For example, an East Coast railcar wheel inspector had a problem. The inspector needed to inspect the wheels to meet required industry standards. The product is a wheel about 10 feet in circumference and has a 4-inch wide "tread" area inspected for defects from the surface of the tread down to 3 inches deep. It is a solid piece of steel with some geometry to it and it will undergo intense pressures that can turn a defect such as a crack along the grain boundary into a catastrophic failure.
In the past, an operator would paint the outside of the wheel with a gel couplant and manually scan the part's circumference with a handheld, digital ultrasonic flaw detector that uses a 1/2-inch diameter, single-element transducer. The operator takes 1/2-inch readings across a 1-inch swath and then moves to the next swath until the entire 4-inch by 10-foot circumference is inspected.