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Many of the types of weathering-induced degradation are not measured in an accurate quantitative manner. The cracking of a coating is a good example of such a parameter. Visual scales are used that can be either a photograph or a representative drawing of the degradation. Visual evaluation of cracking is particularly susceptible to operator bias and may not produce a repeatable or precise result. It is desirable to make a more exact measurement to discriminate more easily between coatings of different performances. It is also desirable to measure quantitatively the initial stages of cracking since these give the ability to shorten test times and develop more effective pass/fail criteria.
Technology is now available to measure cracking in a precise and repeatable way using digitized photographs or other images of the coatings and image analysis software techniques. A set of cracked coatings were measured using both image analysis and multiple visual inspectors, and the results were compared. The coatings used exhibited a wide range of colours and different levels and sizes of cracking defects. The specimens were evaluated by a team of professional coatings inspectors and were rated according to several industry standard scales, such as ASTM, and TNO(Toegepast Natuuwetenschappelyk Onderzok) of Delft, Netherlands. Items of interest were precision of measurement, reduction of operator-induced bias, and the speed of repetitive measurement. The data were analyzed statistically and the results presented.
image, digizitzed, visual scale, image analysis
Manager, Research & Development, South Florida Test Service,
Materials Engineer, Research & Development, South Florida Test Service, Miami, Fl