PSI - Issue 13

P.N.B. Reis et al. / Procedia Structural Integrity 13 (2018) 1999–2004 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Self-reinforced thermoplastic composites (SRCs) combine oriented polymer fibres or tapes of high ductility with a matrix of the same polymer (Tabatabaei et al. , 2015). In this context, self-reinforced polypropylene (SRPP) has potential for many commercial products, because they provide a balance of strength, modulus and chemical resistance at a relatively low cost. On the other hand, the higher strength and stiffness provided by the self reinforcement does not compromise their recycling potential (Alcock et al. , 2006; Alcock et al. , 2007a). Polymer-based composites are sensitive to the strain rate but, if this subject for metals is widely studied over a wide range of strain rates, open literature is not so abundant for polymer composites (Jacob et al. , 2004). Studies developed by Zhou and Mallick (2002) show, for neat PP, that the yield stress and modulus increase with increasing strain-rate while the failure strain decreases. A remarkable strain-rate dependency was observed by Ohashi et al . (2002) on the deformation of polypropylene under monotonic compression and bending. In terms of SRPP composites, studies of McKown and Cantwell (2007) found that the modulus, yield stress, and strength all increase linearly with the natural logarithm of the strain-rate, whilst the failure strain decreases. Similar tendency was observed by Alcock et al . (2007b). Therefore, this work intends to study the time-dependent behaviour of self-reinforced composites under bending loads, because no studies about the strain rate effects and stress relaxation in self-reinforced polypropylene composites were found in the literature. For this purpose, flexural tests were performed over a range of different strain rates and the flexural properties (modulus, strength and failure strain) will be analysed as a function of the strain rate. Stress relaxation tests will be carried out in the bending mode at different strain values. SRPP composite made of PP tape fabric with an overfed twill 2/2 weave pattern, supplied by Propex Fabrics GmbH (Gronau, Germany), was used in this study. In the hot compacted form this material is known under the trade name of Curv ® . According to Tang et al . (2018) and Swolfs et al . (2013), the areal density of the SRPP fabric is 130 g/cm 3 and the density of this grade polypropylene is 0.92 g/cm 3 , which generates an equivalent thickness of 141 µm. Six plies of SRPP, all aligned in the same direction, were put in a closed copper mould and the overall dimensions of the plates were 275 × 135 × 0.85 (mm). To obtain a complete consolidation of the composite, the system was previously heated to 188ºC in a hot press machine (Fontijne Grotnes LabPro 400) and then compacted at 39 bars for 5 min. The process finishes with the cooling down until the room temperature at 20ºC/min. This high pressure was maintained during the cooling process to prevent shrinkage of the PP tapes. Three point bending (3PB) static tests were performed using specimens cut nominally to 40×10×0.85 mm 3 dimensions and tested with a span length of 20 mm according to the recommendations of the European Standard EN ISO 178:2003. An Instron 4467 universal testing machine equipped with a 1kN load cell was used and the tests were carried out at room temperature with a displacement rate of 200, 20, 2, 0.2 and 0.02 mm/min, which correspond to strain rates of 4.25×10 -2 , 4.25×10 -3 , 4.25×10 -4 , 4.25×10 -5 and 4.25×10 -6 s -1 , respectively. For each condition, five specimens were tested. The bending strength was calculated as the nominal stress in the middle span section obtained using maximum value of the load. The stiffness modulus was calculated by the linear elastic bending beams theory relationship, and it was obtained by linear regression of the load-displacement curves considering the interval in the linear segment with a correlation factor greater than 95% (Ferreira et al. , 2013). Finally, the flexural strain was calculated according to the European Standard EN ISO 178:2003. The tests of stress relaxation were carried out on the same machine used for the bending tests (Instron 4467). In these tests a fixed deflection (strain) was applied and the stress was recorded during the loading time. The specimen geometry was similar to those used in the flexural tests (40×10×0.85 mm 3 ). The strain values used were 0.64%, 1.41% and 2.71%, which correspond to 25% (19 MPa), 50% (38 MPa) and 76.5% (58 MPa) of the average maximum bending stress obtained for the strain rate of 4.25×10 -4 s -1 , respectively. 2. Material and experimental procedure