Assessing the bond strength in thermoplastic fiber composite fabricated by an open mold process

Abstract

Fiber reinforced polymer (FRP) composites have become popular in marine, aerospace, and structural applications due to their high specific stiffness and strength as well as their ability to conform to complex shapes without the need to intensive subtractive machining. Until recently, thermoset FRP composites have been considered as superior to their thermoplastic counterparts. However, the advent of new thermoplastics that offer high melting temperatures and reduced temperature sensitivity motivated the industry to adopt thermoplastic composites in their products. Thermoplastic FRP composites are mendable and amendable, do not produce harmful emissions during their consolidation, and can potentially be recycled. Generally, two major hurdles slow thermoplastic fabrication: preparation of freeform and forced consolidation at elevated temperatures and pressures in closed molds. To overcome these hurdles and to automate the fabrication processes of thermoplastic composites, automated tape placement (ATP) coupled with in-situ consolidation is used. During this process, composite tapes are simultaneously heated, layered, and consolidated following a layer-by-layer build-up process that resembles filament deposition type of 3D printing. The layering rate, consolidation pressure, and temperature can affect the bond strength between the composite layers and the overall quality of the produced composite. Therefore, it is instrumental to characterize the coupling between these process parameters and the inter-laminar bond strength of thermoplastic composites used in ATP. In this work, plies of GFPP are stacked to process by utilizing an open mold processing approach that simulates ATP process. The layers are consolidated using different ranges of consolidation pressure, temperature, and time. The quality of the bond in the produced specimens were assessed through measuring their Interlaminar shear strength (ILSS) according to short beam ASTM standard. Microscopy imaging along with digital image correlation (DIC) were used to investigate the failure location in the tested specimens. Results show that consolidation pressures exceeding 10 bar do not improve bond strength, consolidation can occur at temperatures below the melting temperature of the processed thermoplastic, and consolidation times between 7 to 13 minutes can result in acceptable bond strengths that can potentially reach 50% of the bond strength resulting from conventional processing techniques.