Plastic piping systems are increasingly replacing conventional metal systems for transport of liquids and gases in industry, utilities and building technology. Today, several welding processes are available, but, compared to metals, the qualification and analysis of plastic joints is still done by destructive testing methods. The development of non-destructive evaluation (NDE) methods for the inspection of the joint quality is considered to be mandatory. Of specific interest is the detection of material internal flaws that are invisible to the eye. In this project the focus is set to the development of novel ultrasonic wave based methods. Specifically with regard to cleanroom applications the non-contact air-coupling approach will be studied. Furthermore liquid coupled phased array technology and complementary digital X-ray inspection will be assessed. The project includes numerical simulations of the propagation of ultrasonic waves, dedicated mechanical and software developments as well as advanced signal and image processing.