Palmqvist fracture toughness of a new wear-resistant weld alloy
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INSPIRED by the published suggestions of Ammann and Hinnuber[1,2] nearly a decade earlier in the early 1950s, Palmqvist experimented in the 1960s with the idea that surface crack lengths originating at the four corners of a Vickers hardness indentation generated by an applied load would be a measure of the critical stress intensity factor KIC, more commonly known as fracture toughness, in cemented carbides.[3,4,5] This technique is characterized as an indentation method, where a Vickers diamond pyramid indenter is impacted into the surface of a specimen by a known load that is greater than a critical threshold load[6,7] in order to cause cracks at the corners and subsequently the crack lengths are measured. It is thought that cracks are nucleated and propagated by unloading the residual stresses generated by the indentation process.[8] As would be expected, when the indentation load is increased, the total length of cracking also increases. It is significant that Palmqvist was able to identify the important experimental factors such as hardness that contribute to the fracture toughness in order to measure it. He observed that there was a linear relationship between the applied load to the indenter and the total crack length after the indenter was removed. The linear relation was confirmed by Dawihl et al.[9] and Exner.[10] The slope of this linear relation, along with the hardness, is used to compute the fracture toughness. In spite of his insight and the success of this research, the technique was not widely used as attested to by a review article published in 1975 by Lawn and Wilshaw,[11] which states, ‘‘The full potential of this method as a quantitative tool . . . has yet to be realized.’’ It appears that this technique was not widely known until the work of Evans et al.[12] and Venables et al.[13] in 1976 and 1977, respectively. Evans’ research discovered that there was a universal relation between the fracture toughness and crack length that was independent of all materials. Specifically, their analysis B.E. MEACHAM, Director, M.C. MARSHALL, Research Engineer, and D.J. BRANAGAN, Chief Technical Officer, are with The Institute of Nanomaterials Research and Development, The NanoSteel Company, Idaho Falls, ID 83402. Contact e-mail: [email protected] Manuscript submitted January 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
could fit a single equation to data taken on polycrystalline ceramics such as B4C, SiC, and Si3N4 and cermets such as WC-Co. Venables’ work was only on WC-Co and showed how the Palmqvist toughness W, defined as the ratio of load to crack length, changed with Co binder content. This research seems to have started the wide application of the Palmqvist method in ceramics and cermets. One of the limitations, as pointed out by Peters,[14] was a lack of any ‘‘theoretical correlations between the Palmqvist toughness and conventional mechanical parameters.’’ Eventually, two theoretical models were developed and applied to experimental data. The first theoretical model that was developed, and l
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