Introduction
Toughness and hardness are two key properties of materials. In this section we will look at how they can be determined using various tests, with the aim that by the end of the section you will have an understanding of impact toughness testing and the properties that can be determined from it. To do this we will introduce the concept of toughness and show how it can be determined from a tensile test. We will also Introduce Charpy and Izod toughness testing. Finally, you will also Be aware that standard sized specimens are used and that temperature affects the toughness of some metals.
In the second part of this section we will look at hardness testing, with the intention that you gain an understanding of hardness testing and the properties that can be determined from it. In this part we will introduce the concept of hardness, give an overview of the different types of hardness testing, look at the hardness of some common engineering materials and examine the equivalence between hardness and tensile strength.
Thanks to Dr Gareth Bradley of Perth College UHI for the materials upon which these notes are based.
Toughness from a Tensile Test
In a tensile test, a material is stretched by an ever increasing force until the point it breaks. A graph of force against extension of the material can then be plotted. The area under a tensile test plot is related to a materials toughness, the greater the area, the tougher the material as shown in the diagram.
Source, public domain
Impact Testing
Alternatively, Impact testing is used as an indicator of toughness in a material – remember, do not confuse strength and toughness, they are not the same property.
Impact toughness is the ability of a material to withstand suddenly applied (impact) loads. Simple impact tests using falling weights can be used, and Accurate results need to show the energy of fracture. This is given in Joules (or newton metres)
Most impact testers use a weight attached to a pendulum to strike a specimen, the weight is drop and strikes a sample specimen held at the bottom of its swing. After striking the specimen the pendulum continues on, and the height the pendulum reaches after fracturing the specimen equates to the energy remaining. As you may expect, the tougher the material the more energy absorbed by the specimen in fracturing and therefore the lower the height gained. The energy absorbed is known as the impact energy or notch impact energy if the specimen is notched.
Impact specimens are 55 x 10 x 10 (length x breadth x height) mm and usually have a 2 mm deep “v” notch machined on one face. Two types of test are used, Charpy and, less frequently, Izod. Whichever test is used the specimen is the same size. The diagrams show a schematic of a test sample, and a simplified testing device.
Schematic Diagram of a test sample (UHI)
Before the hammer is released it has potential energy:
Epot = mgH
After the impact it has potential energy:
Epot = mgh
This means we can calculate the notch impact energy using:
KV = mgH - mgh
Where:
m = mass of the hammer
g = acceleration due to gravity
H = initial height
h = final height
If you have an internet connection available then a video of a charpy impact test can be viewed by following the link:
Brittle to Ductile Transition
One point worth noting, is that the notch impact energy of some body centred cubic (BCC) metals, including some steels, is affected by temperature. At low temperatures the metal behaves in a brittle manner. A higher temperatures the metal behaves in a tough manner. The graph shows how this transition can occur as temperature increases:
Gareth Bradley, Perth College UHI
Hardness testing
Hardness is the ability of a material to resist deformation. It is often tested by indentation, but scratch and abrasion tests may also be used. Hardness is not a fundamental material property as the resistance to indentation depends on the shape of the indenter and load applied, meaning that hardness figures must also include details of the test undertaken.
There are a range of common hardness tests including Rockwell, Brinell, Knoop, Shore, Vickers and microhardness. The value of a materials hardness is often used as a general indication of its wear resistance, but it can also be used to measure surface hardening processes, coating thicknesses and softening/hardening resulting from various manufacturing processes. There is a positive correlation between hardness and strength, but it is not possible to accurately convert from one to other.
Vickers Hardness Test
The Vickers harndess test is carried out using a tetrahedral (pyramid) shaped diamond indenter, which is pressed against the surface of the sample. The material sample under test is indented using loads from 1 to 100 kgf and then the indent measured across the corners using a microscope. The diagram shows this in schematic form.
Hardness tables take into account the load used, so you must be careful to check the load quoted when using them. Unlike some other materials tests, Some level of operator skill required both in carrying out the test and in measuring and interpreting the results. Finally to get accurate figures the surface finish needs to be good.
If you have an internet connection available, a video of a Vickers hardness test can be found be following the link below:
Knoop Hardness Test
The Knoop Hardness test is similar a similar process to the Vickers test, but using an elongated diamond indenter. Because of this elongation, thin materials can result in the test recording the hardness of the machine platen/table, for this reason it is vital to check the minimum thicknesses recommended for the particular test. The Knoop test indenter is designed for thin, hard materials or thin coatings and produces an elongated diamond indentation as shown in the diagram.
Gareth Bradley, Perth College UHI
Brinell Hardness Test
Again the Brinell test is similar to the Vickers in that a force is used to indent the surface of a material. This time however, ball indenters made from hardened steel or tungsten carbide in a range of sizes are used. The material under test is indented and then the diameter of the indent measured.
Hardness tables take into account the indenter in terms of material and size, and also the load (up to 3000 kgf) used. It is important to use the correct indenter size and load for the material under test. If not, the test may not be carried out correctly leading to problems such as those shown in the diagram, which in turn will lead to incorrect results.
If you have an internet connection available you can view a video of a Brinnell hardenss test by going to the link below:
Gareth Bradley, Perth College UHI
Rockwell Hardness Test
In the Rockwell hardness test, two different types of indenters can be used – ball and diamond. Ball indenters are made from hardened steel or tungsten carbide in a range of sizes. Diamond indenter for hard materials.
The material under test is indented with a minor load, followed by a major load. The major load is then removed, and the depth of the indent is then measured. A direct reading of hardness is indicated by the machine. Unlike some other hardness tests, little operator skill is required, and the surface finish of the material can be poor.
Gareth Bradley, Perth College UHI
The link below will show a video of a Rockwell test if you have an internet connection available:
Microhardness Test
Microhardness testing involves indentation with either a Vickers or Knoop indenter, but with loads not exceeding 1kgf. The procedure is comparable to a standard Vickers test, except that it is done on a microscopic scale using higher precision instruments.
The surface finish of the material needs to be good, for small loads a metallographic (polished) finish is required. Precision microscopes are used to measure the size of the indentation – it would not be possible to measure with the naked eye. The micrographs show the results of a microhardness test.
Hardness Conversion Between Test Methods
Unfortunately, it is not possible to accurately convert between the methods by making use of a simple equation in the way that it is possible to convert between different temperature scales for example. However, conversion tables and equations exist to convert between the results obtained from the various methods, although this is not an exact process due to the variation in the shape and materials of the indenters, magnitudes of the applied loads and loading times.
The diagram gives a comparison of some scales along with where some materials can be found on the scales.
Image: Dr Gareth Bradley
One final point to note is that there is an equivalence between hardness and tensile strength. This varies with the material types.
