Question

Ductile fracture occurs due to

• A. Direct stress
• B. Shear stress
• C. Compressive stress
• D. Bending stress

Hint:

Brittle fracture is controlled by normal stress, while ductile fracture is governed by shear stress.

Answer 1

The Correct Option is B

Ductile fracture happens because dislocations glide along slip planes, and that glide is only possible under a resolved shear component of the applied load.

1. Direct stress: pulls planes apart but does not itself cause the sliding motion of slip.
2. Shear stress: directly drives dislocation glide on slip planes, exactly what produces the necking and 45 degree cup-and-cone surfaces seen in ductile fracture.
3. Compressive and bending stress: both can generate internal shear components, but it is that resolved shear, not the compressive or bending load itself, that causes the slip.

So ductile fracture occurs due to shear stress.

Answer 2

A revealing way to confirm this is to look at what a typical ductile tensile fracture surface actually looks like and match that appearance to the stress responsible for it.

1. Direct stress: a fracture governed purely by direct (normal) stress tends to produce a flat surface perpendicular to the load, the signature of brittle cleavage, not the fibrous, cup-and-cone surface typical of ductile failure.
2. Shear stress: ductile tensile specimens neck down and fail along surfaces inclined at roughly 45 degrees to the loading axis, the orientation of maximum resolved shear stress. This slanted, dimpled fracture surface is the visible fingerprint of shear-driven slip and void coalescence.
3. Compressive stress: a compressive load tends to cause bulging or barreling rather than the tensile necking and fracture pattern associated with ductile fracture in typical test specimens.
4. Bending stress: bending failures show tension cracking on the convex face, but the actual local yielding and slip at the crack front is still governed by shear, not by the bending stress distribution itself.

The 45 degree, dimpled fracture surface characteristic of ductile failure is the direct fingerprint of shear stress at work.

Therefore, the correct answer is Shear stress.