8.3.2 Fracture Toughness Characterization of Printed Roads The first few trial specimen blanks were tested without a notch and a starter crack but they all failed at the drilled hole edges. As such it was decided that further testing would be performed with notched specimens. A ‘natural’ crack was then produced in each specimen by placing a single-edged razor blade in the notch and tapping the back edge of the razor blade with a small hammer. The initial crack length ‘a’ was then measured for all specimens. (The standard calls for ‘a’ to be from 0.45 W to .55 W from the centerline of the loading holes. In almost all of the specimens the value of ‘a’ was less than the value called for in the standard.) Several attempts were made to fatigue the specimen to meet standard value, but all attempts caused the premature failure of the specimen. Therefore, the ‘best’ 6–8 specimens from each plaque were selected for testing. All specimens were tested on a 500 lb capacity MTS Insight electromechanical test frame at a crosshead displacement rate of .05 in/min. The specimen was loaded until a drop in load occurred. Table 8.4 lists the specimen dimensions along with the maximum load recorded for all the specimens tested. The plane strain fracture toughness was computed using Eq. (8.1), where PQ is the maximum load, t is the thickness, w is the width and a is the crack length. KQ ¼ PQ=tw1=2 :f a=w ð Þ ð8:1Þ where, f a=w ð Þ¼ 2þa=w ð Þ 0:886þ4:64a=w 13:32a2=w2þ14:72a3=w3 5:6a4=w4 ð Þ 1 a=w ð Þ3=2 The average plane strain fracture toughness for specimens with low, nominal and high road density is measured as 417.72 105.87, 1337.08 161.48, and 1990.46 261.89 psi in.0.5, respectively. Thus, not only the pore volume fraction decreases and the contact length between adjacent roads increases with decreasing air-gap distance, the fracture toughness of the printed material increases by approximately five times on reducing the lateral raster-to-raster distance Fig. 8.7 Image analysis of “high” road density specimen yielding a pore volume fraction of 4.725 % Table 8.1 Pore volume fraction evaluation using image analysis Specimen ID Pore volume fraction (%) Pore size (μm2) Low density 15.02 4.7 (104) Nominal density 9.83 1.3 (104) High density 4.73 3.8 (103) 68 G.P. Tandon et al.
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