Thin ply composites open up new opportunities to exceed the limits of conventional composite materials by improving first-ply/ first-damage criteria, fatigue life and ultimate strength [1]. By definition, individual plies with a ply thickness of less than 0.100 mm can be called a thin ply [2,3]. The size effects [1] and design freedom as it allows smaller pitch angles for a specific thickness [4] make thin ply composites superior to conventional ply composites. Obtaining thin plies is possible spreading conventional tows via techniques based on airflow, ultrasonic vibration, and mechanical means as the common alternatives [5]. Among these, mechanical tow spreading is based on pulling dry tows through bars/ pins with a certain tension. An example of a lab-scale tow spreading line, developed at TU Delft, including static spreader bars and tension sensors can be seen in Fig. 1 (a). Wrap angle, the pre-tension of the tow, relative friction between the spreading bar and tow, temperature and pulling speed are some of the parameters influential on spreading. Therefore, we propose an experimental framework to assess and quantify the effect of individual mechanisms on tow spreading. In the present study, we first investigate the effect of tension and wrap angle on tow spreading under the static condition schematically shown in Fig. 1 (b). Then, the effect of an actively controlled bar on tow spreading is tested under static conditions. Spreading bar rotation is controlled with a motor that reveals the influence of relative motion between the bar and the tow and thus the induced friction. Tow spreading is a continuous process, where filaments’ reorganization is time dependent. That means the time spent on a spreading bar, defined by the pulling speed, is influential on spreading, noting that the pulling speed is also intrinsically related to friction. Thus the third experimental method is based on observing tow spreading while the tow is pulled with various pulling rates, as schematically shown in Fig. 1 (d). During the talk, we will report our findings regarding the extent of spreading of different fiber types under different process conditions obtained by controlling the wrap angle, relative speed between the tow and bar(s) as well as by using bars that either are heated and/or exhibits different surface roughness. The analyses will be further enriched by microstructural analyses of specimens. @en

The main bottleneck of using composites for cryogenic storage of clean hydrogen fuel is the permeation of gas molecules. In this work, the permeation of hydrogen gas through thermally cycled thermoplastic composite laminates with two different stacking sequence is investigated. The experimental study is based on a methodology of cryogenically cycling the composite specimen and measuring the permeability in a dedicated hydrogen permeation setup. An optical microscope and X-ray computed tomography scanner are employed to investigate the existence of cracks. The results reveal that thermal cycling does not have a profound influence on permeability, while the stacking sequence has a considerable effect. Laminates with dispersed 0° layers resulted in lower permeation values compared to the laminate with grouped 0° layers at the laminate’s core. The imaging techniques did not reveal any observable crack which supports the hypothesis that permeation is mostly driven by bulk diffusion in the polymer. @en

The effect of thermal contact resistance (TCR) correlated to the degree of intimate contact (DIC) between the incoming tape and the substrate on the temperature history during laser-assisted fiber placement (LAFP) was investigated. A novel experimental methodology was designed to understand the effect with a non-contact method which did not influence the local consolidation quality. To assess the influence of TCR numerically, a three-dimensional optical-thermal model was developed. Experimental results indicated that, for the same tape temperature near the nip point, an increase in the compaction force resulted in a decrease in the temperature at the roller exit and the following cooling phase, in correlation with an increase in the final DIC. Also, the effect of the laser power on the final DIC was less pronounced than the compaction force. In the thermal model, when TCR at the tape-substrate interface was not considered, the temperature predictions underestimated the experimental measurements.

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Laser-assisted tape winding (LATW) is a highly automated process for manufacturing tubular-like fiber-reinforced thermoplastic composites such as flywheels and pipes. One of the crucial parameters in the LATW process is the temperature of the nip point at which the incoming prepreg tape is bonded with the substrate by a compaction roller. Therefore, the temperature evolution of the nip point plays a significant role to have a proper consolidation at the tape-substrate interface. The nip point temperature is highly affected by the time-dependent geometry of the substrate and roller during continuous LATW of thick composite rings. In this paper, a critical assessment of the substrate and tape surface temperature evolution is investigated experimentally by means of a thermal camera during the LATW process of a 26-layers thick carbon/PEEK composites. A three-dimensional (3D) optical model is coupled with a thermal model in which the substrate computational domain is updated with respect to the deposited tapes. A good agreement is found between the measured and predicted tape and substrate temperatures. The total absorbed heat and heated length of the substrate and tape are described based on the roller deformation. An increase in tape and substrate nip point temperatures is found with an increase in roller deformation during consecutive winding. It is also found that the consolidation pressure and contact length at the roller-substrate interface increases during the winding process. Accordingly, the heat transfer coefficient at the roller-substrate interface is studied using the developed process model.

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A new global kinematic-optical-thermal (KOT) model is proposed to provide a proper understanding and description of the temperature evolution during laser-assisted tape winding and placement (LATW/LATP) on any arbitrary shaped tooling geometry. Triangular facets are utilized in the kinematic model to define a generic tooling together with a user-defined fiber path and time-dependent process settings such as the tape feeding rate. The time-dependent heat flux distribution on the surfaces is calculated by the optical model and subsequently coupled to the thermal model. The numerical implementation of the developed KOT model is first verified for process simulations of the LATP on a flat tooling by comparing the temperature predictions with the available literature data. To validate the KOT model, a total of four pressure vessels are manufactured with in-line temperature measurements. The process temperature predictions are found to agree well with the measured temperature during the helical winding. The influence of the changing tooling curvature and process speed on the process temperature is found to be significant as shown by the experimental and numerical findings.

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The non-uniform temperature and crystallinity distributions present in carbon fiber–reinforced PA12 composite pipes, produced via laser-assisted tape winding (LATW), are investigated in this paper. The width of the laser source is usually larger than the substrate width which causes multiple heating and cooling of some regions of the (neighboring) substrate and hence temperature and crystallinity gradients during the adjacent hoop winding. A kinematic-optical-thermal (KOT) model coupled with a non-isothermal crystallinity model is developed to capture the transient temperature and crystallinity distributions for growing substrate thickness and width. The predicted temperature trends are validated with thermocouple and thermal camera measurements. The substrate temperature varies in the width direction up to 52%. This will lead to extra polymer remelting and possible degradation. The maximum variation of the crystallinity degree across the width is found to be 270% which shows agreement with the trend of the measured crystallinity degree. It is found that a more realistic description of the melting behavior of the matrix is needed to obtain a more accurate prediction of the crystallinity distribution.

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Tubular structures of fiber-reinforced polymer composites are utilized in various applications such as risers in the oil and gas industry and hydrogen pressure vessels in the automotive sector. The laser-assisted tape winding process presents an automated and efficient solution for the manufacturing of these structures out of thermoplastic composites. However, in order to guarantee reliable and high-quality process results, the temperature distribution within the laminate governing the consolidation between successively wound layers has to be understood and taken into account for process design. In an experimental setup, thermocouples were embedded between the layers in multiple spots along the perimeter during the manufacturing of pipe samples with five layers wound on a pure thermoplastic liner. This enabled capturing the through-thickness temperature distribution at different transversal locations. In addition to the temperature data recorded by the thermocouples, a stationary infrared thermographic camera focused on the laser-heated area was mounted on the tape winding head. The temperature data points of both sources were contrasted to evaluate how the through-thickness temperature distribution reflects the temperature input on the surface. Furthermore, the experimentally determined temperature distribution was compared with the results of a numerical process model, drawing conclusions with regard to the modelling and control of the multi-variable process.

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A new inverse kinematic-optical-thermal (IKOT) model is introduced to control the process temperature in laser assisted tape winding and placement processes. The optimum time-dependent laser power distribution is obtained by employing a grid of independent laser cells while keeping the temperature of substrate and tape at the target temperature. Multi-layer cylindrical hoop winding with laser grids of 1 × 1, 28 × 1, and 28 × 11 and helical winding of a pressure vessel with laser grids of 22 × 1 and 22 × 11 are considered. It is found that the optimized laser power distribution pattern remains the same during the consecutive hoop winding process while the total power reduces to compensate the heat accumulation. A more non-uniform laser power distribution is obtained for the helical winding because the substrate curvature changes drastically at the dome section of the pressure vessel. The change in the optimum total laser power is found to be almost constant for the helical winding case. Finally, the IKOT model is evaluated by analyzing the effect of the computational parameters on the optimized process temperature.

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Laser-assisted tape winding/placement (LATW/P) is a process in which fiber-reinforced thermoplastic prepregs are heated by a laser source and in-situ consolidated by a compaction roller. Maintaining a constant temperature along the prepreg width prior to the nip point is utmost necessary to manufacture fiber-reinforced thermoplastic composites with proper bonding quality. The heat flux distribution on the incoming prepreg tape and already placed substrate is affected by the local geometry and variation in process parameters during the process. In order to maintain the processing temperature distribution at the desired conditions, a new process optimization approach for the laser power distribution is presented. A laser source with variable power distribution is considered in the optimization scheme which can improve the bonding quality of the final product by achieving the desired constant nip point temperature distribution. The variable laser power distribution can be realized practically with the Vertical-Cavity Surface-Emitting Laser (VCSEL) technology. An inverse optical and thermal process models are developed in which an ideal laser power distribution is calculated based on the surface temperature distributions on the tape and substrate described as an input to the process model. Two different optimization studies are performed based on two different input temperature profiles namely stepwise and linearly ramp profile. First, the inverse thermal model is used to calculate the required heat flux distribution which is forwarded as an input to the inverse optical model. The obtained heat flux distribution is transformed into the laser power distribution using the optical model in which the ray-tracing method is used. The relation between the surface temperature distribution and laser power distribution is investigated. The obtained laser power distribution using the inverse optical model is compared with the calculated laser power distribution as an input for the already developed optical-thermal model.

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The laser-assisted tape winding (LATW) is an automated process for manufacturing fiber-reinforced thermoplastic tubular products, such as pipes and pressure vessels. Multi-physical phenomena such as heat transfer, mechanical bonding, phase changes and solid mechanics take place during the process. These phenomena need to be understood and described well for an improved product reliability. Temperature is one of the important parameters in this process to control and optimize the product quality which can be employed in an intelligent model-based inline control system. The incoming tape can overlap with the already wounded layer during the process based on the lay-up configuration. In this situation, the incoming tape can step-on or step-off to an already deposited layer/laminate. During the overlapping, the part temperature changes due to the variation of the geometry caused by previously deposited layer, i.e. a bump geometry. In order to qualify the temperature behavior at the bump regions, an experimental set up is designed on a flat laminate. Artificial bumps/steps are formed on the laminate with various thicknesses and fiber orientations. As the laser head experiences the step-on and step-off, the IR (Infra-Red) camera and the embedded thermocouples measure the temperature on the surface and inside the laminate, respectively. During the step-on, a small drop in temperature is observed while in step-off a higher peak in temperature is observed. It can be concluded that the change in the temperature during overlapping is due to the change in laser incident angle made by the bump geometry. The effect of the step thickness on the temperature peak is quantified and found to be significant.

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The laser assisted tape winding (LATW) is an automated process for manufacturing fibre-reinforced thermoplastic tubular products, such as pipes and pressure vessels. This process consists of several simultaneous physical phenomena including kinematic, optical and thermal behavior. Among these, the in-situ temperature-dependent consolidation of the tapes plays a vital role in the overall quality of the final product. Currently, a new model-based global approach is under development for industrialized production as a part of EU funded Horizon2020 ambliFibre project. There is a need for a highly efficient thermal model to capture the temperature distributions during the process. Therefore, in this study, the capabilities of the various thermal models are evaluated. The actual laser heat flux distribution is estimated from a comprehensive optical model and then subsequently coupled with the thermal model in order to obtain precise temperature distribution. The transient one-dimensional (1D) and 2D models are developed in a Lagrangian framework. The surface boundary conditions are moving with winding velocity and the mesh remains stationary. The transient approach is used by conventional finite volume (FV) method. The hoop winding process is quantified for various winding speeds using coupled optical-thermal models which have never been done up to now. Since the real LATW process includes many considerations that disturb the spatial temperature field, e.g., various winding angle and non-uniform laser heat influx, a 2D transient model is, therefore, convenient for the LATW process simulation. However, for hoop winding the 1D model is almost as accurate as the 2D model and less computationally expensive. The history of nip point temperature for 10 layers of the continues hoop winding is predicted. Also, the peak in the nip point temperature history which is a physical phenomenon taking place during the LATW process is explained.

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A micromechanical approach is used to study the damage initiation and nonlinear behavior of SiC/Ti composites subjected to a general complicated off-axis loading at elevated service temperatures. The effects of stress relaxation, interface damage together with fiber coating are considered. A cohesive zone model is applied to define the imperfect interface between the fiber and matrix. Introducing a unique failure criterion for all loading angles at different elevated service temperatures may be considered as the main contribution of this study. In order to simultaneously apply a combination of mechanical loadings and thermal residual stresses, appropriate periodic boundary conditions are imposed on the model. Predictions of the presented model show acceptable correlation with the reported experimental data at elevated temperatures. Results reveal that by increasing the service temperature, the strength of the composite is linearly degraded at fast rate especially at loading angles of 20° and 30°.

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A finite element, micromechanical model is developed to predict the inelastic behavior of SiC/Ti composites subjected to off-axis loading using a three-dimensional representative volume element (RVE). The model includes the effects of manufacturing process thermal residual stresses together with interface damage and fiber coating. The cohesive zone model is used to consider the imperfect interface between the fiber and matrix. Introducing a unique failure criterion for various off-axis angles is the main novelty of this study. Apart from interface damage, plastic deformation of the matrix is also considered as another source of nonlinearity. Appropriate boundary conditions are imposed on the RVE to allow simultaneous application of a combined normal axial and transverse and axial shear loading plus thermal residual stresses. Results of the presented finite element model are compared with experimental data for stress-strain response, initiation of nonlinearity, and ultimate strength in various off-axis angles which show good agreement. Moreover, parametric studies are conducted to examine the effects of thermal residual stress and fiber volume fraction (FVF) on the mechanical response of the material.@en