## 1 Introduction  
Stainless-steels are an important class of alloys considering their plentiful applications in daily life. The durability and anticorrosive properties of stainless-steel attract huge budget allocation by countries such as China in its large-scale production and consumption [1]. It is one of the most widely used materials in producing items ranging from cooking utensils [2] to high-rise buildings [3], from small toys to huge air crafts, and from small containers to huge ships and even space vehicles [4]. Application areas of steel include construction, transportation, nuclear and chemical industries, medical equipment, communication and computing devices, roads and bridges, etc. [5, 6]. In fact, its omnipresence in our daily life makes it difficult to set out all concomitant applications. Due to its widespread applications, it is direly important to ensure its good structural integrity at the time of manufacturing and with the passage of time to cater to aging effects. Micro-cracks and defects in steel due to mechanical strain or environmental conditions have always attracted researchers to devise reliable and quick methods to achieve timely, successful, and precise detection [7, 8, 9, 10, 11, 12]. In a report card for American infrastructure 2021, the American Society of Civil Engineers (ASCE) compared the condition of different infrastructures in the USA for several years and estimated the cost of upgrading them to safe limits for current and future usage. Most of these infrastructures incorporate extensive use of steel, including dams, bridges, ports, roads, rail, and aviation, and were found to not be in good condition and require high-cost renovation [13]. In comparison to destructive techniques for defect monitoring, nondestructive testing (NDT) techniques have several benefits: good sensitivity, cost-effectiveness, sample usability, possibility for non-contact, and easy setup, which requires no transducer arrangements, possibility of deployment in harsh and sensitive environments, broad sensing range,and capability to test huge, moving, and remote targets [14; 15; 16]. Among NDT methods, visual inspection is the oldest method for defect analysis and is currently widely used. Currently, due to technological advancements, these methods have a magnification setup or an electronic visual sensor [17; 18]. This technological advancement has enabled humans to develop state-of-the-art robotic systems that incorporate vision sensors (digital cameras, lenses, and lights) to continuously acquire visual details in hazardous environments [19]. Such vision-based visual inspection techniques have also proven their benefit in civil structure crack detection and amazing characteristics of automatic collection and analysis of visual data [20]. Visual inspection often helps to perform routine inspection and maintenance of bridges, although it is time-consuming, requires significant expert personnel, and is consequently expensive. Vibration-based NDT damage detection techniques have been used by engineers and researchers, particularly in the aerospace and offshore oil industries, since the late 1970s [21]. Using these techniques, damage sizes and locations are estimated by comparing the modal data of an intact structure and a damaged structure [22]. Common parameters in vibration-based methods are natural frequency and displacement/strain mode shape. However, vibration-based techniques are limited due to their low sensitivity and accuracy, especially when the damage size is small. In industry, one of the most commonly observed material degradation mechanisms due to prolonged cyclic stress is fatigue. With the passage of time, this phenomenon reduces the effective service life of engineering structures and components [23]. Fatigue causes almost 90% of industrial component failures that suddenly occur without warning [24]. Therefore, NDT of industrial structures has always inspired researchers to innovate methods for more effective and precise results [25].