**4. Corrosion fatigue damage mechanism and damage evolution model of galvanized parallel steel wire**

#### **4.1 Steel wire mesoscopic view analysis based on image grayscale**

In order to obtain a quantitative evaluation index, the corrosion degree of the steel wire is evaluated by a gray image-based method to calculate the corrosion pit density of the steel wire, and the degree of corrosion is defined by the percentage of corrosion pit density. Use GSA image analyzer software to perform grayscale analysis of eroded images. GSA Image Analyzer is an image analysis tool that supports all types of 2D images with comprehensive functions to identify and count the number of objects; supports length and area calculation; supports patterns from files, scanners, and microscopes; and can be used in fields such as cell analysis and materials engineering. Since the image formed by the electron beam scanning of the corroded steel wire is a true color image and cannot be recognized directly on the computer, it is digitized and converted into a grayscale image using GSA Image Analyzer. The principle of digital processing is to convert a true-color image into an array of pixels that can be represented numerically, with the value of each dot being the grayscale value of that dot. The spatial resolution of the original image is related to the number of pixel dots there are, the more dots, the higher the pixel resolution is.
