Testing and analysis of composites with 3D woven reinforcement
Time: Wed 2023-11-29 10.00
Location: D2, Lindstedtsvägen 5, Stockholm
Subject area: Vehicle and Maritime Engineering
Doctoral student: Tomas Ekermann , Lättkonstruktioner, Farkostteknik och Solidmekanik
Opponent: Associate Professor Lars Pilgaard Mikkelsen, Technical University of Denmark
Supervisor: Malin Åkermo, Lättkonstruktioner, Farkost- och flygteknik, Farkostteknik och Solidmekanik; Stefan Hallström, Lättkonstruktioner, Flygteknik, Lättkonstruktioner
Composites with three-dimensional (3D) reinforcement have several benefits, compared to laminated composites. Their through-thickness reinforcement increase the out-of-plane properties significantly and could eliminate problems with delaminations. Also, these composites have proven to have great damage tolerance and energy absorption. However, their complex yarn architectures make it challenging to predict their mechanical response and performance.
In this thesis different aspects of composites with 3D woven reinforcement are explored. The focus is on a specific yarn architecture, called fully interlaced 3D weave. The results are however not only limited to that specific 3D reinforcement but could to a certain extent also be applicable to 3D reinforcement in general.
Preforms with fully interlaced 3D weaves were manufactured and impregnated with epoxy. These were then examined in great detail with computer tomography (CT) to study the internal yarn architecture after impregnation. Analysis showed that the yarns were quite significantly distorted by the through-thickness compression during impregnation. The distortion was attributed to the relatively sparse weave, not supporting the through-thickness reinforcement, which therefore distorts and brings the rest of the yarns along with it. In parallel, a simulation model of the internal geometry of the manufactured material was developed. The simulation model was however not designed to include the distortions encountered in the physical material.
The manufactured material and its corresponding model was tested in a tensile test setup. Two different thicknesses of the material was manufactured as well as a corresponding composite with two-dimensional (2D) reinforcement. Results showed that the material with 2D reinforcement was stiffer and stronger than the ones with 3D reinforcement, which was attributed to the lower crimp in the 2D reinforcement. A difference in stiffness between the two 3D weaves was also found and addressed to the larger amount of surface layer in the thinner weave, where vertical weft yarns are aligned with the warp direction and contributing to the overall stiffness in that direction. Failure analysis of specimens tested in the warp direction showed that initial cracks form in the boundaries of vertical weft yarns, close to the material surface. For specimens tested in the horizontal weft direction, initial cracks were found through the vertical weft yarns at the surfaces. Both these findings were supported by results from the simulation model.
An application for composites with fully interlaced 3D weave was also explored, where it was integrated as a fillet in a composite T-joint. The scope here was to make a 3D reinforced fillet, having low transverse thermal expansion which would decrease the residual stresses in the fillet after curing. T-joints with conventional fillets and fillets with 3D woven reinforcement were manufactured and tested in a pull-off test. Results showed that T-joints with conventional fillets had higher strength, but also higher spread, than the ones with 3D reinforcement. The higher strength of T-joints with conventional fillets was attributed to their better ability to adapt to the T-joint cavity, while the fit was not as good for the 3D fillets. The lower spread in strength of the T-joints with 3D fillets was attributed to their lower sensitivity to minor flaws such as voids inside the fillet.