PSI - Issue 61

Jose Beltra Mira et al. / Procedia Structural Integrity 61 (2024) 156–163 Author name / Structural Integrity Procedia 00 (2024) 000–000

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toughness K IC . However, it has also been observed in some studies that the sample size itself does not seem to have a large e ff ect on the material performance even when the samples are not in plane strain [6–14]. Therefore, the conditional fracture toughness K Q may be equivalent to a true K IC for at least some of these materials. The question over whether this standard is a realistic option for collecting useful data related to AM materials is certainly an important topic to explore further. There is no consensus in the literature about the appropriate size to use for either of the sample options in ASTM D5045 (compact tension (CT) and single-edge notched bending (SENB)). To judge the appropriate size to test that would present a reasonable comparison to the existing literature, a short review was done for CT tests on FFF processed materials; the results are shown in Table 1.

Table 1. Examples of CT testing on FFF-processed materials in the literature.

Study Material

W (mm)

B (mm)

[2] [3] [4] [5] [6]

PLA PLA ABS PLA

40 30 60 19 70 30 32

8 6

12.7

9.4 7.5

Polyether ether ketone (PEEK) 15

[7, 8]

ABS

4 4

[9]

PC

[10]

Onyx (nylon + carbon fiber)

10

Given how important this consideration is for design data collected using ASTM D-5045, the present study explored this question by completing a series of experiments on three common thermoplastic materials (acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and polycarbonate (PC)) manufactured using the fused filament fabrication (FFF) process. Compact tension specimens were used with three di ff erent sample sizes, none of which met the plane strain criterion specified in the standards) with the test run 3 times for a total of 27 tests. The data were then analyzed statistically, followed by additional validation, to see if the sample size had any major impact on the outcome of the experiments. These experiments were replicated several times, both to create a good dataset for analysis but also to determine the variability in the experiments using this standard. The mission and focus of this study is to specifically test and analyze the standard and make a data-driven con clusion about whether the standard is a valid method for testing FFF-processed materials. A clear answer on this question will be very important for the additive manufacturing and materials research communities, as it provides a good perspective not only on future studies but also the existing literature going back to the 1990s. Compact tension specimens designed according to ASTM D5045 were generated in three di ff erent sizes (Sample 1: W = 30 mm, B = 6 mm, Sample 2: W = 40 mm, B = 8 mm, and Sample 3: W = 50 mm, B = 10 mm). The ± 45 ◦ raster orientation was used with the notches printed and then pre-cracked using the method described in [15]. The specimens were printed in a flat orientation using a Prusa i4 machine (0.6 mm nozzle) with a heated glass bed and inside of an enclosure. All filament was sourced from Hatchbox. The extrusion temperatures were 235 ◦ C (ABS), 210 ◦ C (PLA), and 250 ◦ C (PC), while the bed temperatures were 70 ◦ C (ABS and PC) and 60 ◦ C (PLA). Print speed was 50mm / s in all cases. Figure 1a shows example specimens of the three sizes used for the tests (Figure 1b), while Figure 1c gives an example broken sample with the collected data on the screen. Enough samples were printed for three replications of each case for a total of n = 27. In order to enable calculation of the fracture toughness K Q and evaluate the plane strain condition specified by ASTM D5045, tensile tests were completed to establish the basic properties. Samples were printed in several di ff erent orientations ( ± 45 ◦ and 0 / 90 ◦ rasters, with flat and horizontal print orientations) in order to collect a good “average” value for each parameter. With three materials and five replications, this produced a total of 60 tensile curves. The tensile testing was done according to ASTM D638 with Type IV specimens, an extensometer with l = 25 . 4mm, anda testing speed of 5 mm / min. Figure 2 shows one randomly selected curve from each case. Table 2 gives the calculated 2. Materials and Methods