Question

Q3. Describe with sketches an intergranular fracture and a fatigue fracture and the reasons of their appearance. Explain with materials science and engineering knowledge discussed in the class through Zoom (10 points)

Answer 1

Intergranular fracture: When a fracture occurs through grain boundaries it is called as intergranular fracture.

The reasons for its occurrence are

(i) Depletion of elements at grain boundaries

(ii) Growth of precipitates at the grain boundaries and faster nucleation.

(iii) Due to elevated temperature, there is grain boundary crack and cavity formations which results in an intergranular fracture.

(iv) Weakening of the grain boundaries due to chemical action or oxidation or embrittlement.

(v) Crack growth as a result of the quenching process(rapid grains formation) will also lead to Intergranular fracture.

(vi) Due to insufficient slips when the specimen can't accommodate plastic deformation crack develops across grain boundaries.

Fatigue Fracture: It is defined as a material failure that occurs due to excessive cyclic loading. Prior to the final fatigue fracture, many different microfractures occur and eventually, the repeated dynamic loading helps propagate the cracks. The reasons for its occurrence are:

Intergranular Fracture: 90 -Grains Intergrcaluar Fracture Grain bondaries Path of crack

(i) A fatigue fracture depends prominently on the type and shape of the material.

(ii) Fatigue fracture occurs due to cyclic stresses that are less than the ultimate tensile stress, or even the yield stress of the material.

(iii) Due to the accumulation of dislocations near-surface stress concentrations.

(iv) Due to insufficient slips when the specimen can't accommodate plastic deformation crack develops across grain boundaries.

(v) Due to the formation of sufficient microcracks due to cyclic loads.

(vi) Material properties also determine the fracture resistance ( Aluminum has less fatigue resistance than steel)

(vii) shapes of material like materials with sharp corners are more liable to fatigue fracture, similarly, the materials which have sharp grain boundaries are also liable to fatigue fracture.

Fatigue Fracture? - more streved grains due to Patique Fatique Gact Propagation