Unlike most polymers, which usually experience a large amount of yield and elongation before breaking, most samples tested under the specified parameters of this study displayed a brittle fracture. show the various stress-strain profiles of all of the test samples, grouped by the angle between the layer plane and the load, in figures 11a through 12e. While the loads of each sample’s failure were approximately equal, the extension of the sample only reached just above 2.25 mm before failure, while the other two managed to extend to 3.5 mm and 3.75 mm. Because the load at failure is the only variable being analyzed, this does not imply that Trial 1 at 75° is an outlier.
Trial 2’s specimen at 60° failed at approximately half of the load and extension necessary to rupture the other two trials. This load was therefore considered an outlier, and was not considered in calculations. The behavior of the specimen in Trial 3 at 0° in Figure 3 was particularly noteworthy, because instead of undergoing brittle fracture upon failure, it experienced ductile yielding with an elongation of over 28 mm, and still did not experience complete separation. While its failure behavior was inconsistent with those of other specimens, the failure load was consistent with the other specimens of the set, and thus the strength of this particular sample was considered during calculation.
After obtaining the ultimate strengths of the samples at each layer angle, each set of ultimate strengths was averaged and plotted against the layer Angle relative to the load in Figure 4.
Changing the angle reference from “relative to load” to “relative to the horizontal” axis for later use in the stress transformation equations, these loads were used to find the tensile stresses, and both were plotted in Figure 5.
Using Equation 1 to find the transformed stress σx’ normal to the layer interface and plotting against the layer interface angle in Figure 6 revealed that the stress was not constant at any regions.
The remaining stresses σy’ and τx’y’ were calculated and plotted with σx’ in Figure 7.
Realizing that rather than experiencing a single stress the layers were experiencing combined loading, the maximum principal stress for each layer angle was calculated using Equation 4 and plotted in Figure 8.
This revealed that the maximum principal stress was the constant stress between all test specimens at around 38 MPa, with the exception of the samples tested with layers printed at 75°to the load, which consistently failed at much lower stresses.