Research on process optimization for composite materials
Liquid composite molding (LCM) is a processing method to manufacture high-performance composite parts like aircraft and automotive structural components, prosthetics, sports equipment, and the largest wind-turbine blades. It entails closing a mold on a dry fibrous reinforcement, and then pressure-pushing and/or vacuum-drawing a polymer resin such as epoxy through those fibers, and then curing the resin once the reinforcement is filled. LCM is a lower-cost processing alternative for these applications compared to the more traditional route of autoclave-cured prepreg. BYU’s composites lab is working on various ways to optimize the mechanical properties from LCM-made parts, to further reduce part cost, and any other strategy we can think of to help manufacturers learn the process, enabling the low-cost manufacture of light-weight super-strong components.
OPTIMIZATION OF LIQUID COMPOSITE MOLDING, FOR MINIMAL VOID CONTENT
The main drawback to LCM (compared to autoclave prepreg processing) is the tiny bubbles that are formed during the wet out of the reinforcement, owing from the different velocities of the flow within the yarns, and between the yarns. Once the resin cures, these bubbles become trapped in place and are known as voids which act as stress-concentrators and adversely affect the mechanical properties. This bubble/void formation, and what happens to the bubbles after they form, has been poorly understood because of the difficulties of characterizing tiny bubbles inside a mold during LCM processing. This is especially the case with carbon fiber reinforcements, as carbon fiber is black and opaque. A characterization method has been developed at BYU, however, which allows bubble imaging during LCM processing (“in situ”). The method involves macro-lens photography, thick transparent tooling, UV-sensitive dyes in the resin (for contrast) and black light illumination.
FLOW SIMULATION AND PERMEABILITY MEASUREMENT
LCM flow simulation involves virtually simulating the flow of resin through the fibers, such as epoxy filling a carbon fiber reinforcement. Such simulation can be used to determine optimal processing conditions such as applied pressures and temperatures, and locations for the resin inlets and vents. This can all be done before the expensive tooling is made, and iterations of the process can be done with much less material and time costs compared to doing physical prototyping.
BYU researchers are investigating various facets of such LCM process simulation in order to make the simulation more accurate, and more useful to manufacturers. These include:
- Characterization of the compressibility of composite reinforcements, i.e. how thick will it be under a vacuum bag, and how much pressure will be required from a press to close a mold.
- Viscosity (kinetics and rheometry modeling), i.e. how fast does the viscosity build up as the epoxy is curing, and how long will the manufacturer have to mold the part before the epoxy cures.
- Reinforcement permeability, i.e. how easily does the resin flow through a given reinforcement, such as a carbon fiber weave.
BYU is involved in an international group of researchers tasked with development of an ISO standard for permeability measurement. This involves several parallel studies to determine possible variability in permeability measured caused by different test conditions, and will determine the future standard test method prescribed to everyone working with LCM process simulation.