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13 / ene. / 2016

Thermoplastics modelling. Isabel Martin Hernando

Thermoplastics modelling. Isabel Martin Hernando

Analyzing the aeronautic industry evolution related with the materials employment is detected a noticeable trend focused on the weight reduction. The use of reinforced thermoset composite materials has acted as a key for achieving this goal.  These materials are mainly characterized by their thermal stability once the curing steps are completed, for them it is generally necessary the development of autoclave cycles. Researchers have invested multiple efforts on analyzing processes and materials that permit to maintain or increase certain properties by reducing cost and time because of this conditioning drawback. Processes as automatic lamination and in-situ consolidation with reinforced thermoplastic composite materials suppose a response to these efforts and are the focus of this PhD, developed in collaboration with one Spanish University (Universidad Politécnica de Madrid, Escuela de Ingeniería Aeronáutica y del Espacio).

For these studies, FIDAMC has an automatic lamination and in-situ consolidation machine in its facilities. This machine is the result of the collaboration among Airbus Group, the industrial MTorres (specialized center on process automation) and FIDAMC. The basic operation of this machine is based on the contact placement between two layers of reinforced thermoplastic material, mainly PEEK/FC (although FIDAMC is now working on the incorporation of new high performance materials), by pre-heating over the melting temperature of the matrix and the subsequent pressure application by a compaction roller. The heating source consists of a diode laser with variable optics which permits to modify the heated area. The compaction roller is formed by an elastomer material that permits the contact between material parts just after the melting.

Despite the apparent simplicity of the process, there are a lot of implied elements that require a detailed study to make possible the control of the final quality in the sample. Thermal profiles in the set, intimate contact between parts, chains healing in the joint line, crystallization, partial degradation experienced by excessive heating peaks and residual stresses states in the part, condition the inherent manufacturing methodology.

With the development of this PhD, the aim is deepen in each previously enumerated phenomenon and by comprehending them, make the needed modifications to improve the manufactured parts results, producing a control system on the set by presenting mathematical models which describe approximately the interacting physics.

The key element in the study which acts as a feedback in the rest of theories is the thermal profile analysis in the whole laminate. For predicting the behavior in the laminate during the heating under laser performance, it has been developed simulations with Comsol Multiphysics. These simulations had permitted to extract temperature values in each point of the laminate as a function of the process time.

The experimental validation of the results obtained with the software has been done by thermocouples and fiber optic senses. They were placed in random positions in the width and the length.

Once the temperature profiles on the material are known, it is possible to feed the rest of the developed models highly dependent on the temperature. With respect to the intimate contact models, it has been employed published theories by other authors which describe the material surface as an irregular roughly profile composed by rectangular elements. Mathematically, the equation that predicts the degree of contact between the parts requires the knowing of applied pressures on the set, the material viscosity as a function of the temperature and the geometric initial state on the laminas. Referring to the healing, De Gennes reptation theory permits to explain the movement of polymeric chains, reptation times will stablish a limiting threshold in the possible manufacturing process velocities.

Also, during the heating, the material can experience elevated heating peaks which suppose a degradation effect. Modelling the thermal degradation as a dependent function on the temperature and time permits the detection of defects in the samples and the prevention over their generation.

One common way to determine the goodness in all manufacturing process is the execution of mechanical tests. Good mechanical properties in one automatic process as the one under study is the result of good intimate contact and healing grades, a limited or null degradation effect and the correct interaction of the crystallization degree. The most used thermoplastic in the industry is the semi-crystalline PEEK, it stands out from the others because of its high performances, that’s why it is the example material used in the thesis calculus.

Crystalline materials are very influenced by their cooling profile because it modifies the crystalline state, both in morphology and size. The effect of an elevated and quickly heating couple with high cooling rates produces differences in the final laminated compared with the ones obtained with other techniques (oven, autoclave, press). Evaluate the causes that produce differences in the behavior of the material will permit to modify the process and stablish allowable values.

As a final step in the study, it will be done a simulation on the residual stresses effects over the material as a cause of the cooling shrinkage, the differences between the thermal expansion coefficients and non-homogeneous heating inherent to the process. The control of their presence will suppose a solution to reach the projecting geometries.

Wedge Peeling test.png  Process description.jpg  COMSOL Multiphysics thermal modelling.png  ISC Machine.JPG