Can you briefly describe the origins of liquid penetant (PT) inspection?

Answered by: Michael C. Schantz

The history of liquid penetrant testing can be traced back to the “oil and whiting” method. From the late 1800’s until the 1940’s, this technique was commonly used by railroad companies for finding cracks on steel locomotive parts such as rods, axles, crank pins, etc.   The advent of magnetic particle inspection in the 1940’s proved the “oil & whiting” method to be unreliable.  Until that time the “penetrant” that was used was typically a heavy lubricating oil diluted with kerosene.  The imprecise nature of the “penetrant” solutions, coupled with a lack of set procedures resulted in too many variables to provide consistency, and magnetic particle inspection was considered a more reliable alternative for testing magnetic iron and steels.

 

                The advent of World War II brought a new challenge to the inspection world when the extensive use of non-magnetic materials such as aluminum, magnesium and stainless steels in the aircraft industry.  At the time, radiography and magnetic particle testing was the only truly non-destructive methods available. Radiography was reliable for finding internal flaws in castings and forgings but was much less reliable for finding surface discontinuities. A new method had to be developed.

                Many methods were tested, but proved to be impractical or unreliable until Robert C. Switzer of Switzer Brothers, Inc. (now the Day-Glo Corp. of Cleveland) began working with colored dyes, in particular fluorescents, to locate defects in materials. He was successful in creating a method using fluorescent dyes and a black light which provided a high contrast on the part being tested. In February 1942 an exclusive license was given to the Magaflux Corp. to develop and perfect the method. By July 1942 Magnaflux introduced “Zyglo” the first commercially available product for liquid penetrant testing. Seventy years later, through numerous advancements in techniques and materials, fluorescent liquid penetrant inspection with Zyglo is still a consistently reliable and trusted inspection method that we utilize on a daily basis.