PSI - Issue 56

Dan Micota et al. / Procedia Structural Integrity 56 (2024) 144–152 Dan Micota / Structural Integrity Procedia 00 (2019) 000 – 000

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The chosen wall thickness values to be analyzed, 2 and 3.2 mm, are right in the recommended range for injection molded short fiber reinforced plastic parts, this range being in between 1 and 5 mm and is common knowledge in the plastic injection molding industry based on many years of experience and material suppliers recommendations. Some of the reasons why is not recommended to inject SFRP in molds that produce wall thicknesses lower than 1 mm include high resistance to flow of the material, increased injection pressures, accelerated wear of the molds, risk of unfilled cavities, thus making such a process very difficult from a technological point of view. On the maximum side of the recommended thickness range it is easy to inject SFRP material in molds which produce wall thickness over 5 mm but the resulting parts will have major issues with shrinkage and warpage as the part cannot be effectively cooled throughout it’s cross se ction. From the described plates, dog-bone tensile specimens were cut via CNC machining at different angles to the melt flow direction, according to ISO 527-2 (2016). The relatively small size of the plates has limited the tensile specimen size and the 1BA type was chosen from the standard, Fig. 1 (a). The in-cut finish of the specimens after the CNC routing was very good, so no further preparation was needed before testing. To ensure the best fiber orientation distribution for each specimen, only one was cut from each plate ’s central area. For each of the 2 materials a total of 30 specimens were tested, 5 for each angle/orientation (0°, 15°, 30°, 45°, 60° and 90°) compared to the melt flow direction, all specimen orientation can be seen in Fig. 1 (b).

(a)

(b)

Fig. 1. (a) ISO 527-2 1BA dog-bone tensile specimen with increased thickness (3.2 mm); (b) specimen angles/orientations to the melt flow direction.

All tensile tests were performed on the same machine as for the referred lower wall thickness specimen tests, an Instron 8874 biaxial servo-hydraulic testing system, being capable of axial, torsional and combined axial-torsional loadings, but only the static axial capabilities of the machine were used for the tests. For specimen clamping the machine is equipped with Series 2742-30 kN fatigue rated hydraulic wedge grips, these allow for a precise control of the tightening pressure in order to prevent the specimens from slipping while also not crushing them. The design of the equipped grips also allows for automatic compensation in specimen thickness, maintaining a constant grip force across the test duration. The engineering axial strain of the specimens was measured with the Instron SVE1 (Standard Video Extensometer 1) 2663-822. The rate of loading for these tests was done according to ISO 527-1 (527-1, 2016) with a constant speed of 5 mm/min and all the test were performed at room temperature. Given the high glass fiber content of both materials no necking phenomena was observed during the tests, all of the fractures being quasi-brittle. 4. Results and comparison The results of the previously described tensile tests are presented in terms of engineering stress and strain in Fig. 2 for the PPA-GF33 material and in Fig. 3 for the PPS-GF40 material. For both materials the increased wall