C.Y. Tang

An investigation on the formation and propagation of shear band in fine-blanking process

Abstract An investigation has been made on the formation and propagation of shear band in fine-blanking process. Examinations by optical microscopy and SEM reveal that the highly elongated narrow subgrains extended in the shear direction within the band, while in the other regions, fine equiaxed cell were observed. Both the presence of white-etching trace and the distribution of surface microhardness in the shear band reveal that fine-blanking process involves large deformation and high temperature. Although strain localization is severe in the shear band, especially in the area adjacent to the edges of the punch and die, no cracking has been observed. It is indicated that high hydrostatic pressure, built up by specially designed fine-blanking fixture, plays a significant role to suppress the generation of fracture zones in the sheared surface. On the basis of present findings, the mechanisms of the formation and propagation of shear band in fine-blanking are discussed.

Material damage and forming limits of textured sheet metals

Abstract In this paper, the evolution of preferred grain orientation and its effect on plastic deformation and mechanical properties are discussed based on crystallographic theory and the continuum mechanics of textured polycrystals (CMTP) technique. Mechanical damage due to microdefect development is also studied using continuum damage mechanics (CDM). The forming limit strains in biaxially-stretched textured sheet are predicted using the Marciniak–Kuczynski (M–K) model with modification to take into account the damage due to both texture evolution and the presence of microdefects. The results show that the proposed method can significantly reduce the conventional over-estimation of limit strains in the biaxial stretching state.

A study of the melt flow behaviour of ABS/CaCO3 composites

Abstract The melt flow rate (MFR) is an important characteristic of polymer-processing properties. The MFR of an acrylnitrile–butadiene–styrene (ABS) copolymer filled with two kinds of calcium carbonates (CaCO 3 ) was measured in this article. The results showed that the MFR increased linearly with a rise of temperature, and was a power law function of load. Furthermore, the MFR decreased basically with the addition of the filler content ( φ ), but it was higher than that of the unfilled ABS at φ =10%.

The surface character and substructure of aluminium alloys by laser-melting treatment

A series of laser melted samples of Alue5f8Si alloys with various Si contents were produced using a 2 kW CO2 laser. The surface characteristics of these laser-treated samples, their microstructures and their substructures were investigated. Experimental results showed that hardened zones of specific area and penetration depth on the sample surface can be obtained with a particular laser power. It was found that the laser treatment refined the aluminium alloy grains; that silicon crystal in eutectic was altered morphologically from lath-like into coralline-like; and that the proportion of silicon in the α-Ai was much more than the saturation content of silicon in the aluminium alloys. After solidification, the super-saturated silicon precipitated out in the 〈110〉 direction and in very small rod-like form. X-ray diffraction analysis of the samples illustrated that the lattice parameters of A1 were reduced after laser melting.

Development of a damage-based criterion for ductile fracture prediction in sheet metal forming

Abstract A second-order continuity tensor ψ is proposed to characterize the state of anisotropic damage in ductile sheet metal during a forming process. The continuity tensor can be determined from the effective elastic stiffness matrix and it satisfies the requirement of symmetry for derivation of the effective elastic stiffness matrix and it satisfies the requirement of symmetry for derivation of the effective stress tensor, the effective elastic strain tensor and the effective elastic stiffness tensor. The corresponding anisotropic damage constitutive relations are formulated to model the damage-failure process for sheet metal. Hence, the expressions of the specific damage energy release rate Y , taking into account damage anisotropy and stress triaxiality, have been derived by means of decomposing Y into a hydrostatic and a deviatoric part. The damage evolution equation is established in terms of a Lemaitre’s type plastic damage dissipation potential and plastic stress–strain relation. The minimum principal component of continuity ψ II is taken as the predominant factor governing the damage failure process. A damage-based criterion, which is a non-linear function of the equivalent plastic strain, the triaxiality factor and the continuity threshold, is finally derived for the prediction of fracture strains. An example of the biaxial stretching of steel sheet has been used to demonstrate how the criterion can be applied to predict the fracture limit. The results show that the predicted values are in agreement with experimental values.

Numerical simulation of fine-blanking process using a mixed finite element method

Abstract In order to achieve a more intensive understanding of the forming mechanism of the fine-blanking process, a numerical simulation has been carried out by using a mixed displacement/pressure (u/p) finite element method. According to the special requirement of the fine-blanking technique, the major process attributes, such as the vee-ring, the ejector and the edge radii of tools, have been taken into account in the finite element model. The punch–die clearance was set to 0.5% of the thickness of the workpiece. To verify the effectiveness of the simulation, the equivalent strain on the sheared surface of a SS400 steel specimen has been determined experimentally. The experimental values of the equivalent strain have been estimated by measuring the relative displacements of the local grids pre-etched on the meridian plane of the specimen. The results of the finite element simulation are in proper agreement with the experimental findings. The distributions of the shear stress and the equivalent plastic strain have been computed for discussion. Moreover, a diagram of the blanking force versus the punch penetration has also been constructed. In order to investigate the fracture mechanism in the fine-blanking process, the concept of damage mechanics has been applied. By using a void growth model, the evolution of damage at different stages of the fine-blanking has been evaluated. It has been realized that the compressive hydrostatic stress built up by the fine-blanking fixture plays an important role to suppress the initiation of macrocracks.

Flow and mechanical properties of polypropylene/low density polyethylene blends

Abstract Two types of polypropylene (PP) of different melt flow index (MFI) were blended with a low density polyethylene (LDPE) in a screw injection machine, the flow and mechanical properties of the blends being investigated. For the higher MFI PP (PPH), the melt flow rate (MFR) of the blend being maximum at a blending ratio of 50 50 , whilst for the lower MFI PP (PPL), the MFR of the blend increased with increasing wt.% of LDPE. Basically, the relationship between the mechanical properties, such as tensile strength and Youngs modulus, and the composition of the blends was in agreement with a logarithmic mixing rule. For the dependence of the tensile necking behaviour of the blends versus the blending ratios, some differences were found between the PPH/LDPE blend and the PPL/PP blend.

Surface integrity and modification of electro-discharge machined alumina-based ceramic composite

Abstract Increasingly more applications require ceramic parts or components with good surface integrity. This paper attempts to study the surface integrity of electro-discharge machined engineering ceramics and their surface modification by ultrasonic polishing. Flexural strength is used for evaluating the effect of the two machining processes on the surfaces of the machined specimens. The distributions of the strength data are further analyzed by the Weibull statistical method to give quantitative measures of the surface integrity of the machined specimens.

Experimental and Theoretical Analysis on Formability of Aluminum Tailor-Welded Blanks

This paper presents an investigation on forming limits of tailor-welded blanks (TWBs) made of 5754-O aluminum sheets using both experimental and numerical approaches. TWBs may be composed of two or more welded flat metal sheets of different thicknesses, shapes, or mechanical properties. Due to the existence of weldment and the individual configurations of base blanks, TWBs should be considered as heterogeneous in its structure. The mechanical properties of those base metals and weld metal required for the simulation were measured individually. With the aid of the acquired data, finite element simulations for analyzing the forming process of TWBs were carried out using a general purpose finite element package, LS-DYNA. A localized necking criterion based on the vertex theory was employed to predict forming limit strains and failure locations of the aluminum TWBs. The theoretical predictions were satisfactorily validated with those obtained from the experiments.

A damage-based criterion for fracture prediction in metal forming processes: a case study in Al 2024T3 sheet

Abstract In the present study, a damage-based criterion derived by continuum damage mechanics (CDM) has been used to predict the fracture limit of aluminium alloy Al2024T3 sheet. Microscopic examination has shown that the damage of the aluminium alloy is due to the nucleation and coalescence of micro-voids under high strain. The damage, causes degradation in the effective elastic properties of the material, and may be considered as the first stage of fracture. A second order continuity tensor ψ has been used to quantify the damage of the aluminium alloy. The principal values required for the determination of the continuity tensor have been obtained experimentally under different magnitudes of tensile pre-strain. Thus, the values of the continuity tensor have been calculated using an expression derived recently by the authors. The minimum principal continuity ψII has been taken as a governing parameter to establish the damage evolution equation. Assuming a power-law type material constitutive relation and using a damage-based fracture criterion, the fracture limit curve (FLC) of the Al2024T3 sheet has been computed. A series of experiments has been performed to obtain the FLC of the aluminium alloy sheet. The experimental FLC is found to be in agreement with the predicted one. In addition, the experimental results show that ductile fracture of the aluminium alloy sheet occurs while the minimum principal continuity ψII reaches a critical value.