Question

a) Differentiate between static and dynamic loading? Provide 02 examples for each.

b) Describe the mechanism of crack propagation for both ductile and brittle modes of fracture. Please benchmark at least 05 different research papers where necking (ductile materials) and fracture (brittle materials) have been captured using a scanning electron microscope.

Answer 1

a) Static load:

The load is constant (does not vary) with respect to time. The static load result only in displcement (no variation in velocity or acceleration). Hence static problems are very simple to solve. It has only one solution. eg: constant load, self weight

Dynamic load:

Here the load varies with respect to time. The dynamic load results in displacement, velocity and acceleration. Therefore the dynamic problems are complex to solve. It can have infitely many solutions too. eg: cyclic loading, vibrations, impact loads

b) The fracture mode is termed as ductile or brittle depending on the material's ability to undergo plastic deformaition.

In ductile metals, the material undergoes plastic deformation before failure. The ductile fracture can be explained with the help of below figure. There is a specimen which is pulled (force is applied) in both the directions. Initially there is small dislocation that occurs in the material after which these dislocations expands. Then many dislocations join together and there is a severe necking of the materail. Finally the material breaks. This type of failure is called as cup and cone failure or shear failure.

In brittle material, the material fails without any warning. There is no plastic deformation of the material. A small crack is created within the material which cannot be seen. This cracks propagates and finally a catastropic failure occurs. The sudden failure occurs withour any necking as shown in below figure.