Defects which possess rough surfaces greatly affect
ultrasonic wave scattering behaviour, usually reducing the magnitude
of reflected signals. Understanding and accurately predicting
the influence of roughness on signal amplitudes is crucial,
especially in Non-Destructive Evaluation (NDE) for the inspection
of safety-critical components. An extension of Kirchhoff theory
has formed the basis for many practical applications; however, it
is widely recognised that these predictions are pessimistic owing
to analytical approximations. A numerical full field modelling
approach does not fall victim to such limitations. Here, a Finite
Element (FE) modelling approach is used to develop a realistic
methodology for the prediction of expected back-scattering from
rough defects. The ultrasonic backscatter from multiple rough
surfaces defined by the same statistical class is calculated for
normal and oblique incidence. Results from FE models are
compared with Kirchhoff theory predictions and experimental
measurements in order to establish confidence in the new
approach. At lower levels of roughness excellent agreement is
observed between Kirchhoff theory, FE and experimental data,
whilst at higher values the pessimism of Kirchhoff theory is
confirmed. An important distinction is made between the total,
coherent and diffuse signals and it is observed, significantly, that
the total signal amplitude is representative of the information
obtained during an inspection. This analysis provides a robust
basis for a less sensitive, yet safe, threshold for inspection of
rough defects.