Mechanism of Compaction With Wrinkle Formation During Automatic Stitching of Dry Fabrics and the Size Effect of Compression Molded Discontinuous Fiber-Reinforced Composites

With an ever-increasing demand for composites, more ways of manufacturing them are becoming popular and widely used. Stitching of dry fabrics is an efficient method for improving delamination resistance. Discontinuous fiber reinforced composites can be used as a lightweight alternative material for metals through a process of compression molding, which allows for complex shape manufacturing while offering structural grade mechanical properties. This study demonstrates how the stitching of dry fabrics can be adapted to more complex surfaces. The consequences of stitching of curvilinear surfaces can result in defect formation. Therefore, to understand the physical formation of possible defects, experimental characterization methods were proposed, which considered a compaction test, a roller compaction test with fabric pull to induce a wrinkle formation. A stitching system available at the NASA Langley Research Center was used as a basis for the automated stitching. It was concluded that a purely compaction pressure over 100 kPa would densify the fabric enough that the resin infusion process after stitching could be affected. The pulling of the fabric demonstrated that pressures over 400 kPa along the surface result in permanent fiber damage which will affect the structural properties of the composite after curing. To mitigate the damage of dry fabrics during automated stitching at NASA, a new component called the presser foot was designed to replace its old stiff counterpart. This piece will allow for a more flexible stitching process able to accommodate to more complex surfaces. The carbon/epoxy thermoset was adapted from a continuous pre-impregnated fiber tow to form a discontinuous platelet-based type molding compound. Due to the increasing aspect ratio of the platelets, mechanical properties increase, but after a certain point these properties decrease due to a reduced platelet count in the sample. A premade glass-fiber/nylon thermoplastic composite demonstrated there is a general fiber orientation in the almost randomly oriented composite, which helps control the material behavior. The span of the samples during testing was changed to simulate the change in fiber length of the composite. Both kinds of composites showed there is an increase to material properties as fiber length increases

Effect of Platelet Length and Stochastic Morphology on Flexural Behavior of Prepreg Platelet Molded Composites

Prepreg platelet molding compound (PPMC) can be used to create structural grade material with a heterogeneous mesoscale morphology. The present work considered various platelet lengths of the prepreg system IM7/8552 to study the effect of platelet length on the flexural behavior of PPMC composite. A progressive failure finite-element analysis was used to understand competing failure modes in PPMC with the different platelet length. The interlaminar and in-plane damage mechanisms were employed to describe complex failure modes within the mesostructure of PPMCs. Experimental results of the flexural tests of the PPMC with different platelet length sizes were used to validate the modeling prediction. The experimental and modeling results revealed complex behavior of the flexural mechanical properties (modulus and strength) on the platelet length. The experimental results indicate that PPMC composites processed with a plate length of 12.7 mm have a higher flexural modulus and strength than 25.4 and 6.35 mm. The platelet length effect on the flexural mechanical behavior was attributed to interactions between various damage mechanisms and the stochastic fiber orientation distribution variability in the material