考虑孔隙缺陷的CFRP微观切削仿真与实验研究

碳纤维增强树脂基复合材料(CFRP)在航空航天等领域应用广泛。在CFRP制造过程中难以避免会产生孔隙等缺陷,对后续的切削加工造成一定影响。在考虑了CFRP成型过程形成的孔隙缺陷基础上,运用有限元仿真模拟方法,从纤维-树脂-界面尺度建立了含孔隙缺陷的CFRP微观切削仿真模型,研究了不同孔隙率条件下不同纤维排布方向的CFRP微观切削行为,并通过实验验证了仿真模型的正确性。研究结果表明:孔隙的存在会增加刀具的“空切”现象,从而对CFRP切削过程的切削力、材料破坏及亚表面损伤、材料能量等产生影响。随孔隙率的增加,切削力呈下降趋势,孔隙边缘的纤维产生整体断裂的倾向增加;孔隙对0°、45°和135°纤维排布方向的CFRP切削加工的面下损伤影响不大,在纤维排布方向为90°条件下,孔隙率高于3vol%时对加工表面的面下损伤具有较大影响;在材料内部能量耗散方面,“顺切”(纤维方向角小于90°)时的总耗散能低于“逆切”,随孔隙率增加,总耗散能降低。      

基于粒子群算法优化支持向量机的铝热连轧机轧制力预报

为了提高热轧带材的轧制力预报精度,提出了粒子群算法和支持向量机结合的方法来预报轧制力。根据轧制原理用支持向量机建立轧制力预报的模型,通过粒子群算法优化支持向量机参数来提高预报精度。为了进一步提高轧制力预报精度,还提出了支持向量机网络与数学模型相结合的方法,对某“1+4”铝热连轧厂现场采集的5052铝合金轧制数据进行离线仿真,仿真结果可以看出支持向量机网络与数学模型结合的方法预报轧制力,提高了轧制力预报速度并使其轧制力预报精度控制在7%以内。     

基于有限元的三辊行星轧制力预测及分析

为预测三辊行星轧制力,建立了三维热力耦合有限元模型,用均匀设计法安排有限元模拟样本,采用基于LM算法的BP神经网络学习有限元轧制力结果,确定了轧制参数与轧制力的映射关系,并根据训练后的神经网络分析了摩擦系数、轧辊偏转角和轧辊转速对轧制力影响．预测结果表明：摩擦系数和轧辊偏转角对轧制力影响是多方面的,在高轧辊转速时,较大的摩擦系数有利于降低轧制力．     

Comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials

This paper presents the results of a comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials using the discrete element method. The study used the Particle Flow Code (PFC) to simulate biaxial compression tests in granular materials. To study the effects of rolling resistance, a user-defined rolling resistance model was implemented in PFC. A series of parametric studies was performed to investigate the effects of different levels of rolling resistance on the stress–strain response and the emergence and development of shear bands in granular materials. The PFC models were also tested under a range of macro-mechanical parameters and boundary conditions. It is shown that rolling resistance affects the elastic, shear strength and dilation response of granular materials, and new relationships between rolling resistance and macroscopic elasticity, shear strength and dilation parameters are presented. It is also concluded that the rolling resistance has significant effects on the orientation, thickness and the timing of the occurrence of shear bands. The results reinforce prior conclusions by Oda et al. (Mech Mater 1:269–283, 1982) on the importance of rolling resistance in promoting shear band formation in granular materials. It is shown that increased rolling resistance results in the development of columns of particles in granular materials during strain hardening process. The buckling of these columns of particles in narrow zones then leads to the development of shear bands. High gradients of particle rotation and large voids are produced within the shear band as a result of the buckling of the columns.     

An approximate bending force prediction for 3-roller conical bending process

Roll bending process is an important metal forming process used to produce cylindrical and conical shells and sections for various applications. 3-roller conical bending is one such process. For this process it is important to evaluate the maximum force acting on the rollers during the rolling process for designing the rolling machine as well as for evaluating the coefficient of friction at roller-plate interface. It is observed that maximum force is acting on the roller during the static bending in roll bending process [Gandhi et al. 2008]. In the present study mathematical model for force prediction on the rollers have been developed. Effects of various material properties and geometrical parameters have been studied. It has been concluded that the proposed model can be effectively used to get the roller bending force for given geometrical parameters and material properties. It can also be used to get roller plate interface friction, if the experimental value of roll bending force is available.     

Role of Friction in Cold Ring Rolling

1. Introduction Cold ring rolling is a main technology used to manu- facture various precise seamless ring shape parts. It has been increasingly used in many industrial fields such as bearing, machine, automobile, petrochemicals, aeronau- tics, astronautics and atomic energy because of its many technical superiorities such as considerable saving in en- ergy and material cost, high quality, high efficiency, and low noise, etc. To research and develop advanced precise cold ring rolling technolog…     

Void growth in cyclic loaded porous plastic solid

In low-cycle fatigue, where plastic strains are of great importance, final ductile fracture depends upon the mechanisms of void growth and coalescence of voids. A cell model is used to simulate a periodic array of initial spherical voids and this model is subjected to different loads that include cyclic loading. Three different types of matrix material are simulated: elastic-perfectly plastic, isotropic hardening and kinematic hardening. The cell model results are compared with the approximate constitutive equations for a voided material suggested by Gurson. The simulations show that the unspecified parameters introduced by Tvergaard in the Gurson yield function depend on the hardening behavior of the matrix material. For a perfectly plastic matrix material, the parameters q₁ = 1.5 and q₂ = 1.02 provide very close predictions for a variety of loadings. However, for isotropic or kinematic hardening matrix materials these parameters result in an inferior agreement and a much closer accuracy is obtained by adopting q₁ = 1.5 and q₂ = 0.82. This suggests that the parameter q₂ depends on the hardening behavior of the matrix material. For kinematic hardening of the Gurson model, it is shown that Ziegler's hardening rule is superior to Prager's hardening rule. Finally, the void shape change due to loading is studied and it is found that this change has an insignificant effect on the response.     

Validation of hot ring rolling industrial process 3D simulation

Ring rolling is a hot forming process used in the production of railway tyres, anti friction bearing races and different ring shaped work pieces for automotive energy production and aerospace applications. The advantages of ring rolling process include short production time, uniform quality, closed tolerances, good material quality and considerable saving in material cost. Despite the benefits some problems still exist according to a correct selection of the process parameters. Due to the nature of the process different rolling mills (driving, idle and axial rolls) are involved and the correct selection of the process parameters is not so feasible. Moreover an experimental approach to solve this problem risks to be more expensive. Actually FE codes are available to simulate the non linear problem that characterizes a ring rolling process. In this work a FE model, based on Deform 3D software, was tested versus experimental results acquired from an industrial plant. The accuracy of the FE model was analyzed through a dual comparison: by geometrical and by physical aspects. A good agreement was found between experimental and numerical results for both comparisons and, as a consequence, this code could be used in order to investigate and optimize the process parameters that characterize the ring rolling process in a virtual not expensive environment. The validated model will allow the studies of more environment-friend process configurations.     

Experimental Study on the Surface Densification of Fe-2Cu-0.6C Powder Metallurgy Material

The present work investigated the effects of rolling force (longitudinal feed) and spindle speed on surface performance of Fe-2Cu-0.6C powder metallurgy material. A newly self-designed rolling tool was used to densify the material's surface, and then the surface properties were measured. Results indicated that rolling force was the main effective parameter, and spindle speed was the secondary one. A densified depth and a surface hardness of 335 µm and 320 HV_0.1 were obtained by using a rolling force of 2800 N and a spindle speed of 360 r/min. The tensile strength was improved by 75%. The tensile fracture had mixed-rupture characteristics of dimples and tear ridge. In the surface layer, the ferrite was stretched and refined. The pearlite was deformed and oriented along the rolling direction. The lamellar spacing close to the surface was reduced. It played a significant role in enhancing the strength of material.     

Interaction between anisotropic plastic deformation and damage evolution in Al 2198 sheet metal

Deformation anisotropy of sheet aluminium alloy 2198 (Al-Cu-Li) has been investigated by means of mechanical testing of notched specimens and Kahn-type fracture specimens, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms were investigated via scanning electron microscopy. Contributions to failure are identified as growth of initial voids accompanied by a significant nucleation of a second population of cavities and transgranular failure. A model based on the Gurson-Tvergaard-Needleman (GTN) approach of porous metal plasticity incorporating isotropic voids, direction-dependent void growth, void nucleation at a second population of inclusions and triaxiality-dependent void coalescence has been used to predict the mechanical response of test samples. The model parameters have been calibrated by means of 3D unit cell simulations, revealing the interaction between the plastic anisotropy of the matrix material and void growth. The model has been successfully used to describe and predict direction-dependent deformation behaviour, crack propagation and, in particular, toughness anisotropy.     

Effect of friction stir welding parameters on morphology and strength of acrylonitrile butadiene styrene plate welds

The aim of this study is to examine the effect of main friction stir welding (FSW) parameters on the quality of acrylonitrile butadiene styrene (ABS) plate welds. Welds were carried out in a FSW machine, using a tool with a stationary shoulder and no external heating system. The welding parameters studied were the tool rotational speed which varied between 1000 and 1500 (rpm); the traverse speed which varied between 50 and 200 (mm/min), and the axial force ranging from 0.75 to 4 (kN). The major novelty is to study the influence of the parameter axial force on FSW of polymers. Produced welds have always a tensile strength below the base material, reaching the maximum efficiencies of above 60 (%) for welds made with higher rotational speed and axial force. Good quality welds are achieved without using external heating, when the tool rotational speed and axial force are above a certain threshold. Above that threshold the formation of cavities and porosity in the retreating side of the stir zone is avoided and the weld region is very uniform and smooth. For low rotational speed and axial force welds have poor material mixing at the retreating side and voids at the nugget. For this reason the strain at break of these welded plates is low when compared with that of base material.     

Effect of Load Distribution on Strip Crown in Hot Strip Rolling

In order to establish precision model, a software to calculate the strip crown of four-high hot rolling mill was developedby using affecting function method according to the strip crown calculation theory. The effect of work roll diameter,unit width rolling load, roll bending force, work roll crown, initial strip crown and reduction, etc, on load distributioneffect rate was simulated by using the software. The results show that the load distribution effect rate increaseswith the increase of strip width, work roll diameter, unit width rolling load, roll bending force, work roll crown,initial strip crown and reduction. Based on the simulation results, base value of load distribution effect rate andfitting coefficients of six power polynomial of load distribution effect rate modification coefficient were determinedconsidering all of the above parameters. A simplified mathematical model for calculating load distribution effect ratewas established.     

Effect of viscosity on cavity growth in ductile damage

In this paper the Budiansky approach is used in order to reproduce the void growth measured experimentally by 3D tomography in metals. It permits to avoid the non physical fitting of the alpha coefficient in Huang’s law. This model considers the viscoplasticity of the material to account for the interaction between the voids. It is shown that strain rate sensitivity of the material is playing a key role on growth of cavity and thus on damage of materials. Moreover finite elements calculations are carried out to check the influence of geometrical parameters (fraction of porosity, distance between voids) on the damage behaviour. The results are in agreement with the Budiansky model.     

Mechanical and damage analysis along a flat-rolled wire cold forming schedule

Numerical simulation is used to study patented high-C steel flat-rolled wire cold forming processes. An elasto-plastic power law, identified from mechanical tests, is used into Forge2005? finite element (FEM) package in order to describe the material behaviour during wire drawing followed by cold rolling. A through-process approach has been favoured, transferring residual wire-drawing stresses and strain into the flat-rolling preform. This mechanical analysis, associated with a triaxiality study, points to dangerous areas where fracture may initiate due to high tensile stresses. Lema?tre’s isotropic damage criterion, including crack closure effect, a -1/3 cut-off value of stress triaxiality, and tension/compression damage asymmetry, has been used and has confirmed the previous analysis. A number of non-coalesced voids nucleated on inclusions have been observed in the Scanning Electron Microscopy (SEM), especially in high-deformation zones (“blacksmith’s cross”). Their evolution has been simulated in the FEM model using spherical numerical markers, which deform into oblate or prolate ellipsoids. The deformation-induced morphological evolution of voids observed in the SEM compares well with the geometrical evolution of the markers, which suggests that the morphologies observed do not result from micro-crack propagation, but from material transport of the nucleated voids.     

Influence of transformation induced recrystallisation on hot rolling textures of low carbon steel sheet

Abstract

A series of hot rolling textures of steel sheet with different compositions and hot rolling parameters have been compared. The observed hot rolling textures all belonged to a limited number of different types. These types seemed to depend on the composition, the hot rolling parameters, and the local deformation mode. The influence of these hot rolling textures on the properties of commercial sheet steels after cold rolling and continuous annealing is critically evaluated. For steels with a low content of high temperature precipitates, the texture and microstructure after intercritical hot rolling often appeared to be the result of a transformation induced recrystallisation process with a specific nucleation mode. Understanding of the nature of this type of recrystallisation may facilitate an improvement in the material properties by optimisation of the hot rolling texture.

MST/1327     

An experimental and numerical investigation on micro rolling for ultra-thin strip

The demand for miniaturized parts and miniaturized semi-finished products is increasing nowadays, because microforming processes can improve production rate and minimize material waste due to less forming passes. However, traditional macro metal forming processes and modelling cannot be simply scaled down to produce miniaturized micro parts. In this study, a 2-Hi micro rolling mill has been successfully built. Experimental and numerical investigations on the micro rolling process for ultra-thin SUS 304 stainless steel strip have been conducted. The experimental results show that the micro rolling deformation of ultra-thin strip is influenced by size effect which results from the specimen size difference and this size effect is embodied in the flow stress and the friction coefficient. Analytical and finite element (FE) models in describing size effect related phenomena, such as flow stress, friction, rolling force and deformation behaviour, are proposed. The material surface constraint and the material deformation mode are critical in determination of material flow stress curve. The analysis of surface roughness evolution with rolling conditions has also been performed. The identified analysis on deformation mechanics provides a basis for further exploration of the material behaviour in plastic deformation of micro scale and the development of micro scale products via micro rolling.     

Effect of pressure rolling on the residual stress state of a particulate-reinforced metal matrix composite

The large difference between the coefficients of thermal expansion of the matrix alloy and the particle in a metal matrix composite gives rise to residual stresses in the material. In the present work, the effect of pressure rolling on the residual stress state of a silicon carbide particle-reinforced 2014 aluminium alloy has been investigated. The three-dimensional stress state measured in both phases-matrix and reinforcement-has been determined using an X-ray diffraction technique. A twofold effect of pressure rolling on the residual stresses was observed. On the one hand, compressive macrostresses as large as - 250 MPa were induced. On the other hand a significant reduction in pseudo-macrostresses was measured where the plastic deformation reaches a maximum. A modified Eshelby model was used to predict quantitatively and qualitatively the residual microstresses after heat treatment and pressure rolling respectively.     

基于循环自编码网络的冷轧轧制力建模方法

在钢铁冷连轧生产过程中,轧制力预测结果直接影响带材的轧制精度和产品质量。为进一步提高轧制力预测精度,同时实现模型的在线更新,避免漂移问题,提出了基于循环自编码网络的轧制力模型。首先,使用循环自编码网络对处理好的输入数据进行特征提取,为了加速网络训练,加入了小批量训练方法,然后使用高斯过程回归模型对提取到的特征进行回归拟合。仿真结果表明,该模型预测精度可达3%以内,能够实现轧制力的高精度在线预测。     

MODELLING FOR TRANSCRYSTALLINE AND INTERCRYSTALLINE FRACTURE BY VOID NUCLEATION AND GROWTH

A criterion has been formulated for transcrystalline and intercrystalline fracture caused by the evolution of voids located both in a grain and on grain boundaries. The criterion is based on the idea of plastic collapse for a unit cell that is a regular structural mezovolume of polycrystalline material. The criterion does not require the introduction of any empirical parameters, such as critical void size, critical size of ligament between voids and critical void volume fraction, which are used in most models.
Modelling has been performed for void nucleation and growth in a grain and on grain boundaries for elastic–plastic deformation and under creep conditions. A scheme is proposed to describe the transition from transcrystalline to intercrystalline cavitation fracture as a function of strain rate and temperature.
The effect of stress triaxiality on the critical strain and the lifetime for both transcrystalline and intercrystalline fracture has been investigated. A comparison of the results predicted by the suggested criterion with available empirical data has been performed.