P92钢等离子弧焊接接头原位拉伸的SEM观察研究

针对6 mm厚P92钢等离子弧焊接接头,利用带动态拉伸台的高分辨扫描电镜对接头的不同区域进行原位拉伸实验,通过试样动态断裂过程观察并结合断口分析研究了P92钢等离子弧焊接接头各区的微观断裂机理.结果表明:接头焊缝区在拉伸过程中经历了前期的钝化伸张区之后,裂纹起裂于中间相M23 C6处,而焊缝区的裂纹扩展路径沿着中间相M23 C6前进,并在断面上形成了撕裂脊,属于穿层断裂;热影响区是P92焊接接头的最薄弱环节,裂纹起裂于中间相M23 C6处,其断口起裂的孕育阶段基本没有前期的钝化,即前期的塑性裂纹的扩展长度Xf=0,裂纹以沿层断裂模式进行扩展,在断口处有分层现象,最终断面呈现出裂纹以解理断裂模式进行扩展;母材区的拉伸试样在经历了钝化伸张区之后,前期的裂纹以塑性断裂模式进行扩展,裂纹在扩展过程中遇到较多的中间相M23 C6时形成应力集中,同时在其附近出现空洞,此时裂纹以解理断裂模式进行扩展,属于沿层断裂.     

镍基单晶合金紧凑拉伸试样高温断裂特性的实验研究

采用紧凑拉伸（CT）试样对三种晶体取向[001]，[011]，[111]的镍基单晶合金DD3在950，850℃和760℃下的拉伸以及950℃时的蠕变和疲劳性能进行了实验研究。应用光学显微镜（OM）和扫描电子显微镜（SEM）分别对单晶体裂纹扩展路径和断口表面进行了观察和分析。拉伸实验结果表明：单晶体裂纹扩展路径沿着特定的晶体学矢量方向扩展而在试样自由表明呈现Z字型外观，其扩展方向及形状取决于晶体取向。温度对试样的断裂形式影响较为显著，760℃时的断裂特征为剪切型脆性断裂，断裂平面为沿滑移面的光滑斜断口；950℃下试样断裂逐渐转变为微孔聚集型韧性断裂，断口由粗糙的剪切唇和纤维区构成。蠕变和疲劳实验结果表明：镍基单晶具有明显的蠕变和疲劳性能各向异性，蠕变寿命以[011]，[111]，[001]顺序依次减小，疲劳寿命以[111]，[011]，[001]顺序依次减小；蠕变及疲劳裂纹扩展路径均与加载方向垂直，断裂表面均为光滑的平断口。试样断口细观分析显示，蠕变试样断口由杯锥状韧窝组成，而疲劳试样断口则由疲劳裂纹组成。疲劳损伤比蠕变损伤更有利于裂纹的扩展。     

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。     

T/P92钢焊接接头蠕变损伤的形成

对T/P92钢焊接接头进行高温持久试验,采用金相检验、扫描电镜分析、蠕变孔洞的定量分析等方法,对T/P92钢蠕变损伤的形成进行了分析。结果表明:T/P92钢焊缝细晶区内孔洞的合并并不是一个瞬间完成的过程,而是单独的孔洞持续长大,同时孔洞间距逐渐减小,相临近的孔洞相互连接并扩展成微裂纹,最终T/P92钢焊接接头发生蠕变损伤。     

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。     

P92钢高温拉伸断口形貌的研究

采用热模拟法进行600~1300℃温度区间P92钢的高温拉伸实验。利用SEM,LSCM对不同拉伸温度下的断口形貌及近断口组织进行分析,并对P92钢的力学性能进行研究。结果表明:P92钢拉伸时,抗拉强度由467.32MPa下降到24.32MPa,屈服强度由56.88MPa下降到1.07MPa;不同拉伸温度下,断口表现以韧性断裂为主,韧性与脆性特征共存的现象。在600~900℃时,P92钢发生了动态回复过程,断口形貌表现为韧窝特征。冷却至室温,P92钢近断口处组织均为马氏体+残余奥氏体组织+M7C3+MC+M23C6+M6C+M3C型碳化物。随着温度升高,P92钢发生了动态再结晶,断口形貌以塑孔为主。P92钢近断口处组织为马氏体+残余奥氏体组织+MC+M6C型碳化物。     

S135钻杆钢的拉扭复合加载疲劳行为

采用疲劳实验和回归分析相结合的方法,研究了S135钻杆钢在拉扭复合加载条件下的疲劳行为,并对疲劳断口进行了微观分析。结果表明:当τa/σeq=0.7时,由拉扭应力幅对应的当量应力表示的疲劳寿命公式可很好地描述S135钻杆钢的拉扭疲劳寿命规律;疲劳断口由疲劳源区、疲劳裂纹稳定扩展区和快速瞬断区组成,疲劳裂纹从试样表面萌生,并向试样内部扩展,且常为多疲劳源,不同疲劳源断口的连接和复合加载形成所谓的"屋脊"状特征;拉扭疲劳断裂试样裂纹源区的微观断口特征为明显的河流花样,裂纹扩展区的微观断口特征为疲劳条带与涟波状花样。     

TNM-Ti Al合金室温高周疲劳性能研究

采用升降法与成组法对TNM-TiAl合金试样进行了应力比R=-1的室温拉压疲劳和R=0.1的室温拉伸疲劳试验，得到TNM-TiAl合金的P-S-N曲线，并对断口进行了分析。结果表明:TNM-TiAl合金对应力十分敏感，R=-1和R=0.1时的曲线整体呈较为平直的斜线，R=-1时的疲劳极限为414.7 MPa,R=0.1时的疲劳极限为285.6 MPa。R=0.1的S-N曲线远低于R=-1的S-N曲线;R=-1时，应力幅与疲劳寿命的关系满足Basquin方程。疲劳试件宏观断口较为粗糙，静态拉伸宏观断口平整，两者差异较大。拉伸断口整体分为裂纹萌生区与扩展区，其中起裂源均位于试样表面或板状试件的边角棱线处，起裂源区域包括γ相的解理断裂面、片层团的沿层解理面以及β₀相平整的穿晶断裂平面等特征。疲劳断口整体分为裂纹萌生区、扩展区与瞬断区，其中裂纹萌生区分为表面沿层起裂和γ相起裂。TNM-TiAl合金的疲劳断裂为脆性断裂，主要体现在扩展区上大量的片层团穿层断裂、扭折撕裂、γ相解理断裂和β₀相穿晶断裂。同寿命量级下，R=-1的断口与R=0.1的断口断裂类型...     

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。     

DD6单晶高温合金振动疲劳性能及断裂机理

研究[001]取向的DD6单晶高温合金的室温振动疲劳S-N曲线,并获得了其室温振动疲劳极限。利用体视显微镜、扫描电子显微镜、背散射衍射等手段对DD6单晶高温合金振动疲劳断裂机制进行分析。结果表明:采用S-N法估算得到的[001]取向的DD6单晶高温合金室温振动疲劳极限约为337.5MPa。振动疲劳裂纹断口呈现单个或多个沿{111}晶体学扩展平面组成的形貌特征,断口上分为疲劳源区和疲劳扩展区两个阶段,裂纹在应力最大截面处的表面或内部缺陷处萌生,呈单源特征,疲劳扩展区呈现类解理断裂特征,未出现典型的疲劳条带特征。说明沿{111}晶面滑移是DD6单晶高温合金室温振动疲劳断裂的主要变形机制,断口上的类解理扩展平面以及微观上类解理花样是DD6单晶高温合金室温振动疲劳断裂的主要特征。     

CREEP-FATIGUE CRACK GROWTH AND FRACTOGRAPHY OF A TYPE 304 STAINLESS STEEL AT ELEVATED TEMPERATURE

Abstract— Creep-fatigue crack growth behaviour of a Type 304 stainless steel under four types of reversed loading patterns (P-P, P-C, C-P and C-C) was investigated and the results are discussed in the light of fracture mechanics and fractography. The crack growth rate for all of the four types of loading was successfully correlated in terms of the cyclic integral range λ J. It was unnecessary, for practical purpose, to divide Ay into a fatigue component, λ J _f, and a creep component, λ J _c, as has been done elsewhere. The transition of the correlating fracture mechanics parameter from fatigue to creep was not necessarily associated with the fracture morphology. This was related to the longer transition hold time in morphology in C-C type loading compared to C-P type loading, and was attributed to recovery of grain boundary sliding during the compression hold in the C-C type loading.     

INFLUENCE OF CREEP-FATIGUE INTERACTION ON HIGH TEMPERATURE FATIGUE CRACK GROWTH BEHAVIOR of Ti-1100

Abstract— A series of crack growth experiments has been preformed on the near alpha titanium alloy, Ti-1100, to determine the mechanism of the creep-fatigue interaction. Based on pure creep crack growth results, the increase in the creep-fatigue crack growth rate is not amenable to separate contributions of creep crack growth and fatigue crack growth.
A mechanism has been proposed to account for the increase in creep-fatigue crack growth rate that is based on the planar slip of titanium alloys which results in the formation of dislocation pileups at the prior beta grain boundaries and leads to intergranular fracture. This mechanism has been validated through crack growth experiments preformed on a Ti-1100 that has been microstructurally modified through the precipitation of internal slip barriers. These show that the intergranular fracture and increase in crack growth rate are absent.     

CREEP-FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr-1Mo-0.25V STEEL. PART II: CRACK GROWTH BEHAVIOR AND MODELS

Abstract— Creep-fatigue crack growth (CFCG) behavior of an ex-service 1Cr-1Mo-0.25V steel was investigated for hold times of 100 s, 15 min, and 8 h for a trapezoidal loading waveform at a temperature of 538°C. The correlation of the crack growth rate with ( C _t)_avg for various hold times was significantly improved when an appropriate estimation scheme, previously proposed by the authors, was used to estimate this creep fracture mechanics parameter for this material. Crack growth data under creep-fatigue conditions and analysis procedures used to reduce them are described in detail in this paper.     

The crack initiation and growth under high temperature creep, fatigue and creep-fatigue multiplication

This article concerns some of our recent studies on the crack initiation, early stage crack growth and its subsequent crack growth under high temperature creep, fatigue and creep-fatigue multiplication. The criteria for these and some new ideas are proposed. For instance, the relative notch opening displacement (RNOD) criterion for the crack initiation and the Q^* parameter for the crack growth are critically reviewed. Early stage crack growth and its subsequent crack growth as affected by notch tip acuity were studied. The behaviour of the tail part in the log da/dN vs log C^* curve has been attempted to explain in terms of the curve of the creep behaviour and of the crack length against time. Furthermore it was proposed that early stage crack growth, say, the so-called first stage crack growth in terms of log da/dN vs log K curve may be characterized by the parameter different from those for the so-called second stage crack growth.     

Studies on crack growth rate under high temperature creep,fatigue and creep-fatigue interaction—I: On the experimental studies on crack growth rate as affected by aσg,σg and temperature

Many experimental studies have been reported on the measurements of crack growth rate and the observation of crack growth behaviour under high temperature creep, fatigue and creep-fatigue interaction in literatures. However, many of them have been done in air atmosphere. Furthermore, in many of them the measurements of the crack growth rate have been carried out by interrupting intermittently the running of the testing machine. In such experiments the complex effects due to the atmosphere, the interruption period and the corresponding unloading operation for the crack length measurement might have been involved.In the present paper in order to eliminate such effects, series of experimental studies on the crack growth behaviour under creep, fatigue and creep-fatigue interaction conditions on 304 stainless steel have been carried out by using high temperature microscope and observing the crack length continuously during running the test without interruption in vacuum of 10^?5mm Hg.Among the results, it was found that crack growth rates on a time basis, $\text{da/dt}$, under high temperature creep and creep-fatigue interaction conditions can not be described in terms of solely elastic stress intensity factor $\text{k}{}_{\mathrm{i}}$ or only net section stress σ_net, both independent of gross section stress σ_g. The relation between crack growth rate and stress intensity factor under high temperature fatigue condition changes with some trend according to gross section stress at lower $\text{K}{}_{\mathrm{I}}$ level and it can be approximately described in terms of stress intensity factor $\text{K}{}_{\mathrm{I}}$ only, at higher $\text{K}{}_{\mathrm{I}}$ level. The threshold stress intensity factor and the threshold net section stress under high temperature creep, fatigue and creep-fatigue interaction conditions appears to be almost independent of temperature.     

Creep fatigue crack growth in 0·5Cr–Mo–V steel

Abstract

An investigation is presented of crack growth in a normalised and tempered 0·5Cr–Mo–V steel under cyclic displacement controlled loading conditions at 565–600°C. A transition from fatigue to creep dominated behaviour was observed as the duration of the tensile dwell period in the cycle was increased. This change was a result of a progressive increase in the extent of crack tip grain boundary damage accumulation which, in the long dwell tests, was sufficient to give rise to crack extension directly. Time dependent crack propagation rates during the dwells of the long dwell tests were found to approach those determined for static load conditions. No evidence was found for a significant creep-fatigue interaction and it appears that overall crack growth rates are determined by crack tip oxidation and damage accumulation processes.

MST/756     

CRACK GROWTH MORPHOLOGY AND MICROSTRUCTURAL CHANGES IN 316 STAINLESS STEEL UNDER CREEP-FATIGUE CYCLING

Abstract— A study into microstructural effects and crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550°C within the high strain ranges of 0.9–2.5%, including a 60min hold time, was undertaken on a high-temperature reverse-bending rig. Throughout the tests, surface cracks on both the tensile-hold and the compressive-hold sides were monitored by means of a plastic-strip replication technique. Additional investigations were conducted on failed specimens to examine the crack morphology in the depth direction, and to examine the function of oxidation; also to study changes of fracture surface morphology, changes in dislocation structures and precipitate configurations corresponding to the different strain ranges. These detailed analyses revealed that the predominantly intergranular long cracks on the tensile-hold side and transgranular short cracks on the compressive-hold side are dominant aspects of the investigation. The dislocation structures under creep-fatigue conditions are strain-range dependent, with a clearly defined cell structure at the higher strain ranges and dense dislocation tangles at lower strain ranges. The large reduction in creep-fatigue endurance can be attributed to early crack growth and grain boundary cracking caused by stress relaxation, oxidation, precipitation and, most importantly, the coalescence of the many minor surface short cracks.     

Isochronous creep rupture loci on deviatoric plane and its application to P92 steel creep behaviour under multiaxial stress state

The transformation relationship of the coordinate variables between principal stress space and deviatoric stress plane has been deduced and the isochronous creep rupture loci of disparate criteria have been described on deviatoric stress plane so as to analyze the creep behaviour under multiaxial stress state. The creep experiments of P92 steel smooth and notched specimens subjected to various stresses at 650 °C have been conducted. A modified constitutive model for the creep of P92 steel has been proposed and used to simulate the creep of P92 steel notched specimens with FEA software. The FEA results were consistent with the experimental data and the fracture morphology observation. It was found that the Hayhurst criterion had the best correlation with the experimental results of P92 steel under multiaxial stress state than other criteria through the comparison of the isochronous creep rupture loci on deviatoric plane.     

LIFE PREDICTION OF 316 STAINLESS STEEL UNDER CREEP-FATIGUE LOADING

Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a “combined creep-fatigue loading test” was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.     

Damage model of creep-fatigue interaction

The interaction between creep and fatigue has been studied theoretically by considering a macroscopic crack, interacting with continuously distributed microdamage. This damage is a measure of an assumed deterioration of the material. A Dugdale crack model is used with most deformation and damage concentrated to narrow regions ahead of the crack tips. As studied previously, in the pure creep case, the method predicts the creep rupture curve, well known from creep rupture tests. In the pure fatigue case, the method predicts Paris' law for fatigue crack growth with an exponent approximately equal to four and a finite fatigue lifetime. In the creep-fatigue interaction case, studied here for different material parameters and external load levels, the method always predicts an interaction stronger than the linear creep-fatigue interaction.