Fictitious notch rounding concept applied to sharp V-notches: Evaluation of the microstructural support factor for different failure hypotheses. Part I: Basic stress equations

With the aim to perform a comprehensive and accurate evaluation of the microstructural support factor of sharp V-notches (Neuber’s notch rounding concept), in Part I of this contribution, the indispensable theoretical tools, especially the basic stress equations, are reconsidered and amended in respect of accuracy of results. First, the analytical solution derived by Neuber [Neuber H. Kerbspannungslehre. 2nd ed. Berlin: Springer-Verlag; 1958] for sharp rounded V-notches with an arbitrary flank angle under tension loading is considered. The equation of the normal stress has been obtained with the restriction to the notch bisector. Using the Airy stress function suggested by Neuber, this solution is extended to the region outside the notch bisector, and the complete stress field is derived in this manner. A comparison between Neuber’s solution, a more recent solution due to Filippi et al. [Filippi S, Lazzarin P, Tovo R. Developments of some explicit formulas useful to describe elastic stress fields ahead of notches in plates. Int J Solids Struct 2002;39:4543-65] and highly accurate FE results is performed. Filippi’s equations which include Williams’ solution [Williams ML. Stress singularities resulting from various boundary conditions in angular corners on plates in tension. J Appl Mech 1952;19:526-8] for pointed V-notches, are shown to be superior.     

基于局部应变能密度法的缺口多轴疲劳寿命评估

本文提出了包括实验和数值工作在内的综合分析,阐明了缺口行为对比例多轴疲劳寿命的影响。方法对45钢和45QT钢进行了考虑缺口半径和开口角的多轴疲劳试验,基于平均应变能量密度理论的多轴疲劳分析的分析和计算框架进行了研究,以处理能量梯度。结果显示,大量新的疲劳数据首先通过法向应力和剪应力进行汇总,然后通过凹口尖端周围受控体积中的局部应变能密度重新分析。结论缺口角度对比例载荷下多轴疲劳数据的影响较小,而缺口半径是影响疲劳寿命的主要因素。     

A notch stress intensity approach applied to fatigue life predictions of welded joints with different local toe geometry

In the Notch Stress Intensity Factor (N‐SIF) approach the weld toe region is modelled as a sharp V‐shaped corner and local stress distributions in planar problems can be expressed in closed form on the basis of the relevant mode I and mode II N‐SIFs. Initially thought of as parameters suitable for quantifying only the crack initiation life, N‐SIFs were shown able to predict also the total fatigue life, at least when a large part of the life is spent as in the propagation of small cracks in the highly stressed region close to the notch tip. While the assumption of a welded toe radius equal to zero seems to be reasonable in many cases of practical interest, it is well known that some welding procedures are able to assure the presence of a mean value of the weld toe radius substantially different from zero. Under such conditions any N‐SIF‐based prediction is expected to underestimate the fatigue life. In order to investigate the degree of conservatism, a total of 128 fillet welded specimens are re‐analysed in the present work by using an energy‐based N‐SIF approach. The local weld toe geometry, characterised by its angle and radius, has been measured with accuracy for the actual test series. The aim of the work is to determine if the N‐SIF‐based model is capable of taking into account the large variability of the toe angle, and to quantify the inaccuracy in the predictions due to the simplification of setting the toe radius equal to zero.     

On the Method of Determination of Strain Energy Release Rate During Fatigue Delamination in Composite Materials

In this paper, the problem of calculation of the energy release rate for a fatigue test on composite material has been investigated. The application of the Linear Elastic Failure Mechanics (LEFM) leads to the use of varation of the energy release rate ( G). As the energy release rate is a function of the load squared, the variation of G becomes either a function of variation of the load squared ( G = f((P²))) or a function of the square of the load variation ( G = f(( P)²)).In this paper, we determine, by different fatigue tests, which of the two theoretical results is the best to describe the experiments. These fatigue tests have been made on DCB test-specimen in mode I with different R ratios (R = P_max / P_min) and different maximum loads. The material was a unidirectionnal glass-epoxy.The results show that considering G as a function of ( P)² seems more appropriated to describe a cracking test in fatigue.     

应变能密度比准则在块体非晶合金中的应用

由于块体非晶合金独特的原子排列结构,导致其具有独特的力学行为。多年来,国内外研究人员对块体非晶合金的断裂行为进行了一系列的研究,并取得了一定的成果。本文简单介绍了近年来国内外针对块体非晶合金断裂准则的研究状况,随后比较全面地介绍了应变能密度比失效准则,并推导了遵循该准则时断裂角θT(载荷方向与断裂面的夹角)与α(断裂面剪应力与正应力的比值)应满足的关系,最后利用Zr41.2Ti13.8Cu10.0Ni12.5Be22.5块体非晶合金的拉伸实验和已有的实验结果,证实了块体非晶合金遵循应变能密度比失效准则。     

基于模态应变能耗散率理论的结构损伤识别方法

本提出了基于模态应变能耗散率理论的一种新的结构损伤识别方法。表示结构损伤的损伤变量的概念来自于材料领域，并推广到了构件和结构；在此基础上，结构单元的损伤变量是通过建立模态应变能耗散率和相应的结构损伤前后的应变能变化的关系而获得。该方法所要求的结构损伤前后的振型模态可以是不完备的。对测量噪声的影响具有较强的鲁棒性。最后，通过数值算例的分析结果说明，该方法简便、有效，可定位结构的损伤和识别损伤的程度。     

计算粘弹结构动力学参数的新模态应变能方法

粘弹结构在噪声与振动控制领域已得到广泛应用,然而准确地计算粘弹结构的动力学参数一直存在困难,模态应变能法及其修正方法常用作近似计算。在分析模态应变能方法和已有修正方法的原理及其相互关系的基础上,提出一种基于损耗因子幅值的新模态应变能方法,用于计算粘弹结构的损耗因子与固有频率。新方法的修正因子随粘弹结构对应模态阶次损耗因子幅值变化。取可等价为粘弹性夹层梁/板的四参数原型系统和新型高阻尼航天载荷隔振器为算例,通过与已有方法的对比分析了新方法的准确性。     

裂尖塑性区内方向应变能裂纹扩展归一化准则

裂纹的起始扩展总是沿着裂纹的半径方向,在塑性区内半径方向的应变能能反映出材料的抵抗断裂的能力。对同一材料,不管它处于哪一种裂纹形式,它的断裂韧性参数是一个常量。本文引入了裂纹顶端临界扩展本征区。认为裂纹顶端存在一个决定裂纹扩展本征区,裂纹扩展是因为本征区的应力应变状态或损伤状态达到材料的断裂韧性才发生的。依此针对Ⅰ-Ⅱ...     

Analytical Modeling to Predict the Strain Energy of Circular GLARE Fiber-Metal Laminates Under Lateral Indentation

This article deals with the strain energy calculation of thin circular clamped Glass Reinforced （GLARE） fiber-metal laminates subjected to static indentation by a lateral hemispherical indentor. Using one-, two- and three-parameter Ritz approximations, analytical equations of the strain energy as a function of the central plate deflection are derived. Previously published analytical formulas, concerning the load-indentation response of circular GLARE plates, are used in order to determine the Ritz parameters and the first failure load and deflection due to tensile fracture of glass-epoxy layers. In this study, the membrane and bending strain energy components of aluminum and prepreg layers are determined. Also, the elastic and plastic strain energy absorbed during the indentation loading are calculated. The derived formulas are applied successfully for GLARE 2-2/1-0.3, GLARE 3-3/2-0.4 and GLARE 31 （special lay-up） circular plates subjected to lateral indentation. The strain energy results converge satisfactorily in all examined cases. The predicted strain energy-indentation response is compared with published experimental data and a good agreement is found. No other solution of this problem is known to the authors.     

Dynamic Strain Aging in a Newly Developed Ni-Co-Base Superalloy with Low Stacking Fault Energy

Characteristics of dynamic strain aging(DSA) in a Nie Co-base superalloy were studied by tensile tests at temperatures ranging from 250℃ to 550℃ and strain rates ranging from 3×10-5 to 8×10-4 s-1.Serrated flow in the tensile stress-strain curves was observed in the temperature range from 300℃ to 500℃.Normal DSA behavior was found at temperatures ranging from 300℃ to 350℃,while inverse DSA behavior was observed at temperatures ranging from 400℃ to 500℃.The yield strength,ultimate tensile strength,elongation,work hardening index,and fracture features were not affected by temperature and strain rates in DSA regime.Negative strain-rate sensitivity of flow stress was observed in DSA regime.The analysis suggests that the ordering of the substitutional solutes around some defects like mobile dislocations and stacking faults due to the thermal activated process may cause the serrations on the tensile curves.     

On the Mechanism of Energy Dissipation in a Fatigue Crack

Experimental studies of specimens with a Mode-I edge crack in bending vibration have demonstrated that energy dissipation in the fatigue crack is mainly due to an elastoplastic zone at the crack tip. The absolute level of energy dissipation in the crack is unambiguously governed by the stress intensity factor range and is independent of specimen dimensions and crack location.     

Experimental Verification of the Averaged Strain Energy Density Criterion for Brittle Fracture in Blunt V-Notches under Pure Compression

Strength of Materials - The goal of the present study is to verify experimentally the Averaged Strain Energy Density (ASED) criterion for brittle fracture in blunt V-notches under pure compression....     

10CrNiMo结构钢低周疲劳过程中的应变滞后特性

在6级轴向逐级增加的系列恒应变幅(0.3%~1.0%)控制下,室温空气中,采用光滑圆形横截面试样,研究了10CrNiMo结构钢在低周疲劳作用下的应力-总应变及应力-塑性应变的滞后回线特性。结果表明:在小应变幅(0.3%~0.4%)控制下,虽然弹性应变占绝对优势,但塑性应变并不为零,且逐级增长;而在大应变幅(0.6%~1.0%)控制下,塑性应变增长较快,并逐渐占主导地位。无论应变幅大小,只须几个循环,材料便进入稳定状态,而初始循环与失稳循环却表现出不稳定的特征。     

论岩土塑性体应变与剪应变的相互作用原理

对岩土塑性变形过程中体应变与剪应变的相互作用原理的内涵做了深入的阐述，特别对塑性体应变和剪应变之间的相互作用方式，塑性体应变对抗剪能力的直接控制作用，剪缩和剪胀发生的条件，应力路径相关性是这种相互作用的综合体现及实例验证，临界状态是塑性和弹性体应变都保持不变的纯粹剪切变形过程等方面进行了论证；根据这个相互作用原理，在理论上证明了空间临界状态线的存在性和唯一性以及它与应力是无关的。     

半无限岩体应变能随最小抵抗线变化规律的探讨

利用ADINA计算程序，计算了单孔爆破半无限岩体内应变能的数值，得出了当抵抗线较小时，岩体内应变能分配均匀，有利于岩石破碎的结论。     

Brittle failures from U- and V-notches in mode I and mixed, I + II, mode: a synthesis based on the strain energy density averaged on finite-size volumes

A large bulk of static test results carried out on notched specimens are presented in a unified way by using the mean value of the strain energy density (SED) over a given finite-size volume surrounding the highly stressed regions. In plane problems, when cracks or pointed V-notches are considered, the volume becomes a circle or a circular sector, respectively, with R _C being the radius. R _C depends on the fracture toughness of the material, the ultimate tensile strength and the Poisson's ratio. When the notch is blunt, the control area assumes a crescent shape and R _C is its width as measured along the notch bisector.
About 900 experimental data, taken from recent literature, are involved in the local SED-based synthesis. They have been obtained from (a) U- and V-notched specimens made of different materials tested under mode I loading; (b) U- and V-notched specimens made of polymethyl-metacrylate (PMMA) and an acrylic resin, respectively, tested in mixed, I + II, mode; (c) U-notched specimens made of ceramics materials tested under mode I.
The local SED values are normalized to the critical SED values (as determined from unnotched specimens) and plotted as a function of the R / R _C ratio. A scatter band is obtained whose mean value does not depend on R / R _C, whereas the ratio between the upper and the lower limits are found to be about equal to 1.6. The strong variability of the non-dimensional radius R / R _C (ranging here from about zero to around 1000) makes stringent the check of the approach based on the mean value of the local SED on a material-dependent control volume.     

On the Energy Release Rate at the Crack Tips in a Finite Pre-Strained Strip

The influence of the initial finite stretching or compressing of the strip containing a single crack on the Energy Release Rate (ERR) and on the SIF of mode I at the crack tips is studied by the use of the Three-Dimensional Linearized Theory of Elasticity. It is assumed that the edges of the crack are parallel to the face planes of the strip and the ends of the strip are simply supported. The initial finite strain state arises by the uniformly distributed normal forces acting at the ends of the strip. The additional normal forces act on the edges of the crack. The elasticity relations for the strip material are given by the harmonic type potential. The corresponding boundary-value problem is solved by employing FEM. The numerical results on the influence of the initial finite strain state the values of the ERR and of the SIF of mode I are presented. In particular, it is established that the values of the ERR and of the SIF of mode I decrease (increase) monotonically with an increase (decrease) in the initial stretching (compression).     

NiMnGa和NiMnFeGa单晶磁感生应变的温度依赖性及孪晶界移动的能量损耗

在提拉法生长出Ni52Mn16.4Fe8Ga23.6单晶样品的基础上,测量了其相变应变和磁感生应变。与Ni52Mn24.5Ga23.5单晶样品的实验结果对比分析可知,两种材料都具有较大的自发相变应变量和磁感生应变量,但Ni52Mn16.4Fe8Ga23.6单晶的磁感生应变具有较好的温度稳定性。依据实验结果和利用热动力学原理,分别计算了两种单晶样品磁感生应变升降磁场一个循环过程中孪晶界移动的能量损耗,给出了能量损耗随温度变化的关系,结果表明磁感生应变量可定性反映一个循环过程中孪晶界移动的能量损耗。     

Potential ambiguity in the determination of the plastic strain range component in LCF testing

Most low cycle fatigue testing is carried out under total strain controlled limits and the plastic strain component is determined retrospectively after the test by considering the hysteresis loop closest to the mid-life. Currently there is no universal method for determining the plastic strain range.

It is demonstrated that variability in excess of 30% can arise in the values of the plastic strain range calculated according to the different methods adopted by current Standards and commercial testing software.