Railcar wheels and axles were some of the first items to be inspected using a form of nondestructive testing. To detect surface cracks on the components, the parts were covered in oil, then cleaned and powder was applied. If a crack was present on the component, the oil would seep into it. The powder would stick to the oil in the crack, making it more visible. This is called the "oil and whiting" method. The basic idea of liquid penetrant testing is still the same, but special oils have been created since. The use of ultraviolet light to make the indications more visible is popular as well.
Wilhelm Conrad Roentgen discovered X-rays in 1895 during his experiments with cathode rays. After experimenting with the new technology, he created the first industrial radiograph of weights in a box. His discovery provided the ability to look for internal defects in components. X-ray for industrial uses became popular during the 1930's.
Magnetism for inspection was used as early as 1868 to locate cracks in cannon barrels. They would magnetize the barrel, then run a magnetic compass along the length of the barrel. The movement of the compass's needle would indicate flaws present. Magnetic Particle Testing was also used in the early 1900's to find cracks in the barrels of guns. William Hoke introduced magnetic particles to the inspection method in the 1920's. He discovered that a flaw will cause the magnetic field to distort and the particles will gather in the flaw. This helped make the indications more visible.
Ultrasonic Testing is the most recent inspection method to come into industrial use. Ultrasonic Testing of castings was first proposed after the sinking of the titanic. During World War II, Ultrasonic testing was used to detect laminations in plates and non-metallic inclusions in hot-rolled profiles. Other NDT methods were unable to inspect these objects where Ultrasonics could.
During the 1970's, there was a major change in the field of nondestructive testing. Inspection methods had been improved upon, and smaller flaws could be detected. The industry saw an increase in rejected parts, and people started to evaluate a flaw based on how long it could be in operation before failing. Predicting weather or not a component would fail during use became critical, and now detection is not enough. This lead to the introduction of Quantitative Nondestructive Evaluation (QNDE).
Where is the future of nondestructive testing headed? Globalization and improved technology have impacted the industry and will be major guiding forces going into the future. Methods will continue to be improved on, and maybe new methods will be developed. The future of NDT looks exciting.