Stress- Strain Diagram Part-1

Explain the stress- strain diagram neatly with relevant fig.

To select a material for using it in aircraft, we need to know what its life time is. How much it performs? and many more requirements. All these are for having a safe flight. The life time and every required factors will be known once if we know stress strain diagram.

In order to attain stress strain diagram, we need to have a specimen of desired standards. A standard specimen of the mild steel material is prepared as per BIS standards is given below.

Then we need to fix it properly in the universal testing machine. We can give tensile (widely used) or compressive load to the specimen. Load value has to be adjusted very smoothly. By determining the stress and strain at various magnitudes of the load, we can plot a diagram of stress versus strain.

Some common terminologies 

Load - The force applied to a material during testing.

Strain Gage or Extensometer - A device used for measuring change in length (strain).

Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material at that instant.

Engineering strain- The amount that a material deforms per unit length in a tensile test.

Elastic deformation

When load acts on a material shown, it creates stress. when load is acting on a object, the object undergoes atomic bond changes, when removed it again retains its original bonding. This happens only within elastic limit. Some object is having linear elastic proportion when load applied, some follows non-linear.

Plastic deformation

From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors. After removal of the load, the large number of atoms that have relocated, do not return to original position. Yield strength is a measure of resistance to plastic deformation.

Yield strength Determination:

Typical stress-strain behavior for a metal showing elastic and plastic deformations, the proportional limit P and the yield strength σ_y, as determined using the 0.002 strain offset method (where there is noticeable plastic deformation). P is the gradual elastic to plastic transition.

For more detail Refer part-2