Coherent gradient sensing(CGS)method can be used to measure the slope of a reflective surface,and has the merits of full-field,non-contact,and real-time measurement.In this study,the thermal stress field of thermal barrier coating(TBC)structures is measured by CGS method.Two kinds of powders were sprayed onto Ni-based alloy using a plasma spraying method to obtain two groups of film–substrate specimens.The specimens were then heated with an oxy-acetylene flame.The resulting thermal mismatch between the film and substrate led to out-of-plane deformation of the specimen.The deformation was measured by the reflective CGS method and the thermal stress field of the structure was obtained through calibration with the help of finite element analysis.Both the experiment and numerical results showed that the thermal stress field of TBC structures can be successfully measured by CGS method.
Fused deposition modelling(FDM), a widely used rapid prototyping process, is a promising technique in manufacturing engineering. In this work, a method for characterizing elastic constants of FDM-fabricated materials is proposed. First of all, according to the manufacturing process of FDM, orthotropic constitutive model is used to describe the mechanical behavior. Then the virtual fields method(VFM) is applied to characterize all the mechanical parameters(Q_(11), Q_(22), Q_(12), Q_(66)) using the full-field strain,which is measured by digital image correlation(DIC). Since the principal axis of the FDM-fabricated structure is sometimes unknown due to the complexity of the manufacturing process, a disk in diametrical compression is used as the load configuration so that the loading angle can be changed conveniently. To verify the feasibility of the proposed method, finite element method(FEM) simulation is conducted to obtain the strain field of the disk. The simulation results show that higher accuracy can be achieved when the loading angle is close to 30?. Finally, a disk fabricated by FDM was used for the experiment. By rotating the disk, several tests with different loading angles were conducted. To determine the position of the principal axis in each test, two groups of parameters(Q_(11), Q_(22), Q_(12), Q_(66)) are calculated by two different groups of virtual fields. Then the corresponding loading angle can be determined by minimizing the deviation between two groups of the parameters. After that, the four constants(Q_(11), Q_(22), Q_(12), Q_(66)) were determined from the test with an angle of 27?.
The mechanical properties of plasma-sprayed thermal barrier coating(TBC) play a vital role in governing their lifetime and performance. This work investigated the microstructural and mechanical properties of TBC with high temperature treatment at 1 400℃ by scanning electron microscopy and indentation.We calculated elastic modulus and hardness through the application of Weibull statistics analysis. The results indicate that the microstructure of ceramic coating will change continuously at high temperature, and accordingly the porosity decreases due to the grain growths and crack closes. In addition, the elastic modulus and hardness nonlinearly go up with the heat treatment time and go down with increasing porosity. This demonstrates that the microstructural evolution and porosity of TBC are caused by high temperature treatment, and as a result its mechanical properties are influenced.
Residual stress evolution regularity in thermal barrier ceramic coatings(TBCs) under different cycles of thermal shock loading of 1 100℃ was investigated by the microscopic digital image correlation(DIC) and micro-Raman spectroscopy, respectively. The obtained results showed that, as the cycle number of the thermal shock loading increases, the evolution of the residual stress undergoes three distinct stages: a sharp increase, a gradual change, and a reduction.The extension stress near the TBC surface is fast transformed to compressive one through just one thermal cycle. After different thermal shock cycles with peak temperature of 1 100℃, phase transformation in TBC does not happen, whereas the generation, development, evolution of the thermally grown oxide(TGO) layer and micro-cracks are the main reasons causing the evolution regularity of the residual stress.
In this paper the elastic constants of graphite at elevated temperature were experimentally investigated by using the virtual fields method(VFM). A new method was presented for the characterization of mechanical properties at elevated temperature. The three-point bending tests were performed on graphite materials by an universal testing machine equipped with heating furnace. Based on the heterogeneous deformation fields measured by the digital image correlation(DIC) technique, the elastic constants were then extracted by using VFM.The measurement results of the elastic constants at 500℃ were obtained. The effect on the experimental results was also analyzed. The successful results verify the feasibility of using the proposed method to measure the properties of graphite at high temperature, and the proposed method is believed to have a good potential for further applications.
This paper presents an effective methodology for characterizing the mechanical parameters of composites using digital image correlation combined with the virtual fields method.By using a three-point bending test configuration,this method can identify all mechanical parameters of the material with merely a single test.Successful results verified that this method is especially effective for characterizing composite materials.In this study,the method is applied to measure the orthotropic elastic parameters of fiber-reinforced polymer-matrix composites before and after the hygrothermal aging process.The results indicate that the hygrothermal aging environment significantly influences the mechanical property of a composite.The components of the parameters in the direction of the fiber bundle decreased significantly.From the accuracy analysis,we found that the actual measurement accuracy is sensitive to a shift of the horizontal edges and rotation of the vertical edges.
