Acoustic emission detection and wave propagation analysis of thermal fracture in thermal barrier coatings
Acoustic Emission (AE) is used to detect cracks in solid structures. Analyses of AE signals in different aspects, such as time, frequency, and wave number domains, can provide valuable information in evaluating cracks. ^ Thermal barrier coatings (TBCs) are used to protect metallic materials in high temperature environment. In this work, TBCs which consist of yttria stabilized zirconia (YSZ), a NiCoCrAlY bond coat, and a superalloy substrate, were subjected to thermal shock tests, and AE signals of thermal fracture were obtained. In other cases, YSZ-bond coat composite and YSZ-mullite composite TBCs were tested. ^ The AE signals obtained from the thermal shock tests were characterized by their frequency spectra, and it was found that different crack types can be distinguished by their unique frequency characteristics. At the same time, AE energy levels can be used to estimate crack lengths. ^ In an effort to assess the feasibility of utilizing finite element models in studies of wave propagation, bar type models were developed. From these models, calculated AE signals were examined in terms of wave mode and speed. After finding good agreement with theoretical results, the wave propagation in TBCs was studied using numerical TBC models. AE time signals were calculated from pressure load and thermal load TBC models. AE frequency signals were compared with the frequency spectra of experimental data. Then frequency-wave number spectra were calculated to investigate wave propagation modes. It was found that frequency characteristics of different crack types can be distinguished same as the experimental results. The spatial spectra in frequency-wave number domain showed which wave modes propagated in TBCs. The finite element calculation of multiple surface crack models showed the existence of a side surface crack could be monitored by its spatial spectrum. Additionally, the correlation of crack length and AE energy level was proven from the results of various thermal load models. ^ Finally, analytical solutions of wave propagation in TBCs were calculated. The comparison of spatial spectrum with that of FEM calculation was found to be in good agreement. ^
Major Professors: Klod Kokini, Purdue University, J. Stuart Bolton, Purdue University.