PSI - Issue 14

V. Srikanth et al. / Procedia Structural Integrity 14 (2019) 952–956 V. Srikanth/ Structural Integrity Procedia 00 (2018) 000–000

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Ni and Ti interlayers. (Not to scale)

2. Experimental procedure In order to estimate the magnitude and distribution of the residual stresses in the joints, FEM calculations were performed. Since the geometry, load and boundary conditions employed are symmetric about a particular axis, simulations are performed using axisymmetric model. Geometries of dimension (20 mm X 10 mm) were created which were further partitioned into two and four parts for performing simulations with and without interlayers, respectively. Temperature dependent material properties were assigned to each part as shown in Fig. 1. The temperature dependent properties of joining materials are taken from different sources [7, 8]. The origin in Fig.1 was fixed to prevent rigid body motion. A structural gradient meshing is used for FEM calculations which will become finer at the interface of the joint. Bilinear quadrilateral elements with 4 nodes were considered for all the simulations with linear interpolation and reduced integration. The thermal stresses which will be generated owing to cooling of the joint from 1273 K to 298 K was simulated imposing initial strain of α∆T, where α and ∆T are thermal expansion coefficient and change in temperature, respectively. Spatially uniform cooling and a perfectly bonded joint interface with no pores and openings are two conditions assumed during the simulations. The stress fields were calculated assuming elasto-plastic behavior of the joining materials. 3. Results and Discussion In case of dissimilar joints, there will be generation of residual stresses when the diffusion bonded couple is cooled from bonding temperature to room temperature owing to the difference in the CTE. Out of all normal and shear stress components, σ yy and σ xy in particular are important, since the normal stress component σ yy acts perpendicular to the joint interface and the shear stress component σ xy acts parallel to the joint interface. High values of either of these components can become a cause for initiation of cracks at the joint interface.

Figure 2: Variation of (a) σ x , (b) σ y and (c) σ xy across the interface of the joint bonded between SS 304L and Zr-2.5%Nb without using interlayers.