Fracture toughness SEPB method

Yoshimura HN, Cesar PF, Miranda Jr WG, Gonzaga CC, Okada CY, Goldenstein H. Fracture toughness of dental porcelains evaluated by IF, SCF, and SEPB methods. Journal of the American Ceramic Society 2005 88(6) 1680-1683. 

EN ISO (2003) 15732. Fine ceramics (Advanced Ceramics, Advanced Technical Ceramics) - Determination of Fracture Toughness of Monolithic Ceramics at Room Temperature by the Sin e-edge Pre-cracked Beam (SEPB) Method, International Organization for Standardization. 

In this study, silicon nitrides with various microstructures from fine and uniform to coarse have been prepared and their RCF performances were measured using a ball-on-fiat method in lubricant oil. The obtained results were compared directly with the hardness and fracture resistance determined by the indentation fiacture (IF) method, as well as fiacture toughness from the SEPB method. The ability of the frmction of both hardness and fiacture toughness to rank the RCF performance was discussed in conjunction with their R-curve behavior, etc. 

Figure 6. NP versus fracture toughness determined by the SEPB method. Error bars are 1 stairdard deviation.

Table 1. Four point bending strength, fracture toughness measured by single edge pre cracked beam (SEPB) method, and Vickers hardness of the tested alumina specimens. Standard deviations are in brackets.

The CNB test specimen does not require crack length measurement, thereby providing some convenience to the user. However the SEPB and SCF test methods require quality measurements if accurate fracture toughness estimates are expected. For the SEPB test specimen, the precrack front is very difficult to delineate unless it leaves a crack arrest line. An arrest line is easily detected if the crack tip turns out of plane or is hooked, thereby making the precrack and fast fracture plane non-coplanar, as shown in Figure 7. In one study, some cracks were so planar that the front could not be detected with conventional optical microscopy [12]. In such cases, the use of dye penetrants have been successful as shown in Figure 8, however, in some cases penetrants tend to bleed on the fracture surface and the measurements must be made immediately. Dye penetrants must be used with caution as they may cause environmentally assisted crack growth. 

A large sample set of representative flexural test specimens from each of the billets was tested at NIST by the SCF and PB (SEPB) methods in accordance with C 1421 unless otherwise noted. In addition, a few test specimens from billets C and H were tested by the CNB method at NIST and at National Aeronautics and Space Administration (NASA)-Glenn Center. As we will show, fracture toughness was independent of test method. A substantial number of additional experiments using nonstandardized procedures were conducted but were not used for the SRM database. Data from these dozens of supplemental experiments were very important for establishing the robustness of the individual test methods and for confirming the tolerances and procedures in the ASTM and ISO standards. 

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