A novel specimen for investigating the mechanical behavior of elastomers under multiaxial loading conditions

In this paper we describe the design and manufacture of an axial-torsion test specimen, and provide relationships needed when conducting stress-strain characterization experiments with the specimen. The specimen is a short hollow cylinder of rubber bonded between two steel mounting rings, in which simultaneous axial and shear strains are produced via independently controlled axial and twist displacements. We present calculations for the strain-displacement and stress-load relationships, and strain energy density. These relationships have been established and validated via a combination of analytical and experimental techniques, and finite element analysis. We have investigated the extent and effects of strain and stress field non-uniformity in the test specimen. The specimen design is sufficiently simple that a closed-form expression for the strain-displacement relationship has been successfully developed.     

电子探针定量分析的标准样品——几种多元氧化物单晶

在很高的温度和适宜的生长条件下,分别采用熔盐籽晶法和高温引上法生长了高质量的YAP、NAB、KTP、LN、BBO、SBN等多元氧化物单晶,它们具有优良的物理化学性能,严格的化学比,固定的组成与结构以及较好的化学均匀性和电子束轰击下的稳定性。广泛地用于激光和非线性光学领域。我们选用这些晶体为原材料研制电子探针定量分析的标准样品。经过测量和标定,这些单晶标样符合中华人民共和国国家标准GB 4930-85(电子探针显微分析标准样品通用技术条件)的规定。含有稀土元素的标样如NAB和YAP能发出绿色荧光,是电子显微术中理想的阴极发光材料。     

On the use of edge cracked short bend beam specimen for PMMA fracture toughness testing under mixed-mode I/II

In this paper an inclined edge cracked short beam specimen subjected to symmetric three-point bend loading was designed and examined for conducting mixed-mode I/II fracture toughness experiments. The aspect ratio (i.e. length to width ratio) and the loading span distance are considered much lower than the other conventional cracked bend beam samples. Crack tip parameters such as stress intensity factors and T-stress were computed numerically for this specimen by several finite element analyses and it was demonstrated that the specimen is able to produce full combinations of mode I and II including pure mode II. The practical capability of the short bend beam specimen was studied experimentally by conducting a set of mixed-mode fracture tests on PolymethylMethacrylate (PMMA) as a well-known model brittle material. The critical stress intensity factors, the direction of fracture kinking and the path of fracture trajectory were investigated both experimentally and theoretically using two stress and strain-based fracture criteria. The fracture toughness of tested PMMA was decreased by moving towards mode II case due to the effect of T-stress on the fracture mechanism of the short bend beam specimen.     

Biaxial tensile behavior and strength of architectural fabric membranes

The utilization of composite fabric membrane materials for large-span membrane structures has attracted considerable attention in recent decades due to enhanced material properties. Biaxial mechanical properties with respect to real engineering applications are essential and indispensable in comparison with uniaxial ones. This study focuses on true biaxial characteristics of a typical polyvinylidene fluoride (PVDF)-coated polyester membrane material in terms of stress-strain characteristics and breaking criteria.The true stress-strain curves obtained from an experimental study, i.e. seven loading ratios on the basis of symmetry and typical conditions, are investigated with digital image correlation method. The interpolation of these curves in combination of least square method achieves a three-dimensional strain surface as a function of warp and weft strains, which is useful to assess reasonable structural behavior. A new breaking criteria intended for architectural fabric membrane is proposed in analogy to Tsai-Hill, Yeh-Stratton and Norris failure criteria. The basic constants in the criteria are determined using experimental results. A comparative analysis between available uniaxial and biaxial criteria shows that the new criteria can cover all criteria due to the fact that biaxial mechanical properties are larger than uniaxial ones. Furthermore, a similar but glued specimen is employed to compare welded specimens. It is obtained that observations, values and curve tendency are similar, demonstrating the suitability of using new specimens to identify true biaxial properties.     

基于紧凑拉伸剪切结构的复合型疲劳裂纹扩展研究

针对含I-II型复合裂纹的紧凑拉伸剪切(CTS)试样,研究了不同加载角度下的裂纹扩展路径及裂纹扩展寿命,通过实验数据给出了适宜于CTS试样的等效应力强度因子关系式,并基于此提出了一种新的I-II型复合裂纹扩展模型。研究表明,CTS试样的裂纹沿与加载方向近垂直的方向扩展,基于Tanaka公式的等效应力强度因子更适合于本文CTS试件的裂纹扩展寿命评估。当加载角度处于0°~45°之间时,提出的复合型裂纹扩展模型预测误差控制在5.49%之内,验证了分析模型的可行性和准确性。     

Computation of mixed mode stress intensity factors in a four-point bend specimen

Four-point bend specimen is one of the most important specimens of the fracture mechanics because it can produce mixed modes I and II. Therefore, computation of stress intensity factors in this specimen is of practical interest. Several relations have been suggested that no one of them has completely considered the effects of the loading point and crack geometry. In this paper, mixed mode stress intensity factors of the bend specimen are computed by finite element method (FEM) and after validating by comparing with the available results in the literature, the results will be assessed to determine the effects of different crack location and loading distances from the middle of the specimen. Finally, two new coefficients comprising these effects are introduced.     

Evaluation of hybrid laser-cut and cast miniature fracture specimens

This paper presents a new hybrid laser-cutting method for producing fracture test specimens from thermosetting materials. The hybrid approach combines casting of a sheet of material with subsequent laser-cutting of the test specimens. The new approach was compared to the traditional casting method using a fracture toughness test. For this study, a compact version of the tapered double cantilever beam (cTDCB) was used as a specimen geometry for both manufacturing methods. The cTDCB specimen is crack length independent, and crack length investigations were performed to ensure the crack length independence of the cTDCB specimens. The specimens that were made by the hybrid laser-cut method were found to be comparable to the specimens obtained by the traditional casting method. Moreover, the laser-cut method provides a fast and accurate method to make a significant number of samples in a reasonable time. These tests show that the hybrid laser-cut method could be a good alternative to the traditional casting method.     

一种精确测量液体折射率的方法

本文介绍了一种在迈克尔逊干涉仪光路加入一楔形样品池进行液体折射率测量的方法。     

陶瓷阻力曲线测定的新方法——断裂栅法

将零厚度单向试件栅刻蚀工艺应用于晶须增韧氮化硅陶瓷基复合材料表面，通过记录试件所在桥路的输出电压值，可得到随载荷不断增大裂纹逐渐向前扩展过程中每一时刻的裂尖位置和裂纹长度，从而得到该种材料的阻力曲线。     

侧槽的拘束作用及对断裂韧性参量的影响

本文研究了侧槽对预制疲劳裂纹Ｃｈａｒｐｙ尺寸试样的拘束效应以及其对断裂韧性参量的影响。当侧槽深度大于临界值时，峰值载荷韧性Ｊｍ值与起裂韧性Ｊ１值基本相等。侧槽对试样的加厚作用可用试样的附加厚度表示，对于同样的侧槽深度，不同材料具有不等的附加厚度，试样取得最大附加厚度的侧槽深度为３３％。文中提出用拘束系数定量地估计侧槽对裂纹尖端的塑性拘束作用，能较好地解解放本试验结果。