# Forming Difficulty Evaluation for Curved Hull Plates Based on Grey Relational Analysis
## 1 Introduction  
In recent years, China's shipbuilding industry has been developing rapidly. The forming and manufacturing of outer hull plates is an important part in shipbuilding. Part of the outer hull plate is composed of complex undevelopable curved surfaces, so complex curved plates forming is a key process in ship manufacturing. The forming methods of curved hull plates mainly include mechanical cold bending technology and line heating forming technology. The technology of cold bending technology is very mature in the field of shipbuilding. [PERSON], [PERSON] et al. reviewed the advances and trends in forming techniques of curved extrusion profiles of metal alloys [1]. They proposed a novel method for forming controlled curved profiles/sections, differential velocity sideways extrusion (DVSE), and verified this through physical experiment and numerical modelling [2]. The curvature in the direction of large curvature for the complex curved hull plate by mechanical cold bending technology, and the curvature in other directions, is formed by the hydrothermal bending method. The line heating technology is generally based on the shape of the steel plate to be formed by experienced workers holding the flame heating source and tracking the water cooling [3]. This forming process is named line heating, which is highlydependent on experience. There are problems such as low production efficiency and unstable forming quality. The process of curved plate forming is widely neglected, and the relationship between the forming difficulty of curved plates and plate division is also neglected, which leads to a decrease in the rationality for part of the curved plate division. Therefore, it is very important to realize the integration of the whole process of shipbuilding design and production, shorten the overall manufacturing cycle of ships and improve the competitiveness of shipbuilding enterprises. Most of the current research by scholars on the manufacturing of outer hull plates are focused on the development of the automatic system of line heating [4; 5; 6], prediction of process parameters [7], prediction of reforming parameters and inspection of formed surfaces, etc. [8]. Assessment of the difficulty in forming complex curved plates is not taken seriously. The main considerations in the division of plates are segment integrity, continuity of plate joints and lifting feasibility, etc. The lack of quantitative evaluation means of forming difficulty of curved plates makes it difficult to make reasonable divisions of curved hull plates with reference to forming difficulty, resulting in some curved plates being extremely difficult to form. In the process of line heating for plate forming, workers make judgments based on experience whether the outer hull plate is difficult to form. There is a certain bias in this process. If a piece of curved plate is extremely difficult to form and process or even impossible to process, it will cause the waste of man-hours, time and materials, and is not conducive to the research of automatic forming of line heating for curved hull plates. In order to solve the above problems, it is necessary to study the evaluation of forming difficulty of curved hull plates and provide reference for the division of hull plates when considering the forming difficulty of complex curved plates. In addition, other industrial fields such as metal shells of large equipment in aerospace and double curvature plates in offshore platforms [9; 10] also have the problem of the difficult forming of complex curved plates. The research on the evaluation of the forming difficulty of complex curved hull plates can also provide reference for other fields of curved plate forming and manufacturing [11]. Therefore, this paper conducts a series of studies on the difficulty evaluation of ship complex curved plate forming based on grey relational analysis (GRA).