Krishnan Balasubramanian

In this paper, it is aimed to identify flaws in glass fiber reinforced polymer composites by smart skin graphene nano platelet (GNP) spray coating in infrared thermography technique. The initial resistance of GNP was made to 1 kΩ. Characterization of sensor and beam was done with scanning electron microscopy and computed tomography (CT) respectively. The thermo-elastic behaviour was evaluated in uniaxial test. The surface temperature was studied with IR camera and it was observed that the surface coated GNP sensor upon a damage and without the damage specimen retains heat than without coating the sensor. Hereafter testing with 0.1 mm/min, 0.5 mm/min and 1 mm/min, it was found that without damage specimen, the temperature increased to 112.5%, 13.3% and 40% respectively. And temperature increased to 93.2%, 36.7% and 76.4% in the specimen with the damage. Specimen were also tested for spectrum fatigue cyclic load at 0.1 Hz and 1 Hz. Failure peak of laminates has been analyzed with optical microscopy and CT which was correlated with temperature rise. For 0.1 Hz spectrum loading, the specimen with the damage, with and without GNP coated, temperature rose to 2040% after first laminate failure. Similarly, for 0.1 Hz specimen temperature rose to 15,637.5% in case of without damage specimen, with GNP coated than without GNP coated. And in case of 1 Hz spectrum loading with damage specimen, the temperature rose to 105.73% after GNP coated. Similarly, at 1 Hz loading, the temperature rose to 143.07% in case of without damage specimen after GNP coated. GNP skin coated nano-sensor helps in early detection of temperature signals.

Thermal barrier coatings are extensively used in aircraft engines. During service, the TBC coatings degrade because of erosion and sintering by hot gas flow and also by localized wear due to rubbing of flaps with spacers. It is necessary to assess the condition of the coatings as a function of service life through suitable non-destructive means. Pulse Thermography (PT) and Terahertz-Time Domain Spectroscopy (THz-TDS) techniques are used to evaluate the degree of degradation of the thin Air Plasma Sprayed (APS) TBCs top coat thickness. Infrared thermography has the advantage of fast inspection of a large area. In this work, we used a simplified analytical model aided calibration and development of a regression model to quantitatively analyze the thickness degradation in real-world TBC samples that have endured varying service life. These measurements were later verified using THz-TDS imaging, an emerging technique for accurate thickness measurements. Assuming the refractive index of the topcoat material, Yttria-Stabilized Zirconia (YSZ) as 4.8, the topcoat thickness of the entire specimen has been estimated using THz-TDS reflection mode setup. Results show that, the thickness values are varying between 94.94 μm - 114.96 μm for 500 h serviced samples and 32.5 μm - 91.96 μm in the case of 1000 h serviced samples demonstrating loss of TBC with increased service life. Comparison of the pulse thermography results with THz-TDS reveals a mean relative error of less than 10.3% in TBC thickness estimation. Further the results of both the techniques are cross-validated with Eddy current testing and optical microscopy. The proposed non-destructive techniques for TBC estimation will aid in the accurate creation of engine digital twin and will help in scheduling preventive maintenance measures thus increasing the life and safety of key aircraft engine components.

Crack detection in metallic samples at high surface temperature, hostile and hazardous environments, etc. is a challenging situation in any manufacturing industries. Most of the present NDE methods are suitable only for lower surface temperatures, especially at room temperature. In this situation, we need a fast and non-contact NDT method which can be applied even in high sample surface temperature. Laser thermography is one of the techniques having a high potential in non-contact inspection. As a preliminary investigation, in this article, we have studied the potentiality of laser line thermography in crack detection at room temperature. In laser line thermography, a continuous wave (CW) laser is used to generate a laser line, which in turn is used to scan the metal surface. The heat distribution over the sample surface is recorded by an infrared thermal (IR) camera. Two different approaches are reported in this work. Firstly, a stationary laser line source and its interaction with cracks; secondly, moving laser line source scanning over a surface with crack. When the distance between crack centre to laser line centre increases, crack detectability will decrease; and when laser power increases, crack detectability will increase. A dedicated image processing algorithm was developed to improve the detectability of the cracks. To understand the heat transfer phenomenon, a simplified 3D model for laser thermography was developed for the heat distribution during laser heating and was validated with experimental results. Defects were incorporated as a thermally thin resistive layer (TTRL) in numerical modeling, and the effect of TTRL in heat conduction is compared with experimental results.