PSI - Issue 13

L A Wray et al. / Procedia Structural Integrity 13 (2018) 1768–1773 L A Wray/ Structural Integrity Procedia 00 (2018) 000 – 000

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T-sections will experience a strain range of approximately 0.26% as the temperature is reduced from 60°C to -10°C.

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All Samples 1st Sample

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600µm as welded variant

600µm milled weld variant

300µm as welded variant

300µm milled weld variant

Cycles to first 1mm crack

Figure 3: Cycles to first 1mm crack on thermal fatigue samples of varying DFT and weld condition

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cycle 10 cycle 140 cycle 317

-20 Thermal Strain (%)

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Figure 4: Average thermal strain measurements of 3 shims at thermal cycling intervals during 3 individual cycles

3. Finite Element Calculation of Thermal Strain

3.1. Model development A 2D model was produced in Abaqus using deformable, planar shell elements. The part represented half a welded T-section partitioned to simulate coating/substrate sections. Substrate and coating sections were assigned the material properties shown in Table 1. As the maximum test temperature is below the Tg of the material only the CTE below Tg was input into the model. Principal stress directions were tangential and radial to the coating surface. Load steps were created with an initial temperature of 60°C followed by a ramp to -10°C that was consistent throughout the entire part. Standard, quadratic plane strain elements (CPE8R) were applied and a seed sensitivity study was carried out to identify ideal mesh conditions. A model simulating a 300µm DFT and 2mm weld fillet radius was analysed to understand the maximum principal stresses present in the coating at -10°C after the temperature had been reduced from 60°C. The results in Figure 5(Left) indicate that the largest stresses of 36MPa are in the centre of the radius along the tangential axis. This translates into a maximum strain of 0.39%. Strains present on the flat surfaces in the tangential direction equated to approximately 0.33%. However, this is significantly larger than the measured strain range presented in Figure 4, this discrepancy is currently under investigation. In the radial direction maximum strains were considerably lower, 0.08%. Therefore, in this situation the likely cause of coating failure is due to strains tangential to coating surface. 3.2. DFT and Weld Fillet Radius Analysis The FE model was used to explore the effects of a wide range of coating DFT and weld fillet radii on the local strain concentrations in the coating. The data presented earlier in section 2.2 showed the measured DFT values to be between 300µm and 1000µm and radii to be from 0.5mm to 3mm. It can be concluded from the results that increasing DFT and decreasing radii will result in increased maximum strains in the weld region while strains on flat surface remained constant, independent of DFT. Comparing the strain in the weld fillet to the strain on the flat allowed a strain concentration factor to be determined for each condition. The results presented in Figure 5(Right) show that the strain concentration factor increases with increasing DFT and decreasing weld radius. These values may be applied to the measured experimental values of strain on a flat plate to estimate the maximum principal strains in coated T-sections.