Carbon fibre sheet moulding compounds with high in-mould flow: Linking morphology to tensile and compressive properties

Abstract

In-mould flow during manufacturing of Sheet Moulding Compounds (SMCs) heavily affects the material microstructure and its mechanical properties. This influence is studied here for carbon SMCs on panels compression moulded with limited charge coverage. The high in-mould flow caused severe in-plane tow distortions, while their planarity was preserved. Flow induced fibre orientation plays a paramount role in the material failure, whereas local manufacturing defects had no discernible influence. The properties difference between specimens with preferential orientation of 0° and 90° was 150% for tensile stiffness, 260% for tensile strength, 120% for compressive stiffness and 32% for compressive strength. The compressive strength and failure strain for 45° and 90° specimens were higher than those for tension, and comparable for 0° specimens. Compressive and tensile moduli were similar for specimens with the same orientation. A clear link between SMCs manufacturing and mechanical performance is highlighted, together with its implications on structural design.

Direct Fiber Simulation of a Compression Molded Ribbed Structure Made of a Sheet Molding Compound with Randomly Oriented Carbon/Epoxy Prepreg Strands—A Comparison of Predicted Fiber Orientations with Computed Tomography Analyses

“…Since an anisotropic (transversely isotropic) viscosity model is used, strands that are initially oriented in flow direction tend to translate, while those oriented in cross-flow direction widen [17]. Both effects are qualitatively consistent with physical observations [17,31]. Moreover, strand-strand interfaces [17] and interactions are only implicitly taken into account through the anisotropic viscosity model [21].…”

“…SMCs with those randomly oriented strands (ROS) show a high degree of heterogeneity (variability in intra-and inter-part structure on the meso-and macro-scale) yet seek to reach quasiisotropic mechanical properties [13][14][15]. This material class is also called prepreg platelet molding compound (PPMC) [4,[16][17][18][19][20][21][22][23][24][25][26][27] or tow-based discontinuous composite (TBDC) [28][29][30][31][32][33]. The material system is characterized by a high fiber volume fraction with good impregnation, comparable to continuous fiber layups, and therefore higher mechanical properties are reachable than with traditional SMCs [18].…”

“…However, due to the part design together with the high fiber volume fraction and fiber length in the CF-SMC material, complex anisotropic material flow conditions occur [21], which induce a characteristic microstructure in compression molded components [5,31]. This process-induced microstructure is mainly characterized by locally varying fiber orientations and fiber concentrations.…”

“…This process-induced microstructure is mainly characterized by locally varying fiber orientations and fiber concentrations. Besides defects such as voids (air entrapments), swirls or resin rich pockets in between strands [35][36][37][38][39][40], the flow-induced strand alignments have the biggest impact on the mechanical performance of DFC parts [21,31,41]. Therefore, obtaining realistic 3D information of the local strand orientations in a CF-SMC part is key for a better understanding of its related mechanical behavior and a sophisticated part design.…”

Review of the Numerical Modeling of Compression Molding of Sheet Molding Compound

“…The compression is then held for an appropriate time for the resin to be cured and the finished part can then be removed from the mold. One major issue with SMC parts, however, is that the quality of the finished parts is very sensitive to variations in the process parameters [2], which means that the properties may be difficult to predict with quality and trust. The chopped fibers that initially are orientated in, more or less, random directions may orientate during flow and the final part may get anisotropic mechanical properties as a result.…”