Quadratic programming method in numerical simulation of metal forming process

In this paper, a quadratic programming (QP) model based on a parametric variational principle is proposed for elastic–plastic (EP) finite element analysis of metal forming processes. The contact problem with friction between blank and tools is treated in the same way as in plastic analysis. The penalty factors, which are normally introduced into the algorithm for contact analysis, have a direct influence on accuracy of solution. There is no available rule for choosing a reasonable value of these factors for simulation of metal forming, and they are therefore cancelled through a special technique so that the numerical results can be of high accuracy. The algorithms for contact analysis and plastic analysis are established in one frame and consistent with each other. Compared with the conventional EP FEM, the newly developed method requires no tedious iterative procedures, and has no convergence problems. To apply this method easily to simulation of metal forming, detailed forms of some key matrices or vectors for 2D FEM and 3D FEM are presented, and a parametric loading algorithm for the QP model is developed, which is suitable for QP problem with free variables, and can decrease memory cost by avoiding the introduction of additional slack variables and improve the solution efficiency to some extent. Finally the proposed QP model is validated by two examples, analysis of V-notched tension test and analysis of the drawing of a square box––one of the benchmarks proposed at NUMISHEET93. It can be seen that the accuracy of solution of the new EP FEM based on QP is better than that of the conventional EP FEM based on iteration. To make the new EP FEM more applicable to metal forming industries, It is necessary to develop a more efficient QP algorithm that is suitable for large-scale problems.     

泥浆泵凹槽形仿生活塞寿命试验及耐磨机理分析

为提高泥浆泵活塞使用寿命,模仿自然界生物体表形态,在现有泥浆泵活塞工作表面加工不同尺寸的凹槽形仿生单元体,在相同的试验条件下,对比研究泥浆泵仿生活塞与标准活塞的使用寿命。试验结果表明,合理的凹槽形仿生单元体设计可以明显提高活塞使用寿命,本研究中最大可提高BW-160型泥浆泵活塞寿命69.52%;仿生活塞使用寿命受凹槽宽度的影响大于凹槽间距的影响;凹槽形仿生单元体尺寸过小,活塞寿命提高不明显,凹槽形仿生单元体尺寸过大,反而降低活塞寿命。分析了仿生活塞耐磨机理,即凹槽形仿生单元体的存在,能优化活塞表面受力情况,增大储油空间,改善泥浆泵活塞缸套系统界面润滑条件,是提高泥浆泵活塞使用寿命的重要原因。     

Micro/nano engineering on stainless steel substrates to produce superhydrophobic surfaces

Creating micro-/nano-scale topography on material surfaces to change their wetting properties has been a subject of much interest in recent years. Wenzel in 1936 and Cassie and Baxter in 1944 proposed that by microscopically increasing the surface roughness of a substrate, it is possible to increase its hydrophobicity. This paper reports the fabrication of micro-textured surfaces and nano-textured surfaces, and the combination of both on stainless steel substrates by sandblasting, thermal evaporation of aluminum, and aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). Meanwhile, fluorinated carbon films were used to change the chemical composition of the surfaces to render the surfaces more hydrophobic. These surface modifications were investigated to create superhydrophobic surfaces on stainless steel substrates. The topography resulting from these surface modifications was analyzed by scanning electron microscopy and surface profilometry. The wetting properties of these surfaces were characterized by water contact angle measurement. The results of this study show that superhydrophobic surfaces can be produced by either micro-scale surface texturing or nano-scale surface texturing, or the combination of both, after fluorinated carbon film deposition.     

Food system orientation and quality perception among consumers and producers of organic food in Hedmark County,Norway

To explore the potentials of organic agriculture, it is important to know how consumers, as well as producers, relate to food quality and food system issues. A consumer survey from the Hamar region in Southern Norway provided information on a number of these issues, and a rapid food system appraisal and a seminar revealed concerns among organic farmers in the region. Multivariate analyses showed that traditional food quality aspects such as freshness and taste, called “observation traits,” were important to all consumers. In addition, those who purchased organic foods were more concerned about ethical, environmental, and health issues, called “reflection traits”. Three consumer orientations in the food market were identified. Consumers with a “practical” orientation were less likely, and those with a “local” orientation in the food market were more likely, to buy organic food, while “social” considerations were equally important to all. The results indicate that many interests of organic farmers coincide with concerns among those who buy organic food, and that these are more complex than the formal rules for organic agriculture. This may provide a basis for identifying common goals and improving communication and cooperation between consumers and producers in order to further develop the organic food system. [Note that organic agriculture is called “ecological agriculture” in the Nordic Region.]     

Analysis of wear statistics for polymer composites

The wear rate of polymeric short-fibre composites has been analysed by means of a wear lifetime model using a statistical methodology. The wear lifetime model of a power law relationship has been derived from fracture mechanics concepts and the observed wear-induced microstructures. Experimental wear data have been correlated with the model.     

CVD金刚石条强化孕镶金刚石钻头的试验研究

本文将经过激光切割的CVD金刚石条镶嵌到传统的孕镶金刚石钻头胎体中,研制了一种新型的硬支点强化型金刚石钻头,以提高钻头在坚硬地层中的钻进效率和使用寿命。介绍了该新型钻头的设计及制造工艺,并进行了室内钻进试验。结果表明：与聚晶金刚石柱强化孕镶金刚石钻头相比,CVD金刚石条强化的孕镶金刚石钻头的钻进效率较高,达5．575m／h,且该钻头的磨损程度更小,使用寿命长。在钻进过程中更加稳定.     

Internal stress superplasticity in anisotropic polycrystalline materials

A theoretical model of internal stress superplasticity is developed in a single-phase polycrystalline material with an anisotropic thermal expansion. Quasi-steady-state creep equation during a thermal cycle is derived quantitatively based on continuum micromechanics. The model assumes that the generated mismatch strain is accommodated simultaneously by the plastic flow of the material. The linear creep deformation, which corresponds to internal stress superplasticity, is obtained at low applied stress region, and the creep rate depends on the crystallographic texture of the material. The validity of the model is experimentally verified using polycrystalline zinc which is a typical metal having large anisotropy in thermal expansion. The calculated strain rates using the texture information and the isothermal creep equation agree quantitatively well with the experimental results. The apparent activation energy of thermal cycling creep reveals 1/n (n: stress exponent of isothermal creep) of that of isothermal creep, which is one of the characteristics of internal stress superplasticity. Except for the factors attributable to the material geometry, the thermal cycling creep equation in the polycrystalline material is identical to that in a metal matrix composite.     

The effect of fibre aspect ratio on the stiffness of discontinuous fibre-reinforced composites

The mechanics underlying the stiffness of discontinuous fibre-reinforced composites are well understood. In particular the critical fibre aspect ratio is known to affect the resultant deformation of the composite. This paper will explain how the Cox theory can be used to determine the critical aspect ratio for a given fibre/matrix combination at a given volume fraction. Supporting experimental evidence for the key dependencies influencing the critical aspect ratio are shown at the microscopic level (by a Raman spectroscopic approach) and at the macroscopic level (by a tensile creep approach) for a series of different composite materials.     

Fracture surface interference in shear—I. A model based on experimental surface characterizations

A closed form model for estimating the reduction in Mode II stress intensity factor (SIF) and the induced Mode I SIF resulting from the mismatch of fracture surface asperities during shearing of the fracture surfaces is presented. At a given effective Mode II SIF, discretizations of actual Mode I fracture surfaces are displaced in shear according to the classical √r shear displacement law including crack tip plasticity. The crack faces are then “opened” enough to prevent interpenetration of the shifted surfaces resulting in a single point contact. The induced Mode I SIF is then calculated from the solution for a flat crack, opened by a concentrated normal force assuming a classical √r opening displacement law. The resistance Mode II SIF is found from the corresponding problem with a concentrated tangential force which is proportional to the normal force, and is positive or negative, depending on whether the sliding is uphill or downhill, respectively. The proportionality factor is determined through Coulomb's law of sliding friction applied to the inclined asperity surface at the contact point. The applied Mode II SIF is simply the sum of the effective and the resistance Mode II SIF's. The model predicts large initial resistance to the applied Mode II SIF and an induced Mode I SIF of the same order as the effective Mode II SIF. The secondary structure of the K_IIeff vs K_IIapp curves is directly related to the periodicity of the fracture surface profile. It is suggested that the model should be modified to account for asperity wear and subsequent modification of the fracture surface profile.     

Subcritical fatigue crack growth in alumina—II. Crack bridging and cyclic fatigue mechanisms

A crack re-notching procedure based on the “hinged straight crack” approximation is used to determine the distribution and magnitude of the bridging traction, σ(X), existing over the faces of the fatigue cracks grown in the experiments of Part I. From this distribution, the σ(u) relation between the bridging tractions and the crack opening is obtained and a simple method is tentatively proposed to measure the magnitude of the crack bridging stress intensity factor, K_b. The characteristics of the σ(X) and σ (u) relations are discussed in the light of the microscopical observations of crack profiles and in terms of the distribution of the frictional and elastic ligaments existing along the faces of the cracks. Crack growth rates and behaviour under different values of stress ratio R are compared and mechanisms of fatigue crack growth vs static crack growth are proposed.