Optimizing Autoclave Curing: A Finite Element Approach to Reduce Porosity in Composite Laminates
Topic(s) : Manufacturing
Co-authors :
Andrea DEI SOMMI (ITALY), Giuseppe BUCCOLIERO , Francesca LIONETTO , Fabio DE PASCALIS (ITALY), Michele NACUCCHI , Alfonso MAFFEZZOLI (ITALY)Abstract :
A critical issue during the autoclave curing of composite laminates is the formation of porosity, which arises from various factors, including trapped air during lamination and, notably, absorbed moisture, that leads to void nucleation and growth. Assuming that the latter occurs only before gel point, void growth is only possible as long as the hydrostatic resin pressure remains below the water vapor pressure. Several models have been proposed in the literature to predict void growth [1,2], but they generally tend to overestimate the final void size. Other models, instead, aim to assess the ultimate porosity [3,4]. Due to the limitations of these models in quantitatively determining void size or content, a finite element model has been introduced to evaluate the conditions leading to porosity development in order to properly set the process parameters during composite manufacturing.
The developed multiphysic model describes the main phenomena involved in the curing process: i) heat exchange among the autoclave gas, mold, and laminate, coupled with the exothermic resin reaction; ii) resin viscosity changes as a function of temperature and degree of reaction; iii) desorption of moisture under vacuum bag during heating; iv) determination of hydrostatic pressure of the resin through the laminate thickness. In this way, the time evolution of temperature, resin viscosity and degree of reaction, water concentration, hydrostatic resin pressure, and water vapor pressure can be monitored.
In summary, this model allows determining the possibility and location of porosity across the laminate thickness. Water is always present in a laminate as a consequence of the exposure conditions in the clean room. Subsequently, the actual resin pressure during curing can be calculated and compared with the saturated vapor pressure to verify if porosity development conditions occur. Model validation was achieved by manufacturing neat resin samples, first exposed to different relative humidity conditions, and then cured under different hydrostatic pressure levels (1, 4, and 7 bar) following the manufacturer's suggested cycle. The resin samples were analyzed through optical microscopy and micro-CT, confirming the presence of voids as predicted by the model.
Through this model it is possible to modify the suggested curing cycle by adjusting the process parameters in order to limit the development of porosity, favoring water desorption or keeping the water vapor pressure below the hydrostatic resin pressure until the resin gelation, thus counteracting the void growth.
The developed multiphysic model describes the main phenomena involved in the curing process: i) heat exchange among the autoclave gas, mold, and laminate, coupled with the exothermic resin reaction; ii) resin viscosity changes as a function of temperature and degree of reaction; iii) desorption of moisture under vacuum bag during heating; iv) determination of hydrostatic pressure of the resin through the laminate thickness. In this way, the time evolution of temperature, resin viscosity and degree of reaction, water concentration, hydrostatic resin pressure, and water vapor pressure can be monitored.
In summary, this model allows determining the possibility and location of porosity across the laminate thickness. Water is always present in a laminate as a consequence of the exposure conditions in the clean room. Subsequently, the actual resin pressure during curing can be calculated and compared with the saturated vapor pressure to verify if porosity development conditions occur. Model validation was achieved by manufacturing neat resin samples, first exposed to different relative humidity conditions, and then cured under different hydrostatic pressure levels (1, 4, and 7 bar) following the manufacturer's suggested cycle. The resin samples were analyzed through optical microscopy and micro-CT, confirming the presence of voids as predicted by the model.
Through this model it is possible to modify the suggested curing cycle by adjusting the process parameters in order to limit the development of porosity, favoring water desorption or keeping the water vapor pressure below the hydrostatic resin pressure until the resin gelation, thus counteracting the void growth.