TWI Industrial Member Report Summary 1026/2012
By D Panggabean and C Carpentier
This report presents results from a validation programme being carried out in TWI for the evaluation of the performance of finite element analysis (FEA) for the modelling of inspection of small fatigue cracks or crack-like defects created during the manufacturing stage, using ultrasonic phased array techniques.
Fatigue cracks are common service flaws that occur in structural welds and can cause serious damage if not detected in time. Detection and characterisation of fatigue cracks can present difficult challenges, primarily due to variations in crack orientation, crack roughness and crack size. Ultrasonic techniques have been widely used for the detection of fatigue cracks, and phased array technology could be an efficient technique for the detection and sizing of very small cracks.
Theoretical models for ultrasonic inspection have been developed over a number of years. Modelling is a tool used to support the optimisation, validation and cost-effective understanding of ultrasonic procedures, while reducing the need to manufacture expensive samples.
Over the years, TWI has gained considerable experience in the understanding and use of semi-analytical modelling software such as CIVA, developed by the CEA (French nuclear commission), and the in-house software developed by British Energy (BE). Large validation campaigns versus experimental measurement and other theories have been carried out to establish the limits of validity of these software packages (Carpentier et al, 2009). The two main theories used in semi-analytical models are Kirchhoff theory and the Geometrical Theory of Diffraction (GTD); both these theories have well documented limitations (Schneider and Chapman, 1997). In particular, they are both high frequency theories, and are therefore invalid for flaw dimensions smaller than about two ultrasonic wavelengths.
FEA could be potentially a more accurate approach for modelling cracks with complex geometry (eg branched stress corrosion cracks and rough hydrogen cracks), cracks grouped in clusters (eg stress corrosion and copper cracking) and cracks that are small relative to the ultrasonic wavelength. In this project, the modelling of small cracks involved using transducers with high frequencies in order to optimize the detection of the flaws.
Several FEA packages are available that in principle can solve ultrasonic wave propagation problems. PZFLex, which stands for "Piezo-Electric Fast Large and Explicit", is a finite element software package developed by Weidlinger Associates. The early development of the software was dedicated to the prediction of wave propagation for structural engineering problems where vibrations are of concern. The development was later oriented towards transducer development in the medical environment and for ultrasonic wave propagation in human tissue.
Today the software is also used for NDT applications in order to model inspection problems and piezoelectric sensor specification. This software is based on an explicit time domain solver to resolve wave propagation problem. It can resolve 1, 2 and 3D problems and can be used in transient analysis, ie it allows a time-limited pulse to be input (as opposed to a continuous wave).
One potential application of a model allowing the accurate prediction of signals from small cracks (ie smaller than two ultrasonic wavelengths) is in justifying the capability of ultrasonic testing (UT) performed to a manufacturing standard such as Code Case 2235-9 in the ASME boiler and pressure vessel code. This Code Case specifies the flaw acceptance criteria for different weld thicknesses and flaw aspect ratios. It can be noted that the acceptance criteria for configurations with low wall thicknesses containing long flaws tend to require the rejection of flaws of small height and some of these cases cannot be reliably modelled by semi-analytical models. This limitation is especially important for austenitic coarse grained materials, where low testing frequencies (ie large wavelengths) are required.
This report summarizes the results of the earlier progress report (Carpentier and Panggabean, 2011) and presents further results from the latter stages of the project; the accuracy of the FEA package PZFlex for modelling flaws of different sizes and different angles is studied and the software's performance for modelling phased array transducers is evaluated.
- Investigate the capability of the FEA method for the prediction of corner echoes from smooth misoriented surface breaking notches and the sound beams generated by a phased array transducer.
- Evaluate the domain of validity of PZFlex for the detection of small flaws (ie smaller than two ultrasonic wavelengths) by comparing experimental signals from small vertical notches with the corresponding modelling predictions and the prediction of a phased array sound pressure beam compared with previous studies.
- Studying the software capabilities for modelling phased array transducers, and comparing modelling results for the beam computation to previous studies involving other modelling packages.