Fatigue Crack Growth in Bodies with Thermally Sprayed Coating
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O. Kova´rˇı´k, P. Hausˇild, J. Medrˇicky´, L. Tomek, J. Siegl, R. Musˇa´lek, N. Curry, and S. Bjo¨rklund (Submitted May 22, 2015; in revised form September 14, 2015) Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the localization of crack initiation sites and the dynamics of crack propagation are studied. The resonance bending fatigue test was employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical thermal barrier coating consisting of yttria stabilized zirconia and NiCoCrAlY layers. The strain distribution on the coating surface was evaluated by the Digital Image Correlation method through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e., the changes of material properties, crack initiation, and crack propagation, were identified. The tested coatings strongly influenced the first two phases, and the influence on the crack propagation was less significant. In general, the presented crack detection method can be used as a sensitive nondestructive testing method well suited for coated parts.
Keywords
Thermal barrier coating, Fatigue, Crack growth, Digital image correlation, Failure mechanism
1. Introduction Thermally sprayed coatings are often used to protect critical components from wear, temperature, or aggressive environment. For many of these components, coating or underlying substrate failure may have serious consequences. Therefore, the fatigue resistance of the coatingsubstrate composite subjected to cyclic mechanical loading is an important engineering parameter. During the last few decades, significant research effort has been focused on finding fundamental factors that influence the fatigue life of coated components. The effects of residual stresses (Ref 1-6), interface properties This article is an invited paper selected from presentations at the 2015 International Thermal Spray Conference, held May 11-14, 2015, in Long Beach, California, USA, and has been expanded from the original presentation. O. Kova´rˇ´ık, P. Hausˇild, L. Tomek, and J. Siegl, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Trojanova 13, 12000 Praha 2, Czech Republic; J. Medrˇicky´, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Trojanova 13, 12000 Praha 2, Czech Republic and Institute of Plasma Physics CAS, v.v.i, Za Slovankou 1782/3, 182 00 Praha 8, Czech Republic; R. Musˇa´lek, Institute of Plasma Physics CAS, v.v.i, Za Slovankou 1782/3, 182 00 Praha 8, Czech Republic; and N. Curry and S. Bjo¨rklund, Production Technology West, University West, 461 86 Trollha¨ttan, Sweden. Contact e-mail: [email protected].
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