DEVELOPMENT OF NONLINEAR SLOW DYNAMICAL DAMAGE DIAGNOSTICS (S3D) FOR APPLICATION TO NONDESTRUCTIVE EVALUATION P. A. Johnson + , A. Sutin #+ , and J. TenCate + +Geophysics Group, Los Alamos National Laboratory, Los Alamos, NM, USA #Davidson Laboratory, Stevens Institute of Technology, Hoboken, NJ, USA paj@lanl.gov Abstract The onset and monitoring of the nonlinear material response called slow dynamics can be applied for expedient damage interrogation. The method, called Slow Dynamical Damage Diagnostics (S3D), is based on applying a low-amplitude, pure or swept-frequency probe-signal near or on an eigenmode of a sample. The sample is then mechanically excited by a larger amplitude signal, and, if damaged, material softening and slow dynamics are induced (elastic nonlinear response), changing the probe-wave amplitude and frequency characteristics. A change in the probe wave characteristics can be used to directly infer that a material is damaged. We describe two versions of S3D. We also include several novel, non-contact methods of wave excitation that could be applied to other techniques. Introduction When a member of the class of materials known as ''nonclassical" or ''nonlinear mesocopic" [1,2] is strained by an oscillatory-wave or impulsive source at small amplitudes (10 -6 - 10 -7 ) the elastic wave present in the material distorts, manifest by wave speed decrease and the creation of harmonics and wave modulation. Simultaneously, the material modulus and the quality factor (specific dissipation, Q) decrease. We call this behavior “nonclassical nonlinear fast dynamics” (NNFD) [3]. NNFD is distinguished from “atomic" acoustic/elastic nonlinearity in liquids, most metals, individual crystals, etc., [4] by extreme nonlinearity and hysteresis in the stress-strain relation (the equation of state). NNFD materials exhibit characteristic scaling relations of, for instance, harmonic amplitudes, with applied strain that are different from atomic elastic materials [4-8]. The NNFD materials do not immediately recover to their original states. Instead, they recover to their original, equilibrium values over 10 3 - 10 4 seconds as a function of log(t). This phenomenon is the signature of slow dynamics (SD). SD were first observed in relatively homogeneously nonlinear materials, such as rock and concrete, that have a small volume of elastically soft constituents distributed within a rigid matrix (e.g., grains in a rock), e.g. [9]. In contrast, in damaged materials slow dynamics are due to localized nonlinear elastic features, e.g., a crack [3]. In this work we take advantage of the transition region of NNFD/SD, where fast dynamics dissipate quickly but slow dynamical response persists, as a quick and sensitive damage diagnostic. We call this the method of Slow Dynamics Damage Diagnostics (S3D). Our purpose here is to describe two versions of S3D that are most appropriate for application to high Q and low Q materials, respectively. In order to describe the methods, we will also present brief descriptions of SD characteristics. Methods and Results S3D With a Pure Tone. The method is based on applying a pure tone probe signal near an eigenmode of a sample. When the sample is disturbed by a larger amplitude signal, the eigenmode abruptly shifts, changing the probe-wave characteristics and simultaneously inducing slow dynamics [3]. Such a change in amplitude only occurs in damaged materials. The experimental configuration is shown in Figure 1. Figure 1: Experimental configuration for pure-tone S3D. In the measurement a relatively high-amplitude impulse (strain~5x10 -5 ) is used to produce nonlinear material softening in the presence of a crack, and induce the SD. The impulse is delivered by a mechanical excitation in this case (equivalent in energy to a tap with a pencil). A low-amplitude (strain~10 -7 ), pure-tone probe is input into the sample to monitor material change before, during and after the impulse. As noted, due to material softening the effect on a given mode in a nonlinear material upon excitation is to shift the modal frequency downward, as illustrated in Figure (2) [bottom]. As a result of the mode shift, the probe shows a significant amplitude change (2 in Figure 2, top): it is a slope amplifier. The shift remains for some time due to the effect of slow dynamics and is therefore easily observed well after the impulse excitation has dissipated. Slope WCU 2003, Paris, september 7-10, 2003 129