Here we collect some ideas and quotes about the process of decay. This relates to the themes of cancer, entropy, defence and immunity.

Fibreglass and other fibre composites get much of their toughness from crack-stopping. So do plywood, rip-stop nylon, knit fabrics and felt. The staggered pattern of brickwork is designed to divert and control cracking, whether in a vertical wall or in a paved sidewalk. (Bricklayers are conscious of this, and take steps to prevent cracks ‘running’ in places where the bond-pattern has to break, as at a door frame.) Reinforced concrete is reinforced only partly to give concrete useful tensile strength—to a large degree it’s there to blunt cracking. (That’s why stucco is applied over chicken-wire nowadays, and not wooden battens.) Wire rope is favoured over chain for many uses, precisely because a crack in one strand cannot propagate across the whole cable. Even spaced tank-armour applies a similar principle, though in compression rather than tension.

Engineers certainly try to avoid cracks in the first place, especially in solid metal objects, but it’s been generations since they accepted catastrophic failure as the price of a crack appearing. The mechanisms have been well understood since A A Griffith’s work in the UK in the 1920s. I read about them first in J E Gordon’s ‘The New Science of Strong Materials’ in the 70s.

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