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Making a multitude of measurements

02 April 2012

The extensive range of standard grips, probes, fixtures and accessories transform universal materials testing machines into powerful systems for the broadest possible range of applications. Carl Bramley brings us up to date with modern materials testing technology.

Materials testing is a very well established technique in which a compressive or tensile force is applied to the sample under test in a controlled manner with accurate measurements of the resultant force and extension. The truly universal capabilities of the technique come from the fact that by choosing from a large array of sample grips, fixtures and accessories, the force can be applied in such a way to measure a multitude of parameters - not only compression and tensile strength, but also flexural/bend strength, coefficient of friction, peel strength and many, many more.

With instruments available to make accurate and repeatable force measurements in the range from 0.02N to 150kN, tests can be performed on anything from a human hair to large components as diverse as complete car seats, large cardboard cartons, mattresses and large diameter pipes. Depending on the machine, an elongation of between one micron and 2.5m can be measured. In addition, materials testing can be used throughout the design, development and manufacturing process, from the evaluation of raw materials to the finished product.

Materials testing machines can be used in quality control, production, laboratory and R&D environments, and there is a vast range of internationally recognised standards (ASTM, ISO, FINAT etc) that govern procedures and methods.

Force testers
The materials testing portfolio begins with simple hand held force testers and progresses right the way through to PC-controlled, high force, twin column materials testing instruments. Force testers are designed to make basic force measurements in applications where the main interest is in peak load and extension.

A force tester uses a force gauge as the load measurement device, and it is available in mechanical, motorised or pneumatic versions - pneumatic testers being suitable for test procedures that require intrinsically safe operation. However, the most recent developments have come in the form of digital force testers, which are advanced force testing systems optimised for production, quality control and engineering applications. These can be used for tensile, compression, shear, friction, flexural and complex spring testing.

Digital force gauges, such as the Chatillon DF II Series from AMETEK Test & Calibration Instruments, span the full range of handheld and stand-mounted force gauge applications, from the simplest ‘go-no go’ to the most sophisticated quality control. The high-end DFS II can be equipped with integral load cells or smart remote sensors for load measurement and torque measurement and even provides onboard Bluetooth connectivity for wireless transmission of readings to a computer.

Typically, digital force testers are available with force capacities up to 5kN, and standard test set-ups for load limits, distance limits, time limits, break tests, rupture tests, cycling and loop tests can be included.

Materials testers
Dedicated material testers are much more precise and generally offer greater performance levels than force testers and allow more complex measurements to be made. Universal materials testing machines are available in single or twin column versions and allow precisely controlled tensile or compression forces to be applied to the sample under test over a controlled period of time. Precision load cells measure the material under test as a function of force against extension and time.

Materials testing machines are available in a wide range of sizes, allowing different maximum force ranges to be applied - from as little as 1kN or as high as 150kN. Robust, high stiffness load frames are at the core of each materials testing system, with modern linear guide technology, pre-loaded ball screws and advanced software compensation ensuring high displacement precision.

Extension errors of less than 5 microns at full load are achievable for many compression and flexural tests without the use of an extensometer. Most materials testing machines offer PC control, and sophisticated control and data analysis packages are available.

Software packages will often contain a library of the internationally recognised testing standards, plus a suite of useful functions. Lloyd Instruments’ NEXYGENPlus, for example, offers a complete suite of test set-up,; a video and still picture capture system, as well as full documentation aids such as a security and audit trail utility, statistical process control (SPC) trend and histogram charts and a data export utility for connection to LIMS and third party SPC packages
For specialist testing, test creation wizards are included for tension/compression test, tear and peel test, cycling test, flexural test, friction test and a user configurable test for multi-stage testing.

Testing anything and everything!
The extensive range of standard grips, probes, fixtures and accessories transform universal materials testing machines into powerful materials testing systems for the broadest possible range of applications. Grips and fixtures come in a variety of sizes, gripping surfaces, styles and capacities, while extensometer accessories allow accurate measurement of elongation on high elongation materials such as rubber and elastomers.

Testing can also be carried out over a wide temperature range (-70oC to +1,200oC) when the test machine is used in conjunction with thermal chambers or furnaces. And for the testing of very large samples of up to 2m by 2m footprint, Lloyd Instruments offers a novel ‘pogo’ system, which is mounted beneath the tester. The frames are constructed from aluminium and the sides of the cages can be open or shielded with wire mesh or polycarbonate panels.

So, the universal materials tester and its complement of grips, fixtures and accessories not only allows an enormous range of measurements to be carried out, but it is also equipped to provide traceable test results in accordance with international standards. Materials testing may rely on established techniques, but the technology has come a long way and it is now a powerful and indispensable tool in the product developer’s armoury.

Carl Bramley is with Lloyd Instruments

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