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

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

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

Aluminum alloy 7075 ratcheting and plastic shakedown evaluation with the multiplicative Armstrong-Frederick model

This work investigates experimentally and computationally the uniaxial ratcheting strain and plastic shakedown of aluminum alloy 7075-T6. The experimental results illustrate the existence of both plastic shakedown and cyclic hardening, proving that both kinematic and isotropic hardening should be included in modeling. Although the multicomponent Armstrong-Frederick model with multiplier has demonstrated high accuracy in aluminum ratcheting simulation, as well implementation ease, the present results demonstrate poor performance when plastic shakedown is considered. This is attributed to the limited flexibility in varying the model parameters to balance the loading/unloading branches in the hysteresis loops and decelerate ratcheting pace. To improve the ability of the multicomponent Armstrong-Frederick model with multiplier in simulating plastic shakedown, a modification was made within the framework of the model that includes multiple backstress components, with each obeying its own kinematic hardening. A linear kinematic hardening backstress was added in the formulation, enabling the control of ratcheting pace and the occurrence of plastic shakedown. Simulations with the modified multicomponent Armstrong-Frederick model with multiplier demonstrate a significantly improved capability for ratcheting and plastic shakedown. Moreover, the modified multicomponent Armstrong-Frederick model with multiplier improved the life prediction for an actual aerospace structure. This provides a strong indication of the importance of achieving plastic shakedown accuracy when simulating cyclic elastoplastic behavior. Read More

Medical Supplies Shortages and Burnout among Greek Health Care Workers during Economic Crisis: a Pilot Study

Greece has been seriously affected by the economic crisis. In 2011 there were reports of 40% reduction to public hospital budgets. Occasional shortages of medical supplies have been reported in mass media. We attempted to pivotally investigate the frequency of medical supplies shortages in two Greek hospital units of the National Health System and to also assess their possible impact on burnout risk of health care workers. We conducted a cross-sectional study (n=303) of health care workers in two Greek hospitals who were present at the workplace during a casually selected working day (morning shift work). The Maslach Burnout Inventory (MBI) was used as the measure of burnout. An additional questionnaire was used about demographics, and working conditions (duration of employment, cumulative night shifts, type of hospital including medical supplies shortages and their impact on quality of healthcare. The prevalence of emotional exhaustion, depersonalization and low personal accomplishment was 44.5%, 43.2% and 51.5%, respectively. Medical supply shortages were significantly associated with emotional exhaustion and depersonalization. This finding provides preliminary evidence that austerity has affected health care in Greece. Moreover, the medical supply shortages in Greek hospitals may reflect the unfolding humanitarian crisis of the country

Optimising the multiplicative AF model parameters for AA7075 cyclic plasticity and fatigue simulation

Purpose - This study presents the improvements of the multicomponent Armstrong-Frederick model with multiplier (MAFM) performance through a numerical optimisation methodology available in a commercial software. Moreover, this study explores the application of a multiobjective optimisation technique for the determination of the parameters of the constitutive models using uniaxial experimental data gathered from aluminium alloy 7075-T6 specimens. This approach aims to improve the overall accuracy of stress-strain response, for not only symmetric strain-controlled loading but also asymmetrically strain- and stress-controlled loading. Design/methodology/approach - Experimental data from stress- and strain-controlled symmetric and asymmetric cyclic loadings have been used for this purpose. The analysis of the influence of the parameters on simulation accuracy has led to an adjustment scheme that can be used for focused optimisation of the MAFM model performance. The method was successfully used to provide a better understanding of the influence of each model parameter on the overall simulation accuracy. Findings - The optimisation identified an important issue associated with competing ratcheting and mean stress relaxation objectives, highlighting the issues with arriving at a parameter set that can simulate ratcheting and mean stress relaxation for load cases not reaching at complete relaxation. Practical implications - The study uses a strain-life fatigue application to demonstrate the importance of incorporating a technique such as the presented multiobjective optimisation method to arrive at robust parameters capable of accurately simulating a variety of transient cyclic phenomena. Originality/value - The proposed methodology improves the accuracy of cyclic plasticity phenomena and strain-life fatigue simulations for engineering applications. This study is considered a valuable contribution for the engineering community, as it can act as starting point for further e

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 unified material model to predict ratcheting response in head-hardened rail steel due to non-uniform hardness distributions

The non-uniform hardness distribution, metallurgy and ratchetting behaviour of head-hardened Australian rail steel (AS60 HH) are studied. In a bid to extrapolate material properties across the rail head continuum, a unified material model of the rail head has been developed, in which only the hardness of the material is variable. This simplification is shown to enable the material model to reasonably describe ratchetting behaviour across the rail head for heat-treated pearlitic steels. Finally, homogeneous material ratchetting models are used to study the evolution of plastic strain deformation under realistic rail-wheel contact conditions. The results show that non-uniform material properties may compromise ratcheting response with wear progression, thus impacting lifetime of AS60 HH rail steel and re-railing frequency

An Innovative Structural Fatigue Monitoring Solution for General Aviation Aircraft

This article proposes a novel and effective solution for estimating fatigue life of General Aviation (GA) airframes using flight data produced by digital avionics systems, which are being installed or retrofitted into a growing number of GA aircraft. In the proposed implementation, a flight dynamics model is adopted to process the recorded flight data and to determine the dynamic loadings experienced by the aircraft. The equivalent loading cycles at fatigue-critical points of the primary structure are counted by means of statistical methods. For validation purposes, the developed approach is applied to flight data recorded by a fleet of Cessna 172S aircraft fitted with a Garmin G1000 integrated navigation and guidance system. Based on the initial experimental results and the developed uncertainty analysis, the proposed approach provides acceptable estimates of the residual fatigue life of the aircraft, thereby allowing a cost-effective and streamlined structural integrity monitoring solution. Future developments will address the possible adoption of the proposed method for unmanned aircraft structural health monitoring, also considering the accuracy enhancements achievable with advanced navigation and guidance architectures based on Global Navigation Satellite Systems (GNSS), Vision-Based Navigation (VBN) Sensors, Inertial Measurement Units (IMU) and Aircraft Dynamics Model (ADM) augmentation