Question

1. An alloy of steel has a Young’s modulus of 180 GPa , elastic limit of 0.002 and ultimate tensile strength of 860 MPa . An alloy of aluminium has a Young’s modulus of 70 GPa , elastic limit of 0.004 and ultimate tensile strength of 110 MPa . a) Sketch a stress-strain graph for both of these alloys, plotting them on the same graph b) Label significant positions on the graphs c) Compare and contrast the mechanical properties of these alloys

Answer 1

(a) Here the graph 1 shows the plot of alloy of steel and graph 2 shows the plot of alloy of aluminium. The alloy of steel has higher value of young's modulus, that's why graph 1 is rising quickly with higher slope.

Graph a BI A2 B2 Da Stress Grapha Strain

(b) The description of significant points on the graph,

A₁ and A₂ = This point is known as the "Elastic limit", also known as "Proportionality Limit". After this point the stress is no more linearly proportional to stress.

B₁ and B₂ = This point is known as "Yield Point".

C₁ and C₂ = This point is known as "Ultimate Tensile Strength". After this point the material cannot take any more stress and stress has an opposite effect over strain. Allot of steel has larger value of tensile strength, that's why C₁ is ahead of C₂ along strain axis.

D₁ and D₂ = This point is called "Fracture Point". At this point the material breaks or fractures.

(c)

(i) Clearly, alloy of steel is more elastic. It reserves high elastic energy.

(ii) Because of higher young's modulus very small change in strain needs quite high stress to be applied. Therefore, the tendency of shape retainment is higher in alloy of still than in alloy of aluminium.

(iii) Alloy of steel has a very high tensile strength which implies the strain follows the stress up to a really long range. Therefore, alloy of aluminium is more fragile than alloy of steel.