Stress-Strain Diagrams: Stress-strain diagrams can be generated for axial tension and compression, and shear loading conditions. Tension specimens have a narrow region in the middle along the so-called gage length. Compression specimens are much thicker and shorter than tension specimens with no cross-sectional variations. In either case, data are collected in terms of applied force and the change in the gage length. The normal stress is obtained by dividing the applied force by the cross-sectional area of the specimen, and the normal strain is obtained by dividing the change in gage length by its original value. The plot of stress versus strain gives the stress-strain diagram.
If the original cross-sectional area is used to calculate the stress for every value of applied force, then the resulting diagram is known as the Engineering Stress-Strain Diagram. However, if the applied force is divided by the actual value of the cross-sectional area, then the resulting diagram is known as the True Stress-Strain Diagram. Therefore, in engineering stress-strain diagram the ultimate and failure strength points do not coincide whereas in the true diagram they do. The difference in the two diagrams becomes apparent in the inelastic region of the curve where the change in the cross-sectional area of the specimen becomes very significant.
Things We Can Learn from Stress-Strain Diagrams:
Almost all of the mechanical properties of a material can be obtained from its axial and shear stress-strain diagrams. Poisson's ratio is the only exception, but it can be determined by knowing the change in the cross-sectional area of the specimen at a point along the elastic region of the stress-strain curve. The information we can obtain from the stress-strain curve include:
Axial tension/compression stress-strain diagram:
Shear stress-strain diagram: