Wouter Johannes Bernardus Grouve

Weld strength of laser-assisted tape-placed thermoplastic composites

Laser-assisted tape placement is an attractive manufacturing technology for the aerospace industry as it combines high productivity with low energy consumption. It comprises the automated deposition of fiber reinforced thermoplastic tapes to incrementally build up a structure. The process can also be used to tailor the properties of conventionally manufactured woven fabric reinforced components by locally reinforcing these with unidirectionally reinforced tapes. This thesis focuses on the weld strength between the tape and the woven fabric reinforced component. The principal objective is to develop an in situ processing strategy, combining high productivity and energy efficiency with high weld strength. For this purpose, the important bonding mechanisms, processing parameters and material properties are identified through a combination of experimental work and physical modeling. The interlaminar bonding process comprises the development of intimate contact followed by the interdiffusion of polymer chains. Both mechanisms depend strongly on the interface temperature. A thermal process model is, therefore, proposed specifically taking into account the optical aspects of laser heating. The model is validated experimentally. Based on the developed model, the important processing paramaters and material properties are identified. A mandrel peel test is introduced to quantify the interfacial fracture toughness between the tape and the laminate. The applicability and validity of the method is successfully demonstrated by comparing it to standardized fracture mechanics tests. The interfacial fracture toughness does not only depend on the degree of interlaminar bonding. The crystallinity and structural morphology of the interface also play an important role. This is demonstrated by a comparison between the (fast) tape placement process and a (slow) press-molding process. The tape-placed specimens outperform the press-molded specimens in terms of fracture toughness by almost a factor of two. This is attributed to the high cooling rates and short bonding time during the tape placement process. The former results in a low crystallinity, while the latter prevents the migration of tape fibers into the resin pockets of the laminate and thereby minimizes the fiber–fiber contact. Both the low crystallinity and the presence of resin pockets improve the interfacial fracture toughness. Finally, a processing strategy is proposed, which maximizes productivity and energy efficiency. The strategy involves the distribution of all laser power to the tape. Although the proposed strategy should be tested in practice, the work in this thesis suggests that an excellent weld strength will be achieved.

Consolidation process model for film stacking glass/PPS laminates

Abstract A model is proposed to optimise the processing parameters for the consolidation of glass/polyphenylene sulphide (PPS) laminates using a film stacking procedure. In a split approach, the heating and consolidation phase are treated separately. The heating phase is modelled using the one-dimensional heat conduction equation with variable thermal diffusivities. The model shows good agreement with experimental results. The consolidation phase is modelled using Darcys law to predict the bundle impregnation time. The model predicts an impregnation time in the order of seconds, which is significantly shorter than the typical consolidation time of approximately 15 min used in practice. The impregnation model is validated in a comprehensive experimental programme, which included optical microscopy and mechanical testing. The experiments show that the consolidation time can indeed be shortened significantly for the glass/PPS system under consideration.

On crystallisation and fracture toughness of poly(phenylene sulphide) under tape placement conditions

Abstract Fibre reinforced thermoplastic tapes are subjected to high heating and cooling rates during the tape placement process. Such high cooling rates can significantly inhibit the crystallisation of the thermoplastic polymer and thereby affect its mechanical properties, such as strength or toughness. In the present work, the crystallisation of poly(phenylene sulphide) (PPS) subjected to high cooling rates was investigated using a fast scanning calorimeter. The PPS was found to be unable to crystallise when subjected to cooling rates higher than 20°C s−1. The influence of the degree of crystallinity on fracture toughness was investigated using an essential work of fracture approach. The amount of plastic work during the fracture process was found to decrease after moderate annealing.

Influence of preconsolidation on consolidation quality after stamp forming of C/PEEK composites

Stamp forming is a rapid manufacturing technology used to shape flat blanks of thermoplastic composite material into three-dimensional components. Currently, expensive autoclave and press consolidation are used to preconsolidate blanks. This study investigates the influence of preconsolidation on final consolidation quality after stamp forming and explores the potential of alternative blank manufacturing methods that could reduce part costs. Blanks were manufactured using various blank manufacturing methods and subsequently were stamp formed. The consolidation quality both before and after stamp forming was compared, where the focus was on void content as the main measure for consolidation quality. The void content was characterized through thickness and density measurements, as well as by microscopy analysis. Results indicate that preconsolidation quality does have an influence on the final consolidation quality. This is due to the severe deconsolidation and limited reconsolidation during stamp forming. Nevertheless, the potential of automated fiber placement and ultrasonic spot welding as alternative blank manufacturing methods was demonstrated.

Consolidation quality and mechanical performance of stamp formed tailored blanks produced by rapid AFP

Stamp forming is a rapid manufacturing technology used to shape flat blanks of thermoplastic composite material into three-dimensional components. The combination with rapid AFP as blank manufacturing technology can further extend the applicability of stamp forming by allowing rapid lay-up of tailored blanks and offering partial preconsolidation. In an experimental study it is demonstrated that high quality laminates with good flexural strength can be obtained by following this process route. The consolidation of ply-drop regions is demonstrated by flat laminates with a thickness step. The influence of fiber orientations, blank-tooling misalignments and AFP tolerances is investigated.

Influence of Prepreg Characteristics on Stamp Consolidation

Stamp forming is a rapid manufacturing technology used to shape flat blanks of thermoplastic composite material into three-dimensional components. The development of automated lay-up technologies further extends the applicability of stamp forming by allowing rapid lay-up of tailored blanks and partial preconsolidation. This partial preconsolidation makes the influence of prepreg more critical compared to conventional preconsolidation methods which provide full preconsolidation. This paper aims to highlight consolidation challenges that can appear when stamp forming blanks manufactured by automated lay-up. Important prepreg characteristics were identified based on an experimental study where a comparison was made between various prepreg in their as-received, deconsolidated and stamp consolidated state. It was found that adding up small thickness variations across the width of a prepreg when stacking plies into a blank by automated lay-up can cause non-uniform consolidation. Additionally, deconsolidation of the prepreg does not seem to obstruct interlaminar bonding, while intralaminar voids initially present in a prepreg cannot be removed during stamp forming. An additional preconsolidation step after automated lay-up seems necessary to remove blank thickness variations and intralaminar voids for the current prepregs. Eliminating this process step and the successful combination of rapid automated lay-up and stamp forming requires prepregs which are void-free and have less thickness variation.