Viscoplasticity Of Materials Research Articles

Implementation of a Neural Network into a User-Material Subroutine for Finite Element Simulation of Material Viscoplasticity

Abstract In this study, a neural network is trained to predict the response of a viscoplastic solder alloy based on a reduced data set. The model is shown to accurately describe the behavior of the material for the temperature range from 298 °K to 398 °K and the strain rate range from 2 × 10−5 s−1 to 2 × 10−2 s−1. The model is then implemented in the form of a user subroutine in the finite element code Abaqus to be used for simulations of the material behavior. The implementation requires that the weights and biases of the network are extracted and that its gradients (derivatives of the output with respect to the inputs) are calculated to be passed on to the user subroutine. Finite element (FE) simulations based on the implemented neural network are compared with those based on the physical viscoplastic model of Anand, showing an overall good agreement between both approaches. However, some limitations concerning the neural network ability to predict the transient effects during a strain rate jump or a temperature change are identified and discussed.

Production of Al/NiTi composites by friction stir welding assisted by electrical current

Composite Al structures reinforced with NiTi have been produced by solid-state joining process in order to prevent brittle intermetallics to form. For this, friction stir welding (FSW) was used in both the conventional and the hybrid variant assisted by electrical current. The hybrid process allows for a better bonding along the NiTi/Al interface since the material viscoplasticity promoted by the higher temperatures achieved during the process facilitates the material flow around the reinforcement. Mechanical characterization of the composites showed that upon bending and pull-out tests, the composites produced by FSW assisted by electrical current have increasing mechanical properties. Microstructural characterization using synchrotron X-ray diffraction, revealed that composites produced with the hybrid process exhibited a different transformation temperature of the NiTi reinforcements. The originally fully austenitic NiTi presented both martensite and austenite at room temperature after processing, which can be taken as an advantage for applications where damping capacity of the shape memory alloy is required. The ability to successfully join NiTi to Al may open new structural applications based on these composites.

Friction Stir Welding Assisted by Electrical Joule Effect to Overcome Lack of Penetration in Aluminium Alloys

Friction stir welding assisted by electrical joule effect is a development of conventional friction stir welding with the ability to eliminate or minimize the root defects by lack of penetration. The lack of penetration, mainly relevant in the welding of aluminium alloys, still constitute one major constrain to a wider dissemination of friction stir welding into industrial applications. The concept is based on assisting friction stir welding by an external electrical heat source integrated in the FSW tool. The innovative tool feature enables to increase the temperature in the weld root by Joule effect and improve material viscoplasticity in this region. A new tool was designed, manufactured and implemented. The resulting welds produced were analyzed via metallography and Electrical conductivity measurements that have proven to be a valuable technique to identify the different zones of solid-state welded joints with a good correlation with the microstructure and hardness. The potential of this variant was shown reducing the thickness of weld root defect, even for significant levels of lack of penetration, although affecting the grain size of the HAZ in the vicinity of the root surface.

Friction Stir Welding assisted by electrical Joule effect

This paper presents a variant of Friction Stir Welding (FSW) aiming to minimize or eliminate the root defects that still constitute a major constrain to a wider dissemination of FSW into industrial applications. The concept is based on the use of an external electrical energy source, delivering a high intensity current, passing through a thin layer of material between the back plate and the lower tip of the tool probe. Heat generated by Joule effect improves material viscoplasticity in this region, minimizing the root defects. The concept was validated by analytical and experimental analysis. For the later, a new dedicated tool was designed, manufactured and tested. Numerical simulations were performed to study the electrical current flow pattern and its effect on the material below the probe tip. The potential use of this variant was shown by reducing the size of the weld root defect, even for significant levels of lack of penetration, without affecting overall metallurgical characteristics of the welded joints.

A viscoelastic, viscoplastic model of cortical bone valid at low and high strain rates

The stress-strain behavior of cortical bone is well known to be strain-rate dependent, exhibiting both viscoelastic and viscoplastic behavior. Viscoelasticity has been demonstrated in literature data with initial modulus increasing by more than a factor of 2 as applied strain rate is increased from 0.001 to 1500 s(-1). A strong dependence of yield on strain rate has also been reported in the literature, with the yield stress at 250 s(-1) having been observed to be more than twice that at 0.001 s(-1), demonstrating the material viscoplasticity. Constitutive models which capture this rate-dependent behavior from very low to very high strain rates are required in order to model and simulate the full range of loading conditions which may be experienced in vivo; particularly those involving impact, ballistic and blast events. This paper proposes a new viscoelastic, viscoplastic constitutive model which has been developed to meet these requirements. The model is fitted to three sets of stress-strain measurements from the literature and shown to be valid at strain rates ranging over seven orders of magnitude.

Injection of a viscoplastic material inside a tube or between two parallel disks: Conditions for wall detachment of the advancing front

The injection of a viscoplastic material, driven by a constant pressure drop, inside a pipe or between two parallel coaxial disks under creeping flow conditions is examined. The transient nature of both flow arrangements requires solving a time-dependent problem and fully accounting for the advancing liquid/air interface. Material viscoplasticity is described by the Papanastasiou constitutive equation. A quasi-elliptic grid generation scheme is employed for the construction of the mesh, combined with local mesh refinement near the material front and, periodically, full mesh reconstruction. All equations are solved using the mixed finite element/Galerkin formulation coupled with the implicit Euler method. For a viscoplastic fluid, the flow field changes qualitatively from that of a Newtonian fluid because the material gets detached from the walls. For small Bingham numbers, the contact line moves in the flow direction, so that initially the flow resembles that of a Newtonian fluid, but even in that case detachment eventually occurs. The distance covered by the contact line, before detachment takes place, decreases as the Bingham number increases. For large enough Bingham numbers, the fluid may even detach from the wall without advancing appreciably. In pipe flow, when detachment occurs, unyielded material arises at the front and the flow changes into one under constant flow rate with pressure distribution that does not vary with time. In the flow between disks, it remains decelerating and the material keeps rearranging at its front because of the increased cross section through which it advances. The wall detachment we predict has been observed experimentally by Bates and Bridgwater [Chem. Eng. Sci. 55, 3003–3012 (2000)] in radial flow of pastes between two disks.

An elastic–visco-plastic analysis of ductile expanding ring

We investigate the fragmentation of an elastic–visco-plastic ring in the one-dimensional framework. The governing equations include the effects of large deformation, geometric softening, hydrostatic pressure and material's viscoplasticity. Linear perturbation analysis exhibits a rate-dependent dominant necking pattern. Numerical solution of the governing equations includes wave propagations, and shows that the fragment size is controlled by the unloading wave propagations in the ultimate fragmentation stage. The results are compared with experimental data on the fragmentation of uranium rings under different strain-rates.

Nanoscratch and friction: An innovative approach to understand the tribological behaviour of poly(amide) fibres

The analysis of the wear resistance of polymeric fibres requires a better understanding of both their abrasive scratch behaviour and their frictional response. These aspects have been investigated at the nanometre scale using the resources of a modified surface force apparatus. In an attempt to simulate the abrasive wear losses, nanomachining experiments have been carried out which consists in the repeated scratching of a portion of the fibre surface by the rigid indenter. However, an analysis of the resulting surface topography indicated a significant plastic grooving of the fibre surface with no evidence of wear losses as it was observed at the macroscopic scale. Single pass nanoscratch experiments realised at various sliding speeds also allow discussing the relative contributions of both the material viscoplasticity and the tip/material local interactions on the frictional response. When the sliding speed was incrementally changed during a scratch experiment, it was observed that the associated friction variation was accommodated on a 50 nm distance, independently of the sliding speed.

Analysis of a viscoplastic plate subjected to rapid external heating

A computational analysis is presented herein for an axisymmetric plate subjected to rapid external heating. The analysis is complicated by several forms of nonlinearity, including radiation boundary conditions, material viscoplasticity and geometric nonlinearity. The mechanical constitution is extremely complex, involving history and rate dependence which causes the constitutive equations to be mathematically stiff. Results are obtained for two viscoplasticity models available in the current literature. The solution utilized herein adopts the finite element method in spatial coordinates and standard finite differencing in time. Iterative techniques are used to account for nonlinearity. Results are obtained for a circular plate subjected to a high energy instantaneous heat source applied axisymmetrically to one side of the plate. Sensitivity studies are conducted to determine the effects of various heat intensities and plate thicknesses on temperature, displacements, and stresses. It is found that material viscoplasticity and geometric nonlinearity contribute significantly to the predicted response of the plate.