不同拘束条件下P92钢高温蠕变裂纹扩展速率的有限元模拟

基于蠕变损伤力学,仿真了蠕变裂纹扩展过程。用ABAQUS软件模拟研究了试样几何形状和宽度不同的两种面内拘束条件对P92钢在650℃时蠕变裂纹扩展(CCG)速率的影响,并进行了试验验证。仿真结果表明:试样几何形状和宽度对CCG速率的影响与载荷水平(C*)有关;在低载荷水平区内,C(T)试样的拘束水平大于SEN(T)试样,并且对应的CCG速率较大;在中载荷水平区,随着试样宽度的增大,裂纹尖端拘束和CCG速率增大;在高载荷水平区,CCG速率基本不受面内拘束(试样几何形状、宽度)条件的影响;在条件相同的情况下,试样宽度拘束水平大于试样几何形状。试验结果表明,试验与有限元分析(FEA)的蠕变裂纹扩展结果相符合。     

两种高温合金蠕变断裂行为的研究

一、引 言 从大量的高温故障分析可看到,断口往往是沿晶或混合型的特征。说明材料高温热强性不足,持久、蠕变裂纹成核和扩展是高温构件失效的一种方式。因此,研究材料在蠕变条件下裂纹扩展的规律,对于合理选材,改进工艺,确保安全,延长寿命都有一定的实际意义。 七十年代以来,用断裂力学方法已成功地研究了疲劳断裂问题,现已进入了定量分析,对构件的剩余寿命作出估计的阶段。从高温故障中,人们开始注意到对高温蠕变条件下断裂     

高温蠕变裂纹扩展参量Q*(t)及其应用

针对C^*参数的不足，本研究修正Q^*并得到了一个新的蠕变裂纹扩展速率(CCGR)控制参数Q^*(t)。该参数综合了温度、应力场、激活能等影响因素，很好地表征了在不同应力或温度条件下蠕变裂纹扩展的特性。该参数能完整关联整个蠕变裂纹扩展的不同阶段，并从材料蠕变断裂的机制上反映了蠕变裂纹扩展的本质。     

高温合金CT试样裂纹前沿应力分布及第二相影响的模拟计算

利用有限元方法分析了IN718合金CT试样裂纹区域的应力应变分布、裂纹尖端应力强度因子及第二相状态对它的影响，计算结果与实测数据基本一致，表明采用有限元方法研究含裂纹体构件的蠕变行为是一种较好的研究手段。通过计算认为，IN718合金在时效初期虽然有α-Cr相的析出而不至于促进蠕变裂纹扩展，是由于α—Cr于δ相周围γ″贫化区(微塑性区)内析出而处于微塑性区内使得裂纹尖端钝化，不利于裂纹扩展。     

多耦合拘束效应对P92钢蠕变裂纹扩展行为的影响

基于断裂力学理论,针对不同面内拘束效应下P92钢高温蠕变裂纹扩展(Creep crack growth,CCG)进行了数据模拟与分析。研究发现不同的拘束效应对CCG行为有着各异的影响,主要分析了试样几何形状、尺寸以及初始裂纹深度等对CCG行为的影响,并且着重研究比较这3种耦合拘束效应对CCG行为的影响程度。基于多种耦合拘束对CCG行为的影响程度做了横向比较,研究发现:试样几何形状对CCG行为的影响程度要大于几何尺寸,试样尺寸和初始裂纹深度对CCG行为的影响程度相似,试样形状比初始裂纹深度的影响程度更大。     

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。     

循环载荷对裂纹扩展和持久寿命的影响

对铁基高温合金ＧＨ２１３２在各种最大试验载荷下的纯蠕变和循环蠕变寿命以及裂纹扩展断裂行征进行分析，结果表明，在裂纹扩展不同时期（σ〈σ０．２和σ〉σ０．２）循环加载对裂纹扩展的影响相反，而对最终持久寿命的影响则取决于这两种相反作用之总的效果。     

GH36带缺口试样蠕变裂纹扩展的动态观察及机理的初探

本文通过高温显微镜在连续加载的条件下对 GH36带缺口试样蠕变裂纹扩展过程进行了观察,结果发现裂纹尖端邻近区域晶界变粗处品粒内无变化,而晶粒内出现滑移变形的晶界无变粗迹象,同时发现蠕变裂纹沿晶扩展至锯齿晶界时被迫转为穿晶扩展。本文分析这些微观现象产生的原因,并对蠕变裂纹扩展理论进行初探。     

线弹性断裂力学问题的扩展自然单元法

为了更加有效地求解线弹性断裂问题,提出了扩展自然单元法。该方法基于单位分解的思想,在自然单元法的位移模式中加入扩展项表征不连续位移场和裂纹尖端奇异场。通过水平集方法确定裂纹面和裂纹尖端区域,并基于虚位移原理推导了平衡方程的离散线性方程。由于自然单元法的形函数满足Kronecker delta函数性质,本质边界条件易于施加。混合模式裂纹的应力强度因子由相互作用能量积分方法计算。数值算例结果表明扩展自然单元法可以方便地求解线弹性断裂力学问题。     

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.     

Three‐dimensional analyses of unified characterization parameter of in‐plane and out‐of‐plane creep constraint

Based on extensive three‐dimensional finite element analyses, the unified characterization parameter A_c of in‐plane and out‐of‐plane creep constraint based on crack‐tip equivalent creep strain for three specimen geometries (C(T), SEN(T) and M(T)) were quantified for 316H steel at 550 °C and steady‐state creep. The distributions of the parameter A_c along crack fronts (specimen thickness) were calculated, and its capability and applicability for characterizing a wide range of in‐plane and out‐of‐plane creep constraints in different specimen geometries have been comparatively analysed with the constraint parameters based on crack‐tip stress fields (namely R*, h and T_Z). The results show that the parameter A_c in the centre region of all specimens appears uniform distribution and lower value (higher constraint), and in the region near free surface it shows protuberant distribution and higher value (lower constraint). The parameter A_c can simultaneously and effectively characterize a wide range of in‐plane and out‐of‐plane creep constraints, while the parameters R*, h and T_Z based on crack‐tip stress fields cannot achieve this. The different capabilities of these parameters for characterizing in‐plane and out‐of‐plane creep constraints originate from their underlying theories. The parameter A_c may be useful for accurately characterizing the overall constraint level composed of in‐plane and out‐of‐plane constraints in actual high‐temperature components, and it may be used in creep life assessments for improving accuracy.     

Effects of side‐groove depth on creep crack‐tip constraint and creep crack growth rate in C(T) specimens

The effects of side‐groove depth on creep crack‐tip constraint and creep crack growth (CCG) rate in C(T) specimens have been quantitatively studied. The results indicate that with increasing side‐groove depth, the constraint level and CCG rate increase and constraint distribution along crack front (specimen thickness) becomes more uniform. The constraint and CCG rate of thinner specimen are more sensitive to side‐groove depth. Two new creep constraint parameters (namely R^* and A_c) both can quantify constraint levels of the specimens with and without side‐grooves, and the quantitative correlations of CCG rate with constraint have been established. The mechanism of the side‐groove depth effect on the CCG rate has also been analyzed.     

Effects of creep properties of materials on unified creep constraint parameter Ac for cracked pipes

Extensive finite element analyses of cracked pipes with different crack sizes and orientations have been conducted to investigate effects of creep properties of materials on the unified creep constraint parameter A_c. The results show that the constraint parameter A_c is independent on Norton’s coefficient A, and it is only affected by the creep exponent n of materials. For a given crack size, with increasing n, A_c decreases and constraint level increases. The A_c of lower constraint cracks is more sensitive to n. The unified correlation equations between A_c and n have been obtained for cracked pipes with a wide range of crack sizes and constraint levels. They may be used to estimate the constraint parameter A_c at different positions along the crack fronts in cracked pipes made of materials with different n values. The two-parameter C*-A_c approach for assessing creep life of cracked pipes has also been discussed.     

Inclusion of primary creep in the estimation of the C t parameter

The problem of time-dependent fracture under transient creep conditions is investigated via finite element analyses of fracture specimens with stationary cracks. The constitutive models consist of linear elasticity with combinations of power-law secondary creep and two primary creep laws. Two proposed parameters are studied. One is a contour integral, C(t), which characterizes the crack tip singularity strength. The other one, C _t, is evaluated based on the load line deflection rate and has been used successfully in correlating experimental creep crack growth data.It is evident that accurate constitutive modeling is essential to good agreement with experimental data. The inclusion of primary creep resolves earlier discrepancies between the experimental and analytical load line deflection rates which are used to calculate the respective values of C _t. The loading boundary condition is also an important factor that has been addressed. A more general formulation of C _twhich includes primary creep is presented. In small scale and transition creep, the C _tparameter does not characterize the crack tip stress singularity but rather is related to the crack tip creep zone growth rate. At times past transition time, C _tand C(t) both approach a path-independent integral, C ^*(t), which characterizes the stationary crack tip stress field. The relationship between C _tand C(t) is discussed. The interpretation and estimation of the C _tparameter are given based on the numerical results and analytical manipulations.     

Numerical investigation of creep crack growth in cross‐weld CT specimens. Part II: influence of specimen size

A numerical investigation of the influence of specimen size on creep crack growth in cross‐weld CT specimens with material properties of 2.25Cr1Mo at 550 °C is performed. A three‐dimensional large strain and large displacement finite element study is carried out, where the material properties and specimen size are varied under constant load for a total of eight different configurations. The load level is chosen such that the stress intensity factor becomes 20 MPa √m regardless of specimen size. The creep crack growth rate is calculated using a creep ductility‐based damage model, in which the creep strain rate ahead of the crack tip perpendicular to the crack plane is integrated taking the degree of constraint into account. Although the constraint ahead of the crack tip is higher for the larger specimens, the results show that the creep crack growth (CCG) rate is higher for the smaller specimens than for the larger ones. This is due to much higher creep strain rates ahead of the crack tip for the smaller specimens. If, on the other hand, the CCG rate is evaluated under a constant C * condition, the creep crack growth rate is found to be higher for the larger specimens, except when the crack is located in a HAZ embedded in a material with a lower minimum creep strain rate; then, the creep crack growth rate is predicted to be higher for the smaller specimen. In view of these results, it is obvious that the size effect needs to be considered in assessments of defected welded components using results from CCG testing of cross‐weld CT specimens.     

Effect of loading rate on dynamic fracture initiation toughness of brittle materials

Numerical simulation is carried out to investigate the effect of loading rate on dynamic fracture initiation toughness including the crack-tip constraint. Finite element analyses are performed for a single edge cracked plate whose crack surface is subjected to uniform pressure with various loading rate. The first three terms in the Williams’ asymptotic series solution is utilized to characterize the crack-tip stress field under dynamic loads. The coefficient of the third term in Williams’ solution, A ₃, was utilized as a crack tip constraint parameter. Numerical results demonstrate that (a) the dynamic crack tip opening stress field is well represented by the three term solution at various loading rate, (b) the loading rate can be reflected by the constraint, and (c) the constraint A ₃ decreases with increasing loading rate. To predict the dynamic fracture initiation toughness, a failure criterion based on the attainment of a critical opening stress at a critical distance ahead of the crack tip is assumed. Using this failure criterion with the constraint parameter, A ₃, fracture initiation toughness is determined and in agreement with available experimental data for Homalite-100 material at various loading rate.     

Numerical investigation of creep crack growth in cross-weld CT specimens. Part I: influence of mismatch in creep deformation properties and notch tip location

Creep crack growth (CCG) in cross-weld CT specimens is investigated using two-dimensional finite element simulations. A creep ductility-based damage model describes the accumulation of creep damage ahead of the crack tip where a constraint parameter and the creep strain rate perpendicular to the crack plane are used as characterizing parameters.
The numerical results reveal that, not only the material properties of the region in which the crack is propagating, but also the deformation properties of the surrounding material influence the CCG behaviour. For the specimen configurations investigated, the location of the starter notch in the HAZ of the cross-weld CT specimen has, however, a minor influence on the CCG rate and the value of C *. This applies as long as the crack is propagating within a sufficiently narrow region that has material properties which can be regarded as homogeneous.     

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C^* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30 mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.     

Characterization of the nonlinear fracture resistance parameters for an aviation GTE turbine disc

The effects of crack growth rate model formulation, based on the elastic‐plastic and undamaged/damaged creep crack tip fields on the behaviour of low‐cycle fatigue and creep fracture resistance parameter behaviour, are represented by numerical calculations. The crack growth rate models include the fracture process zone size and damage parameters. An aviation gas turbine engine (GTE) rotating turbine disc is the focus of this innovative application of basic analytical and numerical solutions. For the GTE turbine disc, the constraint parameters, local fracture process zone sizes, and nonlinear plastic (K_p) and creep (K_cr) stress intensity factors are calculated by finite element analysis to characterize the fracture resistance along the semielliptical crack front as a function of the flaw aspect ratio, operation temperature, and disc rotation speed. Predictions of the creep‐fatigue crack growth rate and residual lifetime are given for different combinations of operation loading conditions and damage of the GTE turbine disc.     

Correlation of material constraint with fracture toughness of interface regions in a dissimilar metal welded joint

In this work, the constraint parameter A_p based on crack‐tip equivalent plastic strain was calculated by finite element analyses for the cracks located at different locations in two interface regions in a dissimilar metal weld joint (DMWJ). The capabilities of the parameter A_p for characterizing material constraint and establishing correlation of material constraint with fracture toughness of the interface region cracks have been examined. The results show that the parameter A_p can characterize material constraint effect caused by material mismatch and initial crack positions in the interface regions. Based on the A_p, the correlation lines and formulae of material constraint with fracture toughness of the interface region cracks in the DMWJ can be established, and they may be used for obtaining material constraint‐dependent fracture toughness for the interface region cracks. The results in this work combining with those in the previous studies indicate that the parameter A_p may be a unified constraint parameter that can characterize both geometry constraint (including in‐plane and out‐of‐plane constraints) and material constraint, and it may be used in accurate fracture assessments of welded components with different geometry and material constraints.