Investigations on the interconnection between the polarization rotation and crack propagation are performed for [110J-oriented 74Pb（Mg1/3Nb2/3）O3- 26PbTiO3 relaxor ferroelectric single crystal under electric loading...Investigations on the interconnection between the polarization rotation and crack propagation are performed for [110J-oriented 74Pb（Mg1/3Nb2/3）O3- 26PbTiO3 relaxor ferroelectric single crystal under electric loadings along [001] direction. The crystal is of predominantly monoclinic MA phase with scatter dis- tributed rhombohedral （R） phase under a moderate poling field of 900 V/mm in [00l] direction. With magnitude of 800 V/ram, a through thickness crack is initi- ated near the electrode by electric cycling. Static electric loadings is then imposed to the single crystal. As the applied static electric field increases, domain switch- ing in the monoclinic MA phase and phase transition from MA to R phase occur near the crack. The results indicate that the crack features a conducting one. Whether domain switching or phase transition occurs depends on the intensity of the electric field component that is perpendicular to the applied electric field.展开更多
The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100℃. The crack growth curves as a function of t...The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100℃. The crack growth curves as a function of the number of cycles were plotted. With the increase of peak temperature, the crack initiation life was shortened dramatically. Through optical microscopy （OM） and scanning electron microscopy （SEM） observation, it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles. The primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.展开更多
The numerical analyses of stationary mathematically sharp Mode I crack in FCC and BCC crystals with elastic-ideally plastic(EIP)and fast hardening saturation(FHS)law are carried out in the present paper.From the calcu...The numerical analyses of stationary mathematically sharp Mode I crack in FCC and BCC crystals with elastic-ideally plastic(EIP)and fast hardening saturation(FHS)law are carried out in the present paper.From the calculated re- sults,it is shown that:for the cases of small strain,EIP crystal cracks,the features of concentrated deformation patterns and the stress state in near-crack tip deforma- tion fields are identical to the earlier analytical solutions,but along the angular sector boundaries,there exist narrow complex stress zones.The overall characteristics of de- formation patterns for the cases of EIP and FHS are similar.The behaviours of crack tip opening can be characterized by crack-tip-opening-displacement(CTOD).For the case of FHS,finite deformation BCC crystal crack,our calculations are qualitatively in agreement with recent experimental observations.展开更多
A real-time quantitative optical method to characterize crack propagation in colloidal photonic crystal film（CPCF）is developed based on particle deformation models and previous real-time crack observations. The crac...A real-time quantitative optical method to characterize crack propagation in colloidal photonic crystal film（CPCF）is developed based on particle deformation models and previous real-time crack observations. The crack propagation process and temperature dependence of the crack propagation rate in CPCF are investigated. By this method, the crack propagation rate is found to slow down gradually to zero when cracks become more numerous and dense. Meanwhile, with the temperature increasing, the crack propagation rate constant decreases. The negative temperature dependence of the crack propagation rate is due to the increase of van der Waals attraction, which finally results in the decrease of resultant force. The findings provide new insight into the crack propagation process in CPCF.展开更多
Using the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with asymmetric colfinear cracks in a one-dimensional hexagonal quasi-crystal is solv...Using the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with asymmetric colfinear cracks in a one-dimensional hexagonal quasi-crystal is solved, and the exact analytic solutions of the stress intensity factors （SIFs） for mode Ⅲ problem are obtained. Under the limiting conditions, the present results reduce to the Griffith crack and many new results obtained as well, such as the circular hole with asymmetric collinear cracks, the elliptic hole with a straight crack, the mode T crack, the cross crack and so on. As far as the phonon field is concerned, these results, which play an important role in many practical and theoretical applications, are shown to be in good agreement with the classical results.展开更多
The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are ...The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.展开更多
Finite element （FE） analysis was employed to investigate the casting microcrack and micropore growth in nickel-base single-crystal superalloys DD3. Based on the finite deformation rate-dependent crystallographic con...Finite element （FE） analysis was employed to investigate the casting microcrack and micropore growth in nickel-base single-crystal superalloys DD3. Based on the finite deformation rate-dependent crystallographic constitutive equation, the simulations of casting microcrack and micropore growth in three-dimensional unit cell model were carried out in a range of parameters including stress triaxiality, Lode parameter and type of activated slip systems. The FE results show that the stress triaxiality has profound effects on growth behavior, and the Lode parameter is also important for the casting microcrack and micropore growth. The type of operative slip systems has remarkable effect on casting microcrack and micropore growth, so the life of single- crystal component is associated with the type of activated slip systems, which is related to Schmid factor and the number of activated slip systems. The growth comparison between microcrack and micropore reveals that when the material is subjected to large deformation, the growth rate of microcrack is faster than that of micropore, i.e. microcrack is more dangerous than micropore; the microcrack is easier to result in brittle fracture than micropore. The stress triaxiality and Lode parameter have strong influence on the growth of microcrack and micropore.展开更多
An investigation has been made of the disloca- tion distribution and dislocation free zone near the crack tip in bulk Fe-3% Si single crystal during deformation in SEM.It has been found that a number of dislocations w...An investigation has been made of the disloca- tion distribution and dislocation free zone near the crack tip in bulk Fe-3% Si single crystal during deformation in SEM.It has been found that a number of dislocations were emitted from the crack tip during deformation.After that,the dislocations moved rapidly away from the crack tip,which indi- cated that they were strongly repelled by the stress field at the crack tip.Between the crack tip and the plastic zone there is a region of dislocation-free, which is referred to as dislocation-free zone (DFZ). The length of DFZs is roughly estimated 100 μm which is much longer than that found in thin foil specimen.The variation of dislocation density as a function of the distance from the crack tip was measured,which showed that the dislocations are inversely piled up in the plastic zone.The length of DFZs increased with both the length of pre-crack and the amplitude of applied stress.展开更多
基金supported by the National Natural Science Foundation of China(11272174)
文摘Investigations on the interconnection between the polarization rotation and crack propagation are performed for [110J-oriented 74Pb（Mg1/3Nb2/3）O3- 26PbTiO3 relaxor ferroelectric single crystal under electric loadings along [001] direction. The crystal is of predominantly monoclinic MA phase with scatter dis- tributed rhombohedral （R） phase under a moderate poling field of 900 V/mm in [00l] direction. With magnitude of 800 V/ram, a through thickness crack is initi- ated near the electrode by electric cycling. Static electric loadings is then imposed to the single crystal. As the applied static electric field increases, domain switch- ing in the monoclinic MA phase and phase transition from MA to R phase occur near the crack. The results indicate that the crack features a conducting one. Whether domain switching or phase transition occurs depends on the intensity of the electric field component that is perpendicular to the applied electric field.
文摘The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100℃. The crack growth curves as a function of the number of cycles were plotted. With the increase of peak temperature, the crack initiation life was shortened dramatically. Through optical microscopy （OM） and scanning electron microscopy （SEM） observation, it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles. The primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.
基金The project supported by National Natural Science Foundation of China
文摘The numerical analyses of stationary mathematically sharp Mode I crack in FCC and BCC crystals with elastic-ideally plastic(EIP)and fast hardening saturation(FHS)law are carried out in the present paper.From the calculated re- sults,it is shown that:for the cases of small strain,EIP crystal cracks,the features of concentrated deformation patterns and the stress state in near-crack tip deforma- tion fields are identical to the earlier analytical solutions,but along the angular sector boundaries,there exist narrow complex stress zones.The overall characteristics of de- formation patterns for the cases of EIP and FHS are similar.The behaviours of crack tip opening can be characterized by crack-tip-opening-displacement(CTOD).For the case of FHS,finite deformation BCC crystal crack,our calculations are qualitatively in agreement with recent experimental observations.
基金Project supported by the National Basic Research Program of China(Grant Nos.2012CB932903 and 2012CB932904)the National Natural Science Foundation of China(Grant Nos.51372270,11474333,and 21173260)
文摘A real-time quantitative optical method to characterize crack propagation in colloidal photonic crystal film（CPCF）is developed based on particle deformation models and previous real-time crack observations. The crack propagation process and temperature dependence of the crack propagation rate in CPCF are investigated. By this method, the crack propagation rate is found to slow down gradually to zero when cracks become more numerous and dense. Meanwhile, with the temperature increasing, the crack propagation rate constant decreases. The negative temperature dependence of the crack propagation rate is due to the increase of van der Waals attraction, which finally results in the decrease of resultant force. The findings provide new insight into the crack propagation process in CPCF.
基金supported by the National Natural Science Foundation of China (Grant No 10761005)the Inner Mongolia Natural Science Foundation of China (Grant No 200607010104)
文摘Using the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with asymmetric colfinear cracks in a one-dimensional hexagonal quasi-crystal is solved, and the exact analytic solutions of the stress intensity factors （SIFs） for mode Ⅲ problem are obtained. Under the limiting conditions, the present results reduce to the Griffith crack and many new results obtained as well, such as the circular hole with asymmetric collinear cracks, the elliptic hole with a straight crack, the mode T crack, the cross crack and so on. As far as the phonon field is concerned, these results, which play an important role in many practical and theoretical applications, are shown to be in good agreement with the classical results.
文摘The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.
基金This work was supported by the Postdoctoral Science Foundation of China under Grant No.2004035688.
文摘Finite element （FE） analysis was employed to investigate the casting microcrack and micropore growth in nickel-base single-crystal superalloys DD3. Based on the finite deformation rate-dependent crystallographic constitutive equation, the simulations of casting microcrack and micropore growth in three-dimensional unit cell model were carried out in a range of parameters including stress triaxiality, Lode parameter and type of activated slip systems. The FE results show that the stress triaxiality has profound effects on growth behavior, and the Lode parameter is also important for the casting microcrack and micropore growth. The type of operative slip systems has remarkable effect on casting microcrack and micropore growth, so the life of single- crystal component is associated with the type of activated slip systems, which is related to Schmid factor and the number of activated slip systems. The growth comparison between microcrack and micropore reveals that when the material is subjected to large deformation, the growth rate of microcrack is faster than that of micropore, i.e. microcrack is more dangerous than micropore; the microcrack is easier to result in brittle fracture than micropore. The stress triaxiality and Lode parameter have strong influence on the growth of microcrack and micropore.
文摘An investigation has been made of the disloca- tion distribution and dislocation free zone near the crack tip in bulk Fe-3% Si single crystal during deformation in SEM.It has been found that a number of dislocations were emitted from the crack tip during deformation.After that,the dislocations moved rapidly away from the crack tip,which indi- cated that they were strongly repelled by the stress field at the crack tip.Between the crack tip and the plastic zone there is a region of dislocation-free, which is referred to as dislocation-free zone (DFZ). The length of DFZs is roughly estimated 100 μm which is much longer than that found in thin foil specimen.The variation of dislocation density as a function of the distance from the crack tip was measured,which showed that the dislocations are inversely piled up in the plastic zone.The length of DFZs increased with both the length of pre-crack and the amplitude of applied stress.
文摘声发射(acoustic emission,AE)检测就是利用采集和分析声发射现象所产生的信号从而实现对材料进行无损评价的技术,该技术具有实时监测并定位破坏源的能力但是由于检测过程中声发射源信号未知,因此评价检测可靠性的检测概率(probability of detection,POD)研究仍然是空白。进一步来说,在利用声发射监测损伤过程时,声发射波源直接决定其检测结果的可靠性,因此波源的检测概率可以成为量化声发射检测有效性的一个重要指标。为了确定声发射波源的检测概率,利用声子晶体(phononic crystals,PCs)的特性,通过部署声子晶体形成物理带隙,滤除噪声信号,从而获得铝合金试件在紧凑拉伸试验下由于疲劳裂缝活动而引起的声发射信号,最终确定声发射源的检测概率及其影响因素。结果表明,检测概率会随着疲劳裂缝扩展而提高,但裂缝长度达到一定值后开始下降,最终趋于一个平稳值,另外通过设置阈值和选择适当的滤波器,能有效提高检测概率,保证声发射评价结构损伤的有效性。该研究对声发射源识别和检测可靠性评价具有重要意义。
