Key Engineering Materials Vols. 651-653

Abstract: For automotive gear manufacturers, reducing gear noise while maintaining the gear load-carrying capacity as well as the wear resistance has become more and more important. Macro- and micro-geometrical defects have long been studied in order to explain the vibratory behavior of gears. However, the contribution of the micro-scale roughness of the flanks, essential in the gear contact mechanics, has not yet been fully understood.This paper addresses this issue where gears were manufactured with two industrial finishing processes (grinding and power-honing) while having the same macro-scale characteristics. Tridimensional topographical features of teeth surface were hence measured using a three-dimensional white light interferometer. As manufactured surface topographies are highly complex, irregular, and multiscale, all the teeth surfaces were characterized in the entire wavelength band using a multiscale method based on wavelets transform. Vibration performances of the gears were then tested on a single-stage low powertrain. Results demonstrate the influence of micro-roughness scales on vibrations amplitude.

Abstract: The fretting is responsible for many failures of components in the industry. It is present in assemblies like rivet and screw fixture, dovetailjoint, shaft and hub with key, and all connections of two bodies with a contact force and a small induced relative displacement. Topic of studies for decades, the researchers perform experimental tests with some simplification in order to accelerate the phenomenon, some times using standard devices or creating dedicated machines for better representing the behavior of the desired components. There are a few studies with thin sheets, in which the fretting fatigue has more impact because a small reduction of the cross section due to the wear of crack results in a significant increase of stress and rate of crack propatation, decresing the number of cycles until failure. In this work, it wasbuild a device to generate the fretting fatigue with two different shape of contact pad. The specimen is a stainless steel sheet thickness 0.152mm, which fractures and surfaces were analyzed using the SEM and white light interferometer to understand the fractures.

Abstract: Magneto-rheological fluids represent one of the most recent technologies introduced in many industrial applications to design fast and reliable electro-mechanical devices. Such fluids, which consist of a dispersion of metallic particles in a fluid carrier, may drastically change their rheological properties when subjected to external magnetic fields with short response times and low power consumption. Up to date, the most common applications of MR fluids regard the integration in mechatronics applications mainly in civil and mechanical fields, where they are often used as active lubricants in rotary components such as bearings. However, no investigations can be found in literature on their tribological performances when applied in metal forming processes.The paper aims at investigating the tribological behaviour of magneto-rheological fluids when applied as lubricants in sheet metal forming processes. To this aim, a new experimental apparatus has been developed to test the friction coefficient at the interface between the tools and the metal sheet when different magnetic fields are applied. The stamping of AA alloys was taken as reference case to select the process parameters in terms of contact pressure, magnetic field and sliding speed.

Abstract: In the sheet metal forming industry lubricants are applied in forming processes to expand the technological boundaries by reducing friction and wear. The friction between tool and sheet metal is crucial to the deep drawing process. Due to economic and ecological reasons the aim of the manufacturers is to reduce or even avoid the use of lubricants. Consequently, this approach enables both a shortening of the process chains and an essential saving of resources. The advantages of structured forming tools in lubricated processes concerning the reduction of the coefficient of friction by the appearance of lubricating micro pockets are well-known. However, without using any lubricant this effect does not work. In this case the contact area is reduced by structuring the forming tool which affects the tribological system.In this paper the influence of microstructures with different geometries and surface treatments (uncoated / a-C:H:Si-coating) on the coefficient of friction in dry metal forming of the alloy AA5182 is compared to the frictional behaviour of unstructured forming tools using lubricant as reference. Before coating, the forming tools are machined by milling to generate tribologically effective microstructures. With the use of a strip drawing plant the effects of different surface microstructures and materials on the coefficient of friction are investigated.

Abstract: In many technological processes, friction results strong wear of the tool set and causes defect forgings. Many experimental investigations started in XX century had shown, that there are some laboratory methods with trustable results, which can be applied for friction prediction in real processes of metal forming. Among them such tests like ring compression test, direct and backward extrusion test, double cup extrusion test and T-shape compression test can be selected. Each of them has limitations that do not allow distributing the results of one certain test on all metal forming processes. Moreover, the tests should be done in a proper way to guarantee the adequate results that could be time-consuming. With the help of FE-analysis, the authors investigated numerically the intensity of the pressure distribution on the tool's surfaces and coupled the results with the real situations, observed in the metal forming operations. For the relative young T-shape compression technique oriented on the high contact pressure values authors have examined the changing of the relative deformed material volume as the merit of the friction characterisation.

Abstract: This paper attempts to address current possibilities in the multiscale simulation of polymer nanocomposites (PNCs) in processing. To provide a comprehensive perspective, a number of PNCs were produced by the incorporation of nanoclays in different polymer matrices. The microstructure evolutions of the simulated counterparts of such systems were studied with and without shear flows in a dissipative particle dynamics (DPD) framework spanning from several nanometers up to a few microns. Transmission electron microscopy (TEM) was utilized to contrast the simulations against the actual nanocomposites. A satisfactory precision was achieved in the build-up of the simulated structures. A significant characteristic of anisometric particles was studied, namely the orientation of the particles due to the imposed flows. It was shown that the orientation of such particles could be well described. Finally, opportunities were addressed for the simulations to carry on to the higher scales.

Abstract: High manganese content TWinning Induced Plasticity (TWIP) steels are promising for the production of lightweight components due to their high strength combined with extreme ductility, see [1]. This paper deals with the implementation of a constitutive model for the macroscopic deformation behavior of TWIP steels under mechanical loading with the aim of simulating metal forming processes and representing the behavior of TWIP-steel components – for example under crash loading - with the Finite Element code LS-DYNA^® and refers to our recently published papers: [2],[4],[5]. Within the present paper we focus on the implementation of the model formulated in [2] and its extension to stress dependent twinning effects.

Abstract: Multiphase steels offer impressive mechanical properties. However, their characterization still represents a challenge. In a quenching processes, phenomena such as undesirable strains or residual stress are inevitable and can be the cause for non-admissible final parts. Microstructural phase transformations generally magnify the problem. This fact leads to the need of numerical tools capable of quantifying these residual stresses, due to the non-existence of efficient non-destructive experimental procedure capable of measuring them. In this work, a numerical multiscale transient model, that uses the Asymptotic Expansion Homogenisation (AEH) method combined with finite element method (FEM), is proposed. The implementation of the AEH method is carried out using the commercial program Abaqus, considering an uncoupled and quasi-static transient problem with implicit time integration. Within the homogenisation method, the existence of two distinct scales is assumed, defining a micro and a macroscale. Within the smaller scale, the evolution of a steel periodic microstructure is analysed in detail and an equivalent homogeneous material model is established for macroscopic use. However, the microstructural evolution leads to the need of new equivalent homogeneous models in order to predict the macro response. Consequently, several mechanical, thermomechanical and transient thermal homogenization procedures are carried in order to establish different equivalent homogeneous models.

Abstract: Multiaxial tube expansion tests were performed to precisely measure the work hardening behavior of mild steel sheets with different r-values for a range of strain from initial yield to fracture. The testing machine is capable of applying an arbitrary linear stress path to a tubular specimen using an electrical, closed-loop servo-control system for the axial force and internal pressure applied to the tubular specimen. Tubular specimens with an inner diameter of 44.6 mm were fabricated from the as-received sheet sample by roller bending and laser welding. Nine linear stress paths, σ_x(rolling direction) :σ_y (transverse direction) =1:0, 4:1, 2:1, 4:3, 1:1, 3:4, 1:2, 1:4, and 0:1, in the first quadrant of the principal stress space were applied to the tubular specimens to measure the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work, or equivalently with increasing the reference plastic strain ; the test materials exhibited differential hardening (DH). The general trend of the DH appeared to be affected by the average r-value, or equivalently by the texture, of the materials.

Abstract: This paper describes fully implicit stress integration scheme for Yoshida’s 6th order yield function combined with Yoshida-Uemori kinematic hardening model and its consistent tangent matrix. Cutting plane method was employed for accurate integrations of stress and state variables appeared in Yoshida-Uemori model. In the present scheme, equivalent plastic strain, stress tensor and all the state variables are treated as independent variables in order to handle the 6th order yield function which is not the J2 yield function, and the equilibriums for each variables are solved for the stress integration. Subsequently, exact consistent tangent matrix which is necessary for implicit static finite element simulation was obtained. The proposed scheme was implemented into finite element code LS-DYNA and deep drawing process for aluminum alloy sheet was calculated. The earing appearance after drawing was compared with the experiment.