Ductile failure

In ductile materials, failure is preceded by necking. This is common in most metals at room temperature. This instability is due to the rate of increase in the true stress caused by the reducing cross section exceeding the rate of work hardening as the material is plastically deformed.

Considère’s Construction for the onset of necking is discussed in the Mechanical Testing of Metals TLP.

Plastic deformation, particularly under tension, can easily open up voids at interfaces, inclusions or precipitates. The final fracture surface can consist almost entirely of voids. Microvoid coalescence in the neck leads to crack formation and fracture of the material. 

At the edges, away from the voids, the material may fail entirely by plasticity. Fracture follows the line of greatest plasticity, leaving a 45° surface since maximum shear occurs at 45° to the applied stress (consider Mohr’s circle).

Figure 1: Micrograph 128 from DoITPoMS library - Rough surface of an Al–Mg–Si alloy, failed through microvoid coalescence

In brittle fracture, failure can be hard to predict due to spontaneous and rapid crack propagation. There is no time to replace parts before catastrophic failure occurs. Tougher materials that fail via ductile fracture are favourable due to the ‘warning sign’ of plastic deformation: the crack propagates slowly, absorbing large amounts of energy before fracture.

Failure occurs from flaws in the material. Any surface contact, even between flat surfaces, introduces flaws to a material, assuming there is an edge to at least one of the surfaces. A brittle material used for strength will develop flaws, unable to absorb much energy before fracture. Instead of removing flaws, which is highly impractical, we require mechanisms to improve toughness to prevent crack formation and subsequent growth.