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

Step 1: Understanding ductile fracture.
Ductile fracture is characterized by significant plastic deformation before failure. It usually involves the formation and growth of microvoids.
Step 2: Role of shear stress.
In ductile materials, failure typically occurs along planes of maximum shear stress. This leads to cup-and-cone type fracture surfaces.
Step 3: Analysis of other options.
Direct, compressive, and bending stresses may contribute to loading, but the actual fracture in ductile materials is governed by shear stress.
Step 4: Conclusion.
Therefore, ductile fracture occurs primarily due to shear stress.

Answer 2

Ductile fracture is driven by the atomic-scale mechanism that actually moves atoms past one another, which is slip. Let's check which stress type is responsible for that slip in each option.

1. Direct stress: A direct (normal) stress pulls atomic planes apart perpendicular to the loading axis. On its own it tends to promote a flat, brittle-style separation rather than the sliding of one atomic plane over another that characterizes ductile behaviour.
2. Shear stress: Plastic slip happens when atomic planes glide over each other along a slip plane and slip direction, and that gliding is driven purely by the resolved shear stress on that plane (as captured by Schmid's law). Necking, void coalescence, and the classic 45 degree cup-and-cone fracture surface in a tensile ductile specimen are all direct evidence of shear-driven slip.
3. Compressive stress: A compressive stress squeezes the material together. While it can still produce shear components internally, on its own it opposes the kind of separation seen in tensile ductile fracture and is not the primary driver named in this context.
4. Bending stress: Bending produces a combination of tension on one face and compression on the other, but the actual atomic slip that leads to ductile failure still occurs because of the resolved shear stress within the material, not because of the bending moment itself.

Since dislocation glide, and therefore ductile fracture, is fundamentally driven by resolved shear stress on slip planes, shear stress is the correct answer.

Therefore, the correct answer is Shear stress.