Fracture Resistance of Ceramics: Edge Fracture Method

Fracture resistance of macrohomogeneous linear elastic yttria, scandia, alumina, zirconia, and silicon nitride ceramics was studied upon flaking of the edges of rectangular specimens with the Rockwell indenter. The correlation between the obtained estimates and fracture toughness test results gained by the single-edge V-notched beam method was demonstrated. Loads giving rise to the flaking of a part of the edge and distances from this edge to the chip scar on the specimen surface were measured. The ratio of those values was considered as a flaking toughness characteristic. The data obtained were statististically reliable (based on more than a hundred determinations). This procedure termed the edge fracture method can be used along with other known fracture toughness test methods for ceramics, its application is especially advantageous when ceramic item sizes are comparable with those of the standard specimens or when expensive materials (e.g., nanoceramics) are tested. __________ Translated from Problemy Prochnosti, No. 5, pp. 84 – 92, September – October, 2005.     

Effect of Strain Rate and Preloading on the Fracture Toughness of ZrO2-Based Ceramics

We study the variation of the fracture toughness K_Ic ofZrO₂ - Y₂ O₃ ceramics (density 98%) as a function of the testing machine crosshead speed (0.005–50 mm/min) and preloading at K_I < K_c. The fracture toughness is shown to be practically constant in the speed range from 0.05 to 5 mm/min. At a loading rate of 50 mm/min, the quantity K_Ic substantially decreases (by a factor of more than two), whereas at a rate of 0.005 mm/min it slightly increases. Preloading leads to a 1.5-fold increase in K_Ic. Variation of the fracture toughness is associated with structural transformations.     

断裂方式对氧化铝基复合陶瓷耐磨性的影响

通过磨粒磨损试验测定了氧化铝 陶瓷、氧化铝/碳化硅复合陶瓷和氧化铝/莫来石复合陶瓷的耐磨性. 利用透射电子显微镜(TEM)观察了样品的微观结构, 采用扫描电子显微镜(SEM)分析了样品的断口形貌和磨损表面的剥落情况. 研究了断裂方式对磨损表面剥落和耐磨性的影响. 结果表明: 氧化铝陶瓷的磨损主要由断裂磨损机制控制, 氧化铝/碳化硅复合陶瓷的磨损主要由塑性磨损机制控制, 氧化铝/莫来石复合陶瓷受这两种磨损机制共同作用. 相对于氧化铝陶瓷, 氧化铝/碳化硅复合陶瓷和氧化铝/莫来石复合陶瓷的耐磨性提高2～4倍, 这主 要是由于其断裂方式转变为以穿晶断裂为主, 减少了磨损面的脆性剥落.     

Si_3N_4 和 SiC 陶瓷材料的压痕断裂行为和断裂韧性测量

本文以热压 Si_3N_4和反应烧结 Si_3N_4为对象,研究其在 Vickers 钻石压头的作用下,相应于压痕断裂行为而发出的声发射特征。发现,除了与压痕特征参数的发展相应的准静态声发射之外,尚有三个以上突然增强的发射讯号,分别与径向裂纹的成核、径向裂纹突发至试样表面和裂纹分岔,以及侧向裂纹的萌生等效应相关。与一般力学测试法相比,据据 Evans 表达式(7),以 Vickers 压痕裂纹技术测得的 K_(IC)值具有的负偏差是,热压 Si_3N_4:-25.1%;反应烧结 Si_3M_4:-21.6%;热压 SiC:-29.2%。以含有弹性模量项的 Evans 表达式(8)取代式(7),从而得到上述材料的 K_(IC)值偏差分别从-25.1%降至-13.9%;从-21.6%降至-12.3%和从-29.2%降至-17.9%。作为从终点烧结温度淬冷至室温的必然结果,在热压陶瓷材料中存在的热残余应力似乎助长了热压 Si_3N_4和热压 SiC 陶瓷的 K_(IC)值偏差。     

Strength and Fracture Toughness of Ceramic Materials for Metal-Ceramic Prosthetic Dentistry

The strength in biaxial flexure and the fracture toughness on indentation of the specimens of ceramic materials for the coverage of metal-ceramic prostheses were studied. The strength was established to be mainly determined by their microstructure and sintering conditions. Comparative studies demonstrated that Ultropaline and VITA OMEGA 900 ceramic compositions exhibited more uniform distribution of leucite microcrystals and higher strength and fracture toughness than Duceram Plus and IPS Classic materials.__________Translated from Problemy Prochnosti, No. 3, pp. 128 – 139, May – June, 2005.     

Local Fracture of Nanolaminates: Edge Chipping Test

Strength of Materials - The fracture resistance of domestic and foreign nanolaminate ceramics was studied by the EF (edge fracture) method. This method provides for indentor chipping of rectangular...     

氧化铝瓷的显微结构与断裂韧性关系的研究

制备并收集一系列不同显微结构的９９－Ａｌ２Ｏ３瓷试样，研究晶粒尺寸及其它显微结构参数与断裂韧性Ｋ１ｃ的关系。结果表明：（１）当晶粒平均直径ｄ＞１５μｍ时，Ｋ１ｃ随ｄ的减小而升高，当ｄ＜５μｍ时，Ｋ１ｃ随ｄ的减小而迅速降低。（２）气孔的数量、分布、形貌及晶界相也影响着材料的Ｋ１ｃ值。     

Fracture Toughness of Advanced Ceramics at Room Temperature

This report presents the results obtained by the five U.S. participating laboratories in the Versailles Advanced Materials and Standards (VAMAS) round-robin for fracture toughness of advanced ceramics. Three test methods were used: indentation fracture, indentation strength, and single-edge pre-cracked beam. Two materials were tested: a gas-pressure sintered silicon nitride and a zirconia toughened alumina. Consistent results were obtained with the latter two test methods. Interpretation of fracture toughness in the zirconia alumina composite was complicated by R-curve and environmentally-assisted crack growth phenomena.     

长柱状晶高韧性氧化铝陶瓷的制备与性能研究

针对一般氧化铝陶瓷材料断裂韧性差的缺点 ,通过球磨的方法向初始原料氢氧化铝中添加晶种 ,并采用热压烧结方式 ,使氧化铝生长成长柱状晶粒结构 ,产生了明显的增韧效果 ,使这种氧化铝瓷体的断裂韧性比一般氧化铝陶瓷材料提高一倍以上 ,可达到 6 .70 MPa· m1 /2。对几种不同烧结温度下烧结的瓷体进行了显微结构分析和力学性能测试 ,研究了长柱状氧化铝晶粒的生长过程及力学性能变化特点     

Lessons from the Ocean: Whale Baleen Fracture Resistance

Whale baleen is a keratin‐based biological material; it provides life‐long (40–100 years) filter‐feeding for baleen whales in place of teeth. This study reveals new aspects of the contribution of the baleen's hierarchical structure to its fracture toughness and connects it to the unique performance requirements, which require anisotropy of fracture resistance. Baleen plates are subjected to competing external effects of hydration and varying loading rates and demonstrate a high fracture toughness in transverse loading, which is the most important direction in the filtering function; in the longitudinal direction, the toughness is much lower since delamination and controlled flexure are expected and desirable. The compressive strength is also established and results support the fracture toughness measurements: it is also highly anisotropic, and exhibits a ductile‐to‐brittle transition with increasing strain rate in the dry condition, which is absent in the hydrated condition, conferring impact resistance to the baleen. Using 3D‐printing prototypes that replicate the three principal structural features of the baleen plate (hollow medulla, mineralized tubules, and sandwich‐tubular structure) are created, and the role of its structure in determining its mechanical behavior is demonstrated. These findings suggest new bioinspired engineering materials.     

Effect of Calcium Hydroxide on the Fracture Resistance of Dentin

An increased incidence of fracture has been reported in teeth where root canals were treated with calcium hydroxide. Edge chipping is one test used to measure the resistance of brittle materials to fracture. Presently, no studies have reported on edge chipping in teeth. This study evaluated the fracture resistance of human dentin exposed to calcium hydroxide for up to 60 days using the edge chipping method. Twelve recently extracted teeth were divided into a control group and three experimental groups with varying calcium hydroxide exposures. All teeth underwent pulpectomy via standard protocol. It was expected that the edge chip resistance would decrease as a function of exposure, but the results showed the converse. Chip resistance may reflect both the fracture resistance and the hardness of dentin, a quasi brittle material.     

Fracture resistance of 8 mol% yttria stabilized zirconia

Anin situ technique for the assessment of fracture resistance employing double cantilever beam (DCB) specimens was developed in the present study. The side-grooved DCB specimens were loaded with pure bending moments in a specially designed and fabricated test fixture which went inside the specimen chamber of a scanning electron microscope. The study as conducted on a 8 mol% fully stabilized cubic phase yttria (Y₂O₃) stabilized zirconia (YSZ) ceramic. The powder processed sheets were sintered at 1600°C for 2 h in a zirconia tube furnace. The mode I applied energy release rate, G_I was determined for both pure YSZ and treated YSZ. Two sets of experiments were conducted for the complete characterization of the ceramics. Three fracture toughness values were determined for the pure and treated ceramics, viz. (i) at the onset of the crack initiation,G _ic, (ii) at the arrest of a subcritical crack, G_ia and (iii) at the onset of the fast fracture,G _if. Two analyses of the experimental data were carried out, viz. method of extrapolation and statistical analysis. In case of the pure YSZ, a transgranular mode of the stable crack growth was identified to be predominant. The porous coating treatment appeared to have positive effects as the crack initiation resistance increased due to electrode layers. The stable crack growth behaviours of the ceramics were investigated by monitoring the crack growth velocity as a function of appliedG values. The results obtained were of direct significance in designing and fabrication of SOFC stacks.     

Fracture Resistance of Residually-Stressed Ceramic Laminated Structures

We have studied the effect of residual stresses on fracture resistance and crack arrest behavior of asymmetric ceramic laminated Si ₃ N ₄/Si ₃ N ₄–TiN structures. Using the compliance method, we assessed the technique of R-curve construction for laminar composites. For laminar structures with layers varying by their elastic characteristics we developed an analytical method for calculation of fracture resistance – crack length dependence. The method applicability is verified by calculation of stress intensity factors for laminar specimens with an edge crack. The calculated results are compared to the experimental data.     

Fracture Resistance of Brittle Materials Under Local Loading by Scratching to Edge Chipping. Part 2. The S+EF Relation for Linear Elastic Ceramics

Strength of Materials - Experimental studies to evaluate the fracture resistance of brittle materials by Rockwell scratching of the specimen surface to its edge chipping (S+EF method) are...     

氧化锆层状复合陶瓷表面压应力与相变增韧的关系

利用维氏硬度仪和X射线应力分析仪、X射线衍射仪等手段分别对单层和层状氧化锆陶瓷进行了力学性能测试和分析,研究结果表明,在ZrO2层状复合陶瓷中,压痕裂纹的形成除了因塑性区体积变化产生的残余应力外,还与相变应力和表面压应力有关,表面压应力对表面裂纹具有较大的抑制作用.层状陶瓷断裂韧性提高,主要是通过表面压应力对压痕裂纹区应力强度因子的贡献、提高断裂相变量,强化相变增韧效果、细化晶粒等几个方面来实现的.     

Temperature Dependence of Brittle Fracture Toughness of Reactor Pressure-Vessel Steels upon Ductile Crack Growth

The main mechanisms of brittle fracture upon ductile crack growth are studied on the basis of the probabilistic model of brittle fracture and the deterministic model of ductile fracture, which were put forward by the authors earlier. The investigations are carried out on the reactor pressure-vessel steel 15Kh2NMFAA in the initial and embrittled states. The dependences of brittle-fracture probability on the stress intensity factor and the value of ductile crack growth are calculated for various temperatures. The temperature dependence of brittle fracture toughness in the initial and embrittled states is predicted with and without regard for ductile crack growth. The authors analyze the main factors that govern the above-mentioned relationships. The calculated results are compared to test data for CT-type compact specimens.     

QP980板材边缘开裂的实验研究和数值模拟

目的 冲裁加工后的第3代先进高强钢QP980板材在成形中会因边缘开裂而显著影响汽车结构件安全性,针对这一问题,对QP980板材边缘开裂行为进行研究。方法 使用QP980板材通过钻孔和冲孔2种方法制备不同边缘状态的试样,并进行扩孔和中心孔拉伸试验。分析不同边缘状态试样的扩孔率和断裂应变演化规律。采用DF2015断裂模型对QP980板材的韧性断裂行为进行预测。结果 钻孔试样的扩孔率约为33%,冲孔试样的扩孔率约为24%。与钻孔试样的试验结果相比,DF2015断裂模型的模拟结果显示出了良好的预测性,但DF2015断裂模型无法准确预测冲孔试样的载荷–位移响应、扩孔率和断裂应变。结论 不同的预加工工艺导致QP980板材表现出不同的边缘开裂行为。中心孔拉伸试验结果与扩孔试验结果趋势一致,因此中心孔拉伸试验是研究边缘开裂的良好方法。钻孔预加工工艺可以保持板材的原始性能,而冲裁预加工工艺会导致板材边缘发生严重的预损伤。由于DF2015断裂模型未考虑预损伤,因此无法准确预测冲孔试样的边缘开裂行为。