Question

Experimentally it has been found that the toughness to fracture, varies with the thickness of the test specimen, under what conditions a minimum value (flat effort or flat deformation) is obtained and why ?.

Answer 1

Ans. Toughness is defined as the total energy absorbed by a material upto fracture. This is an important mechanical property in material for metal forming application.

It is given by total area under load vs. elongation diagram upto fracture.

Modulus of toughness is defined as toughness per unit volume.

Modulus of toughness= toughness/ Volume

Units - J/cm^3

Here experimentally we get toughness to fracture varies with thickness of the material.

This is achieved by Tension Test of the material.

In this test once the stress goes beyond the yield strength there will be decrease in cross section area hence stress increases.

This increase in stress is compensated by increase in strength by work hardening. A unique neck will appear at a point of plastic instability where the increase in strength due to work hardening fails to compensate for decrease in cross sectional area.

Gradually specimen fails like cup and come fracture.

Minimum value of flat effort or flat defirmdefor is obtained by yield point phenomenon.

Elastic - plastic transition occurs in specimen when load is applied and abruoabr this yield phenomenon is observed where thickness or area of specimen is minimum.

This is due to plastic deformation is initiated with an actual decrease in stress, continued deformation fluctuate about some constant stress value which is called as lower yield stress.

After yielding stress continued to increase in plastic deformation.

This stress at maximum on stress strain curve is called as tensile strength.

This is maximum stress that can be sustained by a specimen before fracture.

Calculation of minimum thickness and area is based on condition of necking .

Necking takes place at maximum load , so using true tress strain relationship we calculate strain and hence all other related parameters.