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.
We report on field test results conducted during 1999 in Bosnia and at the Army Mine Training School, Fort Leonard Wood, MO, on a ne prototype landmine detection system. In all test, non-metallic, anti-personnel (AP) and anti-tank (AT) landmines were detected via the NQR explosive signature with a probability of detection of 100 percent. The initial false alarm rate for the AP mine test was < 5 percent and was reduced to zero by a subsequent remeasurement. The test included typical burial depths and a variety of ground and weather conditions. In addition, the system can tolerate very high levels of metallic clutter and has repeatedly achieved zero false alarm rate when scanning for buried explosives at an EOD test range.
Nuclear quadrupole resonance (NQR) is a technique that discriminates mines from clutter by exploiting unique properties of explosives, rather than the attributes of the mine that exist in many forms of anthropic clutter. After exciting the explosive with a properly designed electromagnetic-induction (EMI) system, one attempts to sense late-time spin echoes, which are characterized by radiation at particular frequencies. It is this narrow-band radiation that indicates the presence of explosives, since this effect is not seen in most clutter, both natural and anthropic. However, this problem is complicated by several issues. First, the late-time radiation if often very weak, particularly for TNT, and therefore the signal-to-noise ratio must be high for extracting the NQR response. Further, the frequency at which the explosive radiates is often a strong function of the background environment, and therefore in practice the NQR radiation frequency is not known a priori. Finally, at the frequencies of interest, there is a significant amount of background radiation, which induces radio frequency interference (RFI). In this paper we discuss several signal processing tools we have developed to enhance the utility of NQR explosives detection. In particular, with regard to the RFI, we exposure least-mean-squares algorithms which have proven well suited to extracting background interference. Algorithm performance is assessed through consideration of actual measured data. With regard to the detection of the NQR electromagnetic echo, we consider a Bayesian discrimination algorithm. The performance of the Bayesian algorithm is presented, again using measured NQR data.
Nuclear Quadrupole Resonance (NQR) combines the compound specific detection capability offered by chemical offered by chemical detection techniques with the spatial coating capability and convenience of an induction coil metal detector. In this paper we present the first results of the detection of TNT by NQR with sufficient for detection of many antipersonnel mines and essentially all antitank mines. In addition, we review the result of a blind in-field demonstration of the system in detecting RDX in which 28 out of 31 RDX-only targets were found with 1 false alarm in a 110 m test lane, and a second test in which 21 out of 21 RDX mines were found with zero false alarms at a clearance rate of 1.1 m2 per minute.
Nuclear Quadrupole Resonance (NQR) combines the compound specific detection capability offered by chemical detection techniques with the spatial localization capability and convenience of an induction coil metal detector. In the 16 years since NQR was last applied to mine detection in the U.S., there has been considerable improvement in the basic techniques. This paper reviews the progress achieved under a recent initiative to detect landmines by NQR. Two basic technical developments are summarized: the design of a detection coil suitable for probing the ground for landmines buried at typical depths, and an increase in the NQR signal obtained from the explosive TNT. In addition, we report the sensitivity of an NQR detection system to detect the electromagnetic response of metal-cased landmines.