Surface Crack In Flexure Method Research Articles

Fracture toughness ( KIc) of a dental porcelain determined by fractographic analysis

Objectives: Fractographic analysis of indentation cracks is performed following flexure testing as part of the ASTM (1999) standard for fracture toughness, K Ic, determination in advanced ceramics. This method depends on the conduciveness of the material towards fractographic interpretation. The purpose of this study was to evaluate the use of fractography in fracture toughness methods with a feldspathic dental porcelain, in which K Ic was measured fractographically as well as numerically using two controlled-flaw beam bending techniques. Methods: The following methods for K Ic determination were applied to a dental porcelain containing a leucite volume fraction of 15–20%: (1) surface crack in flexure (SCF) (dependent upon fractographic analysis); and (2) indentation strength (IS) at indentation loads of 9.8 and 19.6 N (applying both standard numeric calculations and fractographic analysis). The testing environments were (1) ambient air (IS and SCF) and (2) flowing dry nitrogen (SCF). Results: No significant differences were found between numeric and fractographic K Ic values for the IS technique at both indentation loads (9.8 and 19.6 N) in ambient air, although K Ic values were sensitive to indentation load. Due to the presence of residual stresses, stable crack extension was observed fractographically in all IS specimens, as evidenced by differences between initial ( a initial) and critical ( a critical) crack dimensions. For the SCF method, there was a significant difference in toughness between specimens tested in air versus dry nitrogen, however no fractographic evidence for chemically assisted slow crack growth (SCG) was observed. Significance: The SCF method as described by the ASTM standard was applicable to the feldspathic porcelain and produced very comparable results with the numeric toughness calculations of the IS procedure. However, fractographic analysis of the surface crack was somewhat difficult for this glassy ceramic compared with polycrystalline ceramics. Knowledge about stable crack extension or slow crack growth and its fractographic appearance is essential when estimating the toughness from examination of flaw dimensions on fractured surfaces since large calculation errors may occur if these effects are not taken into account.

Effect of Precrack “Halos” on Fracture Toughness Determined by the Surface Crack in Flexure Method

The surface crack in flexure method, which is used to determine the fracture toughness of dense ceramics, necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient, room‐temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this article, fractographic techniques are used to characterize evidence of stable crack extension, a “halo,” around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high‐purity Al2O3, an AlN, a glass‐ceramic, and a MgF2 reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF2 is due to indentation‐induced residual stresses initiating crack growth. However, for the Al2O3 and the glass‐ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.