George A. Matzkanin

NDE OF MOISTURE ABSORPTION IN COMPOSITES USING NUCLEAR MAGNETIC RESONANCE

There are a number of NDE applications where nuclear magnetic resonance (NMR) can be useful for evaluating organic matrix composites and other polymer based materials. NMR is sensitive to molecular motions and structural changes and can be used to obtain both qualitative and quantitative information on the dynamic environment in which molecules are located. In particular, certain characteristics of NMR signals can provide information on 1) the presence of atoms in different physical states; 2) the type and strengths and interactions between atoms of both the same species and different species; 3) changes in molecular structure and intermolecular binding; and 4) transport (diffusion) phenomena. Although generally limited to detection of hydrogen in composites, NMR has been investigated for characterizing moisture, modulus variations, environmental degradation, aging, diffusion and degree of cure. Materials which have been examined include graphite, polyester and Kevlar composites in both cured and prepreg conditions, neat resins, and adhesives. In addition to laboratory studies, instrumentation approaches have been developed for practical application of NMR to nondestructive inspection problems involving composite or plastic components and structures. These approaches are based on the use of U-shaped magnets and flat detection coils to allow measurements to be made from a single surface. These one-sided NMR approaches also allow localized measurements to be made in order to map out spatially varying quantities such as moisture gradients and material inhomogeneities.

Advanced Quantitative Magnetic Nondestructive Evaluation Methods — Theory and Experiment

The scale of fatigue crack phenomena is reviewed in relation to the size detection capabilities of nondestructive evaluation methods. Several features of the fatigue phenomena which should be considered in developing nondestructive characteriziation methods are tabulated and briefly discussed. A qualitative assessment of such factors in relation to the inspection of ball and roller bearing components suggested that magnetic methods were very promising. The basis of the magnetic methods is magnetic domain phenomena and several aspects are briefly reviewed, including interaction of domains and inclusions and the influence of stress and magnetic field on domains. While magnetic calculations from first principles are extremely complicated when applied to engineering specimens, simplified treatments have been developed and will be reviewed. Experimental results will also be described which indicate that in many instances the simplified calculations can be used to predict many features of the experimental results. A cursory comparison of results predicted by the simple analytic model and other models in which finite element computer analysis predictions have been made do not agree for certain features. Experimental results and analyses obtained on rod-type fatigue specimens which show the experimental magnetic measurements in relation to featues such as crack opening displacement, crack opening volume, crack depth and other features show much promise in providing methods for greatly improved characterization of cracks in relation to fracture mechanics analyses and life prediction.

Nondestructive Characterization of Kevlar Composites Using Pulsed NMR

Nuclear magnetic resonance (NMR) is a branch of spectroscopy based on the interaction between nuclear magnetic dipole moments and a magnetic field. The term “resonance” is used because a natural frequency of the magnetic system, namely, the frequency of gyroscopic precession of the magnetic moment in an applied static magnetic field, is the quantity detected. Typically, the resonance frequency falls in the radiofrequency (RF) region of the electromagnetic spectrum. To implement the NMR method, a static magnetic field is applied to the specimen to polarize the magnetic dipole system and resonance detection is accomplished by coupling a suitable electromagnetic field to the specimen by means of an RF induction coil. Although NMR has been utilized as an investigative tool for many years in physics and chemistry laboratories, only recently has it been seriously considered for NDE of materials.1

Nondestructive Evaluation of Infrastructure Conditions

In rapid transit systems, the integrity of subway structures must be known for the benefit of public safety. New and innovative surveying techniques are required to overcome limitations of inspection methods presently used for determining the integrity of structures in aging subway systems. Nondestructive evaluation (NDE) techniques have been successfully developed and applied in a project funded by the Office of Technical Assistance in the Urban Mass Transportation Administration (UMTA) to assess the structural integrity of brick tunnels and concrete inverts in the New York City area. Promising results have been obtained so far in tests on a brick wall and concrete slab which simulated the subway structures of interest and in initial field tests conducted on a New York City Transit Authority (NYCTA) concrete floor. In subway tunnels over 70 years old there is an increasingly urgent need to develop NDE inspection methods to detect the degradation of structural materials and the presence of voids that could potentially cause failure. Bricklined tunnels require information on axial and radial variations and material properties, such as density and strength, in order to assess the overall structural integrity. Some of the brick tunnels being studied were originally built with an exterior boiler plate for maintaining the proper dimensions of the traffic passageway. Silt infiltration into gaps between the brick and the boiler plate shell could conceivably lead to deterioration of the outer layer of brick over the years. In this type of structure, there is interest in a method for determining whether the outer metal shell exists and whether the brick in the outer portion of the wall is structurally sound. In the case of the NYCTA subways, the primary structural integrity concern is the existence of voids and cavities under the concrete inverts. Voids can form when cracks occur in the invert and the underlying soil is washed out as a result of changes in the water table and the pumping action associated with passing trains. Reconstruction to replace the invert is very costly and involves substantial diversion of traffic. Therefore, identification of voids or cavities under the inverts is important in guiding rehabilitation procedures.