ANS:B - Brittle fracture is caused by the propagation of pre-existing cracks in the material and involves minimum plastic deformation.

Brittle fracture occurs when a material fails suddenly and catastrophically without significant plastic deformation. This type of fracture is characterized by the propagation of cracks through the material, often with little or no warning. Here's an explanation of the statement:

1. Propagation of Pre-existing Cracks: Brittle fracture typically initiates at points of stress concentration or defects in the material, such as microstructural imperfections, voids, or pre-existing cracks. These flaws act as stress concentrators, causing localized regions of high stress where cracks can initiate and propagate.
2. Minimum Plastic Deformation: Unlike ductile materials, which deform significantly before failure, brittle materials exhibit limited or no plastic deformation prior to fracture. Instead, the applied stress causes the cracks to propagate rapidly through the material, leading to sudden fracture.
3. Characteristics of Brittle Fracture:
   • Sudden Failure: Brittle fracture occurs suddenly and without warning, often with little visible deformation or deformation confined to the immediate vicinity of the crack.
   • Cleavage or Transgranular Fracture: In cleavage fracture, cracks propagate across the grains of the material, while in transgranular fracture, cracks propagate through the grains. Both types of fracture result in a flat, shiny surface perpendicular to the direction of crack propagation.
   • Brittle Behavior: Brittle materials typically have limited ability to deform plastically due to their atomic structure and bonding characteristics. As a result, they exhibit brittle behavior under tensile stress, with fracture occurring before significant plastic deformation can take place.
4. Causes of Brittle Fracture: Brittle fracture can be caused by various factors, including:
   • High Stress Concentration: Concentrated stresses at defects or discontinuities in the material can lead to crack initiation and propagation.
   • Low Temperature: Brittle fracture is often more pronounced at low temperatures, where materials become less ductile and more prone to sudden failure.
   • Material Properties: Brittle materials, such as certain ceramics and some types of metals, inherently exhibit limited ductility and are more susceptible to brittle fracture under tensile stress.

In summary, brittle fracture is characterized by the rapid propagation of cracks through a material, typically with minimal plastic deformation before failure. It occurs due to the presence of pre-existing cracks or defects in the material and is associated with sudden, catastrophic failure.