Abstract

With more and more use of composites in engineering applications, the need for automated composites manufacturing is evident. The use of automated fiber placement (AFP) machine for the manufacturing of thermoplastic composites is under rapid development. In this technique, a moving heat source (hot gas torch, laser, or heat lamp) is melting the thermoplastic composite tape and consolidation occurs in situ. Due to the rapid heating and cooling of the material, there are many issues to be addressed. First is the development of the temperature distribution in different directions which gives rise to temperature gradients. Second is the quality of the bond between different layers, and third is the rate of material deposition to satisfy industrial demand. This thesis addresses the first issue. The temperature distribution affects the variation in crystallinity, and residual stresses throughout the structure as it is being built. The end result is the distortion of the composite laminate even during the process. In order to address this problem, first the temperature distribution due to a moving heat source needs to be determined. From the temperature distribution, the development and distribution of crystallinity, residual stresses and deformation of the structure can then be determined.
As the first phase of the work, this thesis investigates the temperature distribution due to a moving heat source for thermoplastic composites, without considering the material deposition. A finite difference (FD) code based on the energy balance approach is developed to predict the temperature distribution during the process. Unidirectional composite strips are manufactured using AFP and fast-response K-type thermocouples are used to determine the thermal profiles in various locations through the thickness of the composite laminate subjected to a moving heat source. It is shown that temperature variations measured experimentally during the heating pass, using thermocouples embedded into the composite substrate, underneath layers of the composite material, are consistent with the generated thermal profiles from the numerical model. The temperature distribution, both in the direction of the tape and through-thickness direction can be predicted numerically.
It should be noted that the convective heat transfer coefficient employed in the thermal analysis of the process plays an important role in the heat transfer mechanism. Information from the literature shows values of h that vary from 80 W⁄(m^2.K) to 2500 W⁄(m^2.K). This large range can provide a great degree of uncertainty in the determination of important quantities such as temperature distributions and residual stresses. The reason for these large differences can be due to the differences in the process parameters in each of the studies. The process parameters can include the flow rate of the hot gas, the gas temperature, the distance between the nozzle exit and the surface of the composite substrate, the angle of the torch with respect to the surface of the substrate etc. The purpose of the final part of this thesis is to investigate the effect of different AFP process parameters on the convective heat transfer coefficient and to propose a procedure for the determination of the h value according to the particular experimental setup.