Issue 43

N. Montinaro et alii, Frattura ed Integrità Strutturale, 43 (2018) 231-240; DOI: 10.3221/IGF-ESIS.43.18 232 Nonetheless, additive manufacturing has shown great potential in many manufacturing sector, especially in automotive, aerospace, military and medical ones. On one hand, the market scenario for AM parts is constantly growing due to ever new, improved technologies, application areas width and customization possibilities. On the other hand, there remain some drawbacks limiting this market, such as material characterization during development, process and integrity control. In-line inspection would be particularly important in those sectors where verification of AM parts has proven challenging up to now. Indeed, pre-existing non-destructive techniques (NDT) are unable to adapt to the complicated geometries usually created by additive manufacturing. Right now, the soundness of AM parts is evaluated by destructive testing or by X-ray computed tomography (CT) [3], which can only be employed after the part completion, causing parts to be rejected at the end of manufacturing process. When critical requirements of quality are involved, with the additional complication of dealing with complex geometries, the best option remains a non-destructive technique to be integrated as in-line inspection allowing for the detection of defects as each layer is added. Finally, a non-destructive technique should also be able to detect micrometric flaws, which are a typical occurrence in AM products. There is previous literature regarding some NDTs used in AM parts. The use of an ultrasonic squirter probe with a standard industrial robot to inspect a 3D metal deposition structure is demonstrated in [4]. Laser-generated surface waves have been used in [5] to investigate laser powder deposition parts, in both stainless steel and titanium, with pores that are simulated using blind holes. Clark et al. in [6] have shown the potential of an all-optical scanning acoustic microscope instrument for online inspection of AM products. The use of ultrasonic laser transmitter and receiver and the interaction of the incident wave with sub-surface and surface defects have been widely investigated for many different applications [7-10]. A recently introduced NDT active thermographic technique is the flying laser spot technique, which has been employed for the surface crack sizing with micrometric aperture. Li et al. in [11] have developed a thermographic imaging technique using the second spatial derivative of acquired flying laser spot and line thermograms, with the aim of characterizing micrometer cracks in metal samples. In [12] the laser spot imaging thermography was employed and, simultaneously, laser-based ultrasonic measurements to unearth surface breaking cracks. Montinaro et al. in [13] have successfully implemented the flying laser spot technique for the detection and characterization of disbonds and delaminations in fiber metal laminates. The authors monitor the thermal footprint left by the moving heat source, revealing thermal anomalies in a region of interest by using statistical means. The aim of this work is to optimize the parameters used to post-process experimentally acquired thermograms in order to enhance defect sensitivity to micrometric near-surface and surface flaws of additive manufactured parts. The thermal behavior of the sample is thus simulated by a Finite element analysis (FEA) through a transient analysis. The outcomes of the analyses will be used to optimize the experimental procedure. Two Inconel 600 samples with micrometric laser drilled defects have been used as reference for the thermographic experiments. Results from the experiments and the numerical model have been compared showing a sound agreement. M ATERIALS AND METHODS Flying Laser Inner-Probing Thermography (FLIPT) lying Laser Scanning in IR-NDT is characterized by employing a collimated laser heat source that is moved over an object surface. The resulting thermal field is then analyzed to identify and characterize surface or near-surface vertical cracks which behave as barriers towards in-plane heat diffusion [11, 14, 15]. The proposed FLIPT technique uses a laser spot moving at a constant speed, and generating a peculiar temperature profile surrounding the heat source. If the material is uniform and sound, this trailing temperature profile remains unmodified under steady state conditions (i.e. constant laser speed). When the heat source crosses an area over a sub-superficial flaw, the surface temperature field is somewhat disturbed, due to the defective zones stopping temporarily the heat flowing in the through the thickness direction. The defect signature is found in the evolution of the Mean (MT) or the Standard Deviation (SD) of the temperature distribution over a region of interest (ROI) placed on the trail of the heat source. The ROI maintains its position with respect to the laser spot, i.e. it travels at the same speed of the heat source. In [13] the authors have successful applied the technique for the detection of delaminations on a particular kind of hybrid composite materials called fibre metal laminates. Fig. 1 shows the set-up used in this work for the laser thermography experiments; Tab. 1 indicates the parameters of the experimental set-up. The sample is installed on a motorized linear micro-slide controlled by PC. The Continuous Wave (CW) laser beam with 1 W of power and 532 nm of wavelength is focused into a 0.75 mm spot on faces A and B of sample 1 and into 0.5 mm on face C of sample 2 (see Fig. 3). The IR camera (see specifications in Tab. 2) is placed perpendicularly F