SEM in-situ investigation on fatigue cracking behavior of P/M Rene95 alloy with surface inclusions

The low-cycle fatigue behavior of powder metallurgy Rene95 alloy containing surface inclusions was investigated by in-situ observation with scanning electron microscopy （SEM）. The process of fatigue crack initiation and early stage of propagation behavior indicates that fatigue crack mainly occurs at the interface between the inclusion and the matrix. The effect of inclusion on the fatigue crack initiation and the early stage of crack growth was very obvious. The fatigue crack growth path in the matrix is similar to the shape of inclusion made on the basis of fatigue fracture image analysis. The empiric relation between the surface and inside crack growth length, near a surface inclusion, can be expressed. Therefore, the fatigue crack growth rate or life of P/M Rene95 alloy including the inclusions can be evaluated on the basis of the measurable surface crack length parameter. In addition, the effect of two inclusions on the fatigue crack initiation behavior was investigated by the in-situ observation with SEM.

Fatigue properties of age-hardened AI alloy 2017-T4 under ultrasonic loading

Fatigue properties of age-hardened Al alloy 2017-T4 under ultrasonic loading frequency （20 kHz） were investigated and compared with the results under conventional loading of rotating bending（50 Hz）.The growth of a crack retarded at about 500μm in surface length under ultrasonic loading,while at about 20μm under rotating bending.Although striations being a typical fracture mechanism were observed under conventional loading,most of fracture surface was covered with many facets under ultrasonic loading.These facets were also observed under rotating bending in nitrogen gas.The difference in growth mechanism depending on the loading frequency and the retardation of a crack growth under ultrasonic loading may be caused by the environment at the crack tip due to high crack growth rate under ultrasonic loading.

An Uncertainty Evaluation on the ΔK_(th) and ΔK_(IC) of Fine-grained Isotropic Graphite

This paper intensively explores the critical issues related to the quantitative and accurate evaluations of FCG behavior in the early stage,macro fatigue fracture toughness,and the critical crack size for damage tolerance in nuclear graphite.To address these issues,scale-span FCG tests were carried out using two typical specimens,CT and SEM in-situ specimens.These results indicate that the FCG threshold and the effective FCG length have a significant correlation with the modified maximum loop stress theory for a mixed I/II mode.In particular,the effective FCG length(a_(eq))and the applied stress threshold of polycrystalline graphite are important parameters for fatigue damage tolerance design in engineering application.The influencing factors of ΔK_(th,eq) and a_(eq) were discussed in detail using the mixed I/II mode,respectively.In addition,the scattered values of ΔK_(IC) for this graphite can be quantitatively estimated using the Weibull distribution equation.The predicated parameters and experimental results demonstrate a strong correlation.

FGHxx系列合金疲劳损伤行为表征、评价与展望

FGHxx系列合金是我国近年来成功开发的损伤容限型粉末高温合金,由FGH95到FGH99系列组成.通过合金化及优化后续的热处理工艺参数,以较低的成本有效地提高FGHxx高温合金服役温度下的强度、断裂韧性、疲劳裂纹扩展阻力.论文通过对比现有文献的研究结果,对FGHxx系列合金的微结构特征,SEM原位测试方法,疲劳小裂纹的萌生和扩展行为,疲劳裂纹扩展速率和门槛值、高温疲劳寿命分散性,疲劳寿命预测模型,损伤容限设计准则进行了分析与讨论.在此基础上,重点关注了典型的FGH96合金.利用原位扫描电镜观察技术,观察了其疲劳小裂纹萌生与扩展行为,结果显示:在室温和400℃下,FGH96合金的低周疲劳小裂纹初期萌生和扩展优先发生在晶界上,当小裂纹扩展长度大于2-3倍平均晶粒尺寸后发生混合断裂模式,强化相的化学元素组成和弥散位置会影响晶界强度进而影响断裂机制;分析了断口形貌,发现微结构差异尤其是孔洞和夹杂是FGH96合金寿命分散性强的主要原因;利用DFR(细节疲劳寿命)方法预测了FGH96合金的具有Weibull分布特征的寿命特征值;建立了考虑温度和缺口系数的FGH96合金的3维疲劳损伤容限(3D K-T)图.最后探讨了该领域急需解决的问题和未来可能的研究方向.

Hierarchical Dragonfly Wing： Microstructure-Biomechanical Behavior Relations

The dragonfly wing, which consists of veins and membrane, is of biological hierarchical material. We observed the cross-sections of longitudinal veins and membrane using Environmental Scanning Electron Microscopy （ESEM）. Based on the experiments and previous studies, we described the longitudinal vein and the membrane in terms of two hierarchical levels of organization of composite materials at the micro- and nano-scales. The longitudinal vein of dragonfly wing has a complex sandwich structure with two chitinous shells and a protein layer, and it is considered as the first hierarchical level of the vein. Moreover, the chitinous shells are concentric multilayered structures. Clusters of nano-fibrils grow along the circumferential orientation embedded into the protein layer. It is considered as the second level of the hierarchy. Similarly, the upper and lower epidermises of membrane constitute the first hierarchical level of organization in micro scale. Similar to the vein shell, the membrane epidermises were found to be a paralleled multilayered structure, defined as the second hierarchical level of the membrane. Combining with the mechanical behavior analysis of the dragonfly wing, we concluded that the growth orientation of the hierarchical structure of the longitudinal vein and membrane is relevant to its biomechanical behavior.

Effects of Dragonfly Wing Structure on the Dynamic Performances

The configurations of dragonfly wings, including the corrugations of the chordwise cross-section, the microstructure of the longitudinal veins and membrane, were comprehensively investigated using the Environmental Scanning Electron Microscopy （ESEM）. Based on the experimental results reported previously, the multi-scale and multi-dimensional models with different structural features of dragonfly wing were created, and the biological dynamic behaviors of wing models were discussed through the Finite Element Method （FEM）. The results demonstrate that the effects of different structural features on dynamic behaviors of dragonfly wing such as natural frequency/modal, bending/torsional deformation, reaction force/torque are very significant. The corrugations of dragonfly wing along the chordwise can observably improve the flapping frequency because of the greater structural stiffness of wings. In updated model, the novel sandwich microstrueture of the longitudinal veins re- markably improves the torsional deformation of dragonfly wing while it has a little effect on the flapping frequency and bending deformation. These integrated structural features can adjust the deformation of wing oneself, therefore the flow field around the wings can be controlled adaptively. The fact is that the flights of dragonfly wing with sandwich microstructure of longitudinal veins are more efficient and intelligent.

Role of Soft Matter in the Sandwich Vein of Dragonfly Wing in Its Configuration and Aerodynamic Behaviors

The microstructure of the main longitudinal veins of the dragonfly wing and the aerodynamic behaviors of the wing were investigated in this paper. The microstructure of longitudinal vein presents two circumferential chitin layers and a protein-fiber soft layer. The dragonfly wing is corrugated due to the spatial arrangement of longitudinal veins. It was found that the corru- gation angle could significantly influence the lift/drag ratio across a range of attack angles by the wind tunnel experiments. The results of the finite element analysis indicate that the protein soft layer of vein facilitates the change of the corrugation angle by allowing substantial relative twisting deformation between two neighboring veins, which is not possible in veins without a soft sandwich layer.

EFFECT OF LOADING TYPE ON FATIGUE CRACK GROWTH BEHAVIOR OF SOLDER JOINT IN ELECTRONIC PACKAGING

Fatigue cracking tests of a solder joint were carried out using in-situ scanning electron microscopy （SEM） technology under tensile and bending cyclic loadings. The method for predicting the fatigue life is provided based on the fatigue crack growth rate of the solder joint. The results show that the effect of the loading type on the fatigue crack growth behavior of a solder joint cannot be ignored. In addition, the finite element analysis results help quantitatively estimate the response relationship between solder joint structures. The fatigue crack initiation life of a solder joint is in good agreement with the fatigue life （N50%） of a totally electronic board with 36 solder joints.