Question

which one is more prone to fatigue: ductile or brittle materials? why?

Answer 1

1. Fatigue is the failure (by fracture) of structures that are subjected to repeated or cyclic loading. By cyclic loading we mean almost any reasonably periodic stress-time variation, e.g., (a) axial tension-compression, (b) reversed bending and (c) reversed torsion or twisting.
2. Undetected flaws or incipient cracks grow to macroscopic dimensions through incremental propagation during each stress cycle and the component quickly undergoes FRACTURE without warning when a crack of critical size is reached.
3. This sequence of events has been repeated in components of rotating equipment such as motor and helicopter shafts, train wheels and tracks, pump impellers, ship screws and propellers, and gas turbine discs and blades.

What is difficult to design against is the fact that under cyclic loading, failure can occur significantly below the tensile or yield stress ?y of the material.

1 Fatigue failure in ductile material:
After a certain amount of plastic deformation, a neck forms in the tensile test specimen, the force for further deformation decreases and, finally, the piece breaks. When fracture occurs after extensive plastic deformation, the test piece fails by ductile fracture.

• Ductile metals neck and display the cup-cone fracture morphology shown in Fig. 1   

Fig 1.0

• The process of ductile fracture may be broadly viewed in terms of the sequential micro-void nucleation and growth processes, schematically indicated in Fig.2

                                               Fig 2

• Te earliest stage of fracture spawns isolated microscopic cavities. These nucleate at inclusions, second phase particles and probably at grain boundary junctions. Micro-voids coalesce then form an elliptical crack that spreads outward toward the periphery of the neck. Finally, an overloaded outer ring of material is all that is left to connect the specimen halves, and it fails by shear.

2 Fatigue failure in Brittle material:

Brittle fracture is the most feared of all. It often occurs under static loading without advance warning of impending catastrophe.
Figure 3 shows a metal bar that failed by brittle fracture.

     Fig 3

• In case of brittle materials subjected to fatigue will display little or no plastic deformation.
• They break without any prior indication, thereby results are Catastrophic.

Conclusion:

Brittle materials subjected to fatigue loading will fail before Ductile materials under same working conditions and fatigue loading.