用声发射预测静疲劳下陶瓷材料的断裂

运用声发射检测技术，检测出受静载荷作用的陶瓷材料内部形成微裂纹和亚临界裂纹扩展的声发射信号，研究了裂纹形成、扩展时所激发的声发射计数率ｄＮ／ｄｔ与静载荷作用时间ｔ的关系。实验发现试样在断裂前２２～３０ｓ所检测到的亚临界裂纹扩展的声发射信号具有明显不同的变化规律，试样所受的静载荷越小，初次检测到亚临界裂纹扩展的声发射信号离发生断裂的时间越长。     

LiNbO_3和LiTaO_3晶体劳埃定向标准极图和取向测定中的问题

鉴于LiNbO_3和LiTaO_3晶体存在伪对称系统,并且文献上所给出的晶体取向坐标系、晶面及晶向指标比较混乱,致使晶体取向测定容易出现错误。本文根据国际上通用的三方晶系转换为六方晶系时的标准正取向坐标系,给出了供晶体劳埃定向用的标有伪对称坐标系及劳埃斑点强度的LiNbO_3和LiTaO_3的详细标准晶面极图及晶向极图。给出了典型的X射线背射劳埃图谱和不同切型晶片的方位及精密取向测定用的衍射数据。讨论了晶体取向与极化和蚀象的关系。     

脆性材料的亚临界裂纹扩展和双向应力的影响

分析了陶瓷,玻璃等脆性材料的断裂机理和表征.指出了脆性材料的失效过程的三种不同的裂纹发展模式.实验研究了玻璃在静载下的亚临界裂纹扩展特征及其受双向应力的影响.从而为脆性材料的可靠性和寿命评价提供理论基础和手段.     

不同掺镁量的LiNbO3质子交换光波导的制备与表征

用质子交换法在不同掺镁量(0、3、4、5、6mol％MgO)的LiNbO_3晶体中制备了质子交换光波导。光学测试表明,晶体中不同的MgO含量并不影响LiNbO_3质子交换波导阶跃型折射率分布的特点,表面折射率的增加约为0.130,扩散特性研究表明,质子交换反应实际上是一个扩散动力学过程,扩散系数不仅与晶体取向有关,而且受到晶体中MgO含量的影响。本实验还确定了5mol％MgO:LiNbO_3晶体X切型和y切型基片的扩散参数和质子交换激活能。     

剪切变形对PPR断裂韧性的影响

采用等通道转角挤压(ECAE)方法,研究了剪切变形对无规共聚聚丙烯(PPR)结构和断裂韧性的影响。实验发现,在挤压温度为45℃时,进行ECAE加工有利于PPR球晶的取向形变和非晶区取向。断裂韧性实验表明:经过ECAE挤压后试样的裂纹扩展方向与球晶的取向方向一致;经45℃挤压后,试样产生单位新断裂表面所需的能量是未挤压试样的3.7倍,且该能量的98%是通过塑性变形来吸收的;断面形态表明由剪切变形产生的层状结构是其断裂扩展所需能量增加的原因。     

聚硫密封剂浸油后拉伸断裂行为分析

研究了聚硫密封剂浸航空煤油后的断裂行为,对在3号航空煤油中浸渍不同时间(0、24、48、120、168h)的聚硫密封剂进行了拉伸性能测试,采用扫描电子显微镜,对拉伸断裂试样进行了断口观察,并对聚硫密封剂的断裂特征形成原因进行了分析.研究表明:密封剂拉伸时,裂纹形成、扩展到最后试样断裂,其过程可分为裂纹慢速扩展和快速断裂...     

低温热压氮化硅的晶界玻璃相结晶化及其对于高温强度和断裂形貌的影响

本文研究了低温热压 Si_3N_4 的晶界玻璃相结晶化及其对于高温强度的影响。X射线行射表明热处理过程中析出X相,热处理后使1450℃和1550℃热压的C-2材料在1200℃的抗折强度分别提高大约一倍和30％,消除了高温试样断口上的宏观粗糙区域。可能由于晶界玻璃相结晶化在一定程度上抑制亚临界裂纹扩展,而使高温强度有所改进,并改变了断裂形貌。     

工程陶瓷材料的高温疲劳行为(Part Ⅱ)

许多研究表明,高温下陶瓷材料的疲劳断裂主要为蠕变断裂和亚临界裂纹扩展,由于这两个过程的应力与时间关系式不同,因此正确地区分这两个过程并分别求出应力和时间的关系式,是高温下进行寿命预测的关键。     

玻璃在平面双向和单向应力状态下慢裂纹扩展特性

采用热弹性力学方法对含裂纹的玻璃薄片在双向和单向平面拉和下的亚临界裂纹扩展进行了观测和研究。该实验方法使得玻璃、陶瓷等危性材料的双向拉伸,直通裂纹预制和裂纹扩展过程记录这３个难度大的实验得以完成。对６０多个玻璃试件的断裂阻力随载荷速度的变化进行了观测和分析,从而较精确地了解了玻璃在平面和状态下的阻力曲线和断裂特性。     

部分烧结陶瓷材料力学特性的DEM模拟

通过开展三维离散单元法数值模拟,考察了部分烧结陶瓷在单轴拉伸和压缩加载条件下的力学响应行为。模拟结果表明,拉伸加载下试样的破坏表现为裂纹“成核”效应,而压缩加载下则呈现为裂纹“聚并”效应;通过追踪固体键的断裂顺序和断裂模式发现,拉伸加载下固体键的破坏主要源于拉伸作用,而压缩加载下则为剪切作用。试样的宏观断裂强度与固体键临界拉伸强度?c,t和临界剪切强度?c,s有关。对于考察的部分烧结Al2O3陶瓷材料,?c,t=?c,s下模拟得到的试样拉伸强度和压缩强度能与实验数据定量吻合。压缩加载下,固体键断裂强度的分布形式和分布宽度对试样断裂强度的影响较小;拉伸加载下,试样断裂强度取决于固体键的分布形式：高斯分布时,试样断裂强度基本不受分布宽度的影响;均匀分布时,试样断裂强度随分布宽度的增大而减小。     

Fracture Behavior of the Space Shuttle Thermal Protection System

Stable crack-growth and fracture-toughness experiments were conducted using precracked specimens machined from LI-900 reusable surface insulation (RSI) tiles of the space shuttle thermal protection system (TPS) at room temperature. Similar fracture experiments were conducted on fracture specimens with preexisting cracks at the interface of the tile and the strain isolation pad (SIP). Stable crack growth was not observed in the LI-900 tile fracture specimens which had a fracture toughness of 12.0 kPaμm. The intermittent subcritical crack growth at the tile-pad interface of the fracture specimens was attributed to successive local pull-outs due to tensile overload in the LI-900 tile and cannot be characterized by linear elastic fracture mechanics. No subcritical interfacial crack growth was observed in the fracture specimens with densified LI-900 tiles where brittle fracture initiated at an interior point away from the densification.     

掺杂对铌酸锂光折变响应速度的提高

在ＬｉＮｂＯ３晶体掺入Ｔｂ离子，可提高晶体的光折变响应速度。Ｔｂ：Ｆｅ：ＬｉＮｂＯ３晶体既保留了Ｆｅ：ＬｉＮｂＯ３良好的光折变性能，同时其光折变响应时间大大缩短，可达十几秒，光折变响应速度的提高。可归结为晶光电导的增大，使得晶体内空间电荷场的建立更加迅速。     

Fracture and Subcritical Crack Growth in Soda-Lime Glass under Combined Modes I and II Loading

Fracture and subcritical crack growth characteristics under combined modes I and II loading were studied using the compact tension shear specimens of soda–lime glass. The maximum normal stress criterion gives a good agreement with the experimental mode I–mode II fracture toughness envelope for initially straight cracks and kinked cracks. Subcritical crack growth characteristics were determined under sustained modes I and II loading in water. The values of K _I and K _II were calculated approximately by replacing the subcritical kinked crack with an assumed straight crack ā, and the K _Imax value based on the maximum normal stress criterion was used to describe this subcritical kinked crack growth. The experimental results show that subcritical crack growth under pure mode I, pure mode II, and various combined modes I and II loading can be well described by the K _Imax value based on the approximate maximum normal stress criterion.     

The Influence of Water on the Fracture of Magnesium Aluminate (MgAl2O4) Spinel

The influence of water on the fracture of magnesium aluminate (MgAl₂O₄) ceramics and spinel single crystals is investigated in order to determine whether adsorption plays an important role during subcritical crack growth as it does for MnZn ferrites, a compound with the same spinel structure. The fracture toughness of porous and dense polycrystalline ceramic and single crystal specimens are determined using a single edge notched beam setup at different crosshead velocities and humidities. Furthermore, in order to investigate whether a preferred plane of fracture is present, electron backscattered diffraction (EBSD) has been performed. It is found that the fracture toughness decreases with increasing humidity for dense ceramics, indicating that adsorption plays an important role as it does for MnZn ferrites. However, porous ceramics only show a sharp decrease between 0% and 2–10% relative humidity (RH) and hardly decreases onwards. This implies that the pores inhibit in some way the effect of adsorption during fracture. The exact mechanism remains unclear. EBSD measurements indicated that a preferred plane is absent. Therefore, the fracture of spinel single crystal along the (100) and (111) planes was chosen for further detailed investigation. It is revealed that between 2% and 40% RH both planes show a reduction in fracture surface energy of ∼46%. Using reported computer simulations results, it can be concluded that the surfaces are partially hydrated when being fractured.     

Biaxial strength and slow crack growth in porous alumina with silica sintering aid

Biaxial strength, fracture toughness and subcritical crack growth are reported for coarse grained porous alumina ceramics. The materials were prepared with a varying amount of a silica sintering aid, which resulted in the formation of a glassy secondary phase at the grain boundaries. Crystalline mullite was additionally found in the material with the highest silica content. The biaxial strength, measured by Ball-on-Ring and Ball-on-3-Balls, was highest for the material without mullite at the grain boundaries, and the biaxial strength decreased with increasing porosity. The fracture toughness of the materials was in the range of 1.7–1.9 MPa m^0.5. Measurements of subcritical crack growth by a modified lifetime method in air and aqueous environments demonstrated a higher crack growth rate in water and acid relative to in air. The effect of porosity and grain boundary phase were discussed in relation to subcritical crack growth and fracture mode in the coarse grained alumina ceramics.     

R-curve effect,influence of electric field and process zone in BaTiO3 ceramics

Double cantilever beam (DCB) specimens were used to measure fracture toughness and R-curve effect of coarse grained BaTiO₃ doped with 0.5 mol% TiO₂ and fine grained BaTiO₃ doped with 1.5 mol% La₂O₃ and 3.3 mol% TiO₂. The coarse grained BaTiO₃ had an average grain size of 20 μm compared to 0.4 μm for the fine grained material. Coarse grained BaTiO₃ showed increasing crack resistance with rising crack length for crack elongations up to 2 mm. The R-curve behaviour can be attributed to mechanisms shielding the crack tip from applied loads, such as ferroelastic domain switching in a process zone ahead of the crack tip, which was observed in situ. Since domain switching events left a pattern on the polished surface of the DCB specimen, it was possible to visualise the domain switching during crack propagation using Nomarski differential interference contrast. The method allowed to measure the size and shape of the process zone in unpoled and poled material. The maximum width of the process zone was 120 μm in both cases but the spatial distribution of switching events showed remarkable differences. An electrical DC field applied to the DCB samples while the cracks were propagating promoted crack growth in coarse grained, unpoled samples. In specimens poled perpendicularly to the crack, a rising toughness was found, if negative electric fields were applied. Fine grained samples were not affected by electrical fields of up to 1 kV/mm.     

Propagation Behavior of Machining Cracks in Delayed Fracture

The propagation behavior of machining-induced cracks in silicon nitride was investigated by conducting constant-stress and constant-stress-rate tests. A dye-impregnation technique that forced a palladium nitrate solution into the cracks gave clear evidence of subcritical crack growth of machining cracks in suspended constant-stress test specimens. The fracture origin before subcritical crack growth usually consisted of a series of small cracks within an elongated semielliptical envelope. The dye-impregnation results gave evidence that some of the elemental cracks had coalesced during subcritical crack growth. It was hypothesized that coalescence might explain an initially higher crack-growth rate suggested by constant-stress-rate tests.     

近化学计量比Fe:Mg:LiNbO_3晶体生长与其存储性能

在LiNbO3(LN)中分别掺入0.5%(摩尔分数,下同),1%和2% MgO,0.03%(质量分数)Fe2O3,配料中n(Li)/n(Nb)=1.38,采用顶部籽晶溶液生长法生长近化学计量比Fe:Mg:LiNbO3(near-stoichiometric Fe:Mg:LiNbO3,Fe:Mg:SLN)晶体。测试了晶体的红外光谱、抗光损伤能力和存储性能。结果表明:Fe:2%Mg:LN晶体的OH-吸收峰移到3535cm-1,抗光损伤能力比Fe:LN提高3个数量级。Fe:0.5%Mg:LN晶体的灵敏度、动态范围和抗光损伤能力比Fe:LN晶体分别高2.5倍,2倍和1个数量级。以Fe:2%Mg:LN晶体和Fe:LN晶体分别作为存储介质,进行大容量存储实验。在一个公共体积内实现1200幅体全息图的存储。Fe:2%Mg:LN晶体的存储质量优于Fe:LN晶体。