The article, Learjet 85: Large Step Out of the Autoclave pertains to the topic of the new technologies of carbon fiber reinforced plastics (CFRPs) and different resin transfer injection (RTI) processes that have stemmed from an original Composite Liquid Molding (CLM) process. More specifically, the article talks about the newly found interest between these newly manufactured CFRPs and how they can be used to innovate the way aircraft structures are designed, as the aerospace industry continues to grow steadily. Many different forms of CLM processes have evolved since they initially were adopted as a widespread use for production in several marine, automotive, industry, and medicine composite applications from the early 60’s and 70’s . The author writes about a project that Bombardier has been developing and manufacturing for the past several years, the project is known as Learjet 85. Learjet 85 is a mid-sized business jet, and is the first aircraft with a fuselage and wing fabricated from composites formed from an evolved RTI process; on April 9th, 2014 Learjet 85 made its first flight [1,3].
Learjet 85 composite materials go through a few different processes of CLM in order to achieve the necessary volume fiber, lightweight, and key mechanical properties that engineers and manufactures desire for applications in the aerospace industry. The main difference between the various different methods of CLM is the use of an autoclave for the curing step during the fabrication process. The autoclave was previously required in order to properly develop and cure these composite materials with as little void density as possible, usually targeting a value of > 1%. Composites initially used in commercial aviation were prone to strength reducing voids that can form within the material, rendering them useless for wide spread use amongst commercial aircrafts . Bombardier has used a more modern RTI molding process in order to create their Learjet, this process replaces the autoclave with a simple conventional oven. The honeycomb composites found previously in aircrafts, are the result of the old in-autoclave process that is shown in Figure 1 (a). At the beginning of this process, the layers of carbon fiber are bonded to one another using an epoxy resin. Due to the fact that the carbon fiber layers must be bonded one at a time with the epoxy resin, this process tends to be time-consuming and very costly due to the autoclave curing step, such a machine can cost approximately $10m . Figure 1 (b) on the other hand shows us the emerging process known as the vacuum-assisted resin transfer molding (VaRTM) process.
This technique uses an out-of-autoclave (OOA) process in order for the composites to achieve the standard for the mechanical properties that are desired. OOA processes require that all the carbon fiber plies to be laid on top one another and then uses a pump to apply the epoxy resin rather than applying them both simultaneously. Since the VaRTM does not require an autoclave for the curing process, this is especially appealing to the aerospace industry due to the fact that not only are aerospace-sized autoclaves going to drive up tooling costs and slow down production efficiency, they can prove to be difficult to apply the appropriate amount of resin into the carbon fiber . However, it is not an easy task for VaRTM processes, Darcy’s Law allows us to take a few variables into account in order to determine the velocity at which we need to apply our resin in order to minimize voids within the structure and maximize our volume fiber percentage, typically in the range of 50-60%. (1)
Where u is fluid velocity (m/s), K is permeability (m^2), μ is fluid viscosity (Pa‧s) and∇P is pressure gradient (Pa/m) . Proper resin impregnation is crucial in order to achieve the targeted mechanical properties. Once the resin is impregnated into the composite structure, instead