Pan Zeng

Finite element simulation of the effect of clearance on the forming quality in the blanking process

Abstract The clearance between the punch and the die plays an important role in the blanking process. The selection of the clearance will influence the life of the die or punch, the blanking force, the unloading force and the dimensional precision. In this paper, the punch–die clearance values for a given sheet material and thickness are optimized, using the finite element technique and Cockroft and Latham fracture criterion. In the study, the shearing mechanism was studied by simulating the blanking operation of an aluminum alloy 2024. The results of the present paper agree with the previous experimental results.

Three-dimensional rigid–plastic finite element simulation for the two-roll cross-wedge rolling process

Abstract In this paper, the process of cross-wedge rolling (CWR) has been simulated and analyzed by the 3D rigid–plastic finite element method. Considering the characteristic of CWR, the static implicit FEM program DEFORM-3D is selected. To simulate all forming stages in the CWR process, dynamic adaptive remeshing technology for tetrahedral solid elements was applied. The stress distributions in the cross-section of the forming workpiece are analyzed to interpret fracture or rarefaction at the center of workpiece. The authors have also analyzed the time–torque curve and the laws of load changing.

Development and testing of a 2.5?kW synchronous generator with a high temperature superconducting stator and permanent magnet rotor

High temperature superconducting (HTS) armature windings have the potential for increasing the electric loading of a synchronous generator due to their high current transport capacity, which could increase the power density of an HTS rotating machine. In this work, a novel synchronous generator prototype with an HTS stator and permanent magnet rotor has been developed. It has a basic structure of four poles and six slots. The armature winding was constructed from six double-pancake race-track coils with 44 turns each. It was designed to deliver 2.5 kW at 300 rpm. A concentrated winding configuration was proposed, to prevent interference at the ends of adjacent HTS coils. The HTS stator was pressure mounted into a hollow Dewar cooled with liquid nitrogen. The whole stator could be cooled down to around 82 K by conduction cooling. In the preliminary testing, the machine worked properly and could deliver 1.8 kW power when the armature current was 14.4 A. Ic for the HTS coils was found to be suppressed due to the influence of the temperature and the leakage field.

Investigation of thermal formability of aluminum alloy 4032

The deformation behavior of a 4032 aluminum alloy by hot compression has been investigated. It was found that the flow stress was strongly dependent on temperature as well as strain rate. The strain rate-sensitive coefficients were calculated at different temperatures. The experimental stress-strain data are fitted by means of the model earlier advanced by Sah et al. The Sellars-Tegart-Garofalo (STG) model is used to obtain activation energy values, which vary with the strain rate and strain.

Experimental investigation on local mechanical response of superelastic NiTi shape memory alloy

In this paper, primary attention is paid to the local mechanical response of NiTi shape memory alloy (SMA) under uniaxial tension. With the help of in situ digital image correlation, sets of experiments are conducted to measure the local strain field at various thermomechanical conditions. Two types of mechanical responses of NiTi SMA are identified. The residual strain localization phenomena are observed, which can be attributed to the localized phase transformation (PT) and we affirm that most of the irreversibility is accumulated simultaneously during PT. It is found that temperature and PT play important roles in inducing delocalization of the reverse transformation. We conclude that forward transformation has more influence on the transition of mechanical response in NiTi SMA than reverse transformation in terms of the critical transition temperature for inducing delocalized reverse transformation.

Comparative analysis between stress-and strain-based forming limit diagrams for aluminum alloy sheet 1060

The forming limit diagram (FLD) and forming limit stress diagram (FLSD) of aluminium alloy 1060 under linear and nonlinear strain paths are investigated. The calculation of FLSD is based on experimental FLD using the method proposed by Stoughton. Different from the FLD that varies with the strain path, the FLSD is not sensitive to the strain path. Therefore, FLSD is convenient as a forming limit criterion for multi-stage sheet forming. The influences of the materials yield criteria on FLSD are also discussed by comparison of the Hills 48, Hills 79 and Hosford non-quadratic criterion. The impacts of material hardening laws (Voce and Swift models) on translation of FLD and FLSD are analyzed. The Voce hardening law and the Hosford yield criterion are appropriate for the FLSD calculation of the aluminium alloy 1060. The stress calculation program and display interfaces of FLD and FLSD are developed on MATLAB, where the strain data can be input from experiment measurement or FEM calculations.

Experimental Investigation on Rate Dependence of Thermomechanical Response in Superelastic NiTi Shape Memory Alloy

Abstract In this paper, we focus on thermomechanical response of superelastic NiTi SMA at different strain rates under uniaxial loading. In situ digital image correlation method is applied to measure the local strain of the specimen. The evolution of the transformation bands is tracked by a CCD camera and the temperature field is recorded by an infrared camera. We show that there exists a strong link among strain rate, average temperature evolution, and the mechanical behavior of the material. It is also observed in the experiments that with the increase in strain rate, the deformation modes of the material transform from localization modes to homogenous modes.

High-accuracy formula for discrete calculation of fourier transforms

The conventional formula for discrete calculation of Fourier transform is to use the rectangular rules to numerically compute the oscillatory integral appearing in the transform over a finite interval. Due to the notable impact of oscillatory kern in integral, the large error by the present formula must occur, even it will lead to wrong results, especially for analysis of the higher order spectra. In order to avoid this fatal fault and to get the better results for the original Fourier transform, this paper tries to derive the modified formula with high-accuracy to the conventional formula. The related procedure includes: (1) to approximately discretize the original signal function at the sampling points, and (2) to use piecewise linear function to represent it within each interval, then (3) to exactly calculate the oscillatory integral of Fourier transform over each interval for each spectrum. The resulted expression (called as modified formula) is particularly simple and useful since only two multipliers are needed to act on the conventional formula. The related formulae and examples are presented in detail.

Experimental Investigation on the Mechanical Instability of Superelastic NiTi Shape Memory Alloy

In this paper, primary attention is paid to the mechanical instability of superelastic NiTi shape memory alloy (SMA) during localized forward transformation at different temperatures. By inhibiting the localized phase transformation, we can obtain the up-down-up mechanical response of NiTi SMA, which is closely related to the intrinsic material softening during localized martensitic transformation. Furthermore, the material parameters of the up-down-up stress-strain curve are extracted, in such a way that this database can be utilized for simulation and validation of the theoretical analysis. It is found that during forward transformation, the upper yield stress, lower yield stress, Maxwell stress, and nucleation stress of NiTi SMA exhibit linear dependence on temperature. The relation between nucleation stress and temperature can be explained by the famous Clausius-Clapeyron equation, while the relation between upper/lower yield stress and temperature lacks theoretical study, which needs further investigation.

Experimental observations on mechanical response of three-phase NiTi shape memory alloy under uniaxial tension

In this paper, the mechanical behavior of three-phase NiTi shape memory alloy (SMA) is examined in a wide temperature range using in situ digital image correlation. By varying the temperature and the cooling/heating history, we get the specimens with initial austenite (A), initial R-phase (R), initial martensite (M), initial mixture of A and R, initial mixture of R and M and initial mixture of A and M. It is observed in the experiments that NiTi SMA exhibits localized A → M transformation and R → M transformation while homogenous R-reorientation and martensitic reorientation. Moreover, the influence of the initial mixed states, i.e. mixture of A and M, mixture of R and M and mixture of A and R, on the mechanical response of NiTi SMA is discussed. Interestingly, we find that the specimens with initial mixture of R and M demonstrate homogenous deformation manner and the emergence of R in M facilitates the transformation of NiTi SMA greatly. The three-phase phase diagram is also established. The thermal dependences of the critical transformation stresses associated with various transformation processes are calculated for further theoretical investigation and simulation.