Abstract
This article provides an overview of the evolution of the in-situ consolidation (ISC) process over time. This evolution is intimately linked with the advancements in each of the steps of the ISC manufacturing process, is additive in nature, and is limited by the orthotropic nature of composite materials and the physicochemical behavior of the thermoplastic matrix. This review covers four key topics: (a) Thermal models—simulation tools are critical to understand a process with such large spatial gradients and fast changes. Heating systems once marked a turning point in the development of industrial ISC systems. Today, lasers are the most recent trend, and there are three key issues being studied: The absorption of energy of light by the material, the laser profile, and the laser focusing. Several approaches have been proposed for the distributed temperature measurements, given the strong temperature gradients. (b) Adhesion—this refers to two subsequent mechanisms. In the first place, the process of intimate contact is one by which two surfaces of thermoplastic pre-impregnated composite materials are brought into contact under pressure and temperature. This enables closure of the existing gaps between the two microscopic irregular surfaces. This process is then followed by the healing or diffusion of polymer molecules across the interface. (c) Crystallinity—mostly influenced by the cooling rate, and strongly affects the mechanical properties. (d) Degradation—this refers to the potential irreversible changes in the polymer structure caused by the high temperatures required for the process. Degradation can be avoided through adequate control of the process parameters. The end goal of the ISC manufacturing process is to achieve a high product quality with a high deposition rate through an industrial process competitive with the current manufacturing process for thermoset composites.
Highlights
The key properties that justify the wide use of composite materials in the aeronautical industry were described in [1] including weight savings due to the increase of specific resistance and stiffness, which leads to reductions in fuel consumption and, allows an increase in aircraft payload
The application of these materials to aeronautical parts has been very progressive until full confidence on the in-service behavior has been demonstrated, and the strict certification process required for the composite structures is fulfilled
For the automatic layup and in-situ consolidation process, the applied thermal profile will determine the development of changes in the material such as melting, chain diffusion, crystallization, the possible thermal degradation and the development of residual stresses
Summary
The key properties that justify the wide use of composite materials in the aeronautical industry were described in [1] including weight savings due to the increase of specific resistance and stiffness, which leads to reductions in fuel consumption and, allows an increase in aircraft payload. It allows the lay-up to be done and simultaneously creates high-quality bonding among layers, suppressing the curing stages in the oven or autoclave Using this idea, the developments that led to the automatic layup process and in-situ consolidation were established, where a heating source heats surfaces of thermoplastic material, subsequently contacting them under the action of a compacting element. In the manufacture of the wingbox, stiffeners designed with a closed omega geometry have been implemented with automatic lamination and in-situ consolidation To facilitate this development, tools made of metal alloys were used, whose melting point is 75 ◦C under the glass transition temperature of PEEK, facilitating their extraction by fusion once the layup has been completed. In [30] the surface finishing results obtained with different cooling solutions for the roller were presented (by external air jet, by thermal contact against a cooled roller, or by flow of internal water) with cooling by internal water being the preferred solution
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