Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 7M

Year: May 2015

• The term deformation refers to any changes in the shape or size of an object due to-
• an applied force (the deformation energy in this case is transferred through work) or
• a change in temperature (the deformation energy in this case is transferred through heat).
• The first case can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting).
• In the second case, the most significant factor, which is determined by the temperature, is the mobility of the structural defects such as grain boundaries.
• Deformation is often described as strain.
• As deformation occurs, internal inter-molecular forces arise that oppose the applied force. If the applied force is not too great these forces may be sufficient to completely resist the applied force and allow the object to assume a new equilibrium state and to return to its original state when the load is removed.
• A larger applied force may lead to a permanent deformation of the object or even to its structural failure.

• In the figure it can be seen that the compressive loading (indicated by the arrow) has caused deformation in the cylinder so that the original shape (dashed lines) has changed (deformed) into one with bulging sides. The sides bulge because the material, although strong enough to not crack or otherwise fail, is not strong enough to support the load without change, thus the material is forced out laterally.

Slip mode of deformation :

• Slip is defined as sliding of blocks of crystal over one another along definite crystallographic planes called slip planes. So it is the relative displacement along a definite direction.
• When slip takes place, one part of the lattice moves with respect to the other.
• Generally slip plane is the plane of highest atomic density and slip direction is the closest packed direction within the slip plane.
• This is because the bond between these planes is weakest, so when force is applied in proper direction, relative movement takes place very easily.
• The shear stress required to produce slip on a crystal plane is called the critical resolved shear stress.
• A slip plane and a slip direction together make a slip system. There are twelve slip systems in each of FCC and BCC crystals, while there are only three in HCP crystal.
• Slip planes are generally fixed for every type of lattice. In FCC Lattice (111) plane, in BCC lattice (110) plane and in HCP lattice (001) planes are the most common slip planes.
• As the magnitude of the applied stress increases, the number of active slips planes and the distance of slip along these planes increases. The extent of slip is limited to an interatomic distance or an integral multiple of that distance.
• Slip in all metals of similar crystal structure will occur along the same crystallographic planes and directions. The slip on each plane may be of the order of several microns.