**3. Ultrasonic non-destructive testing**

During World War I, underwater detection systems using high-frequency acoustic waves and quartz resonators for submarine detection were developed by Langevin [29] as a consequence to the tragic sinking of the Titanic in 1912. In 1928, Sergei Y. Sokolov proposed the use of a through-transmission UT technique for flaw detection in metals [30]. Mulhauser firstly patented an UT device employing separate transmitter and receiver transducers to detect flaws in solids [29]. In 1940, Firestone was the first to realise the UT reflection or pulse-echo technique [31]. In 1948, extensive study of UT medical imaging started in the United States and Japan. One of the first UT testing apparatuses using piezoelectric crystal transducers for the detection of defects was patented by McNulty in 1962. This apparatus was capable of isolating defect signals from high level noise signals and providing an alarm upon occurrence of a defect signal [32]. Since those times, technology improvements led to remarkably enhanced UT non-destructive testing (NDT) allowing to detect surface, subsurface and internal flaws (cracks, delaminations, cavities, pores, inclusions and fractures) in diverse types of materials (metals, composite materials and plastics) [33]. In the manufacturing industries, UT NDT techniques are widely applied for the quality control of components and structures as well as for the characterisation of materials.
