A method to extract slip system dependent information for crystal plasticity models

A tool to implement a length scale dependency to classical crystal plasticity simulations is presented. Classical crystal plasticity models do not include a size effect; therefore, the size of the grain does not influence the simulated deformation. Classical crystal plasticity advancements have been through the inclusion of stress or strain gradient based constitutive models to improve the simulation of length scale dependent deformation. However, this tool presents an alternative to implementing a length scale, where the influence of slip pile-up in the form of dislocations at grain boundaries as a potential to explaining the Hall-Petch effect in materials. This is achieved by calculating the slip distance in adjacent grains for each slip system, by assuming the total slip length spans the grain in the slip direction. These calculations can occur in two ways. The first is the analysis occurs at the start of the simulation, therefore, only occurs once. If this approach is used, the computational cost of this tool is minute. However, if the simulations consider large deformations, during which it is expected that the grains are going to undergo large rotations, then it would be advantageous to the have the tool recalculate the information during the analysis. Consequently, the computational cost would depend on the resolution of the modelled geometry, the number of grains, and the number of slip systems. The tool also provides a capability to develop constitutive models based on complex grain boundary features which can be implemented in classical crystal plasticity models and gradient based crystal plasticity models. The described calculation process is implemented through a Fortran subroutine, which has been designed to be easily used in crystal plasticity simulations. The presented tool also includes Python code designed to link with microstructures built using DREAM.3D to extract the required input data to the Fortran subroutine. The proposed tool is not limited to classical crystal plasticity formulations, instead the data extracted and outputted from the Fortran subroutine can be used to serve alternative purposes in both stress and strain gradient crystal plasticity models. The proposed tool can be modified to extract additional data to that presented. The slip distance in the adjacent grain, the distance from the grain boundary of the current calculation point, and the interaction between slip systems between grains can be used in any crystal plasticity constitutive models

Enhanced non-linear material modelling for analysis and qualification of rollover protective structures

Finite element simulations of a rollover protective structure are an important aspect in its design, as it provides a means of structural integrity qualification prior to the required destructive testing. A good understanding of the rollover protective structure behaviour under simulated loading offers engineering practitioners the opportunity to optimize the design. The testing conditions, which are outlined in the applicable standards, result in plastic deformation of the rollover protective structure, associated with material hardening of various areas of the structure. An accurate description of the material behaviour is important for finite element simulations of the structural response. This research examines some of the hardening models commonly used in simulations of rollover protective structures, which are available in most finite element commercial software, including linear and multi-linear isotropic and kinematic hardening models and non-linear kinematic hardening models. The numerical performance of the plasticity models in representing the material behaviour was compared with the experimental data for commonly used rollover protective structure material. Analysis revealed the potential benefits and drawbacks of the various models. Moreover, a damage-induced softening model was implemented at the structure joints in conjunction with the non-linear hardening models. Enhanced computational results were obtained through this modelling variation, highlighting the importance of material modelling at the primary structure and the joints of a rollover protective structure

Cyclic plasticity of the as-built EOS maraging steel: preliminary experimental and computational results

This short communication offers a preliminary view on ongoing research conducted on the as-built EOS maraging steel 300. The material’s cyclic elastoplastic characteristics under strain-controlled loading have been investigated experimentally. Specimens fabricated under two primary orientations, horizontally and vertically to the build plate, have been tested. The obtained stress–strain hysteresis loops exhibited symmetry, with the vertical specimen showing a higher plastic strain energy dissipation capability than the horizontal specimen. Modelling of the material’s elastoplastic behaviour was performed with a commonly used kinematic hardening rule, combined with both isotropic and anisotropic yield functions and elasticity moduli. The obtained simulations of the hysteresis loops, from the implementation of these two plasticity models, indicate the advantage of the anisotropic modelling approach over the isotropic approach. The anisotropic plasticity model describes in a more representative way the inherent elastic and plastic anisotropy of the as-built material. Further research is underway to explore the low cycle fatigue performance of this additively manufactured metal

Elastoplastic response of as-built SLM and wrought Ti-6Al-4V under symmetric and asymmetric strain-controlled cyclic loading

Purpose The purpose of this paper is to examine the mechanical behaviour of additively manufactured Ti-6Al-4V under cyclic loading. Using as-built selective laser melting (SLM) Ti-6Al-4V in engineering applications requires a detailed understanding of its elastoplastic behaviour. This preliminary study intends to create a better understanding on the cyclic plasticity phenomena exhibited by this material under symmetric and asymmetric strain-controlled cyclic loading.Design/methodology/approach This paper investigates experimentally the cyclic elastoplastic behaviour of as-built SLM Ti-6Al-4V under symmetric and asymmetric strain-controlled loading histories and compares it to that of wrought Ti-6Al-4V. Moreover, a plasticity model has been customised to simulate effectively the mechanical behaviour of the as-built SLM Ti-6Al-4V. This model is formulated to account for the SLM Ti-6Al-4V-specific characteristics, under the strain-controlled experiments.Findings The elastoplastic behaviour of the as-built SLM Ti-6Al-4V has been compared to that of the wrought material, enabling characterisation of the cyclic transient phenomena under symmetric and asymmetric strain-controlled loadings. The test results have identified a difference in the strain-controlled cyclic phenomena in the as-build SLM Ti-6Al-4V when compared to its wrought counterpart, because of a difference in their microstructure. The plasticity model offers accurate simulation of the observed experimental behaviour in the SLM material.Research limitations/implications Further investigation through a more extensive test campaign involving a wider set of strain-controlled loading cases, including multiaxial (biaxial) histories, is required for a more complete characterisation of the material performance.Originality/value The present investigation offers an advancement in the knowledge of cyclic transient effects exhibited by a typical ' martensite SLM Ti-6Al-4V under symmetric and asymmetric strain-controlled tests. The research data and findings reported are among the very few reported so far in the literature

A review of the as-built SLM Ti-6Al-4V mechanical properties towards achieving fatigue resistant designs

Ti-6Al-4V has been widely used in both the biomedical and aerospace industry, due to its high strength, corrosion resistance, high fracture toughness and light weight. Additive manufacturing (AM) is an attractive method of Ti-6Al-4V parts’ fabrication, as it provides a low waste alternative for complex geometries. With continued progress being made in SLM technology, the influence of build layers, grain boundaries and defects can be combined to improve further the design process and allow the fabrication of components with improved static and fatigue strength in critical loading directions. To initiate this possibility, the mechanical properties, including monotonic, low and high cycle fatigue and fracture mechanical behaviour, of machined as-built SLM Ti-6Al-4V, have been critically reviewed in order to inform the research community. The corresponding crystallographic phases, defects and layer orientations have been analysed to determine the influence of these features on the mechanical behaviour. This review paper intends to enhance our understanding of how these features can be manipulated and utilised to improve the fatigue resistance of components fabricated from Ti-6Al-4V using the SLM technolog

Process phenomena influencing the tensile and anisotropic characteristics of additively manufactured maraging steel

The tensile mechanical properties and anisotropy levels of identical test-coupons, fabricated from maraging steel 300 (MS300) using two alternative EOS EOSINT M280 Additive Manufacturing (AM) systems, have been examined. The mechanical performance variations resulting from process differences between the two suppliers and the part's build volume orientation (0°, 45°, and 90°) are investigated. Significant microstructural discrepancies, affecting mechanical performance, plasticity and anisotropy levels, have been observed in the as-built samples obtained from the two suppliers. A difference in the angle of the laser scan strategy, in conjunction with unfavourable powder feedstock characteristics, are understood to have had a profound influence on the plasticity and anisotropy divergences observed in the AM MS300 alloy. Plastic anisotropy levels can be largely reduced through application of aging heat-treatments, however, a degree of transverse strain anisotropy is likely to remain due to the AM alloy's fabrication history. Moreover, in this work both the anisotropic and elasticity tensors for this material are derived. These tensors can be used by researchers working on modelling and simulation of the MS300 mechanical properties

Enhanced non-linear material modelling for analysis and qualification of rollover protective structures

Finite element simulations of a rollover protective structure are an important aspect in its design, as it provides a means of structural integrity qualification prior to the required destructive testing. A good understanding of the rollover protective structure behaviour under simulated loading offers engineering practitioners the opportunity to optimize the design. The testing conditions, which are outlined in the applicable standards, result in plastic deformation of the rollover protective structure, associated with material hardening of various areas of the structure. An accurate description of the material behaviour is important for finite element simulations of the structural response. This research examines some of the hardening models commonly used in simulations of rollover protective structures, which are available in most finite element commercial software, including linear and multi-linear isotropic and kinematic hardening models and non-linear kinematic hardening models. The numerical performance of the plasticity models in representing the material behaviour was compared with the experimental data for commonly used rollover protective structure material. Analysis revealed the potential benefits and drawbacks of the various models. Moreover, a damage-induced softening model was implemented at the structure joints in conjunction with the non-linear hardening models. Enhanced computational results were obtained through this modelling variation, highlighting the importance of material modelling at the primary structure and the joints of a rollover protective structure

Constitutive modeling of additive manufactured Ti-6Al-4V cyclic elastoplastic behaviour

Metal additive manufacturing techniques have been increasingly attracting the interest of the aerospace and biomedical industry. A particular focus has been on high value and complexity parts and components, as there the advantages offered by additive manufacturing are very significant for the design and production organisations. Various additive manufacturing techniques have been tested and utilized over the past years, with laser-based technology being among the preferred solutions – e.g. selective laser melting / sintering (SLM / SLS). Fatigue qualification, as one of the primary design challenges to meet, imposes the need for extensive material testing. Moreover, this need is amplified by the fact that currently there is very limited in-service experience and understanding of the distinct mechanical behaviour of additively manufactured metallic materials. To this end, material modelling can serve as a mediator, nevertheless research particular to additively manufactured metals is also quite limited. This work attempts to identify the cyclic elastoplastic behaviour characteristics of SLM manufactured Ti-6Al-4V. A set of uniaxial stress and strain controlled mechanical tests have been conducted on as-built SLM coupons. Phenomena critical for engineering applications and interrelated to fatigue performance (mean stress relaxation, ratcheting) have been examined under the prism of constitutive modeling. Cyclic plasticity models have been successfully employed to simulate the test results. Moreover, a preliminary analysis has been conducted on the differences observed in the elastoplastic behaviour of SLM and conventionally manufactured Ti-6Al-4V and their possible connection to material performance in the high cycle fatigue regime