20th International Forgemasters Meeting - IFM 2017

Ultrasonic Nondestructive Testing (UT-NDT) is the only technique that allows the inspection of the entire volume of large forgings. For this reason, improvements of the UT-NDT sensitivity are a key goal for the forging industry. Pulse-Echo (PuE) is the reference method for which all the Standards and evaluation procedures have been developed so far, and it is effective in most of the situations. Nevertheless, in the presence of high attenuation (e.g. in austenitic samples) and-or large dimensions of the forgings, the PuE method could not guarantee an adequate signal to noise ratio (SNR) and sensitivity as requested by the Standards or by customers. It is therefore of utmost importance to develop procedures able to outperform the PuE in those critical applications requiring a high sensitivity, as happens with very weak signals or high noise. Fortunately, suitable signal processing protocols can be exploited to enhance the SNR of the procedure. Recently the authors proposed to exploit Pulse Compression (PuC) techniques in combination with the use of two separate transducers, one transmitter -Tx- and one receiver -Rx-, to increase the SNR of the measurement and thus reducing the dimension of the minimum detectable defect. In the present work, the method is further developed and improved by introducing two fundamental improvements: (1) a tool for calculating the AVG curves for an arbitrary Tx-Rx configuration has been developed; (2) a multi-frequency AVG analysis relying on a broadband chirp signal excitation that acts as coded waveform for the PuC procedure. The contextual implementation of (1) and (2) allows the inspection sensitivity to be optimized “depth-by-depth”. In the full paper, details on the experimental procedure, on the digital signals processing and on the AVG calculation will be given and an extensive comparison between PuC and PuE methods will be carried out on various forgings to validate the technique. [1] Ricci, M., et al. "Pulse-compression ultrasonic technique for the inspection of forged steel with high attenuation." Insight-Non-Destructive Testing and Condition Monitoring 54.2 (2012): 91-95.