## 3 Current Non-Destructive Testing Practices for Port Cranes and Their Drawbacks
### Common Non-Destructive Testing Methods  
NDT is critical for ensuring the safety and reliability of port cranes. Current NDT practices for port cranes involve various techniques, each with specific applications and limitations:  
* Visual inspection (VI)--the most basic NDT method, in which inspectors visually examine a crane for signs of wear, corrosion, or damage. Although VI is simple and cost-effective, it is subjective and can detect only surface-visible defects. This requires the inspectors to have extensive experience and expertise. [PERSON] et al. [49] highlighted that inspectors with years of experience are better equipped to identify signs of cracks or rust that may lead to damage in complex steel structures. Damage to container cranes often appears in critical structural areas, which are regions that are most prone to structural weaknesses owing to high loads, stress, and environmental factors. Gantry cranes, for instance, operate 24/7, 365 days a year and load and unload containers next to the sea. These working conditions accelerate material deterioration and necessitate more frequent inspections without prolonged downtime for maintenance because such downtime requires significant resources, time, and costs.
* Ultrasonic testing (UT)--uses high-frequency sound waves to detect internal flaws in a material [50]. It is commonly employed to inspect critical components such as welds, pins, and slewing ring bolts [51]. UT can detect subsurface defects and measure the thickness of a material [52]. However, skilled technicians are required to operate the equipment and interpret the results. In conventional UT, the tested component must typically be cleaned and prepared, and direct contact between the probe and the component is necessary, making the process time consuming. [PERSON] et al. [53] indicated that UT is a widely used NDT method for detecting internal defects in steel structures but noted its limitations, such as the need for specialized personnel and component preparation [54].
* Magnetic particle testing (MPT)--used to detect surface and near-surface defects in ferromagnetic materials [55]. It is often applied to inspect components, such as crane hooks and pins. MPT can identify fine cracks but is limited to ferromagnetic materials. The tested component must be magnetized, and the testing environment should avoid magnetic interference [56]. [PERSON] et al. [57] pointed out that MPT is effective for detecting surface and near-surface defects in ferromagnetic materials but does not apply to non-ferromagnetic materials [58].
* Eddy current testing (ECT)--detects surface and near-surface flaws in conductive materials and is used to inspect components, such as structural welds, pressure vessels and piping [59; 60; 61; 62].
* Radiographic testing (RT)--uses X-rays or gamma rays to inspect internal structures and can detect internal defects, such as cracks and porosity, in components such as welds [63; 64; 65]. RT provides intuitive image results but involves radiation safety hazards. The testing process is complex and time-consuming, and requires strict safety precautions [66; 67].  
Table 1 compares common NDT techniques used for port cranes. It focuses on four key aspects: the detection depth, material limitations, strengths, and limitations. This comparison is crucial, as it allows for a direct assessment of each technique's capabilities and constraints in the context of port crane inspection.