The Izod Impact Test
Measuring toughness by breaking things on purpose.
Origin
Devised by Edwin Gilbert Izod and described to the British Association in 1903, developed in Sheffield at the heart of the British steel industry.
The catch
Hardness and toughness are not the same thing, and confusing them breaks machinery. Glass is hard and shatters; lead is soft and bends. Izod measures the second property by clamping a notched specimen, swinging a pendulum into it, and recording how much energy the break absorbed.
Fractures cleanly with little warning. Cast iron, hardened steel, most ceramics and glass.
Some deformation before failure, but not much. High-carbon steels.
Meaningful energy absorbed. General structural steels at room temperature.
Substantial deformation before fracture. Low-carbon and alloy steels.
Bends rather than breaks. Some specimens do not fracture at all and the test reports only that the pendulum passed through.
The notch matters as much as the material: it concentrates stress at a known point so that results are comparable between specimens. Temperature matters too, and dramatically, because many steels undergo a ductile-to-brittle transition as they cool. That is not an academic point. The Liberty ships of the Second World War, welded rather than riveted, suffered catastrophic fractures in cold North Atlantic water, and impact testing at temperature became a routine requirement afterwards. The American Charpy test does the same job with the specimen supported at both ends rather than clamped at one.