###### Abstract  
To ensure and to demonstrate the mechanical integrity of a welded structure, precise ultrasonic testing (UT) is often mandatory. The importance of the link between nondestructive testing (NDT) and the assessment of structural integrity is recalled. However, it is difficult to achieve great efficiency as the welding of thick and heavy structural part produces heterogeneous material. Heterogeneity results from the welding process itself as well as from the material solidification laws. For thick components, several welding passes are deposited, and temperature gradients create material grain elongation and/or size variations. In many cases, the welded material is also anisotropic, this anisotropy being due to the metal used, for example, austenitic stainless steel. At the early stages of ultrasonic testing, this kind of welded material was considered too unpredictable, and thus too difficult to be tested by ultrasounds without possible diagnosis errors and misunderstandings. At the end of the 1990s, an algorithmic solution to predict the material organisation began to be developed using data included in the welding notebook. This algorithm or modelling solution was called MINA. This present work recalls, in a synthetic form, the path followed to create this algorithm combining the use of solidification laws and the knowledge of the order of passes in the case of shielded metal arc welding (SMAW). This work describes and questions the simplifications used to produce a robust algorithm able to give a digital description of the material for wave simulation code. Step by step, advances and demonstrations are described as well as the limitations, and ways to progress are sketched. Recent developments are then explained and discussed for modelling in the case of gas tungsten arc welding (GTAW), in addition to discussions about 3D modelling for the future. The discussion includes alternative ways to represent the welded material and challenges to continue to produce more and more convincing weld material model to qualify and to make use of UT methods. urlasounds; ultrasonic testing; weld; multipass weld; modelling; stainless steel; nondestructive testing; anisotropic material; heterogeneous material +
Footnote †: journal: Applied Sciences  
damage. Steels still have a predominant role in large mechanical structures, and these structures are frequently connected by welding. Since the 1930s, welding has become an increasingly important part of the manufacturing process. The growing importance of this joining technique began with the Liberty Ships when welds replaced riveted joints, lightening the structure and allowing for smoother hulls [1]. However, they also have their drawbacks: residual stresses, manufacturing or fatigue defects such as cracks, inclusions and porosity. Nondestructive methods applied to weld inspection include a large number of well-known techniques, such as radiography, ultrasound, electromagnetic methods (eddy current, magnetic particle inspection) [2; 3]. The choice of the best method for solving the problem posed (i.e., detection, characterisation or sizing) is the first essential step in any approach aiming at the nondestructive examination of a mechanical structure. For the inspection of welds, and particularly for the inspection of structures on site, ultrasonic inspection is often preferred because of its multiple possible implementations using surface waves, volume waves and guided waves of all types. Ultrasonic transducers generate high-frequency mechanical waves. Several kinds of propagating modes are possible, offering many solutions to component integrity investigation. The development of probabilistic fracture mechanics has completely changed the role of NDT. Thanks to the combination of quantitative nondestructive evaluation with the probabilistic fracture mechanics approach, NDT has become a technology that can reduce costs and improve productivity.