TITLE: NON-CONTACT ULTRASONIC TECHNIOUES FOR PROCESS CONTROL OF COMPOSITE FABRICATION

Abstract:
(Introduction) Organic composite materials are conventionally classified by the matrix material as thermoset or thermoplastic. The primary processing steps in manufacture of fiber-reinforced composites are resin impregnation, layup and consolidation. Composite material structures are normally manufactured by processes such as:

(1) hand layup and autoclave cure,

(2) resin transfer molding,

(3) filament or tow placement with a winding machine (sometimes multiple axis robotics controlled) and cured in place,

(4) multiple-axis tape layup of complex geometries,

The critical process step requiring monitoring is the final consolidation process for both thermoset and thermoplastic composites. Examples of the parameters and properties that can be nondestructively monitored for process control of organic matrix composites are listed in Figure 1. The two parameters currently routinely monitored are the temperature and the pressure applied to the filaments, tows, or laminates. Commonly this is done with conventional thermocouples and pressure gauges. The only other device fairly routinely used to monitor the cure process is a dielectric sensor mounted between two composite plys in autoclave curing which measures the ionic conductivity. Figure 2 lists the potential devices for process monitoring of organic matrix composites. At the present time, no technique exists to optimally monitor the mechanical properties during the curing or melt process, although the mechanical properties are the most important factor for overall integrity of composite structures. Ultrasonic tests can be designed and optimized for measurement of the elastic wave speed and attenuation of the wave propagating through the composite during the consolidation process. The wave speed is a function of the elastic modulus, viscosity and density of the composite, while the attenuation is a monitor of the change in viscosity of the resin, fiber compaction, resin-fiber ratio and porosity. Figure 3 compares NDE techniques available for process monitoring of organic composite materials. Figure 4 shows the results of an experiment where an ultrasonic waveguide embedded in the midplane of 1 6-ply graphite/epoxy panel during the prepreg lay-up process was used for in-process cure monitoring.

Source: Proceedings of the 'NDE applied to Process Control of Composite Fabrication' - Conference, 4-5 Oct 1994 St. Louis, Missouri. Publisher and Organizer: Nondestructive Testing Information Analysis Center (NTIAC) Texas Research Institute Austin, Inc. Austin, Texas [http://www.ntiac.com] [Buying the Proceedings]