Methods for investigating and testing materials

Material testing is used to compare how materials behave and to decide whether they are suitable for a product. AQA requires you to understand how workshop and industrial tests are set up, what is tested, measured and compared, and how the resulting data can be analysed.

The properties specifically required here are tensile strength, toughness, hardness, malleability, corrosion and conductivity.

Making tests reliable

For a fair comparison, important variables should be controlled. Depending on the test, these can include:

the dimensions and shape of the material sample
environmental conditions
the equipment and testing method
the size, position and rate of the applied force
the distance between measurement points
the person or instrument taking the measurement

Tests should be repeated where appropriate so that anomalous results can be identified and a representative result can be calculated.

Tensile strength

Tensile strength is the ability of a material to resist being pulled apart by opposing forces.

Simple workshop test

A material sample can be supported or clamped and subjected to a known load. The amount of deformation is measured and samples tested under the same conditions can be compared. Less deformation under the same load indicates greater resistance to the applied tensile/bending effect.

Tensometer tensile test

A tensometer provides a more controlled industrial tensile test.

A standard test piece is securely clamped at both ends.
One clamp remains fixed while the other moves away, placing the specimen in tension.
The tensile load is increased in a controlled way while the extension of the specimen is measured.
Load and extension data are recorded as the specimen stretches.
Testing continues through elastic deformation, permanent deformation and eventually fracture.
The results can be used to identify behaviour such as the elastic limit, yield point, maximum load and breaking point and to compare materials.

A material that withstands a greater tensile stress before failure has greater tensile strength.

Toughness

Toughness is the ability of a material to absorb energy and resist fracture when subjected to impact.

Simple workshop impact test

Samples of the same dimensions can be struck using the same pendulum or impact arrangement. The starting position, specimen support and impact conditions must remain constant. A tougher material absorbs more impact energy before fracturing.

Izod impact test

The Izod impact test is an industrial test used to compare toughness.

A standard notched specimen is held vertically in the vice of the testing machine.
A pendulum is raised to a fixed starting position, giving it a known amount of potential energy.
The pendulum is released without an additional push.
It strikes the specimen at the notch and causes it to bend or fracture.
The height reached by the pendulum after impact is compared with its starting energy.
The difference represents the energy absorbed by the specimen. A tougher material absorbs more energy, so the pendulum rises less after the impact.

The notch provides a consistent point of stress concentration so different materials can be compared under similar conditions.

Hardness

Hardness is resistance to indentation, scratching and surface wear.

Simple workshop indentation test

A centre or dot punch can be used with a controlled impact to indent different samples. The same punch, specimen dimensions, support, angle and force should be used for each material. A smaller indentation indicates a harder material when the test conditions are identical.

Industrial hardness tests provide much more controlled and repeatable measurements. The three important tests in your notes are Rockwell, Brinell and Vickers.

Rockwell hardness test

The Rockwell test determines hardness from the depth of penetration of an indenter.

The material sample is placed securely on the machine.
An indenter is pressed into the surface using a small preload/minor load. This establishes a datum position and reduces the effect of small surface irregularities.
A larger major load is then applied for a controlled period.
The major load is removed while the minor load remains.
The permanent increase in penetration depth is measured.
The machine converts this depth into a Rockwell hardness value. A shallower permanent indentation indicates a harder material.

Different Rockwell scales use different loads and indenters. A diamond cone is used for harder materials, while ball indenters can be used on softer materials.

Brinell hardness test

The Brinell test determines hardness from the size of an indentation made by a ball indenter.

A hard, standard-size ball indenter is pressed into the material surface.
A known load is applied for a specified period.
The load is removed, leaving a circular indentation.
The diameter of the indentation is measured.
The indentation size is considered together with the applied load and ball size to determine the Brinell hardness value.

Under the same test conditions, a smaller indentation means the material is harder.

Brinell is useful where a relatively large indentation can provide a representative average of the material surface. The test does leave a visible indentation, so it is a destructive or surface-damaging test.

Vickers pyramid hardness test

The Vickers test uses a square-based diamond pyramid indenter and is suitable for a wide range of materials, including very hard materials.

The specimen is positioned beneath the diamond pyramid indenter.
A known load is applied, pressing the pyramid into the surface.
The load is removed, leaving a small square-shaped indentation.
The diagonals of the indentation are measured accurately, normally using a microscope or optical system.
The indentation dimensions and applied load are used to determine the Vickers hardness value.

A smaller indentation for the same load indicates a harder material. Because the indentation can be very small, Vickers is useful where a more localised hardness measurement is required.

Comparing the hardness tests

Test	Indenter	What is measured	Key point
Rockwell	Diamond cone or ball depending on scale	Depth of permanent penetration	Quick direct hardness reading; different scales suit different materials
Brinell	Hard ball	Diameter of indentation	Larger indent; useful for obtaining an average surface hardness
Vickers	Square-based diamond pyramid	Diagonals/size of indentation	Small precise indentation; suitable for very hard materials and localised testing

Malleability and ductility

Malleability is the ability to undergo permanent deformation under compressive forces without cracking. Ductility is the ability to undergo plastic deformation under tension without cracking.

Workshop bend test

A sample can be bent through a controlled angle, such as 90°, and inspected for cracking. Samples must have the same dimensions and be bent using the same equipment and method.

Cracking on the inside of the bend, where the material is compressed, indicates limited malleability.
Cracking on the outside of the bend, where the material is stretched, indicates limited ductility.

Industrial bend test

A standard test piece is supported at both ends in a bending machine.
A mandrel or plunger applies a controlled force at the centre.
The specimen is bent to a specified angle or until failure occurs.
The amount of permanent deformation is observed.
The inside and outside surfaces are inspected for cracks.
Different materials can then be compared using the same specimen dimensions and loading conditions.

Corrosion resistance

Corrosion resistance is the ability of a material to withstand deterioration caused by environmental or chemical attack.

Simple workshop test

Samples of the same size can be placed outdoors in the same location for a fixed period. They are then visually inspected for changes such as rusting, pitting or other surface corrosion.

Controlled corrosion test

For a more controlled comparison, equal samples are exposed to the same simulated corrosive environment for a set time. Conditions such as moisture, temperature and exposure should be kept constant. The amount of surface corrosion or other measurable change can then be compared.

A controlled test is more reliable than simply leaving samples outdoors because environmental conditions can be standardised.

Electrical conductivity

Electrical conductivity describes how easily electrical current passes through a material.

Resistance test

Samples should have the same dimensions.
Two points a fixed distance apart are marked on each specimen.
Multimeter probes are placed at the same positions.
Electrical resistance is measured.
Results are compared under the same conditions.

For equivalent specimens, higher resistance means lower electrical conductivity, while lower resistance indicates greater conductivity.

Thermal conductivity

Thermal conductivity describes how readily heat energy passes through a material.

Simple workshop test

Samples of the same dimensions can be heated at one end using the same heat source. Temperature is measured at a fixed distance from the heated end. The time taken to reach a specified temperature can be compared: under identical conditions, a shorter time indicates greater thermal conductivity.

Heat-flow meter test

A more controlled industrial method can use a heat-flow meter.

A specimen of controlled dimensions is placed between temperature-controlled plates.
A known temperature difference is established across the material.
Heat flowing through the specimen is detected by sensors.
The heat-flow data and specimen dimensions are used to determine or compare thermal conductivity.

This provides a more controlled comparison than an open workshop heat test.

Non-destructive testing (NDT)

Non-destructive testing checks a component or product for faults without making it unusable. It is particularly useful for valuable finished components where destructive testing would be wasteful.

Ultrasonic testing

High-frequency sound waves are sent into the material. Reflections from boundaries or internal discontinuities are detected. Unexpected echoes can indicate defects such as internal cracks or voids.

X-ray testing

X-rays pass through a component and produce an image based on differences in the amount of radiation transmitted through the material. Internal defects that cannot be seen from the outside, such as cracks or voids, may therefore be detected.

NDT differs from tests such as tensile or impact testing because the aim is to inspect the component without deliberately taking it to failure.

Analysing test results

The result alone is not enough. You should be able to interpret what the data means when comparing materials.

For example:

a smaller Brinell or Vickers indentation under equivalent conditions indicates greater hardness
a smaller permanent Rockwell penetration indicates greater hardness
greater energy absorbed during an Izod impact test indicates greater toughness
greater load or stress before failure in a tensile test indicates greater tensile strength
greater electrical resistance in equivalent samples indicates lower electrical conductivity
faster heat transfer under controlled conditions indicates greater thermal conductivity
less corrosion after identical exposure indicates greater corrosion resistance

When comparing test data, check that the specimens and test conditions were equivalent. A difference in sample dimensions, loading, temperature or measurement method could otherwise make the conclusion unreliable.