Structural Design and Life Prediction of Sheet Metal Parts for Gas Turbine Vane

In the cooling design of gas turbine vane, sheet metal parts were used to arrange the impact cooling holes in the platform and inner cavity, and divided the chamber space for impact cooling. During the service period, due to the impact of temperature load and air flow, the sheet metal parts were prone to bulge, crack, fall off, which worsen the cooling effect. In order to ensure the safe and reliable operation of blade and sheet metal parts, it was necessary to design reasonable sheet metal structure and accurately predict the service life. In this paper, material mechanical property test of GH3536 was carried out to obtain its constitutive equation and fatigue performance parameters. The structure of sheet metal parts were designed, then the temperature field of the blade and the sheet metal was simulated by using the fluid-thermal-solid coupling method, and the stress-strain analysis was carried out by using the elastic-plastic constitutive model. Finally, the fatigue life of the sheet metal was obtained, and the service life meet the design requirements. This work was helpful to prolong the life of sheet metal structure and ensure the cooling effect of blade.


Introduction
In the cooling design of turbine guide vanes in gas turbines, impact cooling is one commonly used method.The impact cooling holes for the vane trailing edge and internal cavity are arranged using sheet metal components, which also partition the required chamber space for impact cooling [1][2].The impact cooling hole plate and the deflector are flanged and welded to the vane substrate, maintaining a stable impact cooling gap to achieve optimal impact cooling effectiveness.During the service life of the vanes, the sheet metal components are prone to failures such as bulging, cracking, and detachment due to temperature loads and aerodynamic impacts, which deteriorate the cooling effectiveness of the vanes [3].Therefore, it is necessary to study the performance of high-temperature alloy materials used for sheet metal components, design appropriate sheet metal structures, and accurately predict the fatigue life of the sheet metal components to ensure the safe and reliable operation of both the sheet metal components and the turbine vanes.Based on considerations of high-temperature resistance, oxidation resistance, and weldability, sheet metal components generally use high-temperature alloys, which can be divided into two main categories: nickel-based high-temperature alloys such as GH4169, GH3044, and GH3536, known for their high yield strength and excellent high-temperature oxidation resistance, and cobalt-based hightemperature alloys such as GH5188, GH605, and K640, known for their superior high-temperature endurance.Generally, nickel-based high-temperature alloys have higher yield strength than cobaltbased high-temperature alloys, but they are also more challenging to stamp and form.Therefore, the selection of sheet metal materials primarily involves finding a compromise between high yield strength and formability in the stamping process.Currently, GH3536 is the most widely used sheet metal material in gas turbines.It exhibits good durability and creep strength below 900°C, and it has good performance in terms of hot forming and weldability.There have been numerous studies conducted domestically and internationally on the properties of GH3536 or similar chemicallycomposed H-X materials [4,5].Traditional sheet metal components are formed into the desired shapes using methods such as die stamping and bending.Typically, external forces are applied to induce plastic deformation in the sheet metal to obtain the desired shapes.The forming processes generally include cold stamping, hot stamping, and hot relaxation forming [6,7].With the increasing demand for turbine efficiency, turbine blades are becoming more complex in their three-dimensional profiles, which in turn leads to more complex three-dimensional profiles of sheet metal components.Traditional stamping processes are no longer able to meet these requirements.As a result, a new advanced technology called hightemperature alloy powder laser selective melting 3D printing has emerged.This technology addresses the challenges posed by complex shapes.It allows for localized reinforcement design based on strength requirements, provides better structural design flexibility, eliminates the need for joint welds, and minimizes internal stress issues [8].Currently, most of the research in this field focuses on the material properties of sheet metal components and the manufacturing processes involved.There is limited literature available regarding the structural design and fatigue life prediction of sheet metal components for gas turbine blades [9].Based on this observation, this study takes the typical material GH3536 used in gas turbines as an example and carries out research on the mechanical properties of the material.The study establishes the constitutive equations and fatigue life curves of the material.Furthermore, a sheet metal component structure for turbine guide vanes is designed.A fluid-thermal-solid coupling method is employed to simulate the temperature field of the blade and sheet metal component during operation.An elastic-plastic constitutive model is applied to perform stress-strain analysis.Ultimately, the fatigue life of the sheet metal component is determined, and the life assessment is conducted.

Sample Preparation
The base material used in this study is a strip.The composition of the material was tested, and the content of elements such as Si, Mn, P, Cr, Ni, Ti, Al, Co, Mo, W, Cu, Fe, and B in the alloy was measured using an Inductively Coupled Plasma Emission Spectrometer.The content of C and S in the alloy was measured using a High-Frequency Infrared Carbon and Sulfur Analyzer.The results are shown in Table 1.

Testing Methods
Room temperature tensile testing and high-temperature low-cycle fatigue testing of GH3536 were conducted in the laboratory to obtain the mechanical performance of the material.The dimensions of the room temperature tensile specimens were based on the national standard GB/T 228.1-2010.Three specimens were cut from the sheet material at angles of 0°, 45°, and 90° with respect to the rolling direction, as shown in Figure 1a).
The testing equipment used was a 300kN electronic universal material testing machine, model CMT5305, as shown in Figure 1b).The testing was conducted in accordance with the standard GB/T 228.1-2010.The strain rate for the test was set at 0.001/s.The room temperature tensile specimens were subjected to tension until failure, and the mechanical properties of the material were measured in the three specified directions.

Room temperature tensile testing Results
The engineering stress-strain curves for room temperature tensile tests are shown in Figure3.The tests were conducted under the same conditions and repeated three times [10].It can be observed that the material exhibits minimal performance differences in the three directions, indicating isotropic behaviour.

High-Temperature Cyclic Tensile Results
The high-temperature cyclic stress-strain curves are shown in Figure 4 and Figure5.The cyclic stressstrain curves were fitted using the Ramberg-Osgood equation.
The high-temperature strain-life curve, as shown in Figure 6 and Figure 7, was fitted using the Manson-Coffin equation.

Structural Description
The sheet metal component used for the turbine guide vane in a certain gas turbine is made of GH3536 material.During the structural design, air film holes were arranged on the sheet metal component for the purpose of impact cooling during operation.The left side represents the impact sheet metal component, while the right side represents the flow guide tube.The impact sheet metal component is welded to the vane substrate by flanging, and the two impact sheet metal components are connected by spot welding.The upper end of the flow guide tube is designed with reinforcing ribs and is welded to the vane substrate by flanging, while the lower end is free.The structure of the sheet metal component is shown in Figure 6.

Temperature Field Analysis
The three-dimensional temperature field of the vane substrate and sheet metal component was calculated using the commercial software CFX 19.0.The heat transfer between the gas, metal, and cooling air was coupled together, and the interface between the fluid and solid domains was treated with the conjugate interface boundary condition.The SST turbulence model [11] was employed, and separate meshing was performed for the fluid and solid domains before importing them into the CFX software.Based on the analysis of grid independence, the final number of grids used was 68 million for the fluid domain and 21 million for the solid domain, ensuring that the y+ values on the fluid side wall were close to 1. Temperature distribution curves for the gas-side inlet boundary and the gas-side outlet boundary is set as the distribution of gas outlet static pressure.After iterative calculations with the cooling air flow network, the following configurations were determined: the cooling air inlet boundary adopts the pressure boundary obtained from the matching of both sides.The calculation is terminated when the residual is less than 10 -5 .At this point, in CFX-Pre, the monitored physical quantities such as cooling air flow rate, total pressure at the gas inlet, and average metal temperature of the blade no longer exhibit fluctuations.The calculated temperature field distribution of the blade and sheet metal component is shown in Figure 7. From the analysis results, it can be observed that the metal temperature of the impact sheet metal component is approximately 400°C, and the temperature distribution is relatively uniform.The upper part of the guide vane exhibits higher temperatures, with a metal temperature of around 500°C, while the bottom metal temperature is approximately 400°C.

Fatigue Analysis
The stress and strain of the sheet metal component are calculated using the commercial software ANSYS 19.1.The Solid186 element is used to mesh the blade and sheet metal component.An elasticplastic constitutive model is employed to simulate the plastic behavior of the sheet metal component [12].The bonding between the flanged sheet metal and the base material is modeled using the Bonded option to simulate welding.Additionally, the temperature field and aerodynamic field data are applied to the blade base and sheet metal.The stress and strain distribution of the impact sheet metal component is shown in Figure 8. Apart from stress concentration around the air film hole edges and stress concentration caused by bonding, the areas with high stress and strain are mainly concentrated around the weld seam.The maximum principal stress is perpendicular to the direction of the weld seam, and the corresponding plastic strain is 6.37e-3.The primary failure mode of the impact sheet metal component is low-cycle fatigue failure caused by high strains around the weld seam due to frequent start-stop cycles.Therefore, the Manson-Coffin formula is used to calculate the low-cycle fatigue life at this location, which is determined to be 3485 cycles, meeting the design requirements.
Equivalent stress Maximum principal stress Plastic strain Figure 8. Stress and strain distribution of impact sheet metal parts Flow-induced bending stress distribution in the diffuser tube is shown in Figure 9.According to the calculation results, the maximum flow-induced bending stress is located at the upper end of the diffuser tube and is measured to be 42.7 MPa.The primary failure mode of the diffuser tube is highcycle fatigue caused by frequent impacts from the airflow.Therefore, the S-N curve [13] is used to calculate the high-cycle fatigue life at this location, which is found to be significantly greater than 10 8 cycles, thus satisfying the design requirements.

Conclusion
In this study, the typical material GH3536 used in gas turbines was investigated to understand its mechanical properties.The mechanical performance of the material was characterized at both room temperature and high temperatures.Additionally, a sheet metal structure for turbine guide vanes was designed, and its fatigue life was determined.The specific conclusions are as follows:  Tensile tests were conducted on GH3536 samples taken at 0°, 45°, and 90° orientations at room temperature.The mechanical properties were found to be nearly identical in all three directions, indicating isotropic behaviour;  High-temperature cyclic tensile tests were performed on GH3536 at 400°C and 500°C.The cyclic stress-strain constitutive model and low-cycle fatigue life were obtained;  Through the coupled thermal-fluid analysis using CFX, the temperature distributions of the impact sheet metal and diffuser tube during the blade's operation were obtained.The impact sheet metal exhibited a uniform temperature distribution at approximately 400°C, while the top portion of the diffuser tube reached a higher temperature of 500°C, with the bottom temperature being around 400°C;  The low-cycle fatigue life of the impact sheet metal was calculated to be 3485 cycles, and the high-cycle fatigue life of the diffuser tube was found to be greater than 10 8 cycles.These results indicate that the designed sheet metal structures meet the design requirements.

Figure 1 .
Figure 1.Room Temperature Tensile testThe dimensions of the high-temperature low-cycle fatigue specimens were based on the standard GB/T 15248-2008.The specimen configuration is shown in Figure2a).The experimental setup employed a 250kN fatigue machine, model Landmark 370.25, as depicted in Figure2b).The test conditions included temperatures of 400°C and 500°C, strain ratio of 1 , axial triangular waveform for loading, and a strain rate of 0.002/s.The specimens were cyclically loaded at high temperatures until failure, and the cyclic stress-strain curves as well as the strain-life curves of the material were measured.

Figure 6 .
Figure 6.Structure of sheet parts for turbine vanes(Impact sheet parts, flow guide tube)

Figure 7 .
Figure 7. Temperature distribution of base and sheet metal parts