三维机织复合材料在拉压循环载荷下的疲劳性能

为研究三维机织复合材料在拉伸-压缩循环载荷下的疲劳性能,对材料进行了应力比R=-1的疲劳试验。在不同的载荷水平下,分别进行了纬向和经向两类拉压疲劳试验。试验获得了试样在疲劳载荷下的滞回曲线和全过程中剩余刚度比随寿命的变化曲线。结果表明,在拉伸-压缩循环载荷下,三维机织复合材料的疲劳损伤过程主要包含3个阶段,分别发生基体破坏、纱线横向裂纹扩展和纱线的最终断裂。基体的破碎和开胶、垂直于载荷方向排布的纱线撕裂和沿载荷方向排布的纱线断裂是试样内部的主要失效模式。试验还获得了纬向和经向拉压疲劳的拟合S-N曲线,可应用于工程中对该型材料进行疲劳寿命估算。该型材料的疲劳寿命在低应力区和高应力区均显示出较小的分散性,双对数坐标系下的拟合S-N曲线具有较好的线性度。     

高强度结构钢材单调荷载作用下的本构模型研究

试验研究了国产Q550D高强度结构钢、焊缝的基本力学性能以及横向对接焊缝和非承载横向角焊缝连接件的疲劳性能。介绍了试件的加工方法、焊接工艺、疲劳试验及其数据处理方法,将疲劳试验结果与国内外钢结构设计规范以及他人的研究结果进行了分析对比。研究表明：Q550D高强钢母材及其焊缝具有优良的延伸率和低温冲击韧性,余高磨平的钢板对接焊缝和横向角焊缝连接件的疲劳强度都分别显著地高于国内外钢结构设计规范关于普通强度钢材相应焊接节点的疲劳强度,200万次疲劳强度分别提高了72%~125%和27%~68%。钢板对接焊缝的余高磨平可改善因焊缝的几何形状和缺陷引起的应力集中程度,比原状焊缝具有好的疲劳性能。无论余高磨平或未磨平的钢板对接焊缝,还是横向角焊缝连接件,它们都具有随高强钢屈服强度的增加而疲劳强度提高的趋势。分别提出了Q550高强钢钢余高磨平的钢板对接焊缝和横向角焊缝连接件的S-N曲线,可供抗疲劳设计参考。     

K节点应力集中系数的试验和数值研究方法

工程中常用的评价海洋平台中管节点疲劳寿命的方法是使用S-N曲线。当管节点承受疲劳载荷作用的时候,可以通过数值或者试验方法得到沿着焊缝处的热点应力幅的大小。然后通过S-N曲线,可以预测此节点在破坏前可以承受疲劳载荷的循环次数。应力幅的大小可以由应力集中系数这个参数来确定。对K型节点在承受基本载荷作用下的应力集中系数进行了数值和试验分析,得到了各种基本载荷作用下K节点沿着焊缝处应力分布情况和极值应力点的位置。     

小直径钢丝绳的轴向疲劳断裂特征分析

运用疲劳试验结合声发射技术和显微分析手段 ,对小直径钢绳的拉 -拉疲劳断裂特征进行分析 ,绘制钢丝绳的S -N曲线 ,测定钢丝绳断丝个数和位置 ,对钢丝绳进行失效预报。同时讨论钢丝绳疲劳断裂和组成钢丝疲劳断裂特征与钢丝绳使用寿命的关系     

A GENERAL CONSTITUTIVE RELATION FOR FATIGUE CRACK GROWTH ANALYSIS OF METAL STRUCTURES

Crack growth rate curves are the fundamental material property for metal structures under fatigue loading. Although there are many crack growth rate curves available in the literature, few of them showed the capability to explain various special phenomena observed in tests. A modified constitutive relation recently proposed by McEvily and his co-workers showed very promising capability. This modified constitutive relation is further generalized by (1) introducing an unstable fracture condition; (2) defining a virtual strength to replace the yield stress; and (3) defining an overload and underload parameter. The performances of this general constitutive relation for fatigue crack growth is extensively studied and it is found that this general constitutive relation is able to explain various phenomena observed with particular strong capability on load sequence effect.     

普通混凝土梁用钢筋的疲劳S- N曲线研究

介绍了钢筋疲劳Ｓ－Ｎ曲线的表达方法和影响钢筋疲劳强度的因素,对普通钢筋混凝土粱用光圆钢筋和变形钢筋的疲劳强度进行了试验研究,对铁科院以前做的对接焊钢筋的疲劳试验结果重新进行了分析,然后以应力幅作为疲劳参数,以９５％作为保证概率,提出了适用于疲劳寿命评估的三种钢筋的疲劳Ｓ—Ｎ曲线,并与国外的试验结果进行了比较。这些曲线为有关规范标准的制定和修订以及工程应用研究提供了新的研究依据。     

支管在轴向荷载作用下Q460C高强钢T型圆管节点疲劳性能试验研究

对8个支管承受轴向荷载的Q460C高强钢T型圆管相贯节点分别进行热点应力试验和疲劳试验研究,将热点应力试验结果与现有热点应力集中系数计算公式进行对比验证分析,通过疲劳性能试验观察疲劳裂纹的发展过程和疲劳破坏模式,并对比分析了CIDECT和DNV疲劳设计规范中圆管节点疲劳S-N曲线对于高强钢管节点的适用性。研究表明,疲劳裂缝首先出现在67.5°到90°附近应力集中较大的区域（鞍点附近）,然后迅速沿着相贯线向冠点处发展,并在裂缝末端产生沿主管轴向的横向裂缝。通过试验结果与已有规范的对比分析表明,CIDECT建议的S-N曲线应用于支管承受轴向荷载的Q460C高强钢圆管节点的疲劳寿命预估在某些情况下是偏于不安全的,而DNV规范的计算值过于保守,因此同样也不适合于Q460C高强钢圆管节点的疲劳性能分析。     

Fatigue behavior of tailor (laser)-welded blanks for automotive applications

The drive towards weight reduction in the automotive industry has led to the use of tailor-welded blanks (TWBs) for structural applications. This work is aimed at evaluating the fatigue behavior of the TWBs having different thickness combinations and compositions with and without zinc coating. The results showed that TWBs made from zinc-coated/galvanized steels exhibited a lower fatigue limit as compared with the TWB combination from uncoated bare metals. This was attributed to the intergranular cracking in the galvanized TWBs caused by the presence of zinc penetrating beneath the sheet surface. The fracture surface of the uncoated TWB was basically characterized by the fatigue striations.     

Morphology of step-wise S–N curves depending on work-hardened layer and humidity in a high-strength steel

The present paper describes the effects of work‐hardened layer and humidity on the fatigue response of high strength steel, JIS SNCM439 (AISI4340). Tests were conducted in laboratory air and in dry air using cantilever‐type rotating bending fatigue testing machines. The dew point of dry air was ?60 °C. Specimens with two different surface conditions were prepared, i.e., buff‐finished and electro‐polished specimens. In the buff‐finished specimens, a stepwise S–N curve was seen in both environments, but the transition stress below which subsurface crack initiation occurred shifted to a higher stress level in dry air than in laboratory air. In the electro‐polished specimens, stepwise S–N curve was not observed in laboratory air, but seen in dry air. At stress levels where cracks initiated at the surface, the fatigue lives of the electro‐polished specimens were shorter than those of the buff‐finished specimens. At stress levels where subsurface cracks initiated, the fatigue lives of both specimens were the same, irrespective of surface finishing condition and humidity.     

1Cr17Ni1双相不锈钢室温旋转弯曲疲劳性能

在应力比R=-1,转速为5 000 r/min和室温环境下测试了1Cr17Ni1双相不锈钢光滑旋转弯曲疲劳试样和缺口旋转弯曲疲劳试样的数据,得到了2种试样的S-N曲线.并采用扫描电镜(SEM)分析了2种试样的断口形貌,研究了1Cr17Ni1双相不锈钢高周疲劳特性.结果显示,光滑旋转弯曲疲劳试样的疲劳极限为320 MPa,其疲劳裂纹萌生于试样的表面;缺口旋转弯曲疲劳试样的疲劳极限为137 MPa,其裂纹源萌生于缺口处.经计算得到1Cr17Ni1双相不锈钢的缺口敏感系数为0.67.