Fracture Toughness Techniques for Measuring Printed Materials

Objectives: To test the fracture toughness, K_IC, using four testing methods including ISO6872 and ISO20795
Methods: Two experimental printable materials (low filled and high filled) (n=8) tested using four specimen testing methods: Group-1: ISO6872 (3mmx4mmx17mm) Group-2: ISO20795 (8mmx4mmx39mm), Group-3: Printed notch (8mmx4mmx39mm), and Group-4: Printed notch sharpened with paste (8mmx4mmx39mm). Specimens were printed using a Sprint Ray Pro 2 and clean with alcohol before being cured. Groups 1 and 2 notches were cut using a diamond saw to the appropriate standards depth. Notches were sharpened 100-400µm using a razor blade using diamond paste (Group 1) or glycerol (Group 2). Groups 3 and 4 specimens were printed with 3.0mm V-notches. Group 3 notches were not altered while Group 4 notches were sharpened 100-400µm using a razor blade using paste.

For Group 1, specimens were loaded in a three-point testing fixture(span-15mm) at 0.5mm/min in an Instron testing machine. Groups 2-4 specimens were loaded at 1.0 mm/min(span-32mm). Notches were measured using an optical microscope (50x) and averaged.

For Group 1, fracture toughness (K_IC) was calculated using:

K_IC = g*[(P_f*Sx10^-6)/(t*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)]

P_f=failure load; s=span; t=thickness; w=width; a=average V-notch depth

For Groups 2-4, the maximum stress intensity factor, K_max, was calculated using:

K_max=(f*P_max*l_t/((b_t*h_t^3/2) x √10^-3 MPa-m^1/2

where f is:

f(x)=3x^1/2[1.99-x(1-x)(2.15-3.93x+2.7x²]/[2(1+2x)(1-x)^3/2] and x=a/h_t

P_max is maximum load(N) and h_t, b_t, l_t and a are specimen height, width, span, and depth of precrack(mm)
Results: Data was analyzed using (ANOVA) and Tukey's post hoc for differences(p≤0.05) within each material.
Conclusions: Within the limitations of this study, Group 3 gave the highest values for both materials. For the lower filled material, Groups 1 and 4 were statistically not different while Group 2 was the lowest. For the higher filled material, Groups 2 and 4 had the lowest mean values and were not statistically different.