In this paper, a modified shear-lag model is developed to calculate the surface crack density in thermal barrier coatings(TBCs). The mechanical properties of TBCs are also measured to quantitatively assess their surface crack density. Acoustic emission(AE) and digital image correlation methods are applied to monitor the surface cracking in TBCs under tensile loading. The results show that the calculated surface crack density from the modified model is in agreement with that obtained from experiments. The surface cracking process of TBCs can be discriminated by their AE characteristics and strain evolution. Based on the correlation of energy released from cracking and its corresponding AE signals, a linear relationship is built up between the surface crack density and AE parameters, with the slope being dependent on the mechanical properties of TBCs.
Thermal barrier coatings(TBCs) usually exhibit an uncertain lifetime owing to their scattering mechanical properties and severe service conditions. To consider these uncertainties, a reliability assessment method is proposed based on failure probability analysis. First, a limit state equation is established to demarcate the boundary between failure and safe regions, and then the failure probability is calculated by the integration of a probability density function in the failure area according to the first- or second-order moment.It is shown that the parameters related to interfacial failure follow a Weibull distribution in two types of TBC. The interfacial failure of TBCs is significantly affected by the thermal mismatch of material properties and the temperature drop in service.
Jin-Wei GuoLi YangYi-Chun ZhouLi-Min HeWang ZhuCan-Ying CaiChun-Sheng Lu
In this paper,acoustic emission(AE) and digital image correlation methods were applied to monitor interface cracking in thermal barrier coatings under compression.The interface failure process can be identifie via its AE features,including buckling,delamination incubation and spallation.According to the Fourier transformation of AE signals,there arefourdifferentfailuremodes:surfaceverticalcracks,opening and sliding interface cracks,and substrate deformation.The characteristic frequency of AE signals from surface vertical cracks is 0.21 MHz,whilst that of the two types of interface cracks are 0.43 and 0.29 MHz,respectively.The energy released of the two types of interface cracks are 0.43 and 0.29 MHz,respectively.Based on the energy released from cracking and the AE signals,a relationship is established between the interface crack length and AE parameters,which is in good agreement with experimental results.
Li YangZhi-Chun ZhongYi-Chun ZhouWang ZhuZhi-Biao ZhangCan-Ying CaiChun-Sheng Lu
To study the temperature distribution and thermal-stress?eld in different service stages,a twodimensional model of a turbine blade with thermal barrier coatings is developed,in which the conjugate heat transfer analysis and the decoupled thermal-stress calculation method are adopted.Based on the simulation results,it is found that a non-uniform distribution of temperature appears in different positions of the blade surface,which has directly impacted on stress?eld.The maximum temperature with a value of 1030°C occurs at the leading edge.During the steady stage,the maximum stress of thermally grown oxide(TGO)appears in the middle of the suction side,reaching 3.75 GPa.At the end stage of cooling,the maximum compressive stress of TGO with a value of -3.5 GPa occurs at the leading edge.Thus,it can be predicted that during the steady stage the dangerous regions may locate at the suction side,while the leading edge may be more prone to failure on cooling.
In this paper,a finite element model was developed for a turbine blade with thermal barrier coatings to investigate its failure behavior under cyclic thermal loading.Based on temperature and stress fields obtained from finite element simulations,dangerous regions in ceramic coating were determined in terms of the maximum principal stress criterion.The results show that damage preferentially occurs in the chamfer and rabbet of a turbine blade with thermal barrier coatings and its thermal fatigue life decreases with the increase of thermal stress induced by high service temperature.
