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

A phenomenological approach for fatigue damage accumulation of CF/PEEK laminates under two-stage loading

The life of composite materials (CM) subjected to fatigue conditions is associated with the gradual damage of their structure. Since the damage accumulation is reflected by changes in CM properties, the derivation of damage accumulation curves may be based on test results providing the reduction of the residual strength. Examined in this work is a non-linear fatigue damage accumulation function for cross-ply CF/PEEK laminates. This damage function is specified with respect to damage parameters derived by the strength decrease during fatigue for the CF/PEEK. Taking into account this concept, an improvement of the theory of isodamage lines is proposed. Since the nonlinearity of the derived damage function depends on an exponent that is a function of the fatigue stress amplitude, the effect of the loading order and the previous damage history on the total fatigue life is investigated. Numerical examples of fatigue life prediction under two-stage loading conditions and a damage accumulation map for the CP/PEEK are presented and discussed

Derivation of fatigue damage accumulation curves based on changes in composite material properties

The life of composite materials (CM) subjected to fatigue conditions is associated with the gradual damage of their structure. Since the damage accumulation is reflected by changes in CM properties, the derivation of damage accumulation curves may be based on test results providing the reduction of the residual strength. Examined in this work is a non-linear fatigue damage accumulation function for cross-ply CF/PEEK laminates. This damage function is specified with respect to damage parameters derived by the strength decrease during fatigue for the CF/PEEK. Taking into account this concept, an improvement of the theory of isodamage lines is proposed. Since the nonlinearity of the derived damage function depends on an exponent that is a function of the fatigue stress amplitude, the effect of the loading order and the previous damage history on the total fatigue life is investigated. Numerical examples of fatigue life prediction under two-stage loading conditions and a damage accumulation map for the CP/PEEK are presented and discussed

Cyclic plasticity and microstructure of as-built SLM Ti-6Al-4V: The effect of build orientation

This paper investigates the cyclic elastoplastic anisotropy of Ti-6Al-4V manufactured via selective laser melting, caused by different build orientations. Tensile monotonic and cyclic stepped symmetric strain-controlled tests on coupons manufactured at 0 degrees, 45 degrees, and 90 degrees build orientations were performed for both the SLM material and for a mill annealed material. The microstructure characteristics of the SLM material were examined through optical and electron microscopy revealing a unique alpha' martensite microstructure. The examination of the evolving tensile and compressive maximum stresses identified an interesting phenomenon, that of asymmetric cyclic softening. This phenomenon was observed only in the SLM Ti-6Al-4V, while its wrought counterpart confirmed the findings of past research reported in the literature. The residual stresses presented in the SLM coupons had a significant influence on the cyclic behaviour of the material. Mechanical anisotropy in both monotonic and cyclic tests was noticed with the diagonal (45 degrees) coupon having the largest yield stress in both loading conditions. The findings of this research study can be very useful in engineering applications utilising as-built SLM materials

Characterisation of head-hardened rail steel in terms of cyclic plasticity response and microstructure for improved material modelling

Stress- and strain-controlled tests of heat treated high-strength rail steel (Australian Standard AS1085.1) have been performed in order to improve the characterisation of the said material׳s ratcheting and fatigue wear behaviour. The hardness of the rail head material has also been studied and it has been found that hardness reduces considerably below four-millimetres from the rail top surface. Historically, researchers have used test coupons with circular cross-sections to conduct cyclic load tests. Such test coupons, typically five-millimetres in gauge diameter and ten‐millimetres in grip diameter, are usually taken from the rail head sample. When there is considerable variation of material properties over the cross-section it becomes likely that localised properties of the rail material will be missed. In another case from the literature, disks 47 mm in diameter for a twin-disk rolling contact test machine were obtained directly from the rail sample and used to validate ratcheting and rolling contact fatigue wear models. The question arises: How accurate are such tests, especially when large material property gradients exist? In this research paper, the effects of rail sampling location on the ratcheting behaviour of AS1085.1 rail steel were investigated using rectangular-shaped specimens obtained at four different depths to observe their respective cyclic plasticity behaviour. The microstructural features of the test coupons were also analysed, especially the pearlite inter-lamellar spacing which showed strong correlation with both hardness and cyclic plasticity behaviour of the material. This work ultimately provides new data and testing methodology to aid the selection of valid parameters for material constitutive models to better understand rail surface ratcheting and wear

A modification of the multicomponent ArmstrongFrederick model with multiplier for the enhanced simulation of aerospace aluminium elastoplasticity

The Multicomponent ArmstrongFrederick (AF) model with Multiplier (MAFM) has demonstrated high simulation accuracy for uniaxial and multiaxial loading conditions for a number of different materials. In this study the MAFM model is modified to improve the phenomenological modelling of aerospace aluminium alloys 7075-T6 and 7050-T7451 under uniaxial constant and variable amplitude loading. In order to recognise the experimentally observed strain amplitude dependency of mean stress relaxation rate, the coefficient of the linear kinematic backstress was modified from a constant to a strain amplitude dependent dynamic term. This modification improved the mean stress relaxation capability of the MAFM model. Additionally, the hysteresis loop evolution has been enhanced via further modification of the MAFM model by improving the monotonic stress-strain evolution of the initial loading branch of cyclic load cases by separating the kinematic backstress coefficients into two parts, the contributions from cyclic and monotonic micro-mechanisms. The monotonic coefficients were allowed to decay with continued cycling, which captured the monotonic to cyclic transition of stress-strain development. Finally, the experimentally observed reversibility of the monotonic stress-strain evolution has been also incorporated successfully through the introduction of a decaying strain range memory parameter, which improved the variable amplitude hysteresis loop evolution. Overall, the modified MAFM model has been successful in improving simulation accuracy of the cyclic elastoplastic response exhibited by both aluminium alloys examined

Letter to the Editor: S. Sreenivasan, S.K. Mishra, K. Dutta. Ratcheting strain and its effect on low cycle fatigue behavior of Al 7075-T6 alloy [Mater. Sci. Eng. A 698 (2017) 46-53]

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A simple model to estimate yield stress and hardness variation in railheads

This technical note presents a macroscopic model capable to estimate the variation of hardness and yield stress at different railhead distances (depths) from the running surface. Published data, including results reported in past work by the authors, have been utilised to calibrate and test the validity of the model. From this preliminary investigation it was found that the model can predict accurately the measured hardness and yield stress values, as well as represent the variation profile exhibited in the examined railhead material. This model, subject to further validation, has the potential to be used in practical applications