impact testing
/ Charpy: SHAR-pee; Izod: EYE-zod /
How do you measure whether a steel will shatter or bend when hit suddenly — not slowly squeezed, but struck? You hit it and see how much energy the blow soaks up. Impact testing does exactly that: a heavy pendulum swings down and smashes a small notched bar, and the machine records the energy absorbed in breaking it. A lot of energy means tough; a little means brittle.
The two standard versions are Charpy and Izod. In the common Charpy V-notch test, a square bar with a machined notch is laid horizontally, supported at both ends, and struck behind the notch by the pendulum; Izod clamps the bar vertically and strikes the free end. The pendulum starts at a known height, so its energy is known; whatever height it swings up to on the far side tells you how much energy the specimen absorbed, read straight off the dial in joules. The notch is deliberate — it creates a sharp stress concentration and the tri-axial stress that promotes brittle behaviour, making the test a severe, repeatable screen.
Impact testing is cheap, fast, and the standard way to map the ductile-to-brittle transition: run the same steel at many temperatures and plot absorbed energy versus temperature to find where it turns brittle. Its limitation is that it gives a comparative energy number, not a design property — you cannot plug a Charpy value directly into a stress calculation the way you can with fracture toughness K_IC. It ranks and screens materials and pins down the transition temperature; fracture mechanics does the quantitative design.
A pipeline steel that absorbs 150 joules in a Charpy test at 20 degrees C but only 15 joules at -40 degrees C is telling you it is tough in summer and dangerously brittle in an Arctic winter — so a grade with a lower transition temperature is specified for cold service.
A swinging pendulum snaps a notched bar; the energy it absorbs grades toughness.
Charpy energy is a comparative screening number, not a true material property you can design with. Two bars of the same nominal energy can differ, and you cannot convert joules directly into a critical crack size — that needs the fracture-toughness test.