After working on the knitting techniques at Buck-TSP, VTT found that the manufactured composite knits improved the quality by decreasing carbon fiber damage. No fragments with carcinogenic potential were observed at ETH. With the developed technique there are no limitations regarding the choice of fibers, matrices and fiber impregnation techniques.
Structural and mechanical properties
In a consolidated laminate, knit layers are interlocked. Out-of-plane spread was found by ETH to be 1.5 times higher in bulk than in surface layers. The interaction of knit layers is also assessed by mechanical properties at IVW, ETH and INASMET. Compared to woven fabrics, out-of-plane properties like KIC and interlaminar strength as well as inplane shear are the most promising properties of knitted fabrics.
As expected, the knits showed an excellent deformation behaviour. Drawing ratios ranging from 2.4 to 3.25 were obtained by IVW for out-of-plane deformation. Uniaxial stretching degrees of 200% in wale and 160% in course directions were achieved at ETH. The textile deformation is higher when the matrix is melted as, for instance, during a forming process.
Optimal parameters were found to obtain consolidated panels by hydraulic and double belt pressing. All investigated thermoforming processes allowed complete moulding, although some constratins have been found as deconsolidation and sagging of the previously consolidated sheets during the preheating step. This obligue to use special handling techniques as clamping devices which also guarantee controlled loop deformation. Because of the short cycle time (40 seconds) at ETH and the good part quality, the rubber stamp forming is the most suitable for forming of GF/PET consolidated sheets. A rough surface is obtained on the rubber side.
Design and manufacturing of technical parts
To obtain smooth surfaces in both part sides for automotive applications, a thermoforming process with rigid hot tools was shown to be very suitable at BMW. Cycle times with the conventional thermoforming installed facilities of 20 minutes were achieved, being too long for series mass production. For shorter series in the automotive and aeronautical industry, it can be used.
Manufacturing of the aeronautical double corrugated panel or INASMET shows the factibility of manufacturing very complicated geometries in PEEK/CF composites. Further work is needed to optimize the process in order to get the target of 400 Euro per Kg of finished part. Basis for optimization are clear and known. All other targets for the aeronautical demonstrator (mechanical under environmental effects, surface quality and weight) were fully achieved.
Crash worthiness and structural testing
As shown by testing of crash elements at IVW, the energy absorption potential of knitted fabrics is in the range between SMC and GMT and woven fabrics. The developed model simulates the quasi-static mechanical behaviour of knitted PET/GF.
As shown by BMW, the bending stiffness required for the automotive part can be obtained by beams made of PET/GF knitted fabrics.
CASA found that the failure of the aeronautical demonstrator was due to debonding of stiffener and skin. Besides this fact, a value more than 1.5 times above the required limit value was obtained, showing the actual potential of using PEEK/CF knitted fabrics in secondary structural parts.
The project has also developed the basis for the utilization of other material combinations not analyzed yet in deep but very competitive for the future as envisaged by CASA.
The project "PROTAILTHERM" has the aim, to show the mechanical and economical potential of knit fabric reinforced thermoplastic materials compared to "standard" reinforcements like woven fabrics. The project is subdivided into the following research points:
- Optimisation of the knitting process
- Development of net shape knitting process for a zero waste production
- Optimisation of processing conditions of "knit" reinforced thermoplastic composites.
- Description of the macro-and micromechanical behaviour of the knit reinforced thermoplastics.
- reduction of technical parts as a verification of the theoretical and experimental studies.
- Simulation of the crash behaviour with such structures.
- Verification on real structures in destruction tests.
- Assessment of the process under economical and ecological aspects.
- Well known advantages of knitted fabrics are the drapability (without wrinkles) during the forming process and the low production price (40%-60% cheaper than woven fabrics) of knitted textile preforms.
Especially the last arguments will open the application field of the automotive industry and enlarge the application for secondar structures in aeronautic industry.
Funding SchemeCSC - Cost-sharing contracts
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