The point N represents the maximum stress which the wire can bear and is called ultimate stress/breaking stress. ‘Neck’ or constriction begins to form at a weak point.At this stage, when wire begins to flow, the cross-section of wire decreases uniformly up to N.Beyond the yield point, the curve begins to bend upwards and the portion Y p and N graph is obtained.The value of stress corresponding to the yield point is called yield stress. This means that the extension of wire increases without any increase in stress or load, the wire is said to ‘flow’. If the load is increased further, a point Y p is reached, at which the tangent to a curve becomes parallel to strain axis.If the wire is loaded again, a straight-line graph SE’ is obtained. The strain OS remains permanently in the wire. However, the wire retains its elastic properties as soon as the load is removed.If the wire is strained up to E’ beyond point E and then if the load is removed, the wire is unable to cover its original length.In this case, for a small increase in stress, the strain increases faster and a graph bends towards strain axis.If the stress is increased beyond point E, the graph no longer remains a straight line and Hooke’s law is not obeyed.The point E represents the limit of proportionality between stress and strain. If the load applied to the wire is removed, the wire completely regains its original length.Hooke’s law is obeyed up to point E. The value of stress corresponding to point E is called the elastic limit of the material of the wire. From this, we can see that the initial part OE of the graph is a straight line, which shows that stress is directly proportional to strain.The curve obtained is termed as a stress-strain curve. below in this the graph of stress is along Y-axis and strain along X-axis is plotted. And because of this, the elongation in the wire is measured as shown in fig. Mathematically, true stress(σ T) and engineering stress(σ E) relation is represented asĪccording to Hooke’s law if a load is applied to the wire in steps until the wire breaks. \(Engineering~Stress(\sigma_E)=\)Īs the deformed area will be less than the initial area of the specimen hence, the True stress will always be greater than Engineering stress for the same load.Īlso, the curved traced by True stress will be above the Engineering stress and moves left with increases in load and corresponding strain.īut we cannot measure the change in cross-section area during the process in the universal testing machine(UTM), therefore engineering stress we study. True stress-strain and Engineering Stress-strain Curve:Įngineering stress-strain is lower than true stress-strain because we consider initial area only throughout the process while in true stress-strain curve we consider the deformed area and which is deformed drastically, therefore stress will increase.
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