Influence of damage on the plastic instability of sheet metals under complex strain paths

During the sheet metal forming operation, internal damage occurs as a result of nucleation growth and coalescence of cavities around particles. This phenomenon limits the strains which can be achieved before the appearence of localized necking. In this paper, damage is represented by initially equi-axed cavities and a void growth model is extended and linearized for complex strain paths. For a given void distribution, a statistical study pointed out the existence of weak sections in the material leading to localized plastic flow. The influence of the physical parameters of voids on the forming limit diagrams is shown.     

Geometry and grain size effects on the fracture behavior of sheet metal in micro-scale plastic deformation

The demand on micro-parts is significantly increasing in the last decade due to the trend of product miniaturization. When the part size is scaled down to micro-scale, the billet material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. The size effect phenomena occur in micro-scale plastic deformation or micro-forming and there are still many unknown phenomena related to size effect, including geometry and grain size effects. It is thus critical to investigate the size effect on deformation behavior, especially for the fracture behavior in micro-scale plastic deformation. In this research, tensile test was conducted with annealed pure copper foils with different thicknesses and grain sizes to study the size effects on fracture behavior. It is found that flow stress, fracture stress and strain, and the number of micro-voids on the fracture surface decrease with the decreasing ratio of specimen size to grain size. Based on the experimental results, dislocation density based models which consider the interactive effect of specimen and grain sizes on fracture stress and strain are developed and their accuracies are further verified and validated with the experimental results obtained from this research and prior arts.     

Surface studies of plastic streamer tubes

The structure and chemical composition of plastic streamer tube cathode surfaces were studied. Possible causes for dark currents and continuous discharges observed earlier in these detectors are suggested.     

Some characteristics of plastic streamer tubes

The operating characteristics of plastic streamer tubes chosen as a detector of the DELPHI-LEP hadron calorimeter are described. The work has been performed using a set-up specially designed to investigate a hadron calorimeter prototype on a beam of the Dubna synchrophasotron. The apparatus operated on-line with an ES-1040 computer.     

Application of finite element method for determination of stress intensity factor and plastic zone geometry in an aluminium alloy sheet under uniaxial tension

High strength materials have gained prominence in the fields of aero-structures, space missiles, ship-building, pressure vessels etc. However, high strength materials are often characterised by low values of crack resistance or fracture toughness. Knowledge of stress intensity factor (SIF) is essential to predict their fracture toughness. SIF values can be obtained both theoretically and experimentally. Theoretical methods include analytical techniques as well as the finite element method (FEM). The former is used for simpler geometries and the latter for complicated geometries of engineering structures. The SIF as a function of crack size in an aluminium alloy 2024-T₃ (Al-4·5% Cu, 1·5% Mg, 0·6% Mn) sheet was determined by a computer method. These values were obtained directly from the stresses as well as indirectly from strain energy release rateG andJ integral. The results agree well with the normalised values obtained from an ASTM formula. The size and shape of the plastic zone at the crack tip have been determined as a function of nominal stress for a fixed crack length. The plastic zone has the form of two ellipsoids with their maximum spreads oriented around 69° to the crack axis.     

塑料片材的分切和分切对策

随着社会生产力的不断提高,人们生活水平的飞跃上升,特别是我国加入WTO以来,国际贸易更加繁忙,塑料包装的用量得到了空前的扩大,并呈现出快速发展的趋势。同时由于它们的迅速发展,对它们的要求已越来越严。而塑料包装中除了塑料容器外,几乎是片和膜包装,如热成型制品要用到各种塑料片材;软包装制品和彩印复合要用到各类塑料膜。而在生产塑料     

Creep and plastic collapse lives of scarred boiler tubes

A method for life and failure thickness assessment of boiler tubes with localised scars or pits is described. The method is coded into a computer program coined ‘AUSI-TL1’. To the authors’ knowledge, the ‘AUSI-TL1’ program is the only program for life assessments of boiler tubes with localised scars. Other programs that are currently available assume a uniform loss of tube thickness occurring along the entire tube outside and inside surfaces. The application of the ‘AUSI-TL1’ to a typical reheater and a water-wall boiler tube shows that for short scars, the critical tube thickness at which the failure occurs can be a fraction of a millimetre. This demonstrates the reinforcement that locally exists for a short scar as compared with a long and wide eroded/corroded tube area.     

Determination of plastic collapse loads of rectangular tubes

Summary A simplified theory of yield hinge surfaces usually applied in plastic collapse analysis of thin plates in transverse loading is modified and used to simulate modes of plastic collapse of rectangular tubes in opposed transverse loading. The experimental results of maximum load observed in several cases are compared with the theoretical estimates based on rigid-perfectly plastic material model. The agreement between the theoretical and experimental results was found to be generally satisfactory.Notations H length of the wider face of rectangular tube - h length of the narrower face of the rectangular tube (h=H/2) - M _p plastic moment per unit length - P,P ₁,P ₂ plastic collapse loads - t thickness of the tube walls - u velocity of movement - _y uni-axial tensile yield stress of the material - angular velocity - vector of angular velocity in the directiono toI, etc. With 5 Figures     

Out-of-plane stretching and tearing fracture in ductile sheet materials

Pre-cracked ductile steel sheets are fractured by combined in-plane streching and bending. The deformation mimics the mode of fracture when plating is dented and torn as in ship grounding. Fracture toughness is determined for this mode of tearing. Values are greater than those obtained with DENT testpieces on the same material because of the different mode of crack opening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Localization of plastic deformation and fracture in aluminum polycrystals

The effect of the grain size as a basic structural parameter on plastic strain macrolocalization has been studied for polycrystalline aluminum. The mathematical form of the above dependence has been verified. The limiting cases have been defined both for small-and coarse-grain ranges. The effect of sample dimension on the macrolocalization period has been considered. __________ Translated from Problemy Prochnosti, No. 1, pp. 52–55, January–February, 2008.     

On fracture mechanics in lifting an ice sheet

The penetration of a wedge-like tool into an ice sheet at the interface with its substrate may develop a semi-elliptic interfacial crack. The Griffith energy-balance concept is employed to determine the critical conditions for interfacial crack propagation. The results show that an interfacial crack of any semi-elliptic shape tends to propagate in a stable manner and change the shape to a critical shape at which the propagation becomes unstable. The interfacial crack propagation may be interrupted by transverse fractures in the ice sheet.

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Résumé En faisant pénétrer un outil en forme de coin à l'interface entre un film de glace et le substrat sur lequel il est déposé sur une épaisseur uniforme, il peut se développer dans cet interface une fissure semi-elliptique.On fait appel au concept d'équilibre énergétique de Griffith afin de déterminer les conditions critiques pour que se propage la fissure.Les résultats démontrent que, quelle que soit sa forme, une fissure interfaciale semi-elliptique tend à se propager de manière stable, et à atteindre une forme critique pour laquelle la propagation devient instable.Des ruptures transversales dans le film de glace peuvent avoir pour effet d'interrompre la propagation de la fissure.

     

Modeling plastic deformation and fracture of porous materials

Strain hardening of a porous material was numerically modeled. The corresponding stress-strain (σ-ε) curves, the ultimate strength, and the strain at break were calculated for iron with a relative porosity in the interval from 0 to 30%. Anomalous behavior of these characteristics is observed at a porosity corresponding to the percolation transition from isolated pores to the “infinite” pore cluster. The proposed model adequately describes the available experimental data.     

厚壁管低熔点塑性介质挤胀成形实验研究

采用低熔点塑性材料作为传力介质挤胀成形厚壁空心构件,研究了空心构件低熔点塑性介质挤胀成形机理和主要影响因素,分析了低熔点塑性介质挤胀管坯的成形过程和壁厚分布规律．研究结果表明：低熔点塑性介质挤胀成形时管坯和塑性介质两种材料同时发生塑性变形,管坯的变形流动是塑性介质的内压和冲头轴向挤压共同作用的结果;轴向压力和径向内压力的匹配关系是低熔点塑性介质挤胀成形工艺的关键;管坯胀形区的壁厚有较大的减薄,但与自然胀形相比壁厚减薄的程度较小。