韧性材料的几种断裂形式及判据讨论

考察几种韧性材料的复合型断裂试验过程，用传统强度理论思想认定启裂位置与启裂方向，通过对不同物理机制的断裂形式变化规律的研究，判定韧性材料的主要断裂形式有三类，以空穴成核、扩张、聚合为主导机制的正拉断，和以局部剪切带形成、发展为主导机制的两种不同类型的剪切断裂。局部大塑性变形的出现在材料内引起损伤，空穴的聚合或局部剪切带的出现仅发生在载荷达到某一临界值时。导致材料断裂发生的根本因素，是危险点上某应力参数达到了材料的断裂临界值，以此对不同断裂形式的断裂判据进行讨论，提出新的韧性断裂判据。     

基于二元疲劳失效判据的非线性疲劳损伤累积模型及其强度退化研究

二元疲劳失效判据认为疲劳破坏是由损伤程度和当前作用应力这两个量共同决定的,由此观点出发定义了非线性损伤,并建立了与载荷状态有关的非线性损伤累积模型,推导了在多级加载下的递推公式。经两种金属材料的疲劳试验数据验证表明,用该模型预测疲劳寿命,其结果是令人满意的。从疲劳过程本质上是材料静强度不断退化的过程的观点出发,建立了基于二元疲劳失效判据的非线性强度退化模型,将模型应用于两级及多级载荷下的剩余寿命估算,结果表明,所提出的剩余强度退化模型是合理可行的。

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Hybrid analytical–numerical solution for the shear angle in orthogonal metal cutting — Part I: theoretical foundation

In metal cutting, the shear angle is considered as a fundamental parameter that defines the plastic deformation and the geometry of the process. The present paper presents a further development of the energy method for prediction of the shear angle in case of orthogonal metal cutting. Parallel-sided shear zone model is utilized to describe the geometry of chip formation. The material velocity in the primary shear zone is allowed to change gradually from the bulk material velocity to the chip velocity. The interaction between chip and tool in the secondary shear zone is modeled as sticking to sliding transition. The work material is characterized by an empirical equation, which allows for the influence of temperature, strain, and strain rate as well as their histories. To take into consideration the influence of the temperature on the work material properties, a finite element model (FEM) of heat transfer is employed. The FEM is developed as an adaptive model to reflect the change in the domain geometry. As the work material properties strongly depend on the temperature, an overall iterative calculation procedure including FEM is essential. In Part I, the theoretical basis of the model is described. In Part II the predicted values of the shear angle are compared with data from machining 0.18% C carbon steel over a range of cutting conditions and tool geometry.     

考虑声源非线性影响下材料非线性系数测量研究

采用有限幅值法测量材料非线性系数,提出减少声源非线性影响以提高检测结果精度的方法。利用多元高斯声束模型,将准线性条件下探头产生的基波、二次谐波及声源非线性声场表示为纯平面波解、衰减修正项和衍射修正项组合的形式;通过设备输出的监测信号分析声源非线性值的大小;进一步修正了采用平面波理论计算非线性系数计算公式,得到消除声场衍射、声能衰减及声源非线性波影响下更加准确的计算方法。对6061铝试块进行非线性系数的测量试验,结果显示在消除声源非线性影响下,不同检测电压下的测量结果间误差在10%以内,相比于未考虑声源非线性时的结果,其精度得到了极大的提高。研究将为减少声源非线性影响以提高材料非线性系数的测量精度提供理论帮助。     

不连续切屑特征及形成机理的研究

半不连续切屑的形成过程分为弹性挤压和集中剪切两个阶段。本文推导出这两个阶段的变形能表达式。认为，集中剪切区的塑性变形能与屑段区的弹性能释放有关。每个切屑单元形成后系统能量应保持最低水平。由此得到切屑单元长度Ｌ＿Ｔ与切屑材料机械性能和切削条件和关系。其中，影响半不连续切屑形成的主要因素是切削材料的屈服强度τｓ和弹性模量Ｅ，并用屑形系数ｇ＿ｃ（ｇ＿ｃ＝τ＿ｓ／Ｅ）表达材料的这种切削属性。     

剪切修正GTN模型在小冲杆试验中的应用研究

小冲杆试验作为一种非标准的微试样测试技术,能有效地获取薄板结构的材料参数。而选用合适的损伤模型对准确表征材料变形到断裂的整个过程有着重要影响。基于NAHSHON提出的含剪切修正项的Gurson-Tvergaard-Needleman(GTN)模型,通过有限元软件ABAQUS及用户自定义子程序VUMAT考察不同应力三轴度对断裂失效的影响。采用有限元模拟和拉伸试验获得冷轧硅钢材料的无损伤弹塑性力学参数以及GTN损伤演化模型中的形核参数和临界断裂参数,通过纯剪切试验和数值模拟的对比确定出材料中微孔洞的剪切变形对材料损伤演化的贡献。运用剪切修正的GTN模型对小冲杆试验进行模拟,结果表明,由于修正GTN模型考虑了微孔洞剪切畸变的对材料损伤影响,模拟结果比原GTN模型更接近于试验数据,可更好地应用于小冲杆试验的研究。     

A new roughness parameter to evaluate the near-surface deformation in dry rolling/sliding contact

Within this work, a two-dimensional finite element model for rolling contact of a wheel on a rail is presented that accounts for the roughness of the contact surfaces. The rail material is modeled with elastic–plastic behavior. The maximum of the plastic shear strain is concentrated close to the surface of the rail and is mainly influenced by the surface roughness. A concept is proposed that demonstrates one crucial parameter of the roughness determines surface deformation (based on results of a sinusoidal roughness model). This roughness parameter depicts the ratio between asperity height and width. Numerical validation is achieved for predicting plastic shear strains in rough surfaces. The plastic shear strain is associated with surface damage, such as cracks and wear.     

Ultrasonic torsional guided wave sensor for flow front monitoring inside molds

Measuring the extent of flow of viscous fluids inside opaque molds has been a very important parameter in determining the quality of products in the manufacturing process such as injection molding and resin transfer molding. Hence, in this article, an ultrasonic torsional guided wave sensor has been discussed for monitoring the movement of flow front during filling of resins in opaque molds. A pair of piezoelectric normal shear transducers were used for generating and receiving the fundamental ultrasonic torsional guided wave mode in thin copper wires. The torsional mode was excited at one end of the wire, while the flowing viscous fluid progressively wet the other free end of the wire. The time of flight of the transient reflections of this fundamental mode from the air-fluid interface, where the wire enters the resin, was used to measure the position of the fluid flow front. Experiments were conducted on four fluids with different viscosity values. Two postprocessing algorithms were developed for enhancing the transient reflected signal and for suppressing the unwanted stationary signals. The algorithms were tested for cases where the reflected signals showed a poor signal to noise ratio.     

基于剪切形式的多轴疲劳寿命预测模型

在多轴损伤临界面的基础上,结合多轴疲劳损伤和裂纹萌生与扩展的特点,提出了一种剪切形式的多轴疲劳损伤参量,该参量不含有材料常数,进而建立了一种新的多轴疲劳寿命预测模型,经多轴疲劳试验验证表明,所建立的寿命预测模型可同时适用于多轴比例与非比例循环加载。     

基于PZT结Lamb波方向算法的损伤定位方法

基于Lamb波在板中的传播特性,针对复合材料的损伤检测,提出一种新型的损伤定位方法。该方法采用3个压电陶瓷片( piezoelectric transducers,简称PZT)组成PZT结,通过从各PZT中提取出损伤散射信号,找到其损伤波包到达的时间差,并依据Lamb波的传播速度得到位移差。首先,结合PZT结中各PZT的几何位置,具体推导出损伤方向算法,运用方向算法找到Lamb波在损伤位置发生散射后的其中一个传播方向,通过传播方向的交点来实现损伤位置的判定;其次,分析了互相关理论的基本原理,并运用互相关算法提取出损伤散射信号;最后,在碳纤维增强树脂基复合材料薄板上对该算法进行实验验证,求出了损伤方向以及损伤位置。验证结果表明,该方法能够对复合材料损伤进行有效的定位。     

P91钢蠕变损伤的非线性超声检测方法研究

针对现有的非线性超声评价金属高温蠕变状态的方法中存在的问题,借鉴了高频段能量与低频段能量之比作为非线性参数的定义方式及兰姆波应力波损伤因子表达式,提出利用一定频带范围内非线性参数的累积效应表征金属蠕变损伤程度的新方法。根据非线性能量从低频向高频转移的特性,得到某一激发频率下的高频段能量与低频段能量之比的非线性参量,其次,在考虑基波能量与高次谐波能量随频率变化的基础上,对一定频率范围内得到的非线性参量进行积分,得到累积非线性响应总和。应用该方法对实际现场截取的P91钢主蒸汽管道蠕变程度进行检测（该管道运行温度为550℃,运行压力为3 MPa,运行时间约3万小时）。检测结果与较精确的X射线衍射法测得的残余应力及传统的非线性系数方法作对比,结果表明,累积的非线性参数方法灵敏度较高,稳定性较好,与应力的测量结果符合性较好,能够跟随试样所受应力的改变评价管道的蠕变程度。     

不同损伤参量对橡胶隔振器疲劳寿命预测结果影响的研究

基于橡胶哑铃形试件的疲劳试验结果,分别以最大主工程应变、最大主对数应变、最大主Green-Lagrange应变、八面体切应变、应变能密度、Luo等效应力、Saintier等效应力为损伤参量建立七种橡胶疲劳寿命预测模型。应用此七种模型预测某汽车动力总成橡胶悬置的疲劳寿命,对比不同寿命模型的预测效果。研究结果表明,以最大主工程应变、最大主对数应变、最大主Green-Lagrange应变、八面体切应变、应变能密度为损伤参量建立的寿命模型预测的疲劳寿命均在实测寿命的4倍分散线之内;以Luo应力、Saintiter应力为损伤参量建立的寿命预测模型的预测寿命落在实测寿命的2倍分散线之内。因此,以Luo应力、Saintiter应力为损伤参量建立的寿命预测模型预测效果更好,Luo应力、Saintier应力更适合作为疲劳损伤参量来预测橡胶隔振器的疲劳寿命。     

Study on determination of durability analysis process and fatigue damage parameter for rubber component

Rubber components, which have been widely used in the automotive industry as anti-vibration components for many years, are subjected to fluctuating loads, often failing due to the nucleation and growth of defects or cracks. To prevent such failures, it is necessary to understand the fatigue failure mechanism for rubber materials and to evaluate the fatigue life for rubber components. The objective of this study is to develop a durability analysis process for vulcanized rubber components, that can predict fatigue life at the initial product design step. The determination method of nonlinear material constants for FE analysis was proposed. Also, to investigate the applicability of the commonly used damage parameters, fatigue tests and corresponding finite element analyses were carried out and normal and shear strain was proposed as the fatigue damage parameter for rubber components. Fatigue analysis for automotive rubber components was performed and the durability analysis process was reviewed.     

一种基于熵增理论的疲劳-蠕变交互作用损伤模型及试验验证

根据经典热力学理论,材料疲劳-蠕变交互作用下的损伤过程可视为系统熵增的累积,当熵增积累到临界值时,材料发生失效断裂。按此理论,并基于连续损伤力学和能量守恒定律,以系统熵增的变化来描述材料损伤,建立了一种疲劳-蠕变交互作用的损伤模型。为验证该模型,进行了540℃和520℃环境下1.25Cr-0.5Mo钢应力控制的梯形波加载试验,以材料的残余应变反映熵增积累,选取残余应变的变化作为损伤变量,用上述损伤模型进行了材料疲劳-蠕变交互作用的损伤演化描述,结果表明实测损伤点数据与该模型的损伤演化规律符合较好。     

Method for recognizing wave dynamics damage in high-speed milling cutter

Under the influence of a high-speed, interrupted-cutting impact load, a great difference is existed among the internal load propagation of a milling cutter. Furthermore, the cutter damage caused by partial particle severe vibration has restricted the improvement of a high-speed milling energy efficiency; thus, the essence of wave dynamics damage in milling cutter remains has yet to be revealed. In this paper, through the relation between the systematic whole vibration and the particle motion, the dynamic response of milling cutter’s particle to cutting force load can be solved by the particle motion differential equation which is constructed with a one-dimensional string dynamic system. A combination of Newton’s second law and the constitutive equation of milling cutter material establishes the wave dynamics equation of milling cutter components. An approach for solving the wave front position and wave velocity of milling cutter’s stress wave is proposed, and the propagation path of transient cutting force to the milling cutter is communicated. The attenuation model of stress wave reflection is established to provide a method for revealing the stress wave transmission and distribution in milling cutter. The constitutive relation of milling cutter components under the impact load is obtained by split Hopkinson pressure bar experiment. A force connection method is adopted to make the trans-scale correlation analysis between continuum medium mechanics and molecular dynamics, thereby revealing the wave dynamics damage characteristics of a high-speed milling cutter. The results show that the potential damage position and types of milling cutter can be distinguished by the above method.     

Shear horizontal wave transduction in plates by magnetostrictive gratings

Shear horizontal waves are very useful in nondestructive evaluation applications because of their non-dispersive property. The objective of this research is to investigate frequency-tuned shear horizontal wave transduction in plates by using magnetostrictive nickel gratings. Since wireless energy conversion between magnetic energy and acoustic energy in magnetostrictive material can take place, the generation and measurement of waves by the magnetostrictive principle have recently received much attention. In this work, a grating-type magnetostrictive transducer was considered for SH wave transduction; waves can be effectively tuned at desired frequencies by changing grating size and distance. The present experiments showed that the bias field strength and grating width should be selected appropriately to avoid generating unwanted wave modes such as the S0-Lamb wave modes. It is also confirmed from experiments that the grating distance controls the center (or tuning) frequency of the grating transducer.     

Propagation of transient out-of-plane shear waves in viscoelastic layered media

Propagation of two-dimensional transient out-of-plane shear waves in multilayered viscoelastic media is investigated. The multilayered medium consists of N different isotropic, homogeneous and linearly viscoelastic layers with more than one discrete relaxation time. The top surface of the layered medium is subjected to dynamic out-of-plane shear tractions; whereas, the lower surface is free or fixed. A numerical technique is employed to obtain the solution, which combines the Fourier transform with the method of characteristics. The numerical results are displayed in curves denoting the variations of the shear stresses with time at different locations. These curves reveal clearly the scattering effects caused by the reflections and refractions of inclined waves at the boundaries and at the interfaces of the layers. The curves also display the effects of viscous damping in the wave profiles. By suitably adjusting the material constants, the curves for the case of elastic layers are also obtained as a special case. The curves further show that the numerical technique applied in this study is capable of predicting the sharp variations at the wave fronts.     

Investigation of the effect of cutting tool edge radius on material separation due to ductile fracture in machining

This paper investigates the interaction between cutting tool edge radius and material separation due to ductile fracture based on Atkins’ model of machining. Atkins’ machining model considers the energy needed for material separation in addition to energies required for shearing at the primary shear zone and friction at the secondary shear zone. However, the effect of cutting tool edge radius, which becomes significant at microcutting conditions, was omitted. In this study, the effect of cutting tool edge radius is included in the model and its influence on material separation is investigated. A modification to the solution methodology of Atkins’ machining model is proposed and it is shown that the shear yield stress and the fracture toughness of the work material can be calculated as a function of uncut chip thickness.     

Analytical solution for SH wave propagating through a graded plate of metamaterial

A physical model for the shear horizontal (SH) wave propagating from left-handed material (LHM) through a graded or transition layer to right-handed material (RHM) has been proposed in this paper. After the comparison of the basic wave equations of the electromagnetic, longitudinal, and SH waves, it is found that they take similar differential form. The analytical solutions have been found for power law, hyperbolic, and polynomial profiles. Numerical waveforms of the amplitude and phase of the displacement are obtained for the corresponding profiles. It is found that the waveforms are symmetric for the power law and hyperbolic profiles, and that the waveform for the polynomial profile is shifted and non-symmetric. The shift along with the anti-symmetric profile may provide a way to monitor the wave behavior of the metamaterials.     

An investigation on collapse behavior of shear localization in elasto-thermo-viscoplastic materials

The stress collapse in the formation of shear bands in elasto-thermo-viscoplatic materials is systematically studied within the framework of one-dimensional formulation via analytical and numerical methods. The elastic energy released in a domain is found to play an important role in the collapse behavior of shear localization. A non-dimensional parameter named the stability indicator is introduced to characterize the collapse behavior, with approximate forms of the incremental governing equations. The stability indicator offers useful information regarding the degree of severity of an abrupt change of deformations during the stress collapse. Numerical experiments are carried out to verify the stability indicator by varying material properties.