Fatigue Crack Initiation of Metals Fabricated by Additive Manufacturing—A Crystal Plasticity Energy-Based Approach to IN718 Life Prediction

Abstract

There has been a long-standing need in the marketplace for the economic production of small lots of components that have complex geometry. A potential solution is additive manufacturing (AM). AM is a manufacturing process that adds material from the bottom up. It has the distinct advantages of low preparation costs and a high geometric creation capability. However, the wide range of industrial processing conditions results in large variations in the fatigue lives of metal components fabricated using AM. One of the main reasons for this variation of fatigue lives is differences in microstructure. Our methodology incorporated a crystal plasticity finite element model (CPFEM) that was able to simulate a stress–strain response based on a set of randomly generated representative volume elements. The main advantage of this approach was that the model determined the elastic constants (C11, C12, and C44), the critical resolved shear stress (g0), and the strain hardening modulus (h0) as a function of microstructure. These coefficients were determined based on the stress–strain relationships derived from the tensile test results. By incorporating the effect of microstructure on the elastic constants (C), the shear stress amplitude (Δτ2) can be computed more accurately. In addition, by considering the effect of microstructure on the critical resolved shear stress (g0) and the strain hardening modulus (h0), the localized dislocation slip and plastic slip per cycle (Δγp2) can be precisely calculated by CPFEM. This study represents a major advance in fatigue research by modeling the crack initiation life of materials fabricated by AM with different microstructures. It is also a tool for designing laser AM processes that can fabricate components that meet the fatigue requirements of specific applications.

Microstructure sensitive fatigue life prediction model for SLM fabricated Hastelloy-X

“…Among these studies, in some cases the real microstructures are not considered and simplified random textures are assumed [49,50]. In other cases, the actual microstructures obtained from EBSD data are included in the model [51,46,52,53] but the study is focused only on a particular fatigue regime, either LCF [46,51,53] or HCF [54,55], and a particular loading control, either strain or stress control.…”

Crystal Plasticity

“…Other examples of such works are modeling the local deformation and transformation behavior of cast metal matrix composite by using a continuum mechanics-based crystal plasticity model (Qayyum et al [5]) or mathematical modeling of plastic deformation of a tube from dispersion-hardened aluminum alloy in an inhomogeneous temperature field (Matvienko et al [6]). Ou et al [7] have incorporated a crystal plasticity finite element model for predicting a crack initiation in additively manufactured IN718 alloy, while Kosuge et al [8] have utilized strain gradient plasticity theory based on the finite element method to estimate a behavior of steel structures subjected to large complicated pre-strains. Further examples on crystal plasticity modeling involve a reaction stress model (Zhang et al [9]), statistical crystal plasticity constitutive models of polycrystalline metals and alloys (Shveykin et al [10]) or 3D representative volume element approach (Qayyum et al [11]).…”