A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II, Strength Method (original) (raw)
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Journal of The American Ceramic Society, 1981
The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.
Journal of the American Ceramic Society, 2003
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 MPa⅐m 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.
New developments for fracture toughness determination by Vickers indentation
Materials Science and Technology, 2004
Indentation is a traditional method used to determine the toughness of brittle materials. Different models are used for the calculation depending on the shape of the cracks that are initiated and developed as a result of the indentation. Recently it was observed that a transition between Palmqvist and median cracks is possible when increasing the indentation load. In the present study it is shown that this transition is not as sharp as is generally supposed, but is rather smooth. In these conditions standard calculation procedures cannot be applied. A new methodology is proposed here, which allows the calculation of a unique toughness value on the basis of the determination of the limits of the material cracking tendency.
Evaluation by indentation of fracture toughness of ceramic materials
Journal of Materials Science, 1990
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 (Krc) by indentation is presented. The K~c 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, Kic).
Dental Materials Journal, 2005
This study examined fracture toughness (KIc) measurements obtained using the indentation fracture (IF) method with a view to improving their reliability. The KIc values of five dental ceramics were measured using the IF method with five different formulas, and the single-edge precracked beam (SEPB) method was used as a control. The elastic moduli of the dental ceramics were evaluated by dynamic hardness test. Load conditions of the dental ceramics that produced a median/radial crack for the IF method formulas were investigated. Based on the resultant c/a and P/c1.5 values, the indentation load (P) required for median/radial crack occurrence varied greatly from 29.4 to 196 N depending on the ceramic used. Among the five formulas, none of the KIc values obtained by the IF method with Miyoshi's formula differed significantly (p >0.05) from the values obtained using SEPB method. These results suggested that, after an appropriate indentation load is determined, reliable KIc values for small dental ceramic specimens can be easily obtained using the IF method if Miyoshi's formula is used in combination with the dynamic hardness test.
Journal of Advanced Ceramics, 2013
In this article, a new method has been presented for the estimation of fracture toughness in brittle materials, which enjoys improved accuracy and reduced costs associated with fracture toughness testing procedure compared to similar previous methods, because a vast range of specimens with irregular cracks can be accommodated for testing. Micron-sized alumina powders containing 0.05 wt% magnesium oxide (MgO) nanoparticles were mixed and also together with 2.5 vol%, 5 vol%, 7.5 vol%, 10 vol%, and 15 vol% of silicon carbide (SiC) nanopowders separately. By making and testing various types of ceramics with different mechanical properties, and considering the irregular cracks around the indented area caused by Vickers diamond indenter, a semi-empirical fracture toughness equation has been obtained.