Material development
Project leader
Dr.ir. Harald Bersee
Prof.dr.ir. Stephen Picken
PhD. students
Ir. Daniel Vlasveld
Ir. Patricia Parlevliet
Researcher
Ing. Bert Weteringe
Contactperson
Dr.ir. Harald Bersee
Summary
Background
In the design and manufacturing of composite components boundaries are encountered, which has to be solved by development of new materials or microanalysis of material behavior. This application oriented fundamental material development research is according to the trinity essence in which the material structure is interrelated with the design and process. Two such projects have been started, namely Development of new thermoplastic composites and Micro-cracking of semi-crystalline thermoplastic composites. These projects are being performed in close co-operation with other departements/ universities with a large experience in these area's, i.e. Polymer Materials & Engineering (TU-Delft) for both projects and the Composites group of the University of Twente for micro-cracking of semi-crystalline thermoplastic composites.
New thermoplastic composites
The high mechanical requirements of structural aircraft components necessitate the usage of continuous fibres in composites. The currently available continuous fibre reinforced thermoplastics are APC-2(Carbon fibre reinforced PEEK as UD-tape, Victrex) and Cetex (Aramid, glass or carbon fibre reinforced PEI or PPS as UD-tape and fabric, Ten Cate). These materials have high performance characteristics but are very expensive. The viscosity of PEEK and PEI is very high resulting in difficult impregnation of the reinforcements and difficult deformability of the composite. PPS has a significant lower viscosity, however, is prone to micro-cracking resulting in a critical processing window. New continuous fibre reinforced thermoplastics entering the market are Twintex (Commingled glass fibre reinforced PP as fabric and UD-tape, Vetrotex) and Tepex (Glass fibre reinforced PP, PET or PA6 as fabric, Bond laminates). These composites consist of thermoplastic matrices with relatively low viscosities and thus a very good deformability. Although the material costs are relatively low, so is their mechanical performance. Consequently, there is a large gap between these materials to be filled with new continuous fibre reinforced thermoplastics incorporating either sufficient mechanical performance or reasonable costs. This is illustrated by using the triangle often used to classify thermoplastics, with on the bottom the materials with relative low mechanical properties and low cost and in the top materials with the highest mechanical properties and costs (see Figure 1).
Micro-cracking of semi-crystalline thermoplastic composites
The Cetex composite of Ten Cate Advanced Composites, with a PPS matrix was especially developed for aeronautical applications because of the resistance of PPS against Skydrol in combination with excellent mechanical properties.
The morphology of the PPS is influenced by a lot of processing parameters like the temperature of the material during heating, the total time in the infrared oven, where-in the material temperature is higher than the melting temperature, the die temperature, which influences the cooling rate, transport time, pressing speed and pressure.
When a sheet of material is being processed with the rubber forming process and a sample is taken for microscopic examination, no micro-cracks will be observed. However, once the processed sheet of material is exposed to extreme environmental conditions, like high temperatures and high relative humidity, micro-cracks might suddenly occur. Micro-cracks will always arise at the interface between the PPS and a filament of a fibre and will reduce the mechanical properties of the material. Once micro-cracks are present, the amount of cracks increases rapidly.
The research that has been performed is focussed on the influence of the rubber forming process on the occurrence of micro-cracks during thermal ageing. In order to verify the micro-cracking behaviour of the material Differential Scanning Calorimetry, mechanical tests (In Plane Shear) and microscopic examination has been performed.
