L15. Testing

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Testing

Learning outcomes

By the end of this unit you should have developed a knowledge and understanding of:

the need for materials testing
the mechanical properties that can be tested
simple workshop based tests for materials
industrial/scientific tests for materials
how can you test materials yourself in the workshop environment

Methods for investigating and testing materials

When selecting the most appropriate material for a specific application it is often necessary to perform specific tests on mechanical properties. These tests come in two main categories: simple workshop tests (comparative) and scientific measured tests (industrial). Whichever test is chosen the results need to be comparable (fair) and therefore we must ensure that all possible variables are removed apart from the material being tested.

Variables include:

Environmental conditions
Size of material sample
Testing equipment
Force applied
Method of force application
Position of force application
Individual taking measurements

Industrial Testing

Tensile Testing

A standard test piece of material is placed into a Tensometer machine and held in clamps at each end. One of these moves on a worm drive gear putting the piece under constant tension. The yield or stretch is measured before the piece ‘necks’ and reaches final breaking point.

Toughness testing

Izod impact test: a notched piece of material is held vertically in the vice of the machine. A pendulum is released from a set position. The energy absorbed is calculated from the height the pendulum swings to after it hits the piece. The material that absorbs the most impact is the toughest.

Hardness testing

There are three common tests for hardness: Rockwell, Brinell and Vickers pyramid.

In the Rockwell test a preload is applied to the material sample using a diamond indenter which just breaks through the surface. An additional load is then applied to the test material and held for a predetermined length of time. The distance between the preload and applied load is then measured.

The Brinell test uses a standard sized steel ball which is forced into the materials surface using a pre-set load. The diameter of the indent is measured. The smaller the indent, the harder the material.

Vickers pyramid test is used for very hard materials and uses a diamond square-based pyramid to indent the surface of the material. Diamond is used as it will not deform under load. A microscope is used to measure the size of the indent. The smaller the indent, the harder the material.

Malleability

and

ductility testing

A bend test is used to determine how well a material can withstand cracking during one continuous bend. A ductile material will have lots of plastic deformation prior to failure.

A test piece is placed in a bending machine and held; supported at the ends. A plunger loads the test piece at the centre and bends to a predetermined angle. The material is then inspected for cracks and defects.

Cracks on the outer bend indicate the level of ductility.

Cracks on the inside of the bend indicate the level of malleability.

Non-destructive testing (NDT)

Non-destructive testing (NDT) is usually carried out on products such as large castings where there is a likelihood of internal defects or imperfections which might not be detected by other methods. There are two standard NDT methods:

Ultrasonic testing: a transducer generates sound waves which are pulsed into the material. The intensity of the reflected waves is recorded on a display unit. They can show internal cracks or other defects and can be used on all types of materials.

x-ray testing

X-ray testing: an x-ray beam passes through the material and an image is projected onto a display screen. X-ray testing can be used to inspect integrated circuits (IC’s) and printed circuit boards (PCB’s) and to check for micro-voids in composite materials in products such as Formula One cars. It can also be used to detect faults in turbine blades or aeroplane engine blocks.

Electrical conductivity: a four-point probe method can be used to determine the electrical conductivity of materials. Four small diameter wires are stretched, parallel to each other, across a non-conductive polymer block.

Ohm’s law can be used to calculate the resistance of the material sample.

Thermal conductivity: using a heat flow meter, a square-shaped material test piece is placed between two temperature -controlled plates. The temperature is increased at a controlled rate and the heat flow through the material is measured.

Workshop testing

Practical workshop tests can be easily carried out in a workshop using basic tools and equipment.

Tensile testing

Tensile testing is done by placing a weight on a length of material that is held in a clamp or vice. The weight is applied to the end of the material protruding from the vice. Tension will be applied to the top of the material with compression at the bottom.

Toughness testing

Toughness testing is done by placing the test material in a vice then hitting it with a hammer. The more impact it can withstand, the tougher the material. Brittle materials will withstand less force or even shatter completely. The problem with this test is maintaining a consistent level of force with each blow.

Hardness testing

Hardness testing can be undertaken to assess abrasive wear or resistance to surface indentation.

Abrasive wear can be easily tested by running a file over the surface of the material. The material with the fewest scratches is the hardest.

Another method is using a dot punch which can be placed on the material and hit with a hammer. The larger the surface indentation, the softer the material. Maintaining the same force for each blow is difficult and can lead to inconsistent results.

Malleability and ductility testing

Malleability and ductility testing can be undertaken in the same procedure. A test piece is secured in a vice and then it is attempted to bend to 90 degrees using a hammer or mallet.

Cracks or surface damage on the outside of the bend indicate a lack of ductility.

Cracks on the inside indicate a lack of malleability.

Corrosion testing

Corrosion testing can be done by exposing the material to environmental effects such as rain or sun. The material is simply placed outside in an area exposed to the elements. They can then be visually inspected for signs of surface corrosion. Depending on the material, further testing for hardness, toughness etc. can be undertaken to see if corrosion has affected these properties.

Conductivity testing

Conductivity testing can be done in one of two ways:

Electrical conductivity

Electrical conductivity can be done by using a multimeter with the probes attached at a set distance between them.

The higher the resistance, the lower the conductivity.

Thermal conductivity

Thermal conductivity can be measured with a thermometer which is placed a set distance from one end of the material. A Bunsen burner is lit under the end of the material for a set time. The shorter the time for the thermometer to reach a set temperature, the higher the thermal conductivity.

Summary

Mechanical properties have been covered in previous units, but the tests in this unit are a way of determining those properties for each material.
Workshop tests can be undertaken in any workshop that has simple facilities.
These can include tests for tensile strength, hardness, toughness, malleability, ductility and conductivity.
The Izod test is for toughness Rockwell, Brinell and Vickers pyramid test for hardness
For most of these there is a corresponding industrial test.
Most of these tests are destructive; they will damage the material so they may not be suitable for testing your NEA prototype work
Non-destructive tests (NDT) include ultrasonic, X-ray and electrical and thermal conductivity.

Revision notes

Mechanical properties have been covered in previous units, but you should be familiar with a range of workshop-based tests that can be carried out to assess the mechanical properties of materials that you may use in the course of your studies.

You should also be able to name one industrial process for each test as these may be addressed in the written examination.

Try and make a note of any diagrams in this unit and attempt to replicate them in a simple and easy to understand way in order to support your answers.

This is unlikely to be a major aspect of the exam, but it can support other answers and there have been questions which specifically address materials testing.

A reminder of the mechanical forces and related strengths covered in mechanical properties.

*This is just a silly picture, I hope your work is better!

Lesson Activity

Using notes and diagrams, describe a workshop test for maleability and ductility. You should describe:

How to set up the test
The test procedure
How to determine the results of the test

Prep 15

L15-Testing.ppsx

Extra resources:

Hydraulic press testing

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