STRESS STRAIN CURVE

Stress Strain Curve

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Materials & Manufacturing Engineering

Activity 1

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.

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.

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 ...