Issue 30

W. Tao et alii, Frattura ed Integrità Strutturale, 30 (2014) 537-544; DOI: 10.3221/IGF-ESIS.30.64 541 According to the known area, the computer can calculate the pixel point, and thus obtain the value of the length and width. 4) Perimeter p The contour perimeter level reflects the overall extension of the crack. The number of pixels on the border is calculated after acquiring the boundary contour of the cracks. Then we can obtain the contour perimeter. 5) Circle C Circularity reflects the complexity of the target boundary. The computational formula is as follows. The probability of appearance of the gray value 2 4 C A P   A: When the pixel value of the crack target in a previously assumed binary image is 1 , the area of the connected domain will be P . 6) External Ellipse The external oval reflects the position and range of the area. The center coordinates   , X Y in the ellipse can be represented by the first moment:     , , 1 1 , x y R x y R X x Y y A A       where A: The area of the range; R: The connected domain needs measuring. Example verification This paper chooses 10 underwater bridge crack pictures the size is 640 480  according to the description and analysis of the crack target features. A statistical analysis is performed for a number of typical characteristic quantities of the crack features and the crack area. A gray feature of its external rectangle is performed. The characteristic quantities are shown in Tab. 1. Order number Area Perimeter External rectangle width External rectangle height Aspect ratio Circularity 1 6301 1829.36 639 86 7.43 0.0232 2 2098 1170.11 420 54 7.78 0.0198 3 3591 1640.05 621 59 10.53 0.0174 4 2180 1299.99 395 71 5.56 0.0161 5 2503 1102.27 53 309 5.83 0.0312 6 8053 1750.75 639 40 15.98 0.0322 7 5496 1598.88 419 130 3.22 0.0269 8 6208 2601.47 640 115 5.57 0.0115 9 6697 2259.63 640 113 5.66 0.0159 10 3559 2034.64 640 72 8.89 0.0110 Table 1 : Result of the crack image’s characteristic quantities (unit: pixel).