Question

A brittle material typically exhibits substantial plastic deformation with high energy absorption before fracture.

true or false

For transgranualar fracture, crack propagation is along the grains, whereas for intergranular crack propagation is through the grain boundaries.

true or false

during secondary creep the creep rate is constant

true or false

Answer 1

1.

Soln: B: False.

A brittle material exhibits little or no plastic deformation with low energy absorption before fracture.Takes place without any appreciable plastic deformation and by rapid crack propagation .Direction of crack motion is nearly perpendicular to the direction of applied tensile stress.It occurs when the material is in elastic condition.little plastic deformation and low energy absorption before fracture.

2.

Soln: True

Crack propagation is a process of evolutionary geometry driven by relatively high values and gradients in crack front fields. Crack propagation is also categorized by the crack characteristics at the microscopic level. A crack that passes through the grains within the material is undergoing transgranular fracture. A crack that propagates along the grain boundaries is termed an intergranular fracture. Typically, the bonds between material grains are stronger at room temperature than the material itself, so transgranular fracture is more likely to occur. When temperatures increase enough to weaken the grain bonds, intergranular fracture is the more common fracture mode.Intergranular fractures are brittlein nature, and crack propagation is along grain boundaries.

3.

Soln: True

Creep is a time- dependent deformation under a certain applied load. Generally occurs at high temperature (thermal creep), but can also happen at room temperature in certain materials (e.g. lead or glass), albeit much slower. As a result, the material undergoes a time dependent increase in length, which could be dangerous while in service.

The rate of deformation is called the creep rate. It is the slope of the line in a Creep Strain vs. Time curve.

• Primary Creep: starts at a rapid rate and slows with time.
• Secondary Creep: has a relatively uniform rate.
• Tertiary Creep: has an accelerated creep rate and terminates when the material breaks or ruptures. It is associated with both necking and formation of grain boundary voids.

Creep behavior can be split into three main stages. In primary, or transient, creep, the strain rate is a function of time. In Class M materials, which include most pure materials, strain rate decreases over time. This can be due to increasing dislocation density, or it can be due to evolving grain size. In class A materials, which have large amounts of solid solution hardening, strain rate increases over time due to a thinning of solute drag atoms as dislocations move.

In the secondary, or steady-state, creep, dislocation structure and grain size have reached equilibrium, and therefore strain rate is constant. Equations that yield a strain rate refer to the steady-state strain rate. Stress dependence of this rate depends on the creep mechanism.

In tertiary creep, the strain rate exponentially increases with stress. This can be due to necking phenomena, internal cracks, or voids, which all decrease the cross-sectional area and increase the true stress on the region, further accelerating deformation and leading to fracture