Commonly used methods for non-destructive testing are fluorescence and colour contrast liquid penetrant test (PT), which are perfect methods for detecting surface defects such as cracks. Both methods have their own advantages and disadvantages and require different inspection conditions.
Fluorescence penetration testing is one of the earliest methods of non-destructive material testing, used as early as the 1930s. This method uses ultraviolet (UV) light to visualise defects. A fluorescent penetrant infiltrated into a surface defect, once developed, is indexed by UV light, making it easy to detect indications of defects. It has the advantage of being extremely sensitive, able to detect even the smallest defects down to 0.5 microns. However, it has the disadvantage that special darkened conditions are required to use ultraviolet light effectively, and the long-term health effects of fluorescent materials are known.
Colour contrast penetration testing, also known as red and white testing, became popular in the 1960s. This method uses coloured penetrant materials (green and red used to be common, now typically red) and displays defects in contrasting colours so that they are noticeable during the assessment. The advantage is that it does not require special lighting conditions and can therefore be used flexibly in different environmental conditions as long as the illumination level is at least 1000 lux.
This method is good for detecting volumetric and planar defects, but is not suitable for detecting clustered defects.
Interestingly, non-destructive testing of materials, including penetration testing, also plays an important role in space research. During penetration testing of the Apollo spacecraft, a hairline crack was found in the engine, which ultimately led to the Apollo 13 mission being scuppered in 1970.
In summary, fluorescence and colour contrast liquid penetrant testing each have their own strengths and weaknesses. In their selection, it is important to consider the test conditions, the flaw size to be detected and the available instrumentation. Both methods are important tools for non-destructive material testing and play a significant role in the production of safe and reliable industrial products.
