Comparison of Two Toughness Methods for Glass-Ceramic and Cubic Zirconia

Objectives: To examine SEVN and SEPD fracture toughness methods of two materials.
Methods: Two fracture toughness testing methods were used: Single Edge Precrack Beam(SEPB), and Single Edge V-Notch(SEVN). Two materials were tested: 1) IPS e.max®CAD - Group 1(SEPB) and 2(SEVN) and 2) Katana™ STML - Group 3(SEPB) and 4(SEVN). For each group, eight bars (36mm x 4mm x 3mm after firing) were milled cut using a Buehler ISOMET™1000 diamond saw. The bars were crystallized/sintered according to manufacturer's instructions.

For the SEPB tested method, the fired bars were indented with 3 indents across the bar and then loaded in a precracking fixture (ASTM Standard C 1421) using an Instron® Universal Testing Machine (33R4204) (0.25mm/min crosshead speed). The precracked specimens were loaded in a 30mm 3-point flexure loading fixture (0.5mm/min crosshead speed) until failure.

For the SEVN method, 1mm V-notch was placed using a Diamet diamond saw. The crack tips were sharpened using diamond paste sequence of 6µm, 3 µm and 1 µm. The specimens were loaded to failure in a 30mm 3-point testing fixture to failure (0.5mm/min crosshead speed).

To obtain crack depth, three equally spaced measurements were taken at 80x using an Olympus SZX12 microscope.

The SEPB fracture toughness was calculated using:

K_IC=(P_fS/bW^3/2)f(a/W)
where
f(a/W)=(3/2)(a/W)^1/2(1.964-2.837a/W+13.711a²/W2-23.250a³/W3+24.129a4/W⁴)

The SEVN fracture toughness was calculated using:

K_IC = g*[(P_f*Sx10^-6)/(b*w^3/2)]*[(3(a/w)^1/2)/(2(1-a/w)^3/2)]

g is {1.99-[(a/w)(1-a/w)]*[2.15-3.93(a/w)+2.7*(a/w)²]}/[1+2(a/w)]

Pf is failure load; s is span; b is thickness; w is width; and a is precrack length/average V-notch depth.
Results: The fracture toughness, KIC(MPa●m^1/2), for each group is: Group1: 2.46±0.11; Group2: 2.49±0.09; Group3: 2.50±0.11; and Group4: 3.56±0.20. Using ANOVA (p≤0.05), statistical difference was found between the SEPB and SEVN methods for groups 3 and 4.
Conclusions: Within the bounds of this study, SEVN produced a higher K_IC value of the zirconia material while both testing methods producing similar values for glass-ceramic.