Effects of Geometry and Dimension of Specimen and Rig on Small Punch Creep Property

The purpose of this study was to examine the effects of the geometries and dimensions of the rig and specimen of a small punch (SP) test on the determined creep property. The SP creep property of a specimen was evaluated using finite element analysis by varying the specimen thickness and diameter, diameter of the spherical and hemispherical indentation punches, inner diameter of the lower die, and dimensions of the chamfer of the lower die. We observed that the rupture time decreased with decreasing specimen thickness and punch diameter and with increasing chamfer size and inner diameter of the lower die. Under similar analytical conditions of static average equivalent stress in steady state, the SP creep curves reasonably agreed even in the case where the specimens or rigs have different geometries, which implied the possibility of direct comparison of the test results obtained from specimens and rigs with different dimensions and geometries.     

P91钢中厚壁管焊接残余应力分析

针对管道试件规格不同所要求的焊接工艺不一致问题,讨论P91钢中厚壁管对接接头所要求的双人对称焊形式下的焊接残余应力分布规律．基于ANSYS分析软件,以双移动高斯热源结合单元生死技术,利用顺序耦合方法得到P91钢中厚壁管焊接残余应力．讨论热处理工艺对于P91钢中厚壁管焊接残余应力的影响．结果表明,由于焊接工艺在环向位置上的结构反对称性使得焊接残余应力在环向上呈反对称分布;即便经过较高温度的预热处理,没有焊后热处理时最大等效应力仍超过屈服极限;进行焊后整体热处理的P91钢中厚壁管焊接残余应力得到了有效的改善,平均应力数值下降了60%~70%．     

输油钢管内壁应力检测中的磁测理论与实验研究

本文采用磁致伸缩逆效应(应力对磁化的影响)的磁测技术对常见无缝输油钢管的受力变形进行了两种模拟加载方式的实验研究,给出了适合一定曲率管道"四系数"磁测钢管变形计算方法,并用有限元法对相应的受力状态进行了模拟计算,同时讨论了影响磁测精度的原因和解决方法.有限元计算结果与磁测实验结果基本一致.     

An Extension of the Monkman-Grant Model for the Prediction of the Creep Rupture Time Using Small Punch Tests

Creep tests on a small scale have the potential to be used without significant removal of material or in areas where the available material is limited. In this paper an extension of the modified Monkman-Grant model for the prediction of the creep rupture time using Small Punch Creep Tests (SPCT) is investigated. This test basically consists of punching, under constant load, a small size specimen (10×10×0.5 mm) with the ends fixed. For this purpose an AZ31B magnesium alloy, taking a test temperature of 150?ºC, has been selected. The Monkman-Grant relation is a predictive model, initially developed for uniaxial creep tests, which can be used to predict the rupture time of tests which have been interrupted once secondary stage of creep has been reached. The proposed extension of the Monkman-Grant model for SPCT is based on the definition of the Minimum Relative Punch Displacement Rate. The experimental techniques and data analysis, involving small punch testing, are explained in detail. The proposed predictive model allows the test times of small punch testing to be reduced and it can be directly applied to predict the failure time from an interrupted test at the time when the Minimum Relative Punch Displacement Rate is reached. Good correlations are obtained by comparing the failure time from the proposed Monkman-Grant extension with the experimental failure time.     

Experimental and Numerical Investigation of Second Harmonic Generation by Creep Induced Micro-voids

Background: Identification of damage at an early stage is crucial for critical structural components. Objective: Creep induced micro-voids in heat treated polycrystalline pure copper are experimentally and numerically characterized. Methods: This is accomplished by the use of non-linear ultrasonic waves. Numerically, the study is carried out on an elastic material with randomly located micro-voids. The finite difference in time domain method is used. Results: Experimentally it is found that material damage due to micro-voids that are developed before 40% creep life may effectively be detected with nonlinear ultrasonic waves in the low power regime. Increase in second order nonlinearity parameter with percentage creep life is observed along with the corresponding increase in micro-void concentration. Optical metallography and micro-hardness measurements were used to corroborate the experimental results. For the numerical studies, void dimensions selected for study are much lower than the probing wave length. Concentration of micro-voids is varied from 0.01% to 3%. The second order nonlinearity parameter showed a slight increase in the initial stages and an abrupt increase with higher micro-void concentration in simulations. This agrees with experimental trends. Conclusion: The feasibility of detecting micro-voids in the early stages of creep using nonlinear ultrasonic waves is experimentally demonstrated and a simple model with voids is numerically studied to show the sensitivity of the second order nonlinearity parameter to micro voids.

     

Q345R钢蠕变过程声发射特性实验研究

声发射测试技术由于实时、连续、在线监测的特点,被越来越多地应用于材料性能的研究,但由于蠕变实验温度过高,超过传感器使用温度限制,因此在金属材料蠕变损伤领域还尚属空白。本文以Q345R钢为例,设计蠕变声发射监测专用的夹具导波机构,进行Q345R钢蠕变声发射监测实验。监测结果表明,蠕变损伤过程的声发射活动表现出与蠕变曲线相类似的阶段性特征。蠕变初期撞击数较多,声发射较为活跃;随着损伤的演进,试件进入稳态蠕变阶段,声发射活动渐趋平稳,日平均撞击数趋于稳定值;蠕变后期,能量快速释放,声发射活动加剧,试件发生蠕变断裂。     

在役设备材料断裂力学参数测定方法综述--小冲杆实验力学研究进展之二

20多年以来, 用小型试件的小冲杆实验技术来测量在役设备材料的各种力学参数已经取得了很大进展. 小冲杆实验的试样虽然还没有标准, 但一般均采用直径为3～10mm,厚度为0.1～0.5mm的圆片试样.这个方法已经用来确定材料的弹性模量、屈服强度、塑性性能、抗拉强度、断裂形态转变温度、断裂韧性、蠕变性能以及表示塑性硬化和韧性损伤的微观力学参数和黏塑性性能等各种力学性能.该方法兼具取样方便和几乎无损的优点,非常适用于那些无法取出传统试样或取样不经济的场合,因此引起科技界和工业界的关注.本文综述了小冲杆试验的测量技术及从测量数据来确定材料断裂形态转变温度、断裂韧性和主曲线的移动温度的各种计算方法研究进展.     

Experimental and Numerical Study of Shear Fracture in Brittle Materials with Interference of Initial Double Cracks

A simultaneous experimental and numerical study of shear fracture of concrete-like materials is carried out using Brazilian disc specimens with initial double edge cracks and fourpoint bending beam specimens with double edge-notches.The interference effects of two cracks/notches are investigated through varied ligament angles and crack lengths.It is shown that shear fracturing paths change remarkably with the initial ligament angles and crack lengths.The cracked specimens are numerically simulated by an indirect boundary element method.A comparison between the numerical results and the experimental ones shows good agreement.     

Experimental Analysis of High-Temperature Creep, Fatigue, and Damage. 1. Analysis Methods

Experimental methods used to study high-temperature processes of creep, damage, long-term fracture, and high-cyclic fatigue are reviewed. Emphasis is on nonstandard methods of testing specimens and structural elements, experiment schemes and equipment, and mathematical methods for processing experimental data. A technique of constructing unified deformation and damage diagrams based on the conditions of proportional similarity is substantiated     

含有初始凹陷圆柱壳稳定承载能力的实验研究与数值计算

对含有初始轴对称凹陷的圆柱壳在轴向载荷作用下的稳定承载能力进行了研究。首先,通过实验研究,得到了不同凹陷参数及不同壳长条件下的临界载荷。同时,不考虑凹陷成形过程,仅把凹陷部分当做结构的不规则变化时,对含有凹陷圆柱壳建立三维有限元模型进行稳定性分析。在分析中考虑了初始缺陷(圆柱壳成形加工中的不平直度、椭圆度、初始弯曲等因素引起)幅值f0=0.1t和f0=0.2t(t为壳体壁厚),得到了不同凹陷参数及不同壳长的临界载荷。结果表明:含有凹陷圆柱壳稳定承载能力对初始缺陷不太敏感,即:初始缺陷与凹陷相比,对稳定承载能力的影响极小。数值计算结果与实验结果吻合较好,共同给出了含有初始轴对称凹陷圆柱壳的承载能力随凹陷幅值、长度及圆柱壳长度的变化而改变的规律。     

45号钢氢致脆断试验与GTN模型数值模拟研究

采用电化学方法对试样充氢后再进行单轴拉伸,在此基础上研究45号钢的氢致脆断问题.试验观察到,随充氢时间增加,试样的延伸率和平均断裂应变减小,断口韧窝数目减少,准解理区域增加.但该改变逐渐减缓,充氢大于72 h若继续充氢试样延性和断裂应变几乎不再变化.基于试验观测,将HEDE机制和受氢影响的材料微孔洞演化机制引入GTN本...     

超高速发射实验及其数值分析

采用多介质流体高精度欧拉并行计算程序HVL-MFPPM,对Sandia实验室和中国工程物理研究院流体物理研究所冲击波物理与爆轰物理实验室进行的两个超高速发射实验装置进行数值模拟,前者给出的二级飞片前界面速度历史与VISAR实验测试和HVL-CTH程序计算结果一致,后者给出二级飞片稳定时的速度和飞行姿态与实验测试结果吻合,同时还给出了128个和216个CPU计算的界面形状.为进一步获得更高二级飞片速度的实验装置设计提供了理论和数值分析基础.     

厚度与加载速率对铁素体钢断裂韧性影响的SP压杆法实验研究

采用SP压杆实验方法,在常温下研究小圆薄片断裂韧性的厚度效应及加载速率对断裂特性的影响.实验结果表明,随着厚度的增加,断裂变形能增加,断裂部分的外表面因双向应力状态表现出微突起,微突起四周存在微小裂纹;随着加载速度的增加,断裂变形能增加,剪切断裂表面表现出从密集韧窝到韧窝连接成片特征.考虑试件变形过程中不同部分的能量耗散,从SP试件的整体断裂变形能得到试件的断裂韧性的宏观表达,断裂韧性随着厚度的增加而增加,随着加载速度的增加而减少.采用临界塑性断裂应变作为裂纹起裂判据,单位面积的能量耗散率作为裂纹扩展和失效判据的断裂模型,用有限元方法对SP压杆实验进行模拟,得到与实验结果比较相符的模拟结果.     

Experimental Analysis of High-Temperature Creep, Fatigue, and Damage. 2. Basic Laws

Experimental studies of high-temperature creep, long-term strength, and high-cyclic fatigue are reviewed. Emphasis is on data treatment methods, the substantiation and construction of unified deformation and fracture diagrams, and the development of a system of identifying experiments. New mechanical effects associated with high temperature and vibration are revealed and systematized     

小冲杆试验的有限元模拟中GTN模型参数的确定

GTN模型参数的选择一直是小冲杆模拟中一个至关重要的环节,其正确与否直接影响着模拟得到的载荷-位移曲线的正确性。以16MnR材料作为研究对象,结合拉伸试验与小冲杆试验的实验结果,逐步确定出有限元模拟中的GTN模型参数。本文重点介绍了参数fN,fc,ff的确定方法。结果发现,拟合的载荷位移曲线不具有唯一性。为了验证所用方法的正确性,增加了试样断后减薄率作为另一评判标准。这一做法后来证明是可行的。这说明载荷位移曲线不能作为判断GTN参数正确的唯一准则,而增加的断后减薄率作为另一判断标准,对此方法有很好的辅助作用。上述结果可为小冲杆实验的有限元模拟夯实基础。     

Mapping Multiple Components of the Residual Stress Tensor in a Large P91 Steel Pipe Girth Weld Using a Single Contour Cut

The contour method is applied in an innovative manner to measure the distribution of hoop residual stress in a large martensitic-ferritic steel pipe containing a multi-pass girth weld. First, a novel one-step wire electro-discharge machining cut is conducted to divide the pipe lengthways into two halves. The deformation of the cut halves is then measured and analysed in a way that simultaneously gives maps of hoop stress across the wall thickness on both sides of the pipe and automatically accounts for through-thickness hoop bending effects and how they may vary along the pipe. Finally the contour method results are combined with X-ray diffraction residual stress measurements using the principle of superposition to determine the distribution of the axial and radial residual stresses in the pipe. It is thereby demonstrated how the distribution of three direct components of the residual stress tensor in a welded pipe can be readily determined using a “hybrid” contour measurement approach.     

在役设备材料弹塑性力学参数测定方法综述--小冲杆实验力学研究进展之一

20多年以来,采用小型试件的小冲杆试验技术来测量在役设备材料的各种力学参数已经取得了很大进展,这个方法已经用来确定材料的弹性模量、屈服强度、塑性性能、抗拉强度、韧一脆转变温度、断裂韧度、蠕变性能和黏塑性性能等各种力学性能。由于从小冲杆试验的测量结果来确定材料的力学性能是一个反问题,因此,与此有关的反问题分析方法也得到了相应的发展。本文系统综述小冲杆试验的测量技术及从测量数据来确定材料弹塑性参数的各种经验方法和计算方法,例如有限元分析和参数法、反向有限元法、有限元和反方法、反向识别和人工神经网络、有限元优化和试验变形形状以及杂交反方法等。     

有初弯的刚度非对称转子的动力学特性分析及实验研究

对刚度不对称转子在有初始弯曲时的稳定性和动态响应性特性进行了理论分析和实验研究。重点研究了初弯方向与原始不平衡方向间夹角对系统响应特性的影响，提出了识别转子初弯和刚度不对称情形的特征量和诊断方法。研究表明：初始弯曲转子不仅在低转速时的振幅趋于初始弯曲值，而且随初始弯曲与不平衡间夹角的变化和初始弯曲与偏心距比值大小的变化振幅曲线出现很大的差异。特别是在大夹角情形下，夹角对振幅曲线的影响较大。当初始弯曲小于偏心距时，临界前振幅会随着转速升高而下降然后又上升，而当初始弯曲大于偏心距时，临办后振幅会随着转速升高而下降然后又上升。在临界前刚度不对称轩子的幅频曲线有副临界现象，转子运转频率处于两个自然频率间时系统会失稳，不平衡的增大对两类情况的幅频曲线变化趋势影响较大。     

不同骨料混凝土剪切断裂试验及数值分析

利用四点剪切加载的试验方法对不同骨料混凝土的双边切口试件进行了详细的试验研究。研究表明骨料不同的混凝土试件,其宏观断裂行为存在很大的不同,并且P-Δu曲线呈现出较大的差异,从而为进一步研究混凝土的断裂机理提供了参考。本文在试验研究基础上,建立了与试验模型相对应的三维有限元模型,利用有限元工程分析软件,对混凝土四点剪切的断裂力学行为进行了分析。结果表明,由于混凝土材料本身力学性能的特殊性-其抗拉强度较其抗压抗剪强度要低得多,所以裂纹尖端处的应力场分布状态是导致混凝土纯剪加载下的张拉破坏和剪切破坏的主要原因。