Tag: elastic and plastic substances

Two prominent mechanisms of plastic deformation, namely slip and twinning occur . 
1. Slip involves sliding of blocks of crystal over one other along definite crystallographic planes, called slip planes. During slip each atom usually moves same integral number of atomic distances along the slip plane producing a step, but the orientation of the crystal remains the same.

2. Portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite, symmetrical way. The twinned portion of the crystal is a mirror image of the parent crystal.

Substances that break just after elastic limit is reached are known as brittle substances

The yield point is the point on a stressstrain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Yielding means the start of breaking of fibers.

The correct option is option(a)

Plasticity is defined as the property which enables a material to be deformed continuously and permanently without rupture during the application of force. This deformation determines the viscous behavior of the material and is irrecoverable.

Beyond the elastic limit, permanent deformation will occur. The elastic limit is therefore the lowest stress point at which permanent deformation can be measured. At yield point, permanent deformation occurs or the substance becomes plastic.

The correct option is option (b)

The yield point is the point on a stressstrain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Yielding means the start of breaking of fibers.

Thus, Hooke's law dosent hold good in this region, as stress is not proportional to strain in this region

Modulus of elasticity gives the idea of elasticity. For above materials, values of E are:
Glass: $50-90 GPa$
Rubber:$ 0.01-0.1 GPa$
Steel:$ 200 GPa$
Copper: $117 GPa$