio) or until it breaks.

During the tensile test, data recording is continual from start to finish of the applying force. The following tensile properties are directly measured during a tensile test: Tensile strength, max elongation, and reduction in area. Understanding the above mechanical properties can reveal modulus of elasticity, deformation, yield strength, and strain-hardening characteristics. Click here for an in-depth guide on Tensile Testing

What Data Can Universal Testing Machines Show?

Designers require several essential data sets to ensure their product will be safe to use for its intended purpose. These tests are also essential for quality control purposes.

Ultimate Tensile Strength

The Ultimate Tensile Strength is the maximum stress a test specimen can take before it breaks.

This value is usually required to be above five times any other force applied to the product.

Hooke’s Law

This law states that the extension of a spring is proportional to the load applied, up to the elastic limit.

Products are usually designed with a safety factor never to exceed this limit and break.

Modulus of Elasticity (Young’s Modulus)

The modulus of elasticity measures the stiffness of raw materials; it will determine how much the material will deform when applying force on a linear curve.

Hooke’s Law no longer applies when there is a deviation from the linear curve, as the material has some persistent deformation. Conversely, by removing the tensile load while in the initial linear region, the material will return to its exact condition before the start of the tensile testing.

It’s important to know this value as it helps designers calculate how much stress their product can take without irreversible deformation.

Yield Strength

Yield strength is the maximum amount of force a material can take before it starts to deform permanently.

Designers need this information as they should never design their products to go beyond this limit, making them unsafe for use with their intended purpose.

Offset Yield Strength Method

Offset Yield Strength is the approximation of the materials elastic limit. This is useful for materials that don’t display a clear change in behaviour from elastic to plastic, therefore difficult to determine the yield strength. A parallel line is offset usually by 0.2% at the intersection of this line representing the yield point. 

Strain

The strain or percent of elongation that occurs when a Tensile Test specimen breaks. This value can be used to calculate Tensile Strength and Modulus of Elasticity, Yield Strength etc. Engineering strain and true strain are two additional ways of expressing strain measurements.

Typical Industries That Require Test Data From Tensile Testing

Industries that require data from a tensile tester are:

• Aerospace
• Automotive
• Composites
• Medical Devices
• Plastics 
• Pharmaceutical Industry
• Paper
• Food
• Steel & Metals production and processing 
• Construction
• Consumer Products
• Textiles

Tensile testers are important for quality control in the production of a wide range of products. They ensure that materials being used can withhold the required amount of stress and strain before they hit the market.

Without this test, some faulty materials may make it through to the final product, resulting in a poor user experience, injury, or fatality

Test Specimens can be small or large, and the most common types of materials that undergo a  Tensile Test are metals, plastics, rubbers, composites, and textiles.

Stress/Strain Curve

After the tensile test, the data can be plotted on a graph to show the stress-strain curve.

A stress/strain curve shows the relationship between the engineering stress and engineering strain on a material.

This curve is important as it will help designers understand how their product behaves when force is applied to it.

How does a Tensile Tester Work?

A Tensile Tester works by applying a load to the specimen until it breaks.

The machine will then measure the amount of force applied, the elongation of the specimen, until a break occurs. 

This data is used to create a stress-strain curve which shows how much force and elongation is applied to the specimen 

What Are The Common Components And Parts Of Tensile Testing Machines?

Here are the standard parts you’ll find with all tensile testing machines.

Drive System

The priority of the drive system within a tensile test machine is to feed electricity into the motor at varying amounts to control the speed of the cross head. 

Load Cells

A load cell measures the force applied to the test specimen. The load cell converts tension force into an electrical signal which can be measured and calculated into the relevant force units. 

Extensometer

An extensometer is used to precisely measure the elongation of the specimen. Often referred to as strain measurement. The sensor will be mounted in-line with the test specimen and will measure any movement that occurs.

Specimen Grips

A Tensile Test specimen is clamped into the tensile grips and then pulled until it breaks or permanent deformation occurs. The load applied to the machine will be transferred through these gripping mechanisms, which should always return to their original state after a test has been performed.

Each machine could have several grips and fixtures, allowing the testing of different materials.

Moving Crosshead

The specimen is placed between the grips attached to a crosshead. The moving cross-head moves up and down along the column, pulling on the material under test at a constant speed.

The strain rate (elongation) in a test specimen is related to the crosshead speed.

Electronics

Electronics control moving parts within the tensile testing machine. A microprocessor in the closed-loop servo controller allows the crosshead speed and, consequently, the load rate to be regulated.

Testing Software – Making The Testing Process Easier

The test software, accessed through a computer, allows the controller to configure, run and view the results from the test. The testing process can be viewed on a real time dynamic display assisting the operator in maintaining uniformity and standard compliant test configurations.

Load Frame/Test Frame

A tensile testing machine will either be a single or dual-frame, with the force capacity being the deciding factor. High-capacity frames can withstand greater force and are of a twin column design. 

Tensile Testing Standards

Most tensile testing is carried out to meet standards published by The International Standards Organisation (ISO) or The American Society for Testing and Materials (ASTM).

Both organisations lay out the tensile properties, parameters, and common standards for various raw materials, including metallic materials, plastics, composites, elastomers, and textiles.

There are tensile testing standards for exactly how the test should be performed, the test speeds, methods of calculation of the results etc. 

By ensuring material and products entering the supply chain have met the minimum requirements, end-users can expect the product not to fail if used as intended.