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Introduction to the Testing of Metals
Metals used in the fabrication of mechanical engineering components are numerous. They are selected for the particular qualities required for components under stress, varying loads, high temperatures, and pressure. These metals must be tested and pass rigid standards set by the relative authorities to ensure that they perform their purpose safely and efficiently.
This is the first in new series of articles on metallurgy, which looks at the various tests samples of engineering components are subjected to before their manufacture. Further articles will follow which describe the production of different grades of steel through various smelting methods and the heat treatment of components to induce tempering and hardness qualities.
In this article we shall examine the range of tests carried out to ensure that the mechanical properties of the component are suitable for its purpose.
We start by looking at these mechanical properties.
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Mechanical Properties of Metals
- Strength – Determined through Tensile Testing
This is the load a metal can withstand without fracturing; the greater the load carried determining the ultimate strength of the metal.
- Ductility – Determined through Tensile Testing
This is a property the metal contains which enables it to be drawn easily or stretched to a desired shape without fracturing, whilst retaining the shape.
- Hardness – Determined through Brinell Hardness Test
This is the measure of the metals resistance to corrosion, wear, or erosion.
- Impact Resistance - Determined through Impact Test
This is the measure of resistance to impact of the metals after machining, heat treatment, or casting procedures.
- Elasticity - Determined through Tensile Testing
A metal has elastic qualities if it returns to its original condition after being subjected to a load under its yield point.
- Toughness - Determined through Impact Test
Similar to metals strength, but to withstand varying loads. Some metals can be tough but brittle; a more suitable combination is to have toughness along with strength and pliability.
We shall now examine the equipment used to test these metal properties.
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Tensile Testing Machine - Testing the Strength, Ductility and Elasticity Properties of Metals
The Tensile Testing Machine determines the tensile strength, elasticity, and ductility of the metal.
The sample in the form of a round bar is turned in a lathe, leaving an accurate, predetermined diameter section in between two un-machined larger sections with the finished shape being similar to that of a dumbbell.
The tensile testing machine grips the sample vertically between two vertical circular chucks. Once the sample is securely gripped by both chucks pressure is applied to stretch the sample.
A graphic display which also produces a printout of all the process is visible to the operator and a typical curve is shown below to explain the different stages in the testing process.
As the load is increased the sample starts to elongate directly proportion to the applied load in the elastic stage, until it reaches point A.
From here the sample will continue to elongate without any further load being applied. Point A is known as the Yield Point, and if the load is removed from the sample, it will return to its original length.
The curve between A and B represents the plastic stage and if the load is removed between at point B the sample will retain its elongation.
From point B the load is increased again until it reaches point C where the sample will break. The ultimate load at this point is divided by the original cross-section- area of the machined sample. This calculation provides a figure for the ultimate tensile strength (UTS) of the metal in Newtons/square millimetre (N/mm2)
Dividing the load at point A by the original CSA of the sample will give a very important figure – the metal’s yield point. This figure is used in many design calculations.
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Impact Testing Machine – Testing the Properties of Toughness and Impact Resistance of Metals
The most common machine used to carry out an impact test is a Charpy pendulum machine using a sample known as Single Edge Notch Bend (SENB). This consists of 10 mm square machined bar, 55mm long into which a V notch is cut in the center of one side. It is essential that the notch is machined accurately in the correct location.
The machine consists of a pendulum of known weight which is restrained at a recognized height. The pendulum swings across a horizontal steel table which incorporates a pair of vertical supports designed to hold the sample. The sample is positioned within the supports with the V notch facing away from the pendulum.
The pendulum is released striking and fracturing the sample, continuing its swing after impact. The distance to which the pendulum rises after impact is measured and recorded automatically.
The impact strength is then calculated from the weight of the pendulum, the height of the pendulum at release, and the height that the pendulum reaches after impact with the sample. This represents the amount of energy absorbed by the metal and is therefore measured in Joules.
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Brinell Hardness Testing Machine – Testing the Hardness Properties of Metals
The hardness can be determined using a hardness testing machine, of which there are various types, with some using a hardened steel ball and others using a diamond. They work on the principle of producing an indentation under a known load in a metal sample and measuring the area of the indentation.
In this article we shall examine the operation of a Brinell hardness testing machine, which uses a hardened carbon steel ball to make an indentation on a test sample.
To test for hardness the sample is clamped in a vice under a hydraulically operated ram. The ram incorporates a hardened steel ball of 10 mm diameter. This is lowered onto the horizontal machined surface of the sample and a hydraulic load of up to 3000 kg is applied to the ram for between 10 and 30 seconds. The load and timing are dependent on the material under test.
The ball leaves an indentation in the surface of the sample whose diameter is measured using a magnified scale unit, which is part of the equipment. From this dimension the area of the curved surface of the indentation can be found by consulting the relative tables, or by the calculation shown below. This calculation will give the sample of metal a Brinell Hardness Number. (BHN) Units are not always stated but are in kg/mm2.
1. Technology Engineering, Inc.: Fracture Toughness Testing, Henryk Pisarski
2. CALCE and the University of Maryland: Materials Hardness
3. TWI Engineering: Impact Testing, Gene Mathers