Study of complex ultrasonic NDT cases using hybrid simulation method and experimental validations

The conception, qualification or demonstration of inspection techniques devoted to complex configuration cases usually mostly rely on experimental studies. Simulation tools may also play a major role as they provide,for instance, an efficient way to investigate the influence of various parameters over the inspection performances. In order to assess such objectives, simulation tools need to he both accurate (to predict real actual inspection performance) and within computation times compatible with parametric studies. Although semi-analytical techniques lead to accurate results with reduced computation times and memory burden, their application is somewhat restricted within their range of validity. Pure numerical techniques (finite elements, finite differences) may explore a wider range of applications as they solve the exact wave propagation equations within the whole space of the meshed NDT configuration, but these techniques exhibit higher computation times and memory load, which may be a limitation, especially for 3D applications. In order to get the advantage of both approaches (the computation efficiency of semi-analytical techniques and the genericity of numerical techniques), a hybrid ultrasonic model has been developed by CEA and EDF, which relies on the coupling of two models: beam computation using a semi-analytical model (CIVA, developed by CEA) and flaw scattering computation inside a small region surrounding the flaws, using finite element calculation (ATHENA code, developed by EDF). The coupling of both computations allows the actual response of any arbitrary complex flaw inside a component, inspected in pulse-echo or TOFD mode, to be predicted. Applications of simulation, along with experimental validations, are presented in this paper concerning different configuration cases: the inspection of branched flaws, irregular profiles (rough) flaws and flaws near austenitic welds. The influence of the flaw shape or weld properties (anisotropy, heterogeneity) over the inspection results has been investigated using simulation and compared to experimental tests carried out on realistic mock-ups containing EDM notches. These studies show the ability of simulation tools to efficiently predict the inspection performances as well as to understand, and therefore to optimise, NDT for complex configurations.