Research of effects of bar geometric parameters on initial external force in precision cropping

In precise cropping of the metal bars with different geometrical parameters, it is very important to obtain the reasonable initial external forces required by the bars rapidly and accurately. Stress concentration factor and stress intensity factor are proposed to calculate the initial external force accurately in this paper according to geometrical sizes of metal bars. By means of numerical simulation and linear fitting method, the correctional expressions of stress concentration factor and stress intensity factor of V-shaped notch tip, which include the geometric parameters of the bar, are obtained respectively and the error analyses are also performed. The ratio of the depth of V-shaped notch to the diameter of the bar is presented to be a condition parameter. The numerical analytical results show that the method of stress concentration factor can be used to calculate the initial external force accurately when the ratio of the depth of V-shaped notch to the diameter of the bar is more than 0.05, and the method of the stress intensity factor can be used when the ratio is less than 0.05. The calculating formulas of reasonable initial external forces for the bars with different geometrical parameters are also given. The cropping experimental results have confirmed that it is feasible that the reasonable initial external forces required by the bars with different geometrical parameters are calculated effectively by using the stress concentration factor or the stress intensity factor in precision cropping.     

304不锈钢棒气动式可控旋弯下料工艺及试验

针对金属棒料传统下料工艺存在断面质量差、材料利用率低、能耗高等问题,提出一种综合利用预制环状V型槽缺口效应、起裂微裂纹及扩展宏观裂纹裂尖应力集中效应的新型气动式棒料可控旋弯下料工艺,详细介绍了该下料工艺的工作原理。基于该工作原理研制了金属棒料气动式旋弯下料试验装置,利用材料力学强度理论与线弹性断裂力学理论,建立了弯矩载荷作用下金属棒料下料过程的力学模型,获得了环状“V”型槽根部微裂纹萌生载荷、扩展裂纹起裂载荷及瞬断载荷的理论计算公式。采用三种不同恒应力强度因子幅水平载荷控制曲线对φ30 mm直径的304不锈钢棒料进行下料试验研究,结合所提出的坯料断面评价方法及其评价指标,对下料所得坯料质量进行了量化评价。结果表明,保持裂尖处于恒应力强度因子幅水平,可以实现金属棒料的可控精密下料,断面质量和下料时间随幅值水平提高而降低,当裂尖处△K=0.7K_c时,坯料最终瞬断区最小。该下料工艺利用断裂力学理论建立旋弯下料机理,可以指导实际下料试验,为后续下料工艺数据库的建立提供理论支撑。     

The singular stress field and stress intensity factors of a crack terminating at a bimaterial interface

For the fracture evaluation of inclined cracks terminating at the dissimilar material interface, not only the singularities, but also the detailed stress field and its stress intensity factors are necessary. However, though there are many researches reported on the singularity analysis, the stress field and its stress intensity factors are still not clear. This paper has deduced theoretically the singular stress and displacement fields near the tip of a crack terminating at the interface between bonded dissimilar materials, for both cases of real and oscillatory singularities. From the deduced singular stress field, the stress intensity factors are defined for such a crack, and the corresponding numerical extrapolation methods are also proposed. Through the numerical examinations, it is found that the theoretical stress distributions agree well with the numerical results obtained by the finite element method. Moreover, the proposed extrapolation method shows a good linearity, thus it can be used as an efficient way to determine the characteristics of the stress and displacement fields near the tip of a crack terminating at interface.     

V形切口尖端裂纹起裂方向的主应力判别准则及其验证

围绕V形切口尖端裂纹起裂方向,分析了V形切口尖端裂纹应力场、位移场、应力强度因子,提出了裂纹起裂方向的主应力判别准则。首先,详细给出了V形切口尖端应力应变场的求解方法,通过裂纹尖端场本征值的三次线性拟合及误差分析,确定了V形切口尖端裂纹位移场;然后,建立了V形切口尖端的数值分析模型,运用数值计算方法确定了应力强度因子和切口强度因子,提出了V形切口尖端裂纹起裂方向的主应力判断准则,给出了外推法求解分析过程;最后,以LY8为试验材料,在张角2β＝60°的V形切口情况下,对提出的V形切口尖端裂纹起裂方向计算方法与判别准则进行了试验验证。     

低周疲劳精密下料新工艺及试验研究

传统的金属棒料下料工艺存在着材料利用率低、能耗高、生产效率低以及下料断面质量差等问题。采用一种新的下料工艺-周向加载低周疲劳精密下料技术,利用V形槽的应力集中效应,促使棒料V形槽底尖端处疲劳裂纹的萌生及疲劳裂纹的快速扩展。描述周向加载低周疲劳精密下料机的工作原理。给出在低周疲劳下棒料V形槽根部裂纹是否起裂的判据。采用两种控制曲线对5种材料(20钢、H59、45钢、20Cr和LY12)的棒料进行试验研究,实现加速棒料裂纹的产生、扩展并获得良好的棒料断面质量。试验结果表明,V形槽的应力集中效应可以有效地减小棒料下料过程中的平均应力,在不断增加打击位移的同时减小冲击频率可以保证棒料裂纹的稳定扩展和断裂。     

V形切口复合材料试件拉伸破坏声发射监测

通过对V形切口复合材料试件单向拉伸、分级加卸载拉伸试验及声发射实时监测,研究含切口缺陷复合材料拉伸损伤破坏及其声发射响应特征.结果表明:复合材料切口试件拉伸损伤破坏与声发射特征参量相关;切口尖端的高应力状态导致该区域过早损伤破坏并扩展,且对应较高的声发射相对能量、幅度和急剧增加的撞击数;声发射定位源主要集中在位于试件中部的切口尖端及其扩展区域.     

基于ANSYS的V型切口模型应力强度因子计算

基于ANSYS有限元软件,给出了含有切口尖端应力奇异场模型的有限元建模和分析方法,利用有限元模型的分析结果,分别采用多种方法,计算和分析了V型切口平面模型的切口参数对其应力强度因子的影响,并对所采用的不同方法进行了对比和讨论。结果表明,当利用预制切口对材料进行拉伸断裂的机械加工时,应尽可能加大预制切口的长度和减小切口的张角。     

用切口尖端应力方法分析V形切口的应力强度因子

为了基于传统有限元法分析圆轴中的V形切口尖端的奇异性应力场的高精度数值解,建立一种切口尖端应力方法,用于计算V形切口的应力强度因子.该方法不需要在V形切口尖端采用反映应力奇异性的奇异单元.求解时,首先给定参考问题的广义应力强度因子,然后利用待求问题的应力值与参考问题的应力值之间的比值来求解待求问题的广义应力强度因子.算例采用切口尖端应力方法分析圆轴中的V形切口问题.计算结果表明,该方法计算精度较高,能方便地用于求解相关的工程问题.     

Optimisation Method for Determination of Crack Tip Position Based on Gauss-Newton Iterative Technique

In the digital image correlation research of fatigue crack growth rate,the accuracy of the crack tip position determines the accuracy of the calculation of the stress intensity factor,thereby affecting the life prediction.This paper proposes a Gauss-Newton iteration method for solving the crack tip position.The conventional linear fitting method provides an iterative initial solution for this method,and the preconditioned conjugate gradient method is used to solve the ill-conditioned matrix.A noise-added artificial displacement field is used to verify the feasibility of the method,which shows that all parameters can be solved with satisfactory results.The actual stress intensity factor solution case shows that the stress intensity factor value obtained by the method in this paper is very close to the finite element result,and the relative error between the two is only-0.621％;The Williams coefficient obtained by this method can also better define the contour of the plastic zone at the crack tip,and the maximum relative error with the test plastic zone area is-11.29％.The relative error between the contour of the plastic zone defined by the conventional method and the area of the experimental plastic zone reached a maximum of 26.05％.The crack tip coordinates,stress intensity factors,and plastic zone contour changes in the loading and unloading phases are explored.The results show that the crack tip change during the loading process is faster than the change during the unloading process;the stress intensity factor during the unloading process under the same load condition is larger than that during the loading process;under the same load,the theoretical plastic zone during the unloading process is higher than that during the loading process.     

材料的切口强度与切口敏感性

较全面地总结了高塑性材料,低塑性材料和脆性材料拉伸切口强度的最新研究结果,给出了各种材料切口强度的估算公式。公式考虑了材料的拉伸性能,应力集中和切口根部的应力状态,以及加载条件等。     

Prediction of fatigue regimes in notched components

Fatigue crack growth from notches is described as a two-stage process. Cracks are assumed to initiate in, and their early growth controlled by, the plastic zone due to the notch. Later growth is controlled by the developing crack tip plastic zone which can be described by the elastic stress intensity factor. A simple model is proposed which qualitatively accounts for all observed notch phenomena including non-propagating cracks and size effects. The equation: is shown to quantitatively predict the experimental fatigue failure limit of sharply notched specimens.     

Slip-line field solutions of three-point bend specimens with deep notches

Slip-line field solutions of three-point bend specimens with different notch depths, notch angles and notch root radii are presented. For notch angles less than the critical value the plastic region is localized at the ligament but for larger notch angles the plastic region spreads to the flanks of the notch. It is shown that the critical angle decreases with increasing notch depth and decreasing root radius. Solutions for the constraint factor and maximum hydrostatic stress beneath the notch tip are also obtained as a function of both notch depth and root radius for notch angles above and below the critical value.     

计算切口应力集中系数的无限相似单元法

在边界条件和其他几何条件相同时,可以通过切口的应力集中系数求得相应裂纹问题的应力强度因子。边界条件比较复杂时,应力集中系数较难求得,而应力强度因子可以通过无限相似单元方法求得,提出了计算平面切口应力集中系数的无限相似单元方法。该方法适应性好、精度高、使用方便。用该方法对几个问题进行了计算。     

带环向切口金属杆的断裂强度

对带环向切口的金属圆杆进行了断裂拉伸实验,由实验数据分别计算了净截面应力强度和名义截面应力强度,对切口试件的应力集中问题进行了应力集中系数的讨论和断裂力学的强度因子分析。利用非线性断裂理论推导出计算带环向切口裂纹拉伸杆承载力计算公式,并将数值计算结果与实验结果进行比较。     

Analyses of piezoelectric plates with elliptical notches by special finite element

A novel special finite element with an elliptical notch, based on the complex potential theory and hybrid variational principle, is proposed for mechanical–electrical coupling analyses of piezoelectric plates with notches. The formulations are given and a variety of examples are analyzed. Comparisons are made with existing analytical or numerical data. It is shown that the proposed element is very efficient and accurate. Some new numerical results are given and discussed. It is found, perhaps for the first time according to the authors’ knowledge, that the relationships between the logarithm concentration factor of tangential (θ=0) stress, electrical displacement, and electrical field at the notch tip and logarithm b/a are linear. Some conclusions are drawn based on the results reported herein.     

Experimental and numerical determination of critical stress intensity factor of aluminum curved thin sheets under tensile stress

We determined the fracture toughness of aluminum curved thin sheets using tensile stress tests and finite element method. We applied Linear elastic fracture mechanics (LEFM) and Feddersen procedure to evaluate stress intensity factor of the samples with central wire-cut cracks and fatigue cracks with different lengths to investigate the notch radius effect. Special fixture design was utilized to establish uniform stress distribution at the crack zone. Less than 9 % difference was found between the wire-cut and the fatigue cracked samples. Since generating central fatigue crack with different lengths required so much effort, wire-cut cracked samples were used to determine critical stress intensity factor. Finite element analysis was also performed on one-quarter of the specimen using both the singular Borsum elements and the regular isoparametric elements to further investigate fracture toughness of the samples. It was observed that the singular elements presented better results than the isoparametric ones. A slight difference was also found between the results obtained from finite element method using singular elements and the experimental results.     

Observation of notch effect in Al6061-T6 specimens under tensile loading using digital image correlation and finite element method

The fracture of ductile materials such as Al6061-T6 usually starts from void nucleation. In this study, we investigated strengthening effect and stress triaxiality behavior in notched Al alloy specimens. Various specimens, one un-notched and two with different U-types of notch, one with large diameter (L-notched) and another with small diameter (S-notched), having the same width were selected. The distribution of strain in the full-field was examined using the digital image correlation (DIC) method. Under the tensile test, S-notched specimens showed a more pronounced strengthening effect than the L-notched specimens. The results of DIC confirmed that the notch tip of the S-notched specimen experienced high strain, while the L-notched specimen experienced high strain at the center. The maximum stress triaxiality moved rapidly from the notch tip to the center with the increase in notch radius over 10 mm. The results of the stress triaxiality analysis by finite element method were in good agreement with the above DIC measurement results and predicted the void generation and fracture sites in L-notched and S-notched specimens.     

微动疲劳寿命的估算方法研究

着重分析了零构件由于微动磨损而造成的疲劳失效机制 ,说明了在这种微动疲劳模式下疲劳寿命的组成情况 ,用门槛值应力公式估算了当磨蚀坑根部萌生扩展性裂纹时蚀坑的临界深度尺寸 ,并分析了微动裂纹尖端的应力强度因子 ,得出了计算微动裂纹萌生尺寸的表达式 ,最后用上述方法计算了螺纹联接件的微动磨损寿命与裂纹萌生尺寸 ,用局部应力应变法计算了微动裂纹的萌生寿命 ,所得到的估测寿命与试验值相符 ,由此可见 ,该微动疲劳寿命的估测方法是合理的、有效的     

脆性剖分类零件启裂数值分析

裂解槽作为初始裂解源其尖端的应力状态尤其是厚度方向的应力分布直接影响着剖分类零件的裂解性能,是裂解加工成败的关键。文中利用ABAQUS有限元软件,对脆性剖分类零件(轴承座)启裂进行了分析。分析结果表明:裂解槽应力集中效果明显,随着载荷的增加,裂解槽根部的最大正应力逐渐增加,趋近于材料的断裂强度;最大正应力从裂解槽的中间部位向两侧均匀递减;对于脆性材料,最大正应力出现在裂解槽顶端,启裂发生在工件中心对称面裂尖附近,这种启裂位置的唯一性有利于获得高质量的断裂面,减少裂解缺陷。     

Experimental verification of RV-MTS model for fracture in soda-lime glass weakened by a V-notch

Fracture phenomenon was experimentally investigated in a soda-lime glass weakened by a V-notch under tensile-shear loading. Fracture tests were conducted using a new test sample called the V-notched Brazilian disc (V-BD) specimen. The fracture resistance and fracture initiation angle were obtained initially from the test results. Afterward, a fracture model was utilized to estimate the experimental results. Very good correlation was found between the experimental and theoretical results both for the fracture resistance and the fracture initiation angle in notches having different notch angles and various notch tip radii. Experimental results revealed that for a constant notch tip radius, the failure load under pure tensile loading conditions decreases as the notch angle increases. For a constant notch angle, as the notch tip radius increases, the fracture load in the soda-lime glass V-BD specimens enhances in the whole domain from pure tensile to pure shear loading. Moreover, for a constant notch tip radius, the notch angle has almost no effect on the fracture initiation angle when the specimen is predominantly under tensile loading conditions.