On the estimation of the toughness of surface microcracks

Hardness testing is a method frequently used for evaluating the resistance of body surfaces to the influence of contact loading. The Vickers hardness test is applied for this purpose in a case when the material is sufficiently ductile and no cracks occur in the corners of the indents. Testing of the surface hardness of such materials as glass and ceramics on the basis of the Vickers testing method must take into account the energy that is spent on crack spreading. This paper describes a more exact and accurate method for evaluating resistance to microcrack formation on the surface of a material. To evaluate the conditions for crack spreading, it is necessary to test a specimen under loading in bulk. The suggested procedure involves a bent strip. As bending stresses are known, from the differences in crack length along and across the strip, the resistance of the material surface to crack propagation is determined.     

Hydrogen enhanced crack propagation of SCM440H low-alloy steel under long-term varying load

Crack propagation behavior of SCM440H low-alloy steel enhanced by absorbed hydrogen is investigated. Six materials tempered at different temperatures are used. Effects of stress ratio, loading frequency, hold time and material hardness on the crack propagation rate are examined under long-term varying load. Tests have been performed under continuous hydrogen charging, in which the crack tip has been isolated from the electrolyte and kept dry. An acceleration of crack propagation rate about six times compared to the uncharged material is found in all materials. In addition to this, however, an unexpected acceleration of crack propagation up to 1000 times is experienced under certain conditions. In materials with Vickers hardness higher than 280 tested at low frequency, the above marked acceleration is experienced. The crack surface morphology is quasi-cleavage. This critical hardness (HV = 280) is a little lower than the usually accepted critical hardness for delayed failure (HV = 350). In material with Vickers hardness lower than 268, however, such a marked acceleration is not experienced.     

Frost salt scaling resistance of concrete containing CFBC fly ash

The possibility for using coal combustion by-products in concrete exposed to frost-salt aggression was investigated. The research was aimed to assess an influence addition of circulating fluidized bed combustion (CFBC) fly ash on frost-salt scaling of air-entrained concrete. For evaluation of the resistance of concrete to frost salt scaling the test called “depth sensing indentation” (DSI) was applied. The DSI test method was implemented on a universal testing frame using a standard Vickers indenter. Experimental tests were performed on cement paste specimens and concrete specimens designed with partial replacement of cement with coal combustion by-products. The mass of scaled material in standard frost salt scaling resistance tests on concrete was inversely proportional to the Vickers hardness of the paste containing CFBC fly ash; the best-fit arithmetic relationship is provided.     

Indenter surface area and hardness determination by means of a FEM-supported simulation of nanoindentation

Stress–strain curves are obtained through a finite element method (FEM) simulation of nanoindentation, and the actual indenter tip geometry is determined by additional experimental and FEM-based procedures. Based on such material stress–strain laws and on the actual indenter tip geometry, the following are determined employing the “HANI” algorithm (HArdness determination by means of a FEM-based simulation of NanoIndentation): first, the contact surface due to elastic and plastic deformations during the loading phase of nanoindentation; second the occurring impression geometry after unloading and finally the related hardness values after Martens, Vickers, etc.Moreover, the indenter surface area functions of Vickers and Berkovich indenters are determined experimentally/analytically, by nanoindentations on Si(100) reference material of known Martens hardness. Applying these functions, Martens and Vickers hardness are determined correspondingly for various materials and they are compared to hardness values obtained by the “HANI” algorithm. Significant deviations occur, if the hardness of the reference material is quite different than the hardness of the test pieces.     

角焊缝维氏硬度压痕直线行排列试验的新操作方法

根据设计和标准的要求,在角焊缝中心位置45°角处均匀测量维氏硬度,必须保证硬度压痕间隔为1 mm,角焊缝、热影响区和母材等部位的维氏硬度压痕呈直线行排列。笔者选用了HV-120型维氏硬度计,载荷98 N。为了达到标准要求,在硬度计工作平台上放置两块方型磁铁,以固定试样上、下不可移动,用游标卡尺的精度来保证压痕的间隔为1 mm,用45°角的三角板来确保角焊缝的中心线为45°。用这种方法进行维氏硬度试验,操作简单,压痕成为直线行排列均匀,测试结果准确可靠,满足了设计和标准的要求。     

Crack propagation in a strip of material under plane extension

The problem of a uniformly propagating finite crack in a strip of elastic material is solved using the dynamic equations of elasticity in two-dimensions. Two specific conditions of loading on the strip with finite width are discussed. In the first case, the rigidly clamped edges are pulled apart in the opposite directions. The second case considers equal and opposite tractions applied to the crack surface. By varying the strip width to the crack length ratio, the amplitude of the dynamic stresses ahead of the running crack is determined as a function of the crack velocity. The local dynamic stresses are found to be lower than the corresponding static values for the displacement loading condition and higher for the stress loading condition. This effect becomes increasingly more important as the crack length to strip width ratio is enlarged. Numerical results for the dynamic crack opening displacement are also presented.     

Mechanical properties of and critical conditions for crack initiation in electronic glass

In this paper, the mechanical properties of electronic glass are tested using a combination of the Vickers indentation test and a multiple-loading nanoindentation test to obtain the elastic modulus, Poisson’s ratio and hardness values. The basic mechanical property parameters of the electronic glass and its stress–strain curve are found using atomic force microscopy analysis of the indentation morphology. The critical pressure and depth for crack initiation and the corresponding load and depth can be obtained during vertical loading on the electronic glass. When cracks extend to the surface, the results show that the electronic glass is isotropic. Several loading cycles causes a fatigue effect on the surface of the electronic glass, which decreases its elastic–plastic response. While the loadings are increasing, the elastic–plastic response rates are decreasing bur it rends stability finally. These results can provide a reference and guide for micro machining and surface microstructure machining of electronic glass.     

维氏压痕对常压固相烧结碳化硅陶瓷材料力学性能的影响

为了研究维氏压痕裂纹对常压固相烧结碳化硅陶瓷(SSiC)材料力学性能的影响, 通过扫描电镜观察了0.1~100 N的压痕载荷下产生的表面裂纹及裂纹剖面的状况, 并测试了相应载荷下的力学性质, 探讨了压痕法测量SSiC材料硬度、韧性等力学性质的适当压力载荷. 结果表明, SSiC材料压痕裂纹起始的临界压力载荷介于0.1~0.2 N; 当压痕载荷小于0.5 N时, 裂纹尺寸小于5 μm, SSiC材料的平均弯曲强度受影响程度较小. 此外, 当压痕载荷为10 N以上时, 压痕法测得的维氏硬度值趋近定值, 且所得到的裂纹是半圆形裂纹, 因此, 10 N为采用压痕法准确测量SSiC材料硬度及韧性的最低压痕载荷值.     

基于小波的维氏硬度图快速自动分析新方法

提出一种用小波多分辨率理论来对马氏体材料表面硬度试验压痕图进行自动分析的新方法，采用快速数据采样策略，自适应调整压痕区域的采样间隔，确保分析速度和精度，实现了自动计算维氏硬度。该方法同样能够很好地自动分析一些与马氏体组织在图像纹理结构方面相类似的组织维氏硬度，如回火屈氏体、回火索氏体等。     

Crack propagating in a functionally graded strip under the plane loading

In the present paper, a finite crack with constant length (Yoffe type crack) propagating in the functionally graded strip under the plane loading is investigated by means of the Schmidt method. By using the Fourier transform and defining the jumps of displacement components across the crack surface as the unknown functions, two pairs of dual integral equations are derived. To solve the dual integral equations, the jumps of the displacement components across the crack surface are expanded in a series of Jacobi polynomial. Numerical examples are provided to show the effects of the material properties, the thickness of the functionally graded strip, and speed of the crack propagating upon the dynamic fracture behavior.     

维氏硬度试验中测量不确定度的评定

介绍了硬度测量不确定度的原理和方法。利用无偏差估计方法和考虑硬度计偏差的方法对维氏硬度试验中的测量不确定度进行了评定。并对两种方法评定的不确定度结果做了比较。     

On the Moving Griffith Crack in a Nonhomogeneous Orthotropic Strip

A finite crack with constant length (Yoffe type crack) propagating in the functionally graded orthotropic strip under the plane loading is investigated by means of the Schmidt method. By using the Fourier transform and defining the jumps of displacement components across the crack surface as the unknown functions, two pairs of dual integral equations are derived. To solve the dual integral equations, the jumps of the displacement components across the crack surface are expanded in a series of Jacobi polynomial. Numerical examples are provided to show the effects of material properties, the thickness of the functionally graded orthotropic strip and the speed of the crack propagating upon the dynamic fracture behavior.     

硬度法测定渗氮层深度结果的测量不确定评定及分析

主要以综合评定法对硬度法测定渗氮层深度结果的测量不确定度来源进行了分析,分别讨论了硬度法测定总渗氮层和有效渗氮层深度由于界限硬度值要求不同,导致引入的标准不确定度分量不同,并对每个标准不确定度分量进行了评定。结果表明：在相同试验条件和方法下,硬度法测定总渗氮层深度结果测量不确定度明显小于硬度法测定有效渗氮层深度结果测量不确定度。其原因在于试验测定结果重复性引入的标准不确定度分量是硬度法测定总渗氮层深度结果测量不确定度主要来源;而硬度法测定有效渗氮层深度结果测量不确定度主要来源却是由计算模型中插入的界限硬度值引入的。另外,采用硬度法测定深度,应注意硬度计工作台移动的分度值和垂直度偏差对测量结果的影响。     

空间滑动电接触Au镀层硬度测试方法研究

为了获取一种准确有效的Au镀层显微硬度测试方法以满足航天领域的应用需求,本文采用显微维氏硬度法及纳米压痕硬度法研究了Au镀层在不同测试方法及不同测试载荷下的显微硬度值及其标准方差,并依据相关检测技术标准的要求,就检测方法的可行性进行了分析。结果表明:本文优选的Au镀层显微维氏硬度检测参数及测试数据的判定范围满足标准QJ482,同时还能满足压痕深度小于镀层厚度的1/7,避免了基体对测试结果的影响,测试方法可行,测试数据有效。     

Investigation of Leakage on Water Wall Tube in a 660 MW Supercritical Boiler

The leakage of a supercritical boiler tube has been investigated. The leakage was observed during a working pressure hydrostatic test under 40.1 MPa. The internal geometry of the investigated leaking tube was multi-lead ribbed tube. Various methods for investigation namely radiography, microstructural analysis using optical and scanning electron microscopes, Vickers hardness testing and chemical analysis were conducted to investigate the main cause of leakage. The results revealed that the material tube met the standard for supercritical boiler tube (ASTM A 213 Grade T12). From the microstructural analysis, there was a crack, which initiated from the outer wall adjacent to the weld metal and propagated to the inner wall of the tube. The crack was due to a weld defect known as “excessive melt through” which is due to overheating during the welding process in joining the tubes. It is suggested that control of welding process parameters must be accomplished during fabrication to make a furnace wall, and placing strip of steel between the tubes to reduce the fit-up gap should be considered.     

国内外金属材料焊缝维氏硬度试验的标准化现状

介绍了国内外金属材料焊缝维氏硬度试验标准体系的最新进展,对比分析了我国现行金属材料焊缝维氏硬度试验标准体系中存在的不足.与国际标准相比,我国现行部分金属材料焊缝维氏硬度试验标准标龄较长,需加快修订.此外,要加强标准间相互引用,实现标准之间的方法统一.     

Vickers indentation behavior of several commercial glasses at high temperatures

The Vickers indentation behavior of five commercial glasses has been investigated as a function of temperature. The glasses included: (i) soda-lime-silica Float glass, (ii) lead-alkali silicate, (iii) 7740 Pyrex™ borosilicate, (iv) potassium phosphate, and (v) lanthanum borate. A recording microindentation system was constructed to allow Vickers indentation testing to be conducted at temperatures significantly above room temperature. The Vickers hardness was observed to decrease continuously with increasing temperature for all glasses, with the exception of the 7740 Pyrex™ glass. Decreases in hardness were attributed to decreases in elastic moduli and bond strength with increasing temperature. The lengths of median-radial cracks around indentations in several glasses were observed to first increase, and then decrease, with increasing temperature. The first of this behavior was attributed to initial increases of the crack driving force, characterized by the quantity (E/H), as well as to a decrease in fracture surface energy. Viscous flow at higher temperatures was believed responsible for a reduction in the crack driving force and crack tip stress, resulting in an eventual decrease in crack length. Viscoelastic behavior of the Float glass was characterized by a rate-dependent hardness and indentation crack pattern. Load–displacement traces indicated an increase in the work of indentation and residual indentation depth with increasing temperature for all glasses.     

Retrogression and re-aging of 7010 open die forgings

Abstract

Open die forgings of the aluminium alloy 7010 (DTD5636) have been retrogressed and re-aged at 200 and 240°C to improve the combination of strength and stress corrosion cracking resistance. Vickers hardness, electrical conductivity, tensile, plane strain fracture toughness and constant crack opening displacement stress corrosion cracking test data are reported. A retrogression treatment of 40 min at 200°C produces material with T6 levels of strength and fracture toughness, in combination with T74 levels of stress corrosion cracking resistance.     

ZM6尾减机匣开裂失效分析

某型号飞机尾减机匣试验过程中出现异常,荧光检查发现裂纹.通过故障机匣外观检查、裂纹断口宏微观观察及能谱分析、材料硬度检测、金相检查等试验得出结论,尾减机匣失效属低周疲劳开裂,机匣表面Nd元素偏析是导致机匣疲劳裂纹形成的直接原因,材料韧性不足一定程度上促进了裂纹的疲劳扩展,根据分析结果,给出提高尾减机匣受力部位的疲劳抗力,避免故障再次发生的有效方法.     

Estimation of the elasto-plastic properties of metallic materials from micro-hardness measurements

Although hardness testing is a cost-effective and reliable technique to quickly estimate the overall mechanical properties of materials, hardness is not a fundamental mechanical parameter and it is rarely used in constitutive modeling or in engineering design. This work proposes a procedure to derive the elasto-plastic properties of metallic materials from the Vickers and the Knoop hardness measurements. Through dimensional analysis and finite-element simulations, relationships between hardness testing variables and mechanical parameters (Young’s modulus, yield strength, and strain hardening exponent) are built up. An inverse procedure is developed to derive the mechanical parameters within the framework of genetic algorithm. The method is verified against two bulk steel materials and it is shown the derived stress–strain curves are in good accord with those measured by tensile tests. Then the method is successfully applied to measure the elasto-plastic properties of the pore walls of a metallic foam material. The present method can achieve a performance as good as those based on the instrumented indentation test and it is more readily accessible to the industry considering all the measurements can be performed on a cost-effective micro-hardness tester.