PSI - Issue 28

Anja Gosch et al. / Procedia Structural Integrity 28 (2020) 1184–1192 Anja Gosch/ Structural Integrity Procedia 00 (2019) 000–000

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Table 1. Comparison of the average differences in the measured crack lengths on both specimen sides for the three set-ups: (1) Microscope vs. Microscope, (2) Microscope vs. IRT and (3) Microscope vs. DIC.

Average difference in %

Method

PVC-U

POM

PMMA

Microscope vs. Microscope

4.7 5.5 3.0

1.7 1.7 1.9

2.7 2.7

Microscope vs. IRT Microscope vs. DIC

-

Generally, the determined average difference in the measured crack lengths on the front and backside of the specimens is low for all testing set-ups and thus good results were determined for the examined polymers in this study. The Microscope vs Microscope set-up displayed an average difference of 4.7 % (PVC-U), 2.7 % (PMMA) and 1.7 % (POM) in the measured crack lengths. The varying crack advancement on both specimen sides is due to uneven crack growth during the fatigue tests, which can occur due to residual stresses in the material or the notching procedure. An example for uneven crack growth is shown on the fracture surface of an investigated PVC-U specimen (Figure 4). Directly on the fracture surface, discontinuous crack growth bands, which are represented by lines and which indicate a forward step of the crack during the fatigue test, display uneven crack advancement on the front and backside of the specimen during the experiment.

Figure 4. Fracture surface of PVC-U with stop lines (striations) indicating the uneven crack growth on the front and backside of the specimen.

The determined average difference in the crack lengths of the two further testing methods is presented in Table 1: Microscope vs. IRT and Microscope vs. DIC. For the Microscope vs. IRT method an average difference of 5.5 % (PVC-U), 2.7 % (PMMA) and 1.7 % (POM) was measured. The determined average difference for the Microscope vs. DIC set-up was the lowest in this study, with 3.0 % (PVC-U) and 1.9 % (POM). These two set-ups display similar values of the average difference as the Microscope vs. Microscope set-up. Based on these findings, it is assumed that uneven crack growth is the main influencing factor for the observed differences rather than insufficiencies of the different crack length determination methods. Within one material class, the determined average difference of the used set-up is rather similar. However, the obtained values differ significantly depending on the investigated type of polymer, as shown in Table 1. PVC-U displayed the highest average differences (3.0 % to 5.5 %), PMMA displayed lower values (around 2.7 %) and for POM the lowest average difference was obtained (1.7 % to 1.9 %). This is attributed to the different mechanical behavior of the investigated polymers, as well as the specimen thickness. Since PVC-U displays much bigger plastic