Question

Describe the general theory of stress and strain. Explain the properties and characteristics that can be determined using a tensile test. Give general comparison of the different samples. This should be 1-2 pages typed

Answer 1

Stress is the internal resistive force developed on a unit cross section area when an external load is applied. Strain is ratio of change in length to the original length. The most general and fundamental theory relating stress and strain is Hooke's law. According to his theory stress is directly proportional to the amount of strain a material undergoes till elastic limit. Elastic limit is the limit till which when an object is compressed or elongated by applying external force, it regains its original shape and size. Once, this limit is crossed the material undergoes plastic deformation and elastic behaviour is not observed. At a particular stress material becomes unable to resist the load and hence fracture occurs.

Tensile test helps in determining properties like Young's modulus of material (E), yield stress, ultimate stress, fracture stress, ductility, toughness etc. In a tensile test load is applied gradually and elongation or compression is noted down till fracture. This load Vs elongation data is converted to stress Vs strain curve ( stress = Load / Area ). Then the area of under that curve represent toughness. Percentage elongation represent ductility, and Young's modulus is found by slope of stress strain curve.

The samples can be brittle and ductile. A brittle material will not show a distinct unique yield stress point as fracture occurs before necking. The area under curve for brittle material is less compared to ductile and that's why they are less tough.