【作者】 王岩；

【导师】 罗忠兵；

【作者基本信息】 大连理工大学 ， 材料科学与工程， 2023， 硕士

【摘要】 奥氏体不锈钢（Austenitic stainless steel,ASS）因其优良的强韧性匹配和耐腐蚀性,被广泛应用于化工、核电等工业领域。但由于其极端的工作环境,易导致疲劳等力学损伤,材料性能劣化,从而严重降低构件的承载性能,甚至危及人民生命和财产安全。因此,能否在早期及时发现ASS构件力学损伤并妥善处理是工程安全管理的重中之重。近年来,基于超声的无损检测技术在金属材料力学损伤评价方面应用潜力突出,已经在早期疲劳损伤评价方面取得了一些成果,但对于超声响应与疲劳损伤之间的内在机制尚不明确。本研究基于ABAQUS有限元模拟与试验测试,借助Z2CND18.12N晶粒取向和位错结构演变分析,建立考虑早期损伤的多晶弹性各向异性超声传播模型,结合不同损伤阶段的声学信号分析,阐明超声波在ASS早期疲劳损伤过程中的响应机制。主要内容及结论如下:（1）通过低周疲劳试验,结合电子背散射衍射（Electron back-scatter diffraction,EBSD）和透射电镜（Transmission electron microscope,TEM）分析,发现Z2CND18.12N钢主要经历循环硬化、循环软化和二次硬化三个阶段。EBSD中局域取向差（Local misorientation,M_L）平均值随循环加载周次增加呈先上升后下降最后再上升的趋势,且与塑性应变幅之间呈负相关关系。循环硬化阶段位错快速增殖并相互缠结,循环软化阶段位错缠结程度加深形成位错脉络和位错脉络等,二次硬化阶段材料内形成孪晶和马氏体等组织。（2）利用脉冲反射法和临界折射纵波(Critically refracted longitudinal wave,L_CR)检测法对疲劳试样进行表征,脉冲反射法的声传播速度、声衰减系数与循环加载周次之间为非线性关系,主要受晶界和位错等微观结构演变的协同影响。L_CR波检测法的归一化幅值差(A_dif)在对数坐标系下随循环加载周次呈现一定波动,在初期和后期均呈递增趋势,显示出强的早期损伤敏感性。（3）通过泰森多边形建立了弹性各向异性L_CR波传播有限元模型（随机取向）,优选有限元模型参数,阐明声场成分。对于5 mm厚Z2CND18.12N钢板,其声场主要由顺序进入接收探头的L_CR波、折射纵波经底面反射波和折射横波经底面反射波构成。（4）考虑循环加载导致的晶粒旋转,利用Bond变化法计算对应弹性变化,建立不同疲劳状态下晶粒旋转多晶超声传播有限元模型,研究早期损伤的超声波响应规律。计算结果表明,考虑晶界散射的声衰减系数随循环加载周次的变化也呈先下降再上升后下降的趋势,与实验结果变化趋势一致,表明晶粒旋转是影响超声传播的主要因素之一。对L_CR波模拟信号分别计算提取的A_dif和RR_dif,两者变化趋势大致相同,后者表现出高的灵敏度。（5）基于位错弦模型计算了晶粒内部位错变化导致的声学响应变化,发现随着循环加载周次的增加,基于位错的衰减系数呈现先下降后上升的趋势。综合考虑晶粒内部位错变化与晶粒旋转对超声波传播的叠加效应,计算声衰减系数结果与实验测得L_CR波幅值变化趋势基本一致。说明在Z2CND18.12N早期疲劳损伤中,晶粒旋转和位错演变共同影响超声波的传播行为。更多还原

【Abstract】 Z2CND18.12N nitrogen-controlled austenitic stainless steel is the main steel used for the surge pipeline in pressurized water reactor due to its excellent mechanical properties,corrosion resistance and welding properties.However,the steel is prone to occur fatigue damage under cyclic loading during long-term service in harsh environments.It will lead to structural damage and performance degradation of materials,thereby seriously reduce the bearing performance of components,and even endanger the safety of human beings and property.Therefore,whether the early-stage of components mechanical damage can be detected in time and handled properly is the top priority of engineering safety management.In recent years,non-destructive testing（NDT）technique based on ultrasonic has prominent potential in the application of mechanical damage assessment of metal materials,and some achievements have been achieved in early-stage fatigue damage assessment.However,the underlying mechanism linking ultrasonic response and mechanical damage remains unclear.The ultrasonic propagation law of Z2CND18.12N steel was discussed,based on ABAQUS finite element simulation and experimental testing,combined with the Voronoi diagram and the propagation model of critically refracted longitudinal(L_CR)wave.The main contents and conclusions are as follows:（1）Through low-cycle fatigue tests,combined with electron back-scatter diffraction（EBSD）and transmission electron microscopy（TEM）analysis,it was found that Z2CND18.12N steel mainly undergoes three stages:cyclic hardening,cyclic softening,and secondary hardening.The average value of local misorientation（M_L）in EBSD shows a trend of first increasing,then decreasing,and finally increasing with the increase of loading cycles,and is negatively correlated with the plastic strain amplitude.Dislocations proliferate rapidly and tangle with each other in the cyclic hardening stage,and the degree of dislocation entanglement deepens in the cyclic softening stage to form dislocation veins,etc.Twins and martensite and other structures are formed in the material at the secondary hardening stage.（2）The fatigue specimens were characterized using pulse-echo method and L_CR detection method.The relationship between attenuation coefficient,velocity,and loading cycles of the pulse-echo method is nonlinear,mainly influenced by the synergistic effect of microstructure evolution such as grain boundaries and dislocations.The normalized amplitude difference（Adif）of the LCR wave method shows a certain fluctuation with cyclic loading cycles in the logarithmic coordinate system,and shows an increasing trend in the initial and later stages,with a significant increase in sensitivity to early damage.（3）A finite element model of elastic anisotropic L_CR wave propagation was established using the Voronoi polygon.For a 5mm thick Z2CND18.12N steel plate,the sound field is mainly composed of L_CR waves,refracted longitudinal waves reflected by the bottom,and refracted transverse waves reflected by the bottom.（4）Considering the grain rotation caused by cyclic loading,the Bond variation method is used to calculate the corresponding elastic constant changes.A finite element model of polycrystalline consider grain rotation under different fatigue states was established to study the ultrasonic propagation behavior of early damage.The calculation results indicate that the attenuation coefficient considering grain boundary scattering also shows a trend of first decreasing,then increasing,and finally decreasing with the loading cycles.The pulse-echo simulation signal,after processing,shows a consistent trend with the experimental results,indicating that changes in grain orientation are one of the main factors affecting ultrasonic propagation.The Adif and RRdif extracted from LCR wave simulation signals show a consistent trend with the experimental results,while the latter shows high sensitivity.（5）Based on the dislocation string model,the ultrasonic response of internal dislocation changes in grains was calculated.As the loading cycles increases,the attenuation coefficient based on dislocations shows a trend of first decreasing and then increasing.Considering the superposition effect of dislocation change and grain rotation on ultrasonic propagation,the calculated acoustic attenuation coefficient is basically consistent with the L_CR amplitude value measured in the experiment.This indicates that grain rotation and dislocation evolution jointly affect the propagation behavior of ultrasonic waves in the early-stage fatigue damage of Z2CND18.12N steel.更多还原