Denis J. Politis

Knowledge based cloud FE simulation of sheet metal forming processes

The use of Finite Element (FE) simulation software to adequately predict the outcome of sheet metal forming processes is crucial to enhancing the efficiency and lowering the development time of such processes, whilst reducing costs involved in trial-and-error prototyping. Recent focus on the substitution of steel components with aluminum alloy alternatives in the automotive and aerospace sectors has increased the need to simulate the forming behavior of such alloys for ever more complex component geometries. However these alloys, and in particular their high strength variants, exhibit limited formability at room temperature, and high temperature manufacturing technologies have been developed to form them. Consequently, advanced constitutive models are required to reflect the associated temperature and strain rate effects. Simulating such behavior is computationally very expensive using conventional FE simulation techniques. This paper presents a novel Knowledge Based Cloud FE (KBC-FE) simulation technique that combines advanced material and friction models with conventional FE simulations in an efficient manner thus enhancing the capability of commercial simulation software packages. The application of these methods is demonstrated through two example case studies, namely: the prediction of a materials forming limit under hot stamping conditions, and the tool life prediction under multi-cycle loading conditions.

Examination of the relationship between servant leadership and agency problems: gender matters

The purpose of this paper is to examine the relationship between servant leadership and agency problems. Also, the paper seeks to determine whether gender plays a role in this relationship.,A survey of 276 employees was carried out to investigate the relationship between the characteristics of servant leadership and the constructs of agency problems of 36 male and 22 female managers/supervisors in the banking sector months before the financial crisis in Cyprus. The responses were subjected to a series of correlational and structural equation modelling analyses.,The study revealed three major findings. First, the relationship between servant leadership behaviour and agency problems is near zero and not significant. Second, the servant leadership behaviour of female managers/supervisors brings the desired negative influence on the constructs of agency problems. Finally, the servant leadership behaviour of males has a more positive and significant influence on agency problems than that of females. These findings should be interpreted with caution, because more female than male participants rated the male managers/supervisors, and salient traits might cause followers to feel that female managers display more servant leadership characteristics, regardless of whether they exhibit them or not.,The conclusions cannot be generalised because the research was conducted in a country undergoing a financial crisis. However, this study provides a springboard to further explore whether the findings are valid if the sample is taken from a country not facing financial crisis, and if an equal number of male and female participants involved in rating male managers/supervisors.,These findings provide evidence that servant leadership is not a gender-neutral ethical construct, and practitioners should focus to develop or recruit managers who possess an ethic of care that could bring the desired negative outcome on the constructs of agency problems.,This is the first study empirically investigating the agency problems – ethical leadership relationship in Cyprus. Moreover, the potential role played by gender for the aforementioned relationship was empirically examined.

An Analytical and Numerical Investigation on Flange Wrinkling Behavior in Warm Forming Process of AA5754 Using Macro-Textured Tool Design

In this paper, an analytical buckling model is established to predict the flange wrinkling behavior of deep drawn cylindrical cups of aluminium alloy sheet in warm forming conditions using macro-textured blankholders for the first time. A continuum damage mechanism (CDM) based material model was utilized to reflect the visco-plastic feature of material at elevated temperatures. Forming speed and temperature effects were investigated, and texture ratio and draw ratio effects were also discussed. The developed analytical buckling model was validated by finite element simulations. The increase of forming temperature and forming speed is prone to cause wrinkling for AA5754, but the effects are not as significant as the texture geometry and draw ratio. The analytical model presented in this paper can be used as a design guide to determine tool texture geometry necessary to avoid wrinkling defects in the warm forming processes of aluminium alloy.

A Method of Determining Unified Viscoplastic Constitutive Equations for Hot Forging Simulations

Constitutive equations have been used extensively to accurately describe material properties over a wide range of temperatures and strain rates in numerical simulations. In this paper, an algorithmic method of determining the constants of such constitutive equations is presented. The Genetic Algorithm implementation utilising MATLAB is described, and example fits to experimental data are presented.

An analysis of the tooth stress distribution of forged bi-metallic gears

The work described in this paper is an evaluation of the contact characteristics of bi-metallic gears forged through a novel bi-metallic gear forging process. Finite element analysis of the contact characteristics of single material gears was first performed to validate the tooth contact and tooth root stresses with empirical American Gear Manufacturers Association and British Standard standards. Having verified the validity of the model, simulations were performed for gears comprising lightweight cores with teeth bounded by steel bands of uniform thicknesses, 1 mm, 2 mm, 4 mm, and 6 mm to evaluate the differences in stress distribution and compare to single material gear teeth. The forged profiles obtained experimentally by utilising 2 mm, 4 mm, and 6 mm thickness bands via the bi-metallic gear forging process are also discussed. The uniform thickness model is subsequently adapted to incorporate the experimental forged profiles in order to estimate the contact stress, root stress, and stress distribution within the teeth to identify performance differences between bi-metallic forged gears and traditional single material gears.

The Galling Behavior of Advanced Coating Contacts with Aluminium Alloy during Sliding Wear

This paper investigates the galling behaviour of a range of hard coatings applied to tooling surfaces during the sheet forming of an aluminium alloy workpiece. A total of three types of tooling materials were investigated, two of which were subject to PVD deposited coatings of AlCrN, CrN and DLC applied to the working surface. The third tooling material had undergone induction heating, plasma-nitriding and polishing. To evaluate the galling behaviour of the applied coatings, a tribological evaluation was conducted using a pin-on-disc test set-up at a constant load and varying temperature. The coated discs, replicating the tooling material, were tested against aluminium alloy pins AA6082 and AA7075 representing the workpiece material. This investigation indicated that the friction and galling behaviour of aluminium is highly dependent on temperature, and the use of two different aluminium pins had no significant effect. At room temperature, it was found that carbon-based coatings provide the lowest friction and the best protection against galling, whilst nitride-based treatments exhibit better performance at high temperature. Moreover, at elevated temperatures, coated tools exhibit superior anti-galling properties compared to uncoated tools.

Development of a computational optimisation system to determine constitutive equation constants for metal alloys

Simulations of metal forming processes typically use constitutive equations to describe material properties over a wide range of temperatures and strain rates. Classically, determining a set of values for the large number of constants present in more complex constitutive models presented great difficulty to researchers. In this paper, an algorithmic method of determining the constants of such constitutive equations is presented. The Genetic Algorithm implementation utilising MATLAB is described, and example fits to experimental data are presented. A study is performed to improve the performance of the method by combining the Genetic Algorithm and Particle Swarm optimisation methods into a hybrid algorithm and to exploit the inherent advantages of both methods of solution. Comparative performance between the GA-only and hybrid algorithm are presented, and it is found that such a Hybrid Solver is significantly more computationally efficient and produces more accurate parameter estimations.

Forming limit prediction for AA7075 alloys under hot stamping conditions

In this paper, the forming limits of AA7075 demonstrators with different initial blank shape under hot stamping conditions were predicted using the developed model, named the viscoplastic-Hosford-MK model. The developed model enables the representation of non-isothermal conditions, changes in strain rate and loading path for AA7075 material. Additionally, the developed model was calibrated using the fundamental experiments including uniaxial and formability tests. To assess the capability of the proposed model, the forming limit of AA7075 with different initial blank shape designs were predicted, and the optimal initial blank shape was verified by applying it to the practical forming of a demonstrator AA7075 component.

The friction coefficient evolution of a MoS2/WC multi-layer coating system during sliding wear

This paper discusses the evolution of friction coefficient for the multi-layered Molybdenum Disulphide (MoS2) and WC coated substrate during sliding against Aluminium AA 6082 material. A soft MoS2 coating was prepared over a hard WC coated G3500 cast iron tool substrate and underwent friction test using a pin-on-disc tribometer. The lifetime of the coating was reduced with increasing load while the Aluminium debris accumulated on the WC hard coating surfaces, accelerated the breakdown of the coatings. The lifetime of the coating was represented by the friction coefficient and the sliding distance before MoS2 coating breakdown and was found to be affected by the load applied and the wear mechanism.

An Experimental and Numerical Investigation into Forming Force Reduction in Precision Gear Forging

A significant factor in the cost of industrial machinery for precision forging is the maximum load required to fully forge the final shape of components. Typically in a precision forging process, the required load increases greatly towards the end of the stroke. This study focuses on reducing the final sharp increase in load encountered in a typical closed die forging setup. A technique of reducing the peak load in the forging of gears is proposed, named the Peripheral Relief (PR) method. A gear forging tool set has been designed and manufactured. A number of experimental trials have been performed using model materials to investigate the force reduction technique. An efficient and simplified FE model has been developed to evaluate the effects of the PR method. The experimental load characteristics are compared to the simulated results. The method has been found, both numerically and experimentally, to significantly reduce the peak load encountered at the end of the forging stroke compared to current closed die forging techniques.