Wei Qiu

Gallium nitride (GaN) has wide application prospects in various fields such as light-emitting diode chips, radio frequency chips, high-energy lasers, and energy conversion devices. Complex preparation processes and extreme working environments generate complicated stress/strain states and distributions inside and on the surfaces of the GaN devices, thus affecting their performances and reliabilities. It is a common requirement for the design, manufacturing, and application of GaN devices to develop an experimental method analyzing full-field stress, especially decoupling all components of the unknown stress state. This work established an improved theoretical model of angle-resolved Raman for the stress analysis of GaN, which quantified the birefringence effects and provided the generalized analytic relationship between the three in-plane stress components and the Raman shift of GaN under arbitrary geometric and polarization. This work presented the specific expression forms of the equations in typical situations, and the parameters of the model were calibrated through angle-resolved Raman experiments. Verification experiments employing four-point bending demonstrated close agreements between the measured stress components and the prescribed loadings. Finally, the applicability and precision of the proposed model, for component decoupling analysis of the complex stress states, were systematically evaluated based on the experimental results of the stress fields around a circular through-hole of a compressed GaN sample.

Thermal barrier coating system plays key role in high temperature gas turbine blades. During the manufacture and in-service processes, defects are inevitable due to the inconsistent deformation and fatigue loading. The typical defects, e.g. interfacial debonding and crack in substrate, may significantly degrade the performance, safety and service life of gas turbine. However, it is challenging to non-destructively evaluate the small defects and/or coexistent multi-defects in thermal barrier coating system. Here, we propose a non-destructive method to detect the defects in thermal barrier coating system using the results of both infrared thermography and eddy current testing. Using this method, we detect interfacial debonding as small as 0.7 mm within 0.2 s and identify defect types which are essential for turbine blade fabrication and maintenance. By leveraging multi-physical fields, we achieve effective and quantitative evaluation of coexisting defects in thermal barrier coating system. This method integrates multiple non-destructive evaluation techniques and provides an approach to assess defects in multilayer structures without damaging them.

Thermal barrier coatings (TBCs) are widely used on turbine engines. The failure of TBCs will lead to financial losses or even life hazards, thereby necessitating the employment of non-destructive testing (NDT) methods to ensure structural integrity and safety. The smaller size of defects that can be detected, the more likely it is to prevent potential risks. If multi-defects can be detected simultaneously, the maintenance cycles and costs will be reduced. However, current NDT methods are difficult to detect tiny debonding defects with high accuracy, and there are few methods that can detect multi-defects at the same time. To address this challenge, in this paper, combined with calculation and experiment, an electromagnetic driven based multi-physics fusion detection method is proposed for the first time. This method realizes the precise and fast detection of interfacial debonding with diameter 0.7mm within 0.8 second and the detection of substrate cracks simultaneously. To better simulate the actual debonding defects in real working conditions, the specimens with debonding defects fabricated by salt dissolution method are firstly used for the detection. The multi-physics fusion method leads to the combination of multiple NDT methods, which provides ideas for further improving the detection accuracy and efficiency of multilayer coatings.