The Color of Stress

Engineers typically use strain gauges to measure the stresses on automobile parts, chains, bridges and other weight-bearing objects. But the load stress on this heavy-duty hook was visualized with an infrared camera and processor developed by Stress Photonics, a company in Madison, Wisconsin. The infared camera works by picking up the minuscule temperature changes (down to less than four-thousandths of a degree) that result when a metal is stressed. Stretching cools the metal, just as a gas cools when it expands; compression heats it. The hook, used for lifting cargo, bore loads of up to 700 pounds. In this image, orange regions show where the hook is being stretched by the weight; blue regions show compression; the darkest areas are under no stress. In a poorly designed hook, says Stress Photonics engineer Brian Bartel, you would see a bright orange spot on the inside curve--a spot where the hook would most likely snap with repeated use. Stress Photonics recently signed a contract with the Federal Highway Administration to study using the technology to find cracks and corrosion in bridges.