Pressure bag molding : manufacturing, mechanical testing, non-destructive evaluation, and analysis
Larsen, Erik Barnholt
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It is desirable in the wind turbine industry to use low-cost fiberglass composite materials. However, current manufacturing capabilities for these materials can not keep pace with the increases in size and demands of new wind turbine designs. Process limitations in Resin Transfer Molding (RTM) have been identified that make this otherwise popular process less attractive for wind turbine blades, especially as the size of new blades increases. Other factors such as reliability and maintenance costs also need to ' reduce to allow for the continued competitiveness of these low cost materials. There were three main areas of research addressed in this work which were intended to address these needs. The first was “pressure bag molding”, a variation of RTM which was designed to remedy some of the limitations inherent with RTM. Critical manufacturing process parameters were identified and testing was conducted to compare these parameters for pressure bag molding to those of RTM. Mechanical testing was conducted to compare products of RTM to products of pressure bag molding. The second area of research was a new non-destructive evaluation method for fiberglass materials. This method involves the transmittance of infrared light through a laminate. This optical evaluation method is described in detail. Several exploratory tests were conducted to gain an understanding of the behavior of this method of evaluation. Then, a damage accumulation test was designed to compare damage accumulation properties of products of RTM to those of pressure bag molding. The third research focus was the development of a numerical progressive damage model. Ansys was used to model the complex damage behavior of the layered, angled laminates that were chosen for the damage behavior comparison discussed above. The process parameter tests showed superior performance for pressure bag molding. Mechanical testing of the products showed similar performance for pressure bag molding products, except for slightly reduced performance in the compressive strength test, which was discussed. The progressive damage model seemed to provide reasonable results. However, it was found (and discussed) that the resolution in the mechanical damage accumulation measurement was not adequate to facilitate reasonable comparison to the Ansys model.