PSI - Issue 35

Onkar Salunkhe et al. / Procedia Structural Integrity 35 (2022) 261–268 Onkar Salunkhe, Parag Tandaiya / Structural Integrity Procedia 00 (2021) 000–000

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(a)

(b)

(c)

(d)

Fig. 5. (a) Contour plot of maximum principal logarithmic plastic strain ln λ p

1 for uncoated axisymmetric model at 6% nominal strain, (b) Contour

plot of ln λ p 1 which shows the crack (deleted elements) inside the shear band for coated axisymmetric BMG composite model at 8% nominal strain, (c) Contour plot of equivalent plastic strain in Copper coating at 8% nominal strain and, (d) Comparison of stress-strain curves for axisymmetric uncoated monolithic BMG and Copper coated BMG composite models having 100 µ m thickness of Copper coating.

dominant shear band which is arrested by the Copper coating leading to enhancement in strain to failure for the BMG composite by about 1.5% as seen from Fig. 5(d).

3.5. Corroboration of experimental observations in literature and present 3D simulations

Most of the engineering experiments involve the use of cylindrical specimens to obtain the stress-strain response of materials under tension or compression. Similar experiments were performed by researchers (Sun et al., 2016) who investigated the enhancement in strain to failure of cylindrical BMG composites with copper coating. The experiments were carried out on cylindrical specimens of Vitreloy-105 BMG. The typical Copper coating thickness used was 20 µ m. Both uncoated and coated samples were tested, and their stress-strain response were reported. To understand the experimentally reported e ff ect of coating layer, the deformation and fracture morphologies of both as-cast and coated sample were examined. It can be seen from Fig. 6(a) taken from Sun et al. (2016), the Copper coated BMG composite did not fail catastrophically after deformation but proceeded with gradually decreasing stress-strain curve. In contrast, the as-cast / monolithic BMG specimen failed catastrophically with sudden drop in stress (see Fig. 6(b)). As-cast monolithic BMG showed 4% plastic strain before failure whereas the Copper coated BMG composite showed about 12% plastic strain before failure. This increase in malleability is due to the e ff ect of copper coating confining the BMG matrix leading to arrest of shear bands and eliminating the catastrophic failure. This gradual failure can be explained as the elastic energy released in the final fracture is absorbed by copper coating layer resulting in debonding of copper layer from BMG surface. Also, the breaking of copper layer indicates that a severe plastic deformation has occurred before the debonding of copper from surface. In the present work, models of uncoated monolithic BMG specimen and Copper coated BMG composite cylindrical specimen were created. The finite element mesh used for uncoated and coated cylindrical models of 2 mm diameter and 4 mm height involves using C3D8R elements. The uncoated mesh consists of 35200 elements while the same mesh is coated with 20-micron thickness Copper, which has 5 elements along the thickness with total 49760 elements in the mesh. The BMG matrix in both the models is Vitreloy-105. At room temperature, the mechanical properties of relevance of this BMG are taken from Raut et al. (2018): Young’s modulus, E = 107 GPa, Poisson’s ratio, ν = 0.38, yield strength in compression, σ c y = 1.93 GPa, tensile yield strength, σ t y = 1.87 GPa. The other material parameter values used in the present work are as follows: internal friction parameter, µ = 0.008, initial dilatancy parameter, g 0 = 0.4 in tension and 0.04 in compression as in Anand and Su (2005), initial value of cohesion, c 0 = 980 MPa, saturation value of cohesion, c cv = 960 MPa, maximum plastic dilatation, η cv = 0.005, and strain rate sensitivity parameter, m = 0.02. The values of damage parameters γ c and γ f are taken as 0.18 and 0.28. These values are obtained by calibration such that the experimental and simulation stress strain curves of uncoated / monolithic BMG cylinder specimen agree with each other (see Fig. 7(a)). It can be inferred from the comparison plots of nominal stress-strain curves in Fig. 7(b), the strain to failure of the BMG composite is increased by about 2%. From Fig. 7(c), it can be seen, an inclined shear