Effect of inlet pressure and binder on the void content of vacuum-infused unidirectional GFRP composites
Topic(s) : Manufacturing
Co-authors :
Agustin CANALIS (DENMARK), Robert PIERCE (DENMARK)Abstract :
In the production of large composite structures, such as the skins of wind turbine blades, dry preforms can be used to increase the productivity of the Vacuum-Assisted Resin Infusion (VARI) process by cutting the time required for layup. These preforms typically consist of multiple layers of dry reinforcement fabrics that are tacked together by a binder or tackifier material, thereby producing a stable fabric stack that can be directly placed in the infusion mould [1]. However, the presence of the binder must be thoroughly tested to ensure that it does not adversely affect the manufacturing behaviour, quality, or performance of the final composite.
One of the quality parameters for this comparison is the void content, which is known to be detrimental to the mechanical properties of the material [3]. Previous literature shows that there is an optimum flow front speed at which the void content is minimised [2]. However, most of these studies have focused on woven fabrics in Resin Transfer Moulding processes, typically at relatively high injection pressures compared to the VARI process and with no distribution media [2], [4], [5].
The present study is aimed at exploring the relationship between flow front speed and void content, along with the comparison of reference and preform (pre-consolidated with binder material) fabrics. In each case, four4 layers of a unidirectional non-crimp glass fibre fabric, peel-ply, release foil and distribution media. Infusion was carried out using polyester resin at nominal inlet pressures of 1 atm, 0.5 atm, 0.25 atm, to examine the material quality over a range of flow front speeds.
The speed of the flow front from the top and bottom surfaces of the layup were determined using an image processing script applied to video of the infusion. After curing, multiple samples were cut from the plates at the inlet, centre, and outlet of each plate, to measure the void content by burn-off. Optical microscopy was also performed to assess the shape and location of the voids in the microstructure.
The void content was found to be not only a function of flow front speed, but also location in the plate; in general, the higher speed inlet side of the plates had higher void content than at the slower flowing outlet region. No significant difference in the void content could be observed between similar infusion cases for the different fabrics (with or without binder). Ultimately, the study supports a common infusion strategy of throttling the inlet at the beginning of the infusion as a means of ensuring the quality of the laminate.
One of the quality parameters for this comparison is the void content, which is known to be detrimental to the mechanical properties of the material [3]. Previous literature shows that there is an optimum flow front speed at which the void content is minimised [2]. However, most of these studies have focused on woven fabrics in Resin Transfer Moulding processes, typically at relatively high injection pressures compared to the VARI process and with no distribution media [2], [4], [5].
The present study is aimed at exploring the relationship between flow front speed and void content, along with the comparison of reference and preform (pre-consolidated with binder material) fabrics. In each case, four4 layers of a unidirectional non-crimp glass fibre fabric, peel-ply, release foil and distribution media. Infusion was carried out using polyester resin at nominal inlet pressures of 1 atm, 0.5 atm, 0.25 atm, to examine the material quality over a range of flow front speeds.
The speed of the flow front from the top and bottom surfaces of the layup were determined using an image processing script applied to video of the infusion. After curing, multiple samples were cut from the plates at the inlet, centre, and outlet of each plate, to measure the void content by burn-off. Optical microscopy was also performed to assess the shape and location of the voids in the microstructure.
The void content was found to be not only a function of flow front speed, but also location in the plate; in general, the higher speed inlet side of the plates had higher void content than at the slower flowing outlet region. No significant difference in the void content could be observed between similar infusion cases for the different fabrics (with or without binder). Ultimately, the study supports a common infusion strategy of throttling the inlet at the beginning of the infusion as a means of ensuring the quality of the laminate.