Norma

7 TIPS FOR MATERIALS TESTING

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Introduction
Characterizing material properties is a primary objective for quality control and R&D laboratories. Yet, choosing the right test is not always obvious. There are many different types of static and dynamic tests, including: tensile, compression, shear flexure and fatigue/fracture. The following should be considered when choosing the correcttesting system:
Static materials test system (electromechanical drive) K

Table of Contents
Tip 1: Grip Selection 4-6

Tip 2: Load Cell Selection

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Tip 3: Extensometry Selection

8-9

Test Type: tensile, compression, flexure, torsion, shear, reverse-stress (torsion and compression) Testing Specimens: configuration, size, shape and material Ultimate Test Loads: the highest expected loadexpressed in pounds, newtons or kilograms Ultimate Strain-Extension/Compression: the highest expected elongation expressed as a percentage of initial gauge length Strain Measurements: extensometer gauge length, percentage of travel, other requirements

Tip 4: Optimizing Specimen Alignment

10-11
Dynamic test system

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Tip 5: Accuracy, Resolution and Precision

12-13

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Tip 6: DataRate and Bandwidth

14-15

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Tip 7: Software Function and Performance

16-18

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Static materials test system (hydraulically powered)

This pamphlet aims to offer tips on how to choose the right testing machine based on these and other considerations.

Impact test system

Contact Details

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Table of Contents

Notes

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Introduction

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Tip 1: GripSelection
Successful gripping solutions require the specimen to be held in a way that prevents slippage and jaw breaks and ensures axiality of the applied force. In some cases the gripping requirements are very specific and a purpose-designed grip or fixture is necessary to meet a particular testing standard. However, in most cases, you can use general purpose accessories. General purpose grips andfixtures have the advantage of being able to grip a wide variety of specimen types and materials using a range of options such as different jaw faces, alignment fixtures, etc. Specimen Gripping Solutions Two of the most common problems operators face are specimen slippage and jaw breaks. Slippage occurs most frequently when using mechanical or screw action grips with flat faces. When selecting gripfaces, the surface area should be large enough to cover the tab (for dumbbell samples) or, for a parallel sample, as much of the surface area as possible. With any grip, ensure that the specimen end is gripped by at least 75% of the available jaw face length, otherwise gripping efficiency is reduced and in some cases, the jaw face can be damaged. The important thing to remember if you experiencegripping problems is to experiment. How you arrive at the end result is incidental. Jaw breaks usually occur when the sample inside the grips is damaged by too much clamping force or by serrated faces biting too deep. Below are tips to reduce jaw breaks:
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Hints for Reliable Gripping
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Periodically inspect the grips for defects such as cracks or leaks in hoses. Periodically verify pressuregauges are accurately registering air or oil pressure to the gripping system. Replace jaw faces when the surfaces become worn, damaged or contaminated. Do not use more gripping force than necessary to provide reliable, slip-free gripping. Old grips don't necessarily work with new materials or specimens. ~ You may find that special grips or different jaw face surfaces are needed. ~ You can try avariety of things to modify existing gripping methods including emery cloth, sticky tape, etc.

Screw Action Grips: The operator may be using too much force when tightening onto the sample; use a torque wrench or pneumatic grips. Pneumatic Grips: Drop the pressure, but not to the point where you get slippage. Serrated Faces: Change to faces that have more serrations per inch (less bite) or cover...