小试样高通量蠕变及蠕变裂纹扩展试验装置设计

对于某些取样受限的结构,如在役构件、薄壁件、焊接接头、功能性梯度结构,无法采用传统试样测试获得高温蠕变及裂纹扩展性能,小试样测试方法使得此类构件的高温力学性能的获取成为可能。但现有小试样蠕变试验装置用途单一且存在试样氧化的问题,无法满足试验要求。本文设计一种基于小试样的材料蠕变及蠕变裂纹扩展性能测试装置,装置配备专用夹具和真空系统,可满足不同种类小试样真空环境下的高温试验,避免试样氧化,并可同时完成6个同类或不同类型小试样的蠕变和蠕变裂纹扩展试验。装置采用马弗炉对试样加热,最高试验温度可达1 200℃。采用光栅位移传感器测量小试样变形量,直流电位法测量裂纹长度,提高了变形测量精度。试验结果表明,该装置可以精确测量小试样位移和裂纹长度,用以研究材料蠕变及裂纹扩展性能。     

含有Cu、Mo、Sn的高强度蠕墨铸铁的蠕变行为

研究了一种含有Cu、Mo、Sn的高强度蠕墨铸铁在623~823 K、40~150 MPa的蠕变行为,观察了不同形态的蠕变损伤组织并分析了蠕变变形及断裂机理。当T/T_m＞0.5(T为使用温度,T_m为蠕墨铸铁熔点)、载荷大于150 MPa时这种蠕墨铸铁的蠕变变形显著,且变形主要来自基体变形、蠕变空洞的形核长大以及石墨/基体界面的开裂。随着温度的提高和载荷的增加,蠕变变形逐渐由晶界移动转变为晶内变形。在蠕变过程中有两种开裂机制：(I)微裂纹在石墨/基体开裂处形核长大并优先沿铁素体向基体扩展,与邻近石墨/基体开裂连接而逐渐形成主裂纹;(II)晶界处的蠕变空洞形核长大转变成蠕变裂纹。氧原子通过石墨的连通性向组织内部扩散,造成上述两种裂纹表面氧化。由于,石墨、铁素体、珠光体三者性能的差异,石墨/铁素体界面比石墨/珠光体界面更易发生开裂。另外,在773 K、823 K组织中的珠光体分解明显,层片状渗碳体逐渐转变为短棒状,在晶界附近则以颗粒状为主。     

Ti5Al4Mo4Cr2Sn2ZrNb钛合金在三种热处理状态下的蠕变裂纹扩展

选用三种不同热处理制度，借助于直流电位法对Ｔｉ５Ａｌ４Ｍｏ４Ｃｒ２Ｓｎ２ＺｒＮｂ钛合金进行抗蠕变裂纹扩展性能的研究。结果表明，二次固溶＋时效处理者最佳，合金抗蠕变裂纹扩展的能力最强，抗蠕变断裂应力强度因子Ｋ也最高，蠕变断裂寿命最长，β处理的合金性能最差，退炎处理者介于两者之间。     

蠕变裂纹启裂和扩展的局部损伤理论

根据局部损伤理论和相似原理，给出了蠕谱裂纹启列前的孕育上下限时间并推导了蠕变裂纹扩展率方程。在常载荷下用２．２５Ｃｒ－１Ｍｏ钢作了蠕变裂纹扩展试验，高拘束度使早期局部损伤发展迅速，实验结果与孕育时间下限接近。稳态预测值由于忽略未开裂部分材质劣化而略氏于实验结果值。有限元模拟损伤演变过程。表明稳态裂纹扩展民实验结果吻合。     

基于C(t)积分及参数Ac的P92钢高温蠕变裂纹扩展研究

对于在高温环境下工作的构件,蠕变裂纹扩展是一种主要的失效机制,而裂纹尖端的拘束水平对蠕变裂纹扩展率有很大的影响。通过数值仿真与相关试验数据对比的方法,对裂纹扩展尖端的应力应变率场表征参量C(t)积分进行了相关研究,并基于参数Ac研究了P92材料裂纹尖端的拘束水平对蠕变裂纹扩展的影响。研究结果表明,C(t)积分值随裂纹扩展急剧减小,其数值及变化与积分路径到裂纹尖端的距离相关性很强,并且与拘束水平有一定的关系;拘束水平影响蠕变裂纹扩展率,拘束越大,裂纹扩展速率越快;参数Ac可以有效表征裂纹尖端拘束水平,其在寿命预测方面的应用有待进一步研究,同时在含裂纹的高温工作构件寿命评估方面有重大的意义。     

非匹配焊缝裂纹高温断裂力学参量的估算

焊接接头中的蠕变裂纹扩展受材料组分不均匀和坡口形式等多个因素影响，是一个高度复杂的力学问题。本研究提出了一个能综合反映各影响因素的等效蠕变应变速率概念和计算方法，并在此基础上建立了焊缝裂纹高温断裂力学参量C^*的工程计算式。紧凑拉伸试样的有限元分析表明，本研究方法估算的结果与数值解非常接近。     

P92钢蠕变-疲劳交互作用下的裂纹扩展行为EI北大核心CSCD

在630℃下,对P92钢进行应力控制下的蠕变-疲劳交互作用实验,研究P92钢高温蠕变-疲劳交互作用下的裂纹扩展行为,并结合断口形貌分析蠕变-疲劳裂纹扩展的机理以及a-N曲线的转折点含义。结果表明:P92钢在蠕变-疲劳交互作用下的断裂属于蠕变韧性断裂,应该用(C_t)_(avg)作为裂纹扩展的断裂参量;P92钢在蠕变-疲劳交互条件下,试样的断口主要表现为蠕变孔洞以及微裂纹。此外,发现a-lg(N_i/N_f)曲线以及(da-dN)-N曲线中的拐点,分别对应蠕变-疲劳裂纹萌生区向扩展区转变周次以及扩展区向瞬断区转变的周次。     

GH33A合金在蠕变-疲劳交互作用下的裂纹扩展特性

通过对 GH33A 合金在蠕变与疲劳复合加载条件下的系列试验,发现拉伸保时使蠕变与疲劳发生了交互作用,加快了疲劳裂纹扩展速率,加速裂纹早期进入失稳扩展,大大降低了疲劳寿命。GH33A 合金具有良好的抗蠕变裂纹扩展能力,但疲劳裂纹扩展阻力较低。由此讨论了拉伸保时对裂纹扩展的影响,并对在蠕变-疲劳交互作用下的裂纹扩展模型作了探讨。     

一种含4.5%Re/3.0%Ru的单晶镍基合金的高温蠕变行为

通过对含4.5%Re/3.0%Ru单晶镍基合金进行高温蠕变性能测试,并采用扫描电镜(SEM)、透射电镜(TEM)对不同蠕变期间的试样进行组织形貌观察,研究了该合金的高温蠕变行为。结果表明,本实验所选用的单晶合金在高温蠕变期间具有良好的蠕变抗力,在1040℃/160MPa的蠕变寿命达到725h。高温蠕变初期,合金中γ′相沿垂直于应力轴方向转变成筏状结构,其稳态蠕变期间的变形机制是位错在基体中滑移和攀移越过筏状γ′相。高温蠕变后期,合金的变形机制是位错在基体中滑移和剪切筏状γ′相。位错的交替滑移使筏形γ′相扭曲,并在γ/γ′两相界面发生裂纹的萌生与扩展直至断裂,是合金在高温蠕变后期的断裂机制。     

电站锅炉承压元件的寿命评估方法

综述了国内外迄今为止提出的关于电站锅炉承压元件在运行工况下寿命评估的主要方法.这些方法主要分为以传统持久强度试验外推为主的方法和以蠕变变形和裂纹扩展为主的方法,前者包括持久强度曲线外推法、时间-温度参数法和Robinson寿命消耗法则等;后者包括θ法、C射影法、蠕变曲线逐步外推法以及蠕变损伤开裂及裂纹扩展的评估方法等.     

Proposal of a new concept on creep fracture remnant life for a precracked specimen

Abstract

The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The Ω criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*_L derived from the paremeters Q* and Ω is investigated. The correlation between Q_L* and Ω provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.     

Creep Fracture of Nonlinear Viscoelastic Media Undergoing UV Radiation

Nonlinear viscoelastic relations to describe the mechanical behavior of polymers and polymer based composites are developed. The nominal stress - strain and creep curves for different values of the current coefficient of lateral deformation are constructed. To model some effects associated with lateral deformation, relations for a nonlinear composite formed of elastic and viscous components are formulated. They are applied to creep fracture of polymers subjected to ultraviolet radiation.     

An analysis of the creep embrittlement of Inconel alloy X-750 due to its prior exposure to reduced air pressure at 1150°C

Inconel alloy X-750 test pieces exposed to a reduced air pressure at 1150°C for various times exhibited significant embrittlement during subsequent creep tests in air at 700°C. This embrittlement was characterized by a systematic reduction in time and strain to fracture and an acceleration of the minimum creep rate with increasing time of prior exposure. Earlier work has shown that the near-surface environmental damage in the form of heavily cavitated boundaries and γ-free zones are responsible for this form of embrittlement. The purpose of this investigation is to provide a comprehensive analysis of the effect of the above two forms of damage on the time and strain to fracture and the minimum creep rate. Towards this end, a three-zone creep fracture model has been developed to explain the experimental results.     

Microstructural modelling of creep crack growth from a blunted crack

The effect of crack tip blunting on the initial stages of creep crack growth is investigated by means of a planar microstructural model in which grains are represented discretely. The actual linking-up process of discrete microcracks with the macroscopic crack is simulated, with full account of the underlying physical mechanisms such as the nucleation, growth and coalescence of grain boundary cavities accompanied by grain boundary sliding. Results are presented for -controlled mode I crack growth under small-scale damage conditions. Particular attention is focused on creep constrained vs. unconstrained growth. Also the effect of grain boundary shear stresses on linking-up is investigated through shear-modified nucleation and growth models. The computations show a general trend that while an initially sharp crack tends to propagate away from the original crack plane, crack tip blunting reduces the crack growth direction. Under unconstrained conditions this can be partly rationalized by the strain rate and facet stress distribution corresponding to steady-state creep. This revised version was published online in August 2006 with corrections to the Cover Date.     

蠕变断裂局部损伤方法的有限元模型

提出了蠕变断裂局部损伤方法的有限元模型，通过有局部薄弱环到引入损伤单元，较好地不了多向应力条件下蠕变损伤发生和发展的规律，应用一有限元模型分析缺口圆棒试样的蠕变变形与损伤，得到的计算结果与试验结果能够较好的相吻合。     

Estimation of the fracture front propagation time in structural components

We discuss special features of the fracture front propagation in structural components (variable-thickness disks and plates, internally pressurized thick-walled vessels). The upper and lower bounds of the relative time of the front propagation have been estimated. Some recommendations regarding the application of the results obtained are provided. __________ Translated from Problemy Prochnosti, No. 6, pp. 13–24, November–December, 2007.     

A STUDY ON THE ULTIMATE FRACTURE STRENGTH AND FRACTURE BEHAVIOR OF POLYMERIC MATERIALS——Ⅱ.A KINETIC MODEL THEORY OF CREEP TENSILE STRENGTH

本文对当前蠕变持久强度的表征进行了系统总结,指出了它是高分子材料的可形变性、分子链应力集中情况和应力集中链断链滑移动力学三者统一的体现。基于这种破坏概念,推导出一种能表征半晶态、玻璃态和弹性体等聚合物的蠕变持久强度动力学模型理论和一般化断裂状态方程。当该方程引入一、二级近似条件后,该式就分别还原为准脆性状态下粘弹性蠕变断裂方程,受迫高弹性状态下蠕变断裂方程和高弹性状态下蠕变断裂方程。     

Semianalytic Finite-Element Method in Continuum Creep-Fracture Mechanics Problems for Complex-Shaped Spatial Bodies and Related Systems. Part 3. Practical Applications

We use the developed procedure of solving 3D problems of continuum creep-fracture mechanics to solve practical problems of power machine building. We define mechanisms of the creep process in spatial complex-shaped bodies.     

Semianalytic Finite-Element Method in Continuum Creep Fracture Mechanics Problems for Complex-Shaped Spatial Bodies and Related Systems. Part 2. Analysis of Reliability of the Results and Efficiency of the Method,and Algorithms for Solving the Problems

We perform analyses of convergence and reliability of the results of solving continuum creep-fracture problems and efficiency of the semianalytic finite-element method for heterogeneous circular nonclosed bodies.     

PMMA断口形态及其破坏机理

本文通过对PMMA蠕变试件断口的SEM分析,把材料蠕变破坏过程中主裂纹的发展划分为三个阶段,提出了决定材料承载寿命的因素。