Techniques for quality control and health monitoring of aerospace composite structures must be reliable, nonintrusive and preferably, non-contacting. Quadrupole resonance (QR) spectroscopy can fill this need. Previously, we have demonstrated that Quadrupole Resonance can be used for nondestructive inspection of polymeric fiber-reinforced composites, which can be exploited for both in-service inspection and on-going structural health monitoring.1-6 In this paper we present an extension of this work, applying the QR method to the quality control of composite parts manufactured via pultrusion.
In order to use the QR method for quality control of composite parts they must contain a small amount of tiny crystals of a QR active compound. These crystals are embedded in the part during the manufacture by blending it into the uncured resin. The QR active crystals sense any residual strains that may form inside the part during the manufacturing process. The crystals are interrogated via a single-side coil detector head, which transmits radio frequency (RF) pulses into the composite part. The strain-dependent QR response from the crystals is picked up by the same detector head.
The results presented in this paper demonstrate that the QR method is very successful at distinguishing composites parts manufactured under optimal conditions from those that were manufactured with a misaligned die or at reduced temperatures. Both QR frequency and line widths were used as a distinguishing parameter.
Quadrupole Resonance (QR) has recently been shown to be a feasible method for the non-contact measurement of strain in polymeric fiberglass-reinforced composites. Tiny crystals of a QR active additive are embedded into the composite or are applied as part of a surface coating. Strains in the composite are transferred to the additive crystals. These crystals can be interrogated via radio frequency pulses provided by a single-sided radio frequency coil. Thus, the additive crystals give rise to a strain dependent QR frequency response. The QR frequency and line width from composites containing additive are found to be sensitive parameters for the measurement of tensile strain. The QR active additive that was embedded in the composite matrix was found to be inert and non-intrusive.