NONDESTRUCTIVE EVALUATION OF MOISTURE IN COMPOSITES USING MICROWAVES

THE BROAD OBJECTIVE OF THIS EXPLORATORY RESEARCH IS TO DEMONSTRATE THE FEASIBILITY OF USING INNOVATIVE MICROWAVE SENSORS TO NONDESTRUCTIVELY EVALUATE AND MONITOR MOISTURE WHICH HAS DIFFUSED INTO NONCONDUCTING FIBER REINFORCED CURED COMPOSITE STRUCTURES AND MATRIX PREPREGS. IN CURED COMPOSITES, MOISTURE CAUSES THE MATRIX TO SWELL AND SOFTEN WITH A SUBSEQUENT LOSS OF STIFFNESS AND STRENGTH. IT INDUCES RESIDUAL SWELLING STRAINS AND MICROCRACKING, AND IRREVERSIBLY DEGRADES THE FIBER/MATRIX INTERFACE. IN REPAIR, MOISTURE DIFFUSES INTO THE REPAIR ADHESIVE TO CAUSE DETERIORATION AND BLISTERING. IN UNCURED PREPREGS, MOISTURE SLOWS THE FORMATION OF THE POLYMER NETWORK AND INCREASES THE VISCOSITY AND VOID CONTENT. PRELIMINARY MICROWAVE TESTS HAVE SHOWN THAT DIFFUSED MOISTURE HAS A PRONOUNCED EFFECT ON THE MACROSCOPIC DIELECTRIC PROPERTIES OF COMPOSITES. USING AN INNOVATIVE SENSOR DESIGN, THE BULK EFFECTS OF BOTH THE REAL DIELECTRIC CONSTANT AND THE LOSS FACTOR HAVE BEEN SHOWN TO PROVIDE TWO INDEPENDENT EFFECTS OF DIFFUSED MOISTURE. FROM SUCH INDEPENDENT MEASUREMENTS, IT APPEARS POSSIBLE TO QUANTIFY THE CUMULATIVE MOISTURE AND TO EVEN MAKE SOME ASSESSMENT OF THE MOISTURE IN THE BOUNDARY LAYER WHICH OFTEN OCCURS FOLLOWING TRANSIENT EXPOSURE AND DRYING CYCLES. SPECIFIC TASKS FOCUS ON SENSOR DESIGN, THEIR OPTIMIZATION AND USE, MONITORING MOISTURE DIFFUSION INTO CURED COMPOSITES AND PREPREGS, AND MODELING OF THE DIFFUSION PROCESS FOR CORRELATION OF MOISTURE PROFILES WITH THE MICROWAVE MEASUREMENTS.