Effect of Rapid Laser Deconsolidation on Intimate Contact Development

Abstract

L-AFP is able to place composite tapes in very precise directions, thereby allowing tailored stiffness designs. The laser is used to heat the thermoplastic matrix material to temperatures above its melting point. CF/PEEK is a popular choice for aerospace grade structures due to the high structural performance and high glass transition and melting temperatures. Current L-AFP research is driven by the desire to produce OOA laminates with a performance comparable to autoclave level. This will remove manual, labor intensive, and energy consuming steps, leaving a repeatable and automated process that is more predictable. A key phase for the performance of a laminate during L-AFP is the development of intimate contact. Recent research showed the effects of thermal deconsolidation due to rapid laser heating and questioned the current practice of characterizing the surface of the composite tape material for the intimate contact development models: instead of characterizing the pristine tape surface, the laser deconsolidated surface should be characterized. Additionally, another dominant physical mechanism, driving the intimate contact development was proposed based on the observations of thermal deconsolidation. These ideas are novel and require more extensive understanding of what thermal deconsolidation due to rapid laser heating is, how it affects intimate contact development. The effects of rapid laser deconsolidation were studied by performing (1) a rapid laser deconsolidation experiment where CF/PEEK samples were prepared and (2) an intimate contact development experiment where the specimens were compressed under constant pressure while being subjected to a temperature profile. Three degrees of deconsolidation were manufactured with the laser set-up: (1) zero deconsolidation by using the as-received tape directly as specimens, (2) slightly laser deconsolidated tapes which experienced maximum temperatures between the glass transition and melting region, and (3) highly laser deconsolidated tapes that experienced temperatures above the melting point. All samples were characterized on void content, roughness, and waviness; all of which significantly increased which showed that the state of the tape right before the nip-point during L-AFP is significantly different than the pristine state of the CF/PEEK tape. The intimate contact development experiment was carried out with pressure levels of 10, 50, 100, and 300 kPa reaching maximum temperatures of 363°C. Decreasing the degree of deconsolidation or increasing the pressure resulted in better consolidation. At 10 kPa, most of the characterization showed very little difference from a pristine tape, but a significant amount of DEIC was developed with 36% for the highly laser deconsolidated tape to 46% for the as-received degree of deconsolidation. At the highest pressure of 300 kPa, no significant difference between the degrees of deconsolidation was observed as the average DEIC values are between 85 to 87%. Additional intimate contact development temperature settings below melt showed that no intimate contact was developed below the melting temperature. During the glass transition, a significant compaction occurred which eliminated most of the void content. Between the glass transition and melting temperature, however, void content, thickness, and roughness all remained constant. At the melting point, a second thickness compaction occurred together with the development of DEIC and decrease of roughness which appeared to also relates with resin percolation.