There are 4 types of defects that, according to Tata Steel who are experts on the subject, cover more than 95% of all defect found on rails during production. It is the algorithms for the effective detection of the defects that form the substance of this thesis.
The defects have been referred over the years as rolled in scrap, as can be seen in Figure 3.2 (a), a line on top of the rail, as can be seen in Figure 3.2 (b), tiger stripe, as can be seen in Figure 3.2 (c), and a wire defect, as can be seen in Figure 3.2 (d). All of the mentioned defects appear on the top view of the camera system and on the bottom view and are very difficult to detect by established computer vision techniques.
Figure 3.2: Types of defect, (a) Rolled in scrap, (b) Line on top, (c) Tiger stripes, (d) Wire defect
28 3.3.1 Rolled in Scrap
The rolled in scrap defect is a type of defect that appears when the rail exits the furnace. It appears on the side of the foot of the rail. It is due to the side of the rail being scratched by any sharp surface or material coming out of mill while passing, leading to protruding and/or extruding material on the edge of rail‟s foot. It can be detected by measuring any irregularities at the bottom of the rail edges i.e. deviation from a straight line.
Rolled in scrap can usually be seen from a particular camera viewpoint as indentations at the edge of the rail. Isolating them involves the detection of the edge of the rail followed by detection of non-straight lines or derivatives in the edges, in other words the detection of the defect.
Rolled in scrap can be seen clearly in Figure 3.3. The uneven edge of the side of the foot of the rail is clearly visible, in this image.
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Figure 3.4: Irregularities marked for rolled in scrap manually
3.3.2 Line on Top of Rail
A line on top of rail appears as a straight line at the top of the rail and can have various origins. They are repetitive and short pitch; typically less than 400 mm. They are horizontal straight lines right in the middle of the rail surface. One of the causes might be the worn vertical entry roller guide, in one of the finishing mill stands. It can be avoided by replacing the roller with a clean set and adjusting to give more clearance if possible.
Figure 3.5 and Figure 3.6, represents line on the top of the rail defect, example images. It can be seen from the example images that lines on the top are horizontally running lines, which may break after a certain length and might not be present on the successive images. The random lines present on the rail‟s surface need to have some reasonable length to quality to be a line defect. The lines have to be present across most of the consecutive images to be a valid defect.
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Figure 3.5: Original rail image showing Line on the top of rail type defect highlighted with while color.
Figure 3.6: Another example image of a rail with Line on the top of rail type defect.
3.3.3 Tiger Stripes
Tiger stripes appear as dark areas suggesting patterns on a tiger. They occur when the rail is left for a longer time under the cooling water than required. They occur at the top of the rail as known as crown of the rail. Hence, they are mostly captured by the top camera which is camera 4 or the side ones; camera 3 and 5.
They are also referred as “flow marks” and are actually uneven distribution of metallic outer surface. They might be caused by unequal or uneven spray pattern of cooling water. Even the tilted or too close spray of cooling water on the hot rail surface can cause the tiger stripes to appear on the surface.
The marks are of variable sizes depending on the cause, which makes them difficult to detect. They can either be as large as shown in the Figure 3.7 or as small as shown in Figure 3.8. However, they have characteristic shape, which appears like a roughly drawn less than “<” sign of various shapes and sizes. Moreover, they are generally present on many successive images. These features help in Tiger Stripes detection.
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Figure 3.7: Large tiger stripe pattern – Original image
Figure 3.8: Original rail image with small tiger stripe type defect pattern
Figure 3.9: Small size tiger stripe pattern – Close view of the image displayed in figure 3.8.
3.3.4 Wire Defect
A wire defect is a line type defect that affects the base of the foot in a central region. It appears under the rail and is the most difficult and complicated defect of all to detect from a computer vision aspect. This defect can appear as straight lines, curves
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or even random arbitrary shapes, with varying size.
Wire defect is caused by materials forcing out through the roll collars. Its maximum frequency is 4 instances per rail and it can be up to 0.5 mm deep. This defect can be avoided by rectifying the mill. Its early detection and quantification is necessary to reduce the cost and safety by very large margins. However, the variation in this type of defect made them extremely hard to detect, while maintaining an acceptable level of false positives.
Figure 3.10: Original rail image with wire defect captured by camera 1.