E Murat Sozer

An approach to couple mold design and on-line control to manufacture complex composite parts by resin transfer molding

During impregnation of the resin into a closed mold containing the preform in the resin transfer molding (RTM) process, increased yield of successful parts can be achieved if one could account for the inherent disturbances such as race-tracking and preform variability. One way to address this is to use sensors and actuators to control the resin flow dynamics during the impregnation process to counteract the disturbances. In this paper, we use a mold filling simulation tool to develop a design and control methodology that, with the help of sensors and actuators, identifies the flow disturbance and redirects the resin flow to successfully complete the mold filling process without any voids. The methodology is implemented and experimentally validated for a mold geometry that contains complex features such as tapered regions, rib structures, and thick regions. The flow modeling for features such as ribs and tapered sections are validated independently before integrating them into the mold geometry. The approach encompasses creation of software tools that find the position of the sensors in the mold to identify anticipated disturbances and suggest flow control actions for additional actuators at auxiliary locations to redirect the flow. Laboratory hardware is selected and integrated to automate the filling process. The effectiveness of the methodology is demonstrated by conducting experiments that, with feedback from the sensors, can automate and actively control the flow of the resin to consistently impregnate all the fibers completely despite disturbances in the process.

Compaction of e-glass fabric preforms in the vacuum infusion process: (a) use of characterization database in a model and (b) experiments

Compaction of e-glass fabric preforms (random, woven and biaxial) embedded with a distribution medium (polypropylene) is coupled with 1D resin (polyester) flow during initial application of vacuum, mold filling and fiber relaxation stages of vacuum infusion. In our previous study,1 the compaction characterization procedure had been designed and conducted to realistically model the compaction behavior of fiber preforms in vacuum infusion such that the loading was done on a dry specimen; fiber settling was allowed under constant compaction pressure; unloading was done after the specimen was wetted and the fiber relaxation was characterized at constant pressure. To investigate the effects of characterization components on the part thickness evolution, two compaction models (“unloading only” and “unloading and time-dependent relaxation”) were coupled with two models of flow (“uncoupled” and “coupled” pressure-thickness-permeability). The results of the coupled model of “unloading and time-dependent relaxation” and “coupled pressure-thickness-permeability” was the closest to the vacuum infusion experiments.

Modeling of post-filling stage in vacuum infusion using compaction characterization:

Two-dimensional finite-element method solution of the post-filling stage of vacuum infusion was studied based on mass conservation in an infinitesimal control volume. First, resin pressure distribution at the instant of mold filling was calculated and then used as the initial condition for the transient post-filling stage. Explicit time-marching algorithm was used for the evolution of resin pressure and part thickness, and its stability was ensured by selecting the time step adaptively. Finite-element method solution was verified analytically for one-dimensional case and numerically for two-dimensional cases using global mass conservation. The time that it took for the settlement of pressure and thickness was investigated to compare the effectiveness of different resin-bleeding scenarios where different number and locations of gates were used. It was shown that the settlement time increased exponentially as the dimensions of the mold increased, which proved that process simulation fed with correctly designed material characterization can replace tedious trial-and-error search of control actions to reduce the settlement time and variation in part thickness.

Effect of part thickness variation on the mold filling time in vacuum infusion process

An experimental setup was used to fairly compare mold filling times in vacuum infusion and resin transfer molding, and a 9.5% shorter mold filling time in vacuum infusion was observed than in resin transfer molding. The setup was also used to conduct compaction and permeability characterization experiments, and the results were used in a simplified vacuum infusion model, which is more straightforward to solve than the conventional full and coupled models in the literature. Simulated filling time in vacuum infusion was 31% shorter than in resin transfer molding. The faster resin flow in vacuum infusion is explained by the fact that the thickness in the wetted upstream region increases with time, and thus the effective permeability in that region increases.An experimental setup was used to fairly compare mold filling times in vacuum infusion and resin transfer molding, and a 9.5% shorter mold filling time in vacuum infusion was observed than in resin transfer molding. The setup was also used to conduct compaction and permeability characterization experiments, and the results were used in a simplified vacuum infusion model, which is more straightforward to solve than the conventional full and coupled models in the literature. Simulated filling time in vacuum infusion was 31% shorter than in resin transfer molding. The faster resin flow in vacuum infusion is explained by the fact that the thickness in the wetted upstream region increases with time, and thus the effective permeability in that region increases.

Effect of permeability characterization at different boundary and flow conditions on vacuum infusion process modeling

Permeability characterization of a fabric preform is a key factor that affects the accuracy of process modeling of vacuum infusion. There are various flow types and boundary conditions (such as one-dimensional or radial flow under constant injection pressure or constant injection flow rate during unsaturated or saturated flow regimes) used in permeability measurement experiments in the literature. This study investigates the effect of using different flow and injection boundary conditions in permeability characterization on the results of coupled one-dimensional mold-filling and compaction model. The results of the model are compared with vacuum infusion mold-filling experiments. It is shown that using the permeability measured at constant injection pressure and unsaturated flow results in the closest fill time compared to the experiments for all three types of fabrics investigated in this study.

A novel mold design for one-continuous permeability measurement of fiber preforms

One-continuous permeability measurement experiments allow measuring permeability of a fiber preform within a range of fiber volume fractions by conducting a single unsaturated (a.k.a. transient) flow experiment on a dry specimen at an initial thickness, and a set of saturated flow experiments on the wetted specimen by varying the thickness of the mold cavity. This approach allows quicker database construction and reduces the effect of inherent variation of fabric structure caused by inconsistent labor on permeability. In this study, the drawbacks of previous mold designs are eliminated by using appropriate sealing, gap thickness adjustment mechanism and features that allow straightforward and reliable manual operation. Experiments for three different fabric types are conducted and the results are discussed. It is mainly observed that the unsaturated permeability is higher than the saturated permeability.

Pressure-controlled compaction characterization of fiber preforms suitable for viscoelastic modeling in the vacuum infusion process

A woven fabric’s compaction in the vacuum infusion process is characterized by applying an initial settling under a minor load, compaction, settling under a major load, decompaction and relaxation. The effects of compaction rate, relaxation pressure, wetting and debulking cycles are all investigated. Although wetting helps by increasing fiber volume fraction insignificantly, its contribution is more significant during debulking cycles by increasing the fiber volume fraction to 57.4% as compared to 55.4% for the debulked dry specimens. Recovery during decompaction is much less than the deformation during compaction, and thinning/thickening of the specimens with time under constant pressure, so called settling/relaxation pressures, indicates that fabric specimens are not elastic materials, but viscoelastic. The experimental data of this study will be valuable to compare different viscoelastic and elastic compaction models in our next study.

Examining graduate teaching assistants’ conceptions of and readiness for effective teaching in a non-profit Turkish university

Abstract This study aimed to explore graduate teaching assistants’ (GTAs) perceptions of and readiness for effective teaching in higher education before and after attending an effective teaching training programme that was followed by a short term teaching experience. The study sample consisted of 62 GTAs who participated in an effective teaching training programme in a non-profit Turkish university. First, we administered a survey just before and after the training programme, and six months later when study participants conducted different teaching duties in their discipline specific departments. The results showed that after training and experience, GTAs put more emphasis on the role of instructors as content experts and their conceptions of effective teaching reflected more caring attitudes toward student learning. We also found that for GTAs who actively involved in training and teaching in a real classroom, training and experience have a combined positive effect on their readiness for effective teaching.

Viscoelastic modeling of fiber preform compaction in vacuum infusion process

A woven fabric’s compaction was modeled by using five viscoelastic models – Maxwell, Kelvin-Voigt, Zener, Burgers, and Generalized Maxwell – to reveal the capabilities and limitations of the models. The model parameters were optimized by minimizing the deviation between the model results and experimental data collected in our previous material characterization study mimicking different compaction stages (loading, fiber settling, wetting, unloading, and fiber relaxation) that a fiber structure undergoes during vacuum infusion process. Although Burgers and Generalized Maxwell models have the highest performance due to their almost equal coefficient of determination values, they have diverse characteristics in terms of modeling different stages of compaction. Burgers model allowed modeling the permanent deformation in relaxation stage, but failed in modeling permanent deformation in settling stage. Generalized Maxwell model could do the opposite, i.e. failed in the former and could handle the latter. This study’s major contribution is a holistic numerical approach and its conclusions by modeling all stages of the vacuum infusion process instead of one stage at a time, and thus optimizing only one set of model parameters (constants of springs and dampers) since they do not change with time. The numerical results of different models were fit to the results of a specially designed compaction characterization experiments conducted in our complementary study.