拉伸载荷下陶瓷基高温复合材料损伤状态特征分析

基于声发射(AE)信号和计算机断层扫描(CT)成像技术,开展了高温复合材料的内部损伤状态分析。通过声发射技术获得复合材料损伤时的宏观应力波信号,分析得到声发射信号的频率等波形特征参数;通过计算机断层扫描得到复合材料结构内部微细观三维损伤图像,判断存在的损伤类型。结果表明,不同工艺陶瓷基高温复合材料在拉伸载荷下,内部会产生不同类型的损伤,纤维束与基体之间的脱黏开裂所对应的声发射信号特征频率为44 kHz,基体失效的特征频率为150 kHz,纤维束断裂损伤的特征频率为250 kHz。     

蜂窝夹层复合材料的压缩损伤声发射特征

应用声发射技术对蜂窝夹层复合材料压缩损伤过程进行了试验研究。分析载荷与声发射信号关联图,依据其损伤过程和声发射特征,发现随着加载条件下载荷的增加,复合材料的损伤逐步增大。在加载初始阶段,仅有少量声发射信号,各种表征信号量小幅度增加;在加载中期,声发射信号增多,各种表征信号量不断增大;在加载后期,声发射信号有明显突增,各种表征信号量急剧增加。复合材料压缩损伤破坏与声发射的幅值、能量、撞击、上升时间、持续时间和计数等参量特征相关。根据各阶段特征参量滤波后所得信号分布与实际断裂位置相吻合。     

飞机复合材料拉伸过程损伤的声发射特性分析

利用声发射技术开展对拉伸载荷下飞机复合材料T型结构件的损伤发生过程研究,对复合材料试件拉伸载荷下损伤产生过程的机理进行了初步探讨。采用参数分析方法对拉伸试验采集到的声发射信号进行分析,得到该T型结构件拉伸过程中基体开裂、分层、纤维断裂至完全丧失承载能力时的声发射信号特征及其对应的载荷。结果表明,声发射技术能够准确预测复合材料T型结构件拉伸载荷下的损伤发生过程,可为该类结构件的检测提供参考。     

基于不同频率下疲劳裂纹扩展时声发射特性分析

为了探究Q345R材料在循环载荷作用下的性能,开展不同频率下疲劳裂纹扩展实验,对照疲劳载荷过程中的损伤过程,提取声发射信号特征参数并进行综合分析。研究发现不同加载频率下,材料疲劳裂纹扩展长度与循环次数的关系;同时通过声发射在线监测其过程中的特征参数,发现疲劳裂纹扩展和断裂过程中的规律性,为声发射技术应用于Q345R材料的在线检测提供了参考。     

基于声发射检测的碳布／环氧树脂复合材料压缩损伤评价

用声发射仪器监测二维编织碳布／环氧树脂复合材料的压缩损伤过程。通过载荷与声发射特征关系图,判断出其在损伤过程中的临界失效载荷,作为判断材料临界损伤强度的依据。通过对碳布／环氧树脂复合材料在压缩损伤过程中产生的声发射信号进行小波包分解,并对信号进行频谱分析以及对小波包分解后的各层进行能量分析,得出了声发射源即纤维断裂信号、基体断裂信号和层间开裂信号的频率分布范围。     

SiC维纤增强钛基复合材料的疲劳裂纹扩展速率

用箔-纤维-箔法制备SiC纤维增强Ti-6Al-4V复合材料,研究复合材料在加载频率f=10Hz、应力比R=0.1、最大应力σmax=300MPa条件下的疲劳裂纹扩展速率(da/dN),并采用扫描电子显微镜对疲劳破坏断口进行观察和分析。结果显示,在该加载条件下,复合材料第Ⅱ阶段疲劳裂纹扩展速率符合高斯函数。断口观察表明,复合材料的基体在裂纹稳态扩展区出现明显的疲劳条带,复合材料的疲劳损伤可以分为纤维断裂、基体开裂和纤维/基体界面脱粘等多种形式。     

Fatigue Behavior and Damage Evolution of SiC Fiberreinforced Ti-6Al-4V Alloy Matrix Composites

研究SiC纤维增强钛基复合材料(SiCf/Ti-6Al-4V)室温疲劳行为和损伤演化机制。疲劳试验条件:载荷控制、应力比0.1和加载频率10 Hz。采用疲劳断裂试验建立最大加载应力为600~1200 MPa内SiCf/Ti-6Al-4V的S-N曲线。采用疲劳中止试验以及SEM显微分析研究应力水平对SiCf/Ti-6Al-4V疲劳损伤演化的影响。结果表明,SiCf/Ti-6Al-4V疲劳损伤萌生模式与演化过程与应力水平密切相关。在高应力水平(Smax=1000 MPa),纤维开裂是主要损伤萌生模式。一旦2或3根纤维断裂后,纤维裂纹和基体裂纹开始联接并形成宏观扩展裂纹。在中等应力水平(Smax=800 MPa),基体裂纹萌生与扩展是主要损伤模式。多条基体裂纹萌生于试样外表面棱边和离外表面附近试样内部开裂的纤维基体界面处。基体裂纹均沿垂直于加载方向扩展,且大部分纤维未断裂并纤维桥接基体裂纹。在低应力水平(Smax=600 MPa),仅在C涂层和界面反应层之间和C涂层内部观察到局部界面脱粘现象。     

SiC纤维增强Ti-6Al-4V复合材料的疲劳行为及损伤演化研究（英文）

研究SiC纤维增强钛基复合材料(SiCf/Ti-6Al-4V)室温疲劳行为和损伤演化机制。疲劳试验条件:载荷控制、应力比0.1和加载频率10 Hz。采用疲劳断裂试验建立最大加载应力为600~1200 MPa内SiCf/Ti-6Al-4V的S-N曲线。采用疲劳中止试验以及SEM显微分析研究应力水平对SiCf/Ti-6Al-4V疲劳损伤演化的影响。结果表明,SiCf/Ti-6Al-4V疲劳损伤萌生模式与演化过程与应力水平密切相关。在高应力水平(Smax=1000 MPa),纤维开裂是主要损伤萌生模式。一旦2或3根纤维断裂后,纤维裂纹和基体裂纹开始联接并形成宏观扩展裂纹。在中等应力水平(Smax=800 MPa),基体裂纹萌生与扩展是主要损伤模式。多条基体裂纹萌生于试样外表面棱边和离外表面附近试样内部开裂的纤维基体界面处。基体裂纹均沿垂直于加载方向扩展,且大部分纤维未断裂并纤维桥接基体裂纹。在低应力水平(Smax=600 MPa),仅在C涂层和界面反应层之间和C涂层内部观察到局部界面脱粘现象。     

复合材料拉伸过程的声发射特性研究

为了研究16MnR／0Cr18Ni9Ti复合材料断裂过程的声发射特性,可以利用声发射技术对16MnR／OCr18Ni9Ti复合材料试件的拉伸过程进行全程监测。研究表明,材料拉伸断裂过程中,声发射信号丰富明显,可测性良好,并且不同破坏阶段的声发射信号具有不同的特征。通过对不同拉伸阶段声发射信号的参数分析,可以了解材料不同变形阶段的声发射特性,并据此来分析材料损伤的发生、发展及演变过程。与传统的力学试验方法相比,声发射技术在研究复合材料断裂过程方面具有明显的优势。     

铝合金疲劳裂纹扩展声发射监测

采用声发射(acoustic emission,AE)技术对7N01铝合金单边缺口三点弯曲试样不同应力比、不同峰值载荷下疲劳裂纹扩展过程中声发射信号进行了监测,建立了裂纹扩展速率、声发射计数(count)与应力强度因子之间的关系.结果表明,大部分的声发射信号主要产生于疲劳循环载荷的低应力阶段,这主要是低应力阶段的声发射活动主要与裂纹尖端的塑性变形和裂纹闭合现象有关,声发射计数与应力强度因子之间呈指数增长的关系.基于所建立的声发射计数率与裂纹扩展速率的关系,可以预测疲劳损伤结构的剩余寿命.     

Investigation of the dependency of the upper critical magnetic field on the content x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconducting structures

The Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by using the solid-state reaction technique. Resistivity measurements of the samples were performed in QD–PPMS system under different magnetic fields up to 5 T in zero fields cooling regime. Using the resistivity data, the upper critical magnetic field H_c2(0) at T = 0 K for 50% of R_n was calculated by the extrapolation H_c2(T) to the temperature T = 0 K. The coherence length in T = 0 K were calculated from H_c2(0) and the effects of x in the composition on both the coherence length and the upper critical magnetic field were examined. The results showed that H_c2(0) varied from 84.05 to 122.26 T with the content x. The upper critical magnetic field in the temperature T = 0 K slightly decreased with increasing the content x. Using the content x, the upper critical magnetic field can be controlled and this can be used in the superconductivity applications.     

On the mechanism of current-transport in Cu/CdS/SnO2/In-Ga structures

The structural and optical properties of CdS films deposited by evaporation were investigated. X-ray diffraction study showed that CdS films were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. The study has been made on the behavior of Cu/n-CdS thin film junction on SnO₂ coated glass substrate grown using thermal evaporation method. The forward bias current-voltage (I-V) characteristics of Cu/CdS/SnO₂/In-Ga structures have been investigated in the temperature range of 130-325 K. The semi-logarithmic lnI-V characteristics based on the Thermionic emission (TE) mechanism showed a decrease in the ideality factor (n) and an increase in the zero-bias barrier height (Φ_Bo) with the increasing temperature. The values of n and Φ_Bo change from 8.98 and 0.29 eV (at 130 K) to 3.42 and 0.72 eV (at 325 K), respectively. The conventional Richardson plot of the ln(I_o/T²) vs q/kT shows nonlinear behavior. The forward bias current I is found to be proportional to I_o(T)exp(AV), where A is the slope of ln(I)-V plot and almost independent of the applied bias voltage and temperature, and I_o(T) is relatively a weak function of temperature. These results indicate that the mechanism of charge transport in the SnO₂/CdS/Cu structure in the whole temperature range is performed by tunneling among interface states/traps or dislocations intersecting the space-charge region. In addition, voltage dependent values of resistance (R_i) were obtained from forward and reverse bias I-V characteristics by using Ohm's law for each temperature level.     

Study of the electrical conduction behaviour of the Ba1−xLaxTi1−xNixO3 (x 0.10) system

The electrical conduction behaviour of the Ba_1−xLa_xTi_1−xNi_xO₃ (x 0.10) system has been studied by complex plane impedance analysis and measurements of a.c. conductivity in the temperature range 400–575 K. The values of the bulk resistance for these samples are obtained from a circular arc passing through the origin in their impedance plots. A.c. conductivity obeys the relation σ_a.c.αω⁸ in the temperature range of measurements. These results indicate that conduction occurs in this system because of hopping of charge carriers between localized nickel sites.     

Effect of Ca on the microstructure and magnetocaloric effects in the La1−xCaxFe11.5Si1.5 compounds

The effect of Ca on the microstructure and magnetocaloric effects has been investigated in the La_1−xCa_xFe_11.5Si_1.5 (x = 0, 0.1, 0.2 and 0.3) compounds. The introduction of Ca leads to the appearance of minor α-Fe and Ca-rich phases, which affects the actual compositions of the main phases for the Ca containing samples. With increasing the Ca concentration, the Curie temperature T_C increases from 183 to 208 K, and the maximum magnetic entropy changes |ΔS| at the respective T_C with a magnetic field change from 0 to 5 T are 21.3, 19.5, 16.9, and 11.2 J/kg K for x = 0, 0.1, 0.2, and 0.3, respectively. The nature of the magnetic transition changes from first-order to second-order with an increase in Ca concentration, which leads to a reduction of the hysteresis and a decrease of the magnetic entropy change. However, the relative cooling power for La_1−xCa_xFe_11.5Si_1.5 compounds remains comparable with or even larger than that of other magnetocaloric materials over a wide temperature range. The higher T_C and the smaller hysteresis in comparison with those of the parent compound suggest that the La_1−xCa_xFe_11.5Si_1.5 compounds could be suitable candidates for magnetic refrigerants in the corresponding temperature range.     

Cr/CrN/CrNC/CrC/Cr-DLC梯度膜层的研究

采用中频磁控溅射结合离子源技术沉积Cr／CrN／CrNC／CrC／Cr-DLC膜层。利用扫描电镜（SEM）、电子能谱（EDS）、Raman光谱和俄歇深层剥层分别对膜层微观形貌、成分组成、键结构及梯度成分深层分布进行分析；采用努氏显微硬度计、摩擦磨损试验机、划痕仪及洛氏硬度计测评膜层机械性能。结果表明：采用中频磁控溅射可沉积出大面积Cr／CrN／CrNC／CrC／Cr-DLC梯度膜层；Cr溅射功率对C键结构影响不大，但表层DLC中Cr的含量和膜层厚度随中频功率增大而增大；随DLC中掺Cr量增多，膜层硬度及摩擦因数略有上升；采用梯度过渡层及掺入金属Cr提高了膜层附着力，但过高的中频功率使沉积离子能量偏高，膜层压应力增加反而降低了膜层附着性能。     

Structure and element distribution in the UFe10-xAlxSi2 system

The neutron diffraction studies of powdered alloys withx = 2,2.5, and 3 gave detailed information on crystallographic data of the compounds. The structure refinement at 300 K for allx = 2,2.5, and 3 and at 450 K forx = 2 confirmed the ThMn₁₂ type of structure with uranium located in 2(a) positions, iron in 8(f), 8(i), and 8(j) positions, aluminum atoms in 8(i) and 8(j) positions, and Si atoms in 8(f) and 8(j) positions. Impurity phases, FeAl and UFe₂Si₂ have been detected in all investigated compounds.     

Magnetocaloric effect in the Ce2Fe17−xMnx helical magnets

The Ce₂Fe_17−xMn_x (x = 0-2) compounds demonstrate a complex temperature dependence of the magnetocaloric effect MCE, which is inverse in a narrow temperature interval just below Néel temperature T_N and normal at higher or lower temperatures. The normal MCE exhibits two peaks in the vicinity of temperatures of ferromagnetic ordering Θ_T and T_N for compositions x = 0-0.35, 1.3-2 or one peak near T_N for antiferromagnets with x = 0.5-1. The maximal change of the peak entropy −S_M is about 3 J/kg K in a field of 5 T for the compounds with x = 0-0.5 at T ∼230 K close to T_N. The drastic decrease of the MCE, by half, in the Ce₂Fe_17−xMn_x system is traceable to a decrease of the spontaneous magnetization and the helical type of magnetic states in the compounds.     

Magnetic phase diagrams of the TbRh2−xPdxPdxSi2 and TbRu2−xPdxSi2 systems

The a.c. susceptibility and high field magnetization on TbRh_2−xPd_xPd_xSi₂ and TbRu_2−xPd_xSi₂ compounds were investigated up to 140 kOe. The (T,x) magnetic phase diagrams were determined. For both systems, an increase in the Pd content causes a decrease in the Néel temperature and changes the magnetization curves.     

Influence of the C14 Ti35.4V32.3Fe32.3 Laves phase on the hydrogenation properties of the body-centered cubic compound Ti24.5V59.3Fe16.2

Bcc Ti_24.5V_59.3Fe_16.2 alloys containing 10 and 30% of C14 Laves phase inclusions were prepared by induction melting followed by annealing at 1000 °C. X-ray powder diffraction and BSE microscopy confirmed the presence of the C14 Laves phase (average composition Ti_35.4V_32.3Fe_32.3) embedded in the bcc matrix. The two end members of the series, the C14 Laves phase and the bcc Ti_24.5V_59.3Fe_16.2 alloy, have very different hydrogenation behaviors. The C14 Laves phase does not absorb as much hydrogen as does the bcc phase. No equilibrium plateau and little hysteresis between absorption and desorption were observed at 25 °C for the C14 Laves on the PCI curves whereas those of the bcc sample present one equilibrium plateau and significant hysteresis between absorption and desorption. As a result, the absorption capacity and the length of the equilibrium plateau of the multiphase alloys decrease with the C14 Laves phase content. The hydrogenation properties of an as-cast bcc Ti_24.5V_59.3Fe_16.2 sample were also investigated: the kinetics of the first hydrogenation is found to be slower and the plateau pressures higher for the as-cast alloy than for the annealed sample.