In Wire and Arc Additive Manufacturing (WAAM) and fusion welding, various defects such as porosity, cracks, deformation and lack of fusion can occur during the fabrication process. These have a strong impact on the mechanical properties and can also lead to failure of the manufactured parts during service. These defects can be recognized using non-destructive testing (NDT) methods so that the examined workpiece is not harmed. This paper provides a comprehensive overview of various NDT techniques for WAAM and fusion welding, including laser-ultrasonic, acoustic emission with an airborne optical microphone, optical emission spectroscopy, laser-induced breakdown spectroscopy, laser opto-ultrasonic dual detection, thermography and also in-process defect detection via weld current monitoring with an oscilloscope. In addition, the novel research conducted, its operating principle and the equipment required to perform these techniques are presented. The minimum defect size that can be identified via NDT methods has been obtained from previous academic research or from tests carried out by companies. The use of these techniques in WAAM and fusion welding applications makes it possible to detect defects and to take a step towards the production of high-quality final components.

In conventional photoacoustic tomography, several effects contribute to the loss of resolution, such as the limited bandwidth and the finite size of the transducer, or the space-dependent speed of sound. They can all be compensated (in principle) technically or numerically. Frequency-dependent acoustic attenuation also limits spatial resolution by reducing the bandwidth of the photoacoustic signal, which can be numerically compensated only up to a theoretical limit given by thermodynamics. The entropy production, which is the dissipated energy of the acoustic wave divided by the temperature, turns out to be equal to the information loss, which cannot be compensated for by any reconstruction method. This is demonstrated for the propagation of planar acoustic waves in water, which are induced by short laser pulses and measured by piezoelectric acoustical transducers. It turns out that for water, where the acoustic attenuation is proportional to the squared frequency, the resolution limit is proportional to the square root of the distance and inversely proportional to the square root of the logarithm of the signal-to-noise ratio. The proposed method could be used in future work for media other than water, such as biological tissue, where acoustic attenuation has a different power-law frequency dependence.

Planning of surgical liver tumor resections based on image data from X-ray computed tomography requires correct segmentation of the liver, liver vasculature and pathological structures. Automatic liver segmentation methods frequently fail in cases where the anatomy is degenerated by lesions or other present liver diseases. On the other hand performing a manual segmentation is a tedious and time consuming task. Therefore Augmented Reality based segmentation refinement tools are reported, that aid radiologists to efficiently correct incorrect segmentations in true 3D using head-mounted displays and tracked input devices. The developed methods facilitate segmentation refinement by interactively deforming a mesh data structure reconstructed from an initial segmentation. The variety of refinement methods are all accessible through the intuitive, direct 3D user interface of an Augmented Reality system.

In this article, we show the benefits of a quasi-balanced fringe hopping confocal Fabry–Perot interferometer (CFPI) with broadband common mode rejection ratio (CMRR) for remote ultrasound detection. In laser ultrasound, the ultrasonic information, in general, lies in the phase modulation of laser light which in this case is demodulated using the CFPI at a certain working point on a fringe. By hopping from the positive to the negative slope on the same fringe, the detected ultrasonic signals are inverted. In contrary, interference signals – such crosstalk from the generation, ghosts or noise correlated to pulse laser excitation – are not influenced and hence get rejected by subtracting the signals measured at both slopes. Hence, a minimum of two measurements is needed for common mode rejection. The fringe hopping from the positive to the negative slope is done by changing the distance of the CFPI mirrors with a precise piezoelectric-stack and a fast high-resolution digital controller. As only one photodetector with a transimpedance amplifier is needed, a high CMRR can be accomplished. The CMRR is not affected by the symmetry of the fringe but only by pulse-to-pulse energy fluctuations of the generation laser. We show that with fringe hopping and averaging the signal-to-noise ratio increases much faster than with averaging without fringe hopping. This is due to the correlation of the quasi-noise with the generation cycle.

Compression testing combined with laser ultrasonics enables non-destructive, in situ material characterisation during thermo-mechanical loading, but the shortening and radial expansion of the specimen displace the laser focal points during deformation and therefore require active repositioning of the laser-ultrasonic optics. This work addresses this engineering problem by providing a modular positioning-control system that couples a Gleeble 3800 thermo-mechanical simulator and a laser-ultrasonic measurement unit. The Gleeble provides two analogue shaft-position signals, which are digitised and used as real-time reference signals for stage-position control. The excitation and detection optics are mounted on motorised Zaber linear stages; two stages translate the optics along the compression direction, while a third perpendicular stage adjusts the detection focus to compensate radial specimen expansion and the limited depth of field of the detection optics. The control software was implemented in Rust and computes the stage target positions from the digitised Gleeble voltages output using user-configurable mathematical expressions. It further provides process-data access through an embedded OPC UA server and a browser-based graphical user interface through an integrated HTTP server. A custom ADC-based acquisition chain was introduced because the built-in ADC inputs of the Zaber controllers showed a significant noise level when used for the Gleeble shaft-position signals. Profiling showed that the control-loop runtime was dominated by ADC read/write operations; by reorganising acquisition into a parallel setup, the mean control-loop time was reduced from approximately 130 ms to approximately 27 ms. The resulting system provides an active, configurable, and retrofit-capable link between Gleeble compression testing and laser-ultrasonic measurement. It improves the temporal response available for laser-focus tracking on deforming specimens and provides a foundation for future quantitative validation of tracking error, positioning uncertainty, and LUS signal quality during real compression experiments.