Fracture Toughness of Ceramics by the Vickers Indentation Crack Length Method: a Critical Review

Fracture toughness is an important property that characterizes a material’s brittleness or resistance to fracture. Although some fracture toughness test methods have been refined and even standardized, many researchers have continued to use the Vickers indentation crack length method as an expedient. In this paper, Vickers indentation fracture resistance data for Standard Reference Material SRM 2100 are presented and the weak points of the method are reviewed. INTRODUCTION The state of the art of fracture toughness testing has significantly improved in recent years. Several methods have been painstakingly refined and optimized. ASTM International, Japanese Industrial Standards (JIS), and European Committee for Standards (CEN) formal standard test methods are available for the single-edged precracked beam (SEPB), chevron notched beam (CNB), and surface crack in flexure (SCF) methods. These methods have single well-shaped precrack, a good loading configuration, and an accurate stress intensity factor solution. They are now approved as standards by the International Organization for Standards. International round robins have confirmed that accurate and precise data are obtained with these methods. Other test methods such as double torsion and single-edged V-notched beams (SEVNB) currently are being developed as standards. Those who have developed and refined these test methods have striven to make them technically rigorous, accurate, precise, and practical. In the 1970s and 1980s there was considerable confusion about fracture toughness testing. Results were quite variable and seemed to depend upon the test method. Slow crack growth and R-curve phenomena contributed to the data variability. Some even wondered whether there was such a thing as a set value for fracture toughness, KIc, since all tests seemed to give different outcomes. For example, it was common to hear that “large crack” fracture toughness specimens gave different outcomes than “short crack” specimens. The test methods were gradually refined and the signs of slow crack growth and /or R-curve behavior were recognized. Testing procedures were adapted to either minimize their effects or to study them. Despite these complications, it is now known that many brittle materials do have a specific fracture toughness in the classical sense. These include fine-grained polycrystalline ceramics and coarse-grained ceramics that fracture transgranularly without bridging. For example, pressureless sintered alpha silicon carbide with boron and carbon sintering aides has no boundary phase. It fractures transgranularly and has a fracture toughness of about 2.5 to 2.9 MPa√m depending upon the density and batch and the result does not depend upon the test method. Draft 3 Accepted for publication.