Difference in subsurface damage in indented specimens with and without bonding layer

This paper aims to assess the applicability of the bonded–interface technique (BIT) that has been used for examining sub-surface damage in brittle materials. With the aid of the finite element method, the indentation stress fields in alumina specimens with and without a bonded–interface were analysed. It was found that the bonded–interface greatly alters the stress distribution in the neighbourhood of the interface. The high-stress zone shifts away from the interface, and extends to the surface. Both glue layer mechanical properties and bond thickness play a limited role in the overall stress field of the BIT alumina. Comparisons of theoretical predictions with experimental observations showed that, to a great extent, the BIT presents a different pattern of sub-surface damage. The study clarifies the applicability of the BIT and offers a useful guideline for practitioners.     

Determining the Toughness of Ceramics from Vickers Indentations Using the Crack-Opening Displacements: An Experimental Study

Recently, a method for evaluating the fracture toughness of ceramics has been proposed by Fett based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. To verify this method, experiments have been conducted to determine the toughness of a commercial silicon carbide ceramic, Hexoloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. Although the obtained toughness value of K _o= 2.3 MPa·m^1/2 is within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness display poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on direct observations of lateral subsurface cracks below the indents.     

Indentation Determination of Crack Growth Parameters in Gallium Arsenide

Indentation measurements of the environmentally enhanced crack growth parameter, n , in GaAs have been made in water, acetonitrile, and methanol. Reasonable agreement with n values obtained from double-cantilever-beam tests was obtained. It is concluded that the range of n values which can be measured by the indentation technique is much wider than previously observed, and is strongly dependent upon the experimental configuration employed.     

仪器化压痕试验法测定金属材料的硬度与材料参数

仪器化压痕法是一种新兴的试验方法，不仅可以测量金属材料的硬度，还可以测定金属材料的其他参数。针对该方法的基本原理、特点以及硬度和其他材料参数的测定进行了介绍。     

Frictional contact analysis of composite materials

In this paper, the highly non-linear frictional contact problems of composite materials are analysed. A proportional loading, the potential contact zone method and finite element analysis are used to solve the problems. A tree-like searching method is used to obtain the solution of the parametric linear complementary problem, which may overcome the anisotropic properties of contact equations caused by composite materials. In the frictional contact analysis of composite materials, the distributions of normal contact pressures, tangential contact stresses and relative tangential displacements are presented for different contact material systems and different coefficients of friction. The results show that the solutions in the paper have good agreement with Hertzian solutions. The influence of different contact material systems and different coefficients of friction on the contact stresses and displacements is large. As a numerical example, ball-indentation tests of composite materials are modelled by the three-dimensional finite element method.     

Indentation toughness of Mo5Si3—based alloys

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Effect of curing moisture on the indentation force deflection of flexible polyurethane foam

The moisture effect on the indentation force deflection (IFD) of flexible polyurethane foam was examined through the curing of the foam with different moisture contents. The employed foam was cured at 30, 40, 50, 60, 70, and 80% relative humidity, and the corresponding curing temperature was constant at 23°C. The experiments showed that the measured IFD and support factor of the flexible polyurethane foam decreased, and the hysteresis increased with the curing moisture. A statistical conversion formula was generated experimentally to correct the measured IFD of the flexible polyurethane foam to eliminate the deviation caused by the moisture effect. The measured IFD was inversely exponentially proportional to the curing moisture. The accuracy of the correction formula was verified by the conditioning of the foam again with different moisture contents. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1679–1682, 2007     

柔度标定误差对仪器化压入测试结果的影响

针对两种仪器化压入仪和两种代表性压入测试方法:Oliver-Pharr方法和Ma方法,通过有限元数值模拟分析了仪器柔度标定误差对2种仪器和两种方法测试精度的影响.结果表明仪器柔度的标定精度直接影响压人测试结果的准确度,仪器柔度越小,测试精度越高;就测试方法而言,Ma方法具有比Oliver-Pharr方法更高的精度和更低的仪器柔度敏感性;对同一材料,压入深度越大,由仪器柔度标定误差引入的压入测试结果误差越大;当材料较硬且压入深度较大时仪器柔度的标定尤为重要,小量的标定误差导致测试结果严重偏离真值,甚至为负值.     

残余应力测试技术的进展与动向

针对几种常见的残余应力测试方法的研究进展与应用现状,指出了较为成熟的残余应力测试方法是盲孔法和X射线法,但盲孔法受限于对测试对象有损伤,X射线法受限于测试条件的苛刻,其它的残余应力测试方法在有效性、准确性和操作简便性等方面还不能很好地满足工业需求.而利用压痕法测量残余应力是一个值得关注的研究动向,在局部压载作用下,材料...     

Load dependence of the apparent Knoop hardness of SiC ceramics in a wide range of loads

Silicon carbide (SiC) ceramics is a material with increasing use, due to its excellent mechanical properties, especially high hardness. In order to integrate this material into design process, we need to know its hardness as precise as possible. The Knoop hardness number (HK) is calculated using the expression: HK = α·F/d², where F is the applied load, d is the long diagonal of the resulting 1₀indentation and a is the Knoop indenter geometrical constant. In this paper, the Knoop hardness of SiC ceramics was measured in the applied load range from 4.9 to 98.07 N. For some materials measured “apparent” hardness value decreases with increasing applied test load (normal indentation size effect – ISE), while for some materials measured “apparent” hardness increases with increasing applied test load (reverse indentation size effect – RISE). Obtained results show the measured hardness exhibits the ISE. In the literature several models are given for the phenomenon explanation. We used the following models: Meyer's law (F = K·dⁿ), proportional specimen resistance – PSR (F = a₁·d + a₂·d²) and modified proportional specimen resistance – MPSR model (F = a₀ + a₁·d + a₂·d²). Results of regression analysis for all applied models show they can all be used for ISE analysis. “True” hardness was determined based on the PSR and MPSR model (HK_T = α·a₂). The obtained results were similar. If the specimen surface is carefully prepared and the range of loads is wide, the a₀ coefficient from MPSR model reaches small values and can be excluded. Therefore, for the calculation of SiC ceramics Knoop hardness, the simpler model (PSR) can be used.