Investigation on characteristics of R-curve for engineering ceramics

R-curves for self-toughened HPSN are established by direct measurements of the change in length of Vickers indentation cracks during pure bending test. A modified exponential relationship is selected to describe the experimental results, the physical meanings and dependent factors of specified parameters of R-curve behavior for ceramic materials are discussed. The results show that the new empirical equation is applicable in fitting the R-curves more gracefully, and,the obtained parameters all have distinctive physical meanings. The fitting parameter K-0 represents the threshold crack resistance K-th. The random of the crack growing path and corresponding major toughened mechanisms for different crack are appeared by the obvious dispersion of lambda, but the overall property for the materials is given by its total average value <(<lambda>)over bar>. Whatsmore, it is found that the longer the initial crack length is, the bigger the crack tolerance Deltac* is, and from it, the big crack methods for R-curves research are unified with the little crack methods. The little cracks in the materials are more dangerous than big ones is indicated in further.