Cover: photograph of a leaf taken in Van Heek Park, Enschede. The veins in the leaf do not only serve to transport water and nutrients, but also support the leaf to give it its structure. WELD STRENGTH OF LASER-ASSISTED TAPE-PLACED THERMOPLASTIC COMPOSITES PROEFSCHRIFT SummaryLaser-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. i SamenvattingHet laser-verwarmd tape-placementproces is een aantrekkelij...
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 Darcy's 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.
The combination of rapid automated lay-up and stamp forming has great potential for rapid manufacturing of lightweight load carrying components of thermoplastic composites. However, deconsolidation during blank heating is currently limiting the applicability of rapid lay-up blanks. This experimental work investigates the origin of deconsolidation in blanks produced by advanced fiber placement (AFP) versus traditional press consolidation. The influence of moisture on deconsolidation is investigated through deconsolidation experiments in a convection oven, as well as thermo-mechanical and residual gas analyses. The experiments revealed that thermal expansion of dissolved moisture is the main deconsolidation mechanism for press-consolidated blanks, but not for AFP blanks, which are suggested to deconsolidate mainly due to the release of frozen-in fiber stresses present in the used prepreg.
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