传统压痕法识别陶瓷材料断裂韧性的有效性研究

基于虚拟裂纹闭合法对传统压痕法测试陶瓷材料断裂韧性的数值模型进行计算,以此为基础,分析比较传统压痕法的几种典型公式识别陶瓷材料断裂韧性的测试误差和所测材料的适用范围。结果表明:传统压痕法的Anstis公式较Evans公式,Lawn公式,JISR公式和Niihara公式,在材料比功0.3≤We/Wt≤0.45时所测断裂韧性值与理论计算值较为接近,其最大误差为12.9%,测试结果相对准确;当0.45We/Wt≤0.7时,传统压痕法对陶瓷材料断裂韧性的测试误差随比功增加迅速增大,特别是当We/Wt=0.7时,Anstis公式,Evans公式,Lawn公式,JISR公式和Niihara公式所测断裂韧性值与理论计算值的最大误差分别为70%,148.5%,48.8%,98.7%和166.6%,在此材料比功范围内传统压痕法所测断裂韧性值误差较大。     

The Essential Work of Fracture (EWF) method – Analyzing the Post-Yielding Fracture Mechanics of polymers

The Post-Yielding Fracture Mechanics describe the fracture behaviour of pre-cracked films and thin sheets that show yielding phenomenon at the crack tip during fracture. The Essential Work of Fracture method (EWF) has been used for this type of fracture characterization, determining two parameters: the specific work of fracture, w_e related with the real fracture process area, and the specific non-essential work of fracture, w_p that corresponds with the work done in the outer region of the crack tip.The EWF technique has been successfully employed especially with polymers, allowing the study of the influence of many variables in fracture properties, unavailable using other techniques such us K_IC or J_IC determination. In this work, the fundamentals of the technique and examples of application are reviewed, presenting a brief summary of the most relevant contributions of our group to the EWF method.     

On the Toughness Response of iPP and sPP/MWNT Nanocomposites

Nanocomposites based in polypropylene (both syndiotactic and isotactic) and multiwall carbon nanotubes (MWNTs) directly blended in the melt via micro‐extrusion, were studied. The aim was to determine the role of MWNTs, on the creation of the plastic zone and crack propagation in double‐cracked tensile specimens. Essential work of fracture toughness testing was performed on these nanocomposites. In all cases, the effect of MWNT weight fraction on the above properties was extensively studied. Data obtained have shown a non‐linear relationship of almost all properties with MWNTs content. Scanning electron microscopy revealed that a micro crack/craze bridging mechanism can be detected in polymer matrix/ MWNT nanocomposites. This mechanism was observed in the fibrillated areas of the specimens' fractured surfaces. The results obtained showed a promising role of the nanotubes as tougheners for future nanocomposites' applications. Ductile‐to‐brittle transition was confirmed for MWNT contents above 1 wt%.     

Polymer pre-notched small punch specimens to evaluate the essential work of fracture (EWF) parameters

Standardized deeply double edge notched tensile specimens (DDEN-T) have been usually used to assess fracture properties in thin polymer plates under plane stress conditions. The methodology is based on a multi specimen technique and is well known as the essential work of fracture (EWF) method. In some special circumstances these standard specimens cannot be machined as could be the case if the material is limited. A possible solution proposed in this paper could be the use of pre-notched small punch test specimens, in which the aim is to establish the feasibility of using pre-notched SPT specimens for evaluating the essential work of fracture parameters. The results obtained with these miniature specimens have been compared with those obtained from standard DDEN-T specimens.     

Prediction of Temperature Dependence of Fracture Toughness as a Function of Neutron Fluence for Pressure-Vessel Steels by Using the Unified Curve Method

Using the Unified Curve method as proposed by the authors earlier and a probabilistic model based on the local approach and physical background of the influence of impurities and carbide size on the local strength of material, a method has been elaborated for predicting a variation of the temperature dependence of fracture toughness with neutron fluence. The results calculated by this method are compared to the available experimental data.__________Translated from Problemy Prochnosti, No. 3, pp. 22 – 36, May – June, 2005.     

Fracture Toughness of Veneering Ceramics for Fused to Metal (PFM) and Zirconia Dental Restorative Materials

Veneering ceramics designed to be used with modern zirconia framework restorations have been reported to fracture occasionally in vivo. The fracture toughness of such veneering ceramics was measured and compared to that of conventional feldspathic porcelain veneering ceramics for metal framework restorations. The fracture toughness of the leucite free veneer was measured to be 0.73 MPa m ± 0.02 MPa m, which is less than that for the porcelain fused to metal (PFM) veneering ceramic: 1.10 MPa ± 0.2 MPa. (Uncertainties are one standard deviation unless otherwise noted.) The surface crack in flexure (SCF) method was suitable for both materials, but precrack identification was difficult for the leucite containing feldspathic porcelain PFM veneer.     

激光区熔Al2O3/Er3Al5O12共晶自生复合陶瓷的组织与断裂韧性

采用激光区熔快速定向凝固技术制备出Al2O3/Er3Al5O12(EAG)共晶自生复合陶瓷,研究了不同激光扫描速率下材料的凝固组织特征及其演变规律,并对其力学性能和增韧机制进行了分析.结果表明:激光区熔Al2O3/EAG由连续的Al2O3和EAG两相组成,两相相互交错分布,形成均匀的三维网状结构;共晶间距细小,在0.2～2.1μm之间,并随着扫描速率的增加规律性地减小.在低的扫描速率下,微观组织呈现典型的层片状非规则共晶组织;当扫描速率增至800μm/s时,出现了胞状及EAG树枝晶.共晶陶瓷的硬度和断裂韧性分别为18.7GPa和2.45MPa.m1/2.微观组织高度细化以及裂纹扩展过程中沿两相界面偏转、分叉等机制提高了材料的韧性.     

Influence of femtolaser notch sharpening technique in the determination of essential work of fracture (EWF) parameters

The fracture behaviour of a 0.5 mm thick ethylene-propylene block copolymer is analyzed using the essential work of fracture method, using DENT-type specimens. The influence of three experimental parameters in the technique is evaluated: the effect of the notch sharpening technique, the use of a videoextensometer for monitoring the deformations and the measurement of ligament lengths before or after fracture tests were carried on.The results showed that the femtosecond pulsed laser ablation technique (femtolaser) produced sharp notches with no plastic deformation ahead of the notch tip, which yield smaller specific essential work of fracture (w_e) values than in the cases where the notches were sharpened with razor blades. The use of a videoextensometer has allowed removing the viscoelastic energy from the plastic work, with lower values of βw_p. The measurement of the ligament lengths before or after the test did not affect the results.