The consequences of something breaking can be a pest, or utterly disastrous, as when the pedal drops off one’s bike, but without it, biting and crunching, breaking into crisp packets, pulverizing coal, oil drilling and many other processes would be impossible. The most dramatic failures are catastrophic, but sometimes they can be very gradual even in the most brittle materials.
Here we discuss what determines when a material will break, and whether failure will be catastrophic or more gradual. We show cracking is controlled by the energy changes that occur - it is not the stress at the crack tip that is important..
The emphasis here is on brittle fracture, and although all of this is relevant to metals, we do not discuss the details of ductile fracture.