Abstract

Porosity and voids are a type of defect that may appear during the manufacturing of composite structures. Its possible presence is checked in production parts by different non-destructive techniques based on ultrasonic inspection. It is important to perform this evaluation  considering that a high content of these defects decrease the mechanical properties compared to the original design. In recent years, optical microscopy of composite materials has become a very powerful tool, which is  available for the industry to inspect and look at the internal structure and help to reach conclusions in the field of quality, failure analysis and I+D+I. In addition, the use of automatic scanning techniques of wide cross sections has opened the possibility of quantifying the percentage of porosity/voids, as well as become an alternative to more laborious testing procedures such as acid digestion of the resins. This study contributes to existing test standards within the aeronautical sector, in the sample preparation strategy. For this purpose, samples of epoxy matrices with carbon fibers, both unidirectional and fabric tapes have been analyzed to evaluate the influence of the quality of sample preparation, the stacking sequence and the initial porosity levels, the repeatability of the measurements and the quality of the results. An attempt has been made to correlate the results with standar physical / chemical analysis.

Abstract

Manufacturing quality in composite laminates is one of the key aspects to have in consideration when producing aerostructures. For composite structures quality evaluation, an iterative process of qualification of the first items manufactured is often followed. Thus, the manufacturing process parameters can be tuned until achieving an incremented maturity level that ensures the structure mechanical properties present values within the defined quality standards.

Volume fraction of voids (pores), resin and fibre is part of the set of properties that must be verified, especially in novel and/or low cost manufacturing processess that can affect the structural behaviour. The ability to obtain results depending on the technique used for testing becomes very important at this point.

This study proposes a comparative analysis of the measurement of voids, fibre and resin percentage on carbon fibre reinforced polymers by automated image analysis technique and acid digestion method, varying parameters of the test procedure with the purpose of evaluating their influence in the correct determination of volume fractions. The use of different methods and parameters allows to analyze comparatively the different results obtained and shows advantages and disadvantages of both methods as well as provides valuable information on the best practices for correct test execution following each procedure.

Abstract

In this work, experiments of RTM processes have been carried out. The composite manufacturing have been visualized using Synchrotron X-ray Computed Tomography (S-XCT) in the Swiss Light Source (SLS) facilities (Villigen, Switzerland).  The main goal of these tests is to study the behaviour of the resin in a porous medium such as a dry fibre fabrics at different capillary numbers, i.e. resin velocity. According to the theory, the capillary number of a process is an indicator that shows whether the resin flows preferably the fibre bundles or the channels between the bundles and, therefore, where the voids are mainly generated.  To study this relation, a characterized (viscosity, surface tension and contact angle) epoxy resin has been injected into a closed mould containing six layers of dry E glass fibres at constant and controlled flow using a commercial syringe pump. Due to the high resolution and the low acquisition time of the tomograms, the movement of resin between and within the bundles, the void formation and the void transport have been captured in the images. In this article, it is shown the effect on these phenomena depending on the resin velocity.

Abstract

The inclusion of reliable void formation and transport models in process models for LCM is paramount to guarantee the reliability of the manufactured parts, since it allows the estimation of part in-situ void content after mould filling, as well as the estimation of the ideal bleeding time.

The incorporation of void dynamics into mould filling simulations has traditionally been under the form of unsaturated flow, where a continuous saturation field is advected. However, these models lack the prediction of the morphological properties of voids, which are known to influence their mobility through the reinforcement, as well as the final part mechanical properties. Also, direct numerical simulation methodologies, which are fundamentally based on the Volume of Fluid (VoF) method, can provide void morphological data but are too computationally expensive to apply directly into an entire mould filling domain.

Particle tracking methodologies have extensively been used in computational fluid dynamics (CFD), to solve problems in which the fluid flow carries solid particles, such as in sediment deposition. This methodology could prove to be a computationally efficient way to deal with the different void morphologies registered, both during void generation, as well as transport, since each void is taken as a discrete particle, thus possessing its own set of properties.

As of today, mould filling simulation software do not encompass such a methodology to allow mould design optimization. This work discusses the implementation of such a methodology, envisioning a streamlined application into process design.