超声疲劳试验方法及其应用

超声疲劳是一种加速的疲劳试验方法,它的测试频率(20kHz)远远超过了常规疲劳测试频率(小于200Hz)．超声疲劳试验研究表明50^#车轴钢和40Cr钢直到10^10个应力循环后仍会发生疲劳断裂,并不存在常规疲劳试验曲线所示的“疲劳极限”,因此用10^7周次的疲劳试验数据进行疲劳强度设计并不安全．50^#车轴钢和40Cr钢超声疲劳性能优于常规疲劳性能．扫描电镜分析表明,超长寿命阶段50^#车轴钢裂纹萌生于次表面夹杂．介绍了超声疲劳试验系统、工作原理及超声疲劳试样的设计．     

高温超声拉压疲劳试件设计与温度自动控制

本文较深入地开展了高温环境条件下的超声拉压疲劳实验系统研究,深入分析了试件温度高精测量与自动控制和高温试件设计的关键问题。实验系统可以实现高温环境条件下大于1010周次超高周次的疲劳加载。本实验系统利用传统超声疲劳试验机,使用非接触式红外测温仪对试件温度进行实时动态监测,采用间歇振动的方法保证温度波动小于±3℃。实验结果显示,采用本系统得到的实验结果与传统疲劳加载方式的结果一致性很好。本文工作为进一步开展高温环境条件下金属材料高温超高周(1010周次)疲劳损伤和疲劳破坏机理研究提供了新的有效的平台。     

比例超载对铝合金圆孔薄板试件低周拉-拉疲劳寿命的影响

对铝合金圆孔薄板试件进行了超载和常辐的对比实验,取六种超载比,对每一超载比各进行了不同超载周次的实验,从这些实验结果看出,对每一超载比,超载试件的疲劳寿命和常辐疲劳试件的寿命比值先是随超载周数的增加而较快的增加,之后在一个相当大的超载周数范围内,超载寿命比的变化不大。当超载周数超过一定值时,超载寿命比随超载周数的增加而略有减少。本文的实验中,在超载60％的情况下,超载周数从20周到90周的范围内,超载试件的平均寿命比常辐疲劳试件的寿命约高3.5倍。     

GCr15钢超声微动疲劳性能研究

由微动产生的裂纹萌生对钢组件的疲劳强度具有重要影响。本文选择GCr15轴承钢,在20kHz超声疲劳试验机提供的循环载荷作用下,测试其超长寿命微动疲劳性能。试验结果显示,在109循环周次下微动疲劳强度影响因子达到0.37。通过电子扫描电镜观察试件磨损面和微动疲劳断口,并分析了高频超长寿命微动疲劳断裂机理。高周疲劳裂纹通常会在磨损面的粘着区与滑移区交界处萌生,超高周疲劳裂纹在粘着区内萌生。微动磨损面的面积以及磨损面的粘着区都随着试件疲劳寿命的增加而增加。     

焊接接头超高周疲劳实验研究

本文基于超声疲劳振动技术,设计了三种焊接接头试样(圆形对接焊接试样及其喷丸处理试样和板状十字焊接试样),并利用超声疲劳试验系统测定了其超高周疲劳性能,实验应力比为-1,频率20kHz,实验在室温条件下进行。实验结果表明,圆形对接焊接接头的疲劳性能高于板状十字焊接接头,喷丸处理能提高焊接接头的疲劳强度。将焊接接头的疲劳性能与对应形状的母材进行对比分析,发现焊接接头的疲劳性能远低于母材。在相同疲劳寿命的条件下,圆形焊接接头试件的疲劳强度仅为母材的45%,十字焊接接头试件仅为母材的29%;圆形对接接头在5×106周次以后,试件仍然发生疲劳断裂,而板状十字焊接接头在超高周区域(107～109周次)存在疲劳极限。超声疲劳断口的扫描电子显微镜分析结果显示,圆形焊接接头试件断口位置主要位于熔合区的焊趾处或焊接接头表面几何非连续处,十字接头试件断口位于焊趾处;焊接接头试件裂纹萌生于焊接缺陷、试样表面夹杂或熔合区的不连续处;喷丸处理对焊接接头的裂纹萌生机制没有显著影响。     

TC4钛合金超高周次轴向振动疲劳试验

应用基于压电超声疲劳试验技术开发的20kHz轴向振动疲劳试验系统,完成了室温下TC4钛合金超高周疲劳试验,获得了TC4合金在107～109周次范围内的轴向振动疲劳寿命曲线(S-N曲线);运用C.Paris推导公式预测了TC4合金材料的寿命,得到各应力水平下破坏率为50％、95％、99％的安全寿命.结果表明:在疲劳循环大于107周次时,试件仍会发生疲劳断裂,疲劳强度随循环次数的增加而下降,并不存在明显的疲劳极限.TC4合金的S-N曲线在107～109周次的范围内呈连续下降型.在轴向振动超高周疲劳试验中,试件的裂纹扩展寿命只占其在50％破坏率下疲劳安全寿命的一小部分,其疲劳寿命主要由试件的裂纹萌生寿命决定.     

孔周弹塑性变形与疲劳循环次数的相关性

应用白光散斑相关技术研究了孔周弹塑性变形与疲劳循环次数的相关性,以便探索疲劳寿命的预测方法.疲劳实验在单边带缺口的拉伸板状试件上进行,试件由GH4169材料制成,试件表面上喷涂了白光散斑图.疲劳加载采用0→Pmax→0的循环加载方式,加载波形为正弦波,加载频率为10Hz.实验中,按一定的疲劳循环周次在零载及最大载荷两种状态下停机、采图.应用数字图像相关技术求出孔周附近的残余位移场,并应用逐点最小二乘法求出应变场.实验中获得了一些对于疲劳寿命研究很有意义的结果.     

CrMoW转子钢高温超高周裂纹扩展及疲劳行为研究

本文研究了超超临界汽轮机CrMoW转子钢在常温与600℃条件下的超高周旋转弯曲疲劳行为。实验中利用位移传感器原位监测试件挠度变化,研究裂纹的萌生与扩展特性。研究结果发现,在常温下,疲劳裂纹主要萌生于试样表面,但也发现超高周次疲劳破坏的裂纹萌生于内部的情形。600℃时,S-N曲线呈现直线下降的趋势;大多试样裂纹萌生区和初始扩展区发现非金属夹杂物,其裂纹萌生是表面裂纹起源和亚表面夹杂物相耦合的结果。裂纹萌生寿命占整个疲劳寿命的90%以上,裂纹萌生寿命百分比(Ni/Nf)随疲劳寿命的延长而增大,并且温度对裂纹萌生寿命百分比没有影响。     

沥青混合料疲劳过程的损伤力学分析

采用损伤力学方法研究沥青混合料的疲劳失效问题。针对悬壁梁弯曲疲劳试件,推导出疲劳过程中应力场、损伤场和疲劳裂纹形成寿命的工程封闭公式。根据沥青混合料特点,提出一种模拟疲劳裂纹扩展的特征单元失效模式,从而将疲劳裂纹形成与扩展两个阶段统一用损伤力学理论进行描述和分析。本文对沥青混合料试件的疲劳裂纹形成寿命与扩展寿命分段进行了预测,还对疲劳过程中刚度衰减及位移幅值的演化过程进行了数值模拟计算。理论预期与实验结果吻合良好。     

GH536高温合金光滑试件在800℃时变载疲劳试验研究

通过对GH536合金光滑试件在800℃、0－σmax－0情况下的疲劳寿命实验研究和分析，说明在该情况下试件的断裂是由于材料产生颈缩而引起的。这一结果对于相关金属材料疲劳特性的研究具有一定的参考价值。     

一种基于直接计算高阶奇异积分的断裂力学双边界积分方程分析法

相对于有限元法,边界单元法在求解断裂问题上有着独特的优势,现有的边界单元法中主要有子区域法和双边界积分方程法.采用一种改进的双边界积分方程法求解二维、三维断裂问题的应力强度因子,对非裂纹边界采用传统的位移边界积分方程,只需对裂纹面中的一面采用面力边界积分方程,并以裂纹间断位移为未知量直接用于计算应力强度因子.采用一种高阶奇异积分的直接法计算面力边界积分方程中的超强奇异积分;对于裂纹尖端单元,提供了三种不同形式的间断位移插值函数,采用两点公式计算应力强度因子.给出了多个具体的算例,与现存的精确解或参考解对比,可得到高精度的计算结果.      

汶川地震诱发文家沟巨型滑坡 碎屑流基本特征及成因机制初步分析

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2008年5月12日,汶川M80地震在四川省绵竹市清平乡文家沟内诱发一巨型滑坡。通过现场调查得知,滑坡前后缘高差455m,厚度20～30m,滑面为基岩层面,初始方量2750×107m3。滑体在运动中转化为碎屑流。滑坡-碎屑流总的水平运动距离为4022m,垂直运动距离为1443m,遗留的堆积物体积达5×107m3。滑坡距映秀—北川断裂仅36km,位于其下盘,地震烈度达XI度。滑坡导致文家沟中48人遇害,并形成一条完整的地震次生地质灾害链。初步分析表明滑坡启动速度快,滑坡向碎屑流转化过程明显、地点明确。碎屑流运动过程复杂,伴有强烈的“气垫效应”和“前缘气浪冲击效应”。作者认为,文家沟滑坡的高启动速度是长持时强烈地震动作用的结果,与山体的猛烈碰撞是导致滑体解体并转化为碎屑流的原因。     

Preface: Orbits around asteroid

One of the core issues in modern celestial mechanics is the orbital dynamics in the near-regime gravitational field of as- teroids, which provides deep insights into the mathematical nature of a class of nonlinear systems, and plays as a critical basis for in situ explorations of different science goals. Lots of efforts have been made to reveal the characteristics of orbital motion in the vicinity of asteroids, and to improve the skills of asteroid research in methodology.     

Effet de l'anisotropie élastique cristalline sur la distribution des facteurs de Schmid à la surface des polycristauxInfluence of crystalline elasticity anisotropy on Schmid factor distribution at the free surface of polycrystals

Experimental studies of the plasticity mechanisms of polycrystals are usually based on the Schmid factor distribution supposing crystalline elasticity isotropy. A numerical evaluation of the effect of crystalline elasticity anisotropy on the apparent Schmid factor distribution at the free surface of polycrystals is presented. Cubic elasticity is considered. Order II stresses (averaged on all grains with the same crystallographic orientation) as well as variations between averages computed on grains with the same crystallographic orientation but with different neighbour grains are computed. The Finite Element Method is used. Commonly studied metals presenting an increasing anisotropy degree are considered (aluminium, nickel, austenite, copper). Concerning order II stresses in strongly anisotropic metals, the apparent Schmid factor distribution is drifted towards small Schmid factor values (the maximum Schmid factor is equal to 0.43 instead of 0.5) and the slip activation order between characteristic orientations of the crystallographic standard triangle is modified. The computed square deviations of the stresses averaged on grains with the same crystallographic orientation but with different neighbour grains are a bit higher than the second order ones (inter-orientation scatter). Our numerical evaluations agree quantitatively with several observations and measures of the literature concerning stress and strain distribution in copper and austenite polycrystals submitted to low amplitude loadings. Hopefully, the given apparent Schmid factor distributions could help to better understand the observations of the plasticity mechanisms taking place at the free surface of polycrystals. To cite this article: M. Sauzay, C. R. Mecanique 334 (2006).     

Nonlinear behavior of matrix-inclusion composites under high confining pressure: application to concrete and mortar

This paper is devoted to a micromechanics-based simulation of the response of concrete to hydrostatic and oedometric compressions. Concrete is described as a composite made up of a cement matrix in which rigid inclusions are embedded. The focus is put on the role of the interface between matrix and inclusion which represent the interfacial transition zone (ITZ). A plastic behavior is considered for both the matrix and the interfaces. The effective response of the composite is derived from the modified secant method adapted to the situation of imperfect interfaces. To cite this article: T.H. Le et al., C. R. Mecanique 336 (2008).     

Dynamics and Breakup of Viscoelastic Liquids (A Review)

The phenomena of hydrodynamic breakup of liquid jets, drops, films, bridges, and filaments are reviewed for liquids with viscoelastic properties. The reasons for breakup are capillary instabilities, collisions with rigid obstacles, and other forms of dynamic action. The relationship between the properties of the liquids and the features of the breakup process is discussed.     

Flow stability analysis and excitation using pulsating jets

Classical flow stability applied to transition from laminar to turbulent flow may also describe the behavior of vorticity fluctuations created by a pulsating jet placed along a solid boundary. A numerical laminar flow experiment involving a pulsating jet placed along the surface of a duct with flow separation downstream, resulted in eliminating most part of the separated flow region. Applying the same approach to a turbulent flow, it was possible to develop a turbulent stability flow formulation and apply successfully turbulent pulsating jet flow separation control. To cite this article: D. Skamnakis, K. Papailiou, C. R. Mecanique 333 (2005).