Evaluation by Vickers indentation of fracture toughness of a phosphate biodegradable glass

Indentation tests are commonly used for the evaluation of fracture toughness of brittle materials, particularly glasses and ceramics, because this technique requires only a small polished area on the specimen surface from which a large number of data points can be generated rapidly. However, a wide variety of equations for the calculation of fracture toughness of ceramic materials by means of Vickers indentation are available. Such equations are obtained phenomenologically and their parameters adjusted in such a way that the K_IC values obtained are in good agreement with those obtained by conventional methods. This is the reason why it is necessary to check which type of equation reproduces more accurately the results obtained by means of conventional methods for the material which is going to be investigated. In the present work seven different fracture toughness equations widely used in glass and ceramic studies are considered and the results are compared with those obtained by conventional methods, such as single-edge notch beam (SENB) specimens tested in three-point bending. The role played by the applied indentation load is considered. © 1999 Kluwer Academic Publishers     

Vickers indentation fracture toughness test Part 2 Application and critical evaluation of standardised indentation toughness equations

Abstract

The standardised indentation fracture toughness equations formulated in Part 1 have been applied to a range of brittle materials: namely, glass ceramics, aluminas, zirconias, and WC–Co cermets. Analysis of the results has enabled a critical assessment of (i) the ability of the nineteen equations to yield the same fracture toughness values as a conventional fracture toughness test and (ii) their ability to rank materials in order of fracture toughness. Also, specific equations have been recommended as being the most appropriate equations to use.

MST/1050b     

Vickers indentation fracture toughness test Part 1 Review of literature and formulation of standardised indentation toughness equations

Abstract

There is considerable interest in determining the fracture toughness of brittle materials by measuring the extent of cracking associated with a Vickers indentation because of the ease of specimen preparation and the simplicity of the test. However, confusion has been engendered by the multitude of models and equations in the literature relating the degree of cracking to the fracture toughness. In Part 1 of this work, nineteen of these equations are reviewed and then modified in a standard manner for both experimental convenience and direct comparison.

MST/1050a     

X-ray diffraction and TEM analysis of Fe–Al alloy layer in coating of new hot dip aluminised steel

Abstract

The phase structure in the Fe-Al alloy layer of a new hot dip aluminised steel has been researched by means of electron probe microanalysis, X-ray diffraction and TEM, etc. The test results indicated that the Fe-Al alloy layer of the new aluminised steel was composed of Fe₃Al, FeAl, a few Fe₂Al₅ and α-Fe (Al) solid solution. There was no brittle phase containing higher aluminium content, such as FeAl₃ (59.18 wt-%Al) and Fe₂Al₇ (62.93 wt-%Al). The tiny cracks and brittlement, formerly caused by these brittle phases in the conventional aluminium coated steel, were effectively eliminated. There was no microscopic defect (such as tiny cracks, pores or loose) in the coating. This is favourable for resisting high temperature oxidation and corrosion of the aluminised steel.     

Vickers压入识别氮化硅断裂韧性

依据仪器化Vickers压入氮化硅断裂韧性实验获得的有关压痕裂纹参数,通过有限元数值分析方法识别出氮化硅的弹性模量和屈服强度,进一步采用虚拟裂纹闭合法确定其裂纹尖端的应力强度因子KI。以此为基础,与氮化硅断裂韧性标准值对比,分析有限元仿真KIC结果和基于L-E-M模型建立的3种典型陶瓷断裂韧性压入测试方法的准确度。结果表明:基于Vickers压入有限元数值分析结果的最大误差仅为2.38%,Anstis公式最大识别误差为2.65%,而Lawn公式和Miyoshi公式的识别误差的绝对值均超过10%,因此Vickers压入测试具有较高测试准确度。     

Evaluation by indentation of fracture toughness of ceramic materials

A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials. A new expression to determine the fracture toughness (K _IC) by indentation is presented. The K _IC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load. This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, K _IC).     

Critical evaluation of indentation fracture toughness measurements with Vickers indenter on yttria‐stabilized zirconia dental ceramics

The purpose of this study was to investigate and analyze fracture toughness (K_Ic) of yttria stabilized tetragonal zirconia (Y‐TZP) dental ceramics by the Vickers indentation fracture test. In order to determine fracture toughness, the Vickers indenter was used under the load of 294.20 N (HV30). The cracks, which occur from the corners of a Vickers indentation, were measured and used for fracture toughness determination, through five mathematical models according to (I) Anstis, (II) Evans and Charles, (III) Tanaka, (IV) Niihara, Morena and Hasselman and (V) Lankford. Morphology of indentation cracking was determined by scanning electron microscope. The most adequate model for determination of fracture toughness (K_Ic) of yttria stabilized tetragonal zirconia dental ceramics by the Vickers indentation fracture test is Lankford model.     

Effect of strain rate on fracture toughness of mild steel

Abstract

The effect of strain rate on the yield strength and fracture toughness of mild steel was studied in the strain rate range 10^-5–10² s^-1. The lower yield strength was found to be independent of strain rate in the range 10^-5–10^-3 s^-1. Beyond this strain rate, the lower yield strength was found to vary linearly with logarithm of strain rate up to a value of 10² s¹. Fracture toughness was also found to increase with strain rate up to a value of 10^-3 s^-1 and subsequently decrease gradually with increasing strain rate. However, a drastic reduction in fracture toughness was observed at a strain rate of 10² s^-1. This reduction was attributed to the change in fracture mode from ductile to cleavage.     

The relationship between Palmqvist indentation toughness and bulk fracture toughness for some WC-Co cemented carbides

The Palmqvist indentation toughness (W) has been determined for a series of WC-Co hardmetals containing up to 10 wt % cobalt. Using a correlative parameter (the cobalt mean-free path) the Palmqvist toughness has been compared with true fracture toughness data (G _IC values) for similar materials, obtained by conventional testing techniques. It has been shown that a linear relationship betweenW andG _IC can be predicted, on the basis of a recent theory of the fracture process in hardmetals. The validity of such a relationship has been experimentally confirmed, over a limited range of hardmetal compositions and microstructures. The experimental difficulties and expense associated with fracture toughness determination for brittle materials such as hardmetals, by quasi-conventional techniques, are well known. It is concluded that the Palmqvist test provides a useful method of measuring fracture toughness on a routine basis, since it is simple to perform, non-destructive, and does not require a specialized testpiece geometry. Its application in hardmetal quality control is, therefore, indicated.     

Glass ceramics of high hardness and fracture toughness developed from steel fly ash

Abstract

A study has been carried out on the feasibility of using a high iron content fly ash, generated during steel making, as a raw material for glass ceramic production. The fly ash was mixed in different proportions with soda–lime glass cullet and sand. The devitrification of the parent glasses produced from the different mixtures was investigated using differential thermal analysis, X-ray diffraction and scanning electron microscopy. The mechanical properties of the glass-ceramics were assessed by hardness and indentation fracture toughness measurements. A glass ceramic with composition of 60 wt-% fly ash, 25 wt-% sand and 15 wt-% glass cullet and containing magnetite in a marked dendritic morphology exhibited the best combination of properties, namely, hardness 7·9 GPa and fracture toughness 3·75 MPam^0·5. These new hard glass-ceramics are proposed for wear resistant tiles and paving for heavy duty industrial floors. In powder form, the glass ceramics are candidate materials for blasting and abrasion applications.     

压痕测试法的新发展

介绍了压痕测试法的最新发展,指出了确定超薄薄膜t<10 nm及很硬的薄膜t<100 nm的硬度及模量是对计量学中纳米范围的挑战.以压痕测试法替代显微维氏实验可显著地降低测量不确定度及实现自动化.压痕测试法在纳米范围及显微范围内可为微电子学、微电机械系统、技术光学等领域采用.     

Determination of fracture toughness of 15Kh2NMFA vessel steel by kinetic indentation

A specimen free technique based on kinetic indentation for determination of the fracture toughness of materials is proposed. It is based on the iteration calculation of the specific indentation work which is taken to be equal to the fracture toughness of a material controlled at a defined indentation depth. This technique makes it possible to estimate the degree of embrittlement of materials of long-operating constructions by means of periodic natural nondestructive testing. Examples of the utilization of the technique for steels of nuclear reactor vessels are presented.     

Quasi-static fracture toughness testing of a single CT specimen at two test conditions

Abstract

It is proposed that a single CT specimen can be used for determining J ₀.₂ at two testing conditions, provided it can be ensured that the crack tip plastic zones for the two tests do not interfere. This is achieved by extending the crack at the end of the first fracture test by fatigue cycling at ambient temperature to obtain the starting crack for the second test. This method has been validated by testing thermally aged CT specimens of modified 9Cr - 1Mo steel at 653 K and 803 K. The J-Δa values obtained by a multispecimen method at a specific temperature were on a single curve irrespective of whether the data were generated from the first test or second test on that sample. Also, the J-Δa curves obtained using a single specimen normalisation method from data on first and second tests were within the expected specimen to specimen variation.     

Scratch test model for the determination of fracture toughness

We revisit the scratch test within the framework of linear elastic fracture mechanics. In the analysis, we employ an Airy stress function approach to determine stresses and displacement in the vicinity of the scratch-blade–material interface, which serve as input for the evaluation of the energy release rate by means of the J-Integral. In contrast to previous models, the energy release rate thus found scales with the sum of the applied forces squared. This entails a linear relation between the applied forces and , where w is the scratch width and d the scratch depth. This analytical scaling is validated using experimental scratch data on cement paste and sandstone, which shows that the proposed approach provides a convenient way to determine the fracture toughness from scratch tests carried out with different scratch widths and depths.     

A model for the evaluation of indentation crack arrest fracture toughness of supported films

A simple model is proposed for the evaluation of crack-arrest fracture toughness K _Ic0 of thin films by Vickers indentation. This approach applies to films thinner than the penetration depth of the Vickers indenter. The model equations are provided in closed form, even though they are so complex that they must be integrated numerically in practical applications. The problem of the evaluation of K _Ic0 for thin films and substrates is derived in general form and applied to three cases: (i) evaluation of K _Ic0 for the film in the case that the depth of the crack in the film is smaller than the film thickness, (ii) evaluation of K _Ic0 for the film in the case that the crack emanating from the film either crosses the film/substrate interface or is stopped by it, (iii) evaluation of K _Ic0 for the substrate in the case that the crack emanating from the film crosses the film/substrate interface. The model was tested with original and literature experimental data: (i) revision K _Ic0 values of electroless Ni-P thin films were re-evaluated, (ii) K _Ic0 of electroless Ni-P thin films of various thickness with various loads were measured (original data) and computed, (iii) K _Ic0 of electroless Ni-P substrates coated with electrodeposited Au-Cu were measured (original data) and computed.     

A method for dynamic fracture toughness determination using short beams

This paper deals with dynamic fracture toughness testing of small beam specimens. The need for testing such specimens is often dictated by the characteristic dimensions of the end product. We present a new methodology which combines experimentally determined loads and fracture time, together with a numerical model of the specimen. Calculations are kept to a minimum by virtue of the linearity of the problem. The evolution of the stress intensity factor (SIF) is obtained by convolving the applied load with the calculated specimen response to unit impulse force. The fracture toughness is defined as the value of the SIF at fracture time. The numerical model is first tested by comparing numerical and analytical solutions (Kishimoto et al., 1990) of the impact loaded beam. One point impact experiments were carried out on of commercial tungsten base heavy alloy specimens. The robustness of the method is demonstrated by comparing directly measured stress intensity factors with the results of the hybrid experimental-numerical calculation. The method is simple to implement, computationally inexpensive, and allows testing of large sample sizes, without restriction on the specimen geometry and type of loading.     

The determination of fracture toughness for a porous elastic-plastic solid

The near tip stress and porosity fields in a plane strain mode I crack under small scale yielding (s.s.y.) conditions have been studied using a constitutive model for porous elastic-plastic solids. These fields have been used in conjunction with criteria for fracture of tensile specimens to predict values of K_Ic for a mild steel over a range of temperatures. The regimes of dominance of fracture by slip-induced cleavage and void coalescence mechanisms have been determined. The predictions are shown to compare well with experimental data.

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Résumé On a étudié les champs de porosités et des contraintes au voisinage de l'extrémité d'une fissure soumise à un état plan de déformation de Mode I et présentant des conditions de plastification à faible échelle, en faisant appel à un modèle constitutif pour les solides élasto-plastiques. Ces champs sont utilisés en combinaison avec les critères de rupture d'éprouvettes de traction, afin de prédire les valeurs de K _Ic d'un acier doux dans une certaine gamme de température.On détermine les conditions de prépondérance dans la rupture des mécanismes de coalescence des lacunes et de clivages entrainés par glissement. II se trouve que les prédictions concordent bien avec les résultats expérimentaux.