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75. Mesoscopic model of hydrogen embrittlement in particle strengthened materials

Author

Barrera, Olga and Cocks, Alan

Abstract

This work focuses on the constitutive modelling of damage development in particle strengthened materials in the presence of hydrogen. We apply the model to an area at the interface of a dissimilar weld of 8630 steel/IN625 nickel alloy which is known as the ’featureless region’. This region contains an array of M7C3 carbides each measuring about 40nm. Cleavage-like fracture has been observed only in the presence of hydrogen and it is attributed to a combination of two types of hydrogen embrittlement mechanisms: hydrogen-induced decohesion (HID) along the M7C3-matrix interface of and a ductile-type fracture (Hydrogen Enhanced Local Plasticity, HELP). Modelling the constitutive behaviour of this region at a continuum level is not appropriate as the major role in the material response is the interaction of the carbide particles with dislocations which is captured at the mesoscopic level. Here we propose a constitutive model of the ”featureless zone” that accurately represents the effect of hydrogen on the constitutive response of the M7C3 region at the mesoscopic scale. Hydrogen enhances the evolution of dislocations around the M7C3 carbides, therefore the stress locally increases affecting the interfacial strength. The result is that, in the region of high hydrogen concentration, the material exhibits softening.

Published
2021

77. Characterisation of slip and twin activity using digital image correlation and crystal plasticity finite element simulation: Application to orthorhombic $��$-uranium

Author

Grilli, Nicol��, Earp, Philip, Cocks, Alan C. F., Marrow, James, and Tarleton, Edmund

Subjects
Mesoscale and Nanoscale Physics (cond-mat.mes-hall), Materials Science (cond-mat.mtrl-sci), FOS: Physical sciences
Abstract

Calibrating and verifying crystal plasticity material models is a significant challenge, particularly for materials with a number of potential slip and twin systems. Here we use digital image correlation on coarse-grained $��$-uranium during tensile testing in conjunction with crystal plasticity finite element simulations. This approach allows us to determine the critical resolved shear stress, and hardening rate of the different slip and twin systems. The constitutive model is based on dislocation densities as state variables and the simulated geometry is constructed from electron backscatter diffraction images that provide shape, size and orientation of the grains, allowing a direct comparison between virtual and real experiments. An optimisation algorithm is used to find the model parameters that reproduce the evolution of the average strain in each grain as the load is increased. A tensile bar, containing four grains aligned with the load direction, is used to calibrate the model with eight unknown parameters. The approach is then independently validated by simulating the strain distribution in a second tensile bar. Different mechanisms for the hardening of the twin systems are evaluated. The latent hardening of the most active twin system turns out to be determined by coplanar twins and slip. The hardening rate of the most active slip system is lower than in fine-grained $��$-uranium. The method developed in the present research can be applied to identify the critical resolved shear stress and hardening parameters of other coarse-grained materials.

Published
2020
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81. 2020 roadmap on solid-state batteries

Author

Pasta, Mauro, primary, Armstrong, David, additional, Brown, Zachary L., additional, Bu, Junfu, additional, Castell, Martin R, additional, Chen, Peiyu, additional, Cocks, Alan, additional, Corr, Serena A, additional, Cussen, Edmund J, additional, Darnbrough, Ed, additional, Deshpande, Vikram, additional, Doerrer, Christopher, additional, Dyer, Matthew S, additional, El-Shinawi, Hany, additional, Fleck, Norman, additional, Grant, Patrick, additional, Gregory, Georgina L., additional, Grovenor, Chris, additional, Hardwick, Laurence J, additional, Irvine, John T S, additional, Lee, Hyeon Jeong, additional, Li, Guanchen, additional, Liberti, Emanuela, additional, McClelland, Innes, additional, Monroe, Charles, additional, Nellist, Peter D, additional, Shearing, Paul R, additional, Shoko, Elvis, additional, Song, Weixin, additional, Jolly, Dominic Spencer, additional, Thomas, Christopher I, additional, Turrell, Stephen J, additional, Vestli, Mihkel, additional, Williams, Charlotte K., additional, Zhou, Yundong, additional, and Bruce, Peter G, additional

Published
2020
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90. Crystal plasticity finite element simulations of cast α-uranium

Author

Grilli, Nicolò, Cocks, Alan C. F., Tarleton, Edmund, Grilli, Nicolò, Cocks, Alan C. F., and Tarleton, Edmund

Abstract

α-uranium, the stable phase of uranium up to 670◦C, has a base-centred orthorombic crystal structure. This crystal structure gives rise to elastic and thermal anisotropy, meaning α-uranium exhibits complex deformation and fracture behaviour. Understanding the relationship between the microstructure and mechanical properties is important to prevent fracture during manufacture and usage of components. The lattice of α-uranium corresponds to a distorted close-packed-hexagonal crystal structure and it exhibits twins of both the 1st and 2nd kind. Therefore, detailed examination of the behaviour of α-uranium can also contribute to the general understanding of the interaction between plasticity, twinning and fracture in hcp crystals. Plastic deformation in α-uranium can be accommodated by 4 slip systems and 3 twin systems, previously identified by McCabe et al. These deformation modes are implemented into a crystal plasticity finite element (CPFE) material model. A temperature dependent, dislocation density based law is implemented to describe the critical resolved shear stress on the different slip/twin systems. The strong anisotropic thermal expansion behaviour is taken into account to simulate the development of internal residual stresses following casting of the material. During cooling, the internal stresses in α-uranium are sufficient to induce plasticity. This effect is quantified using polycrystal simulations, in which first the temperature is decreased, then plastic relaxation takes place, followed by application of a mechanical load. The asymmetry between mechanical properties in tension and compression, due to the presence of twins, is investigated. The model is calibrated using stress strain curves and the lattice strain found from published neutron diffraction experiments carried out on textured samples at ISIS. The strength of the slip systems is found to be lower than in fine grained material, while the strength of the twin system is similar to single crystals. The CP

Published
2019

98. An interval finite element method for the analysis of structures with spatially varying uncertainties

Author

Sofi, Alba, Romeo, Eugenia, Barrera, Olga, Cocks, Alan, Sofi, Alba, Romeo, Eugenia, Barrera, Olga, and Cocks, Alan

Abstract

Finite element analysis of linear-elastic structures with spatially varying uncertain properties is addressed within the framework of the interval model of uncertainty. Resorting to a recently proposed interval field model, the uncertain properties are expressed as the superposition of deterministic basis functions weighted by particular unitary intervals. An Interval Finite Element Method (IFEM) incorporating the interval field representation of uncertainties is formulated by applying an interval extension in conjunction with the standard energy approach. Uncertainty propagation analysis is performed by adopting a response surface approach which provides approximate explicit expressions of response bounds requiring only a few deterministic analyses. Then, the whole procedure is implemented in ABAQUS’ environment by coding User Subroutines and Python scripts. 2D plane stress and bending problems involving uncertain Young's modulus of the material are analyzed. The accuracy of the proposed IFEM as well as response variability under spatially dependent uncertainty are investigated.

Published
2018

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