Micro-crack ultrasound scattering in anisotropic composite laminates

A computational model of ultrasound scattering by micro-cracks in fiber reinforced polymer laminates is presented, foundational to study of micro-crack induced ultrasound attenuation. A model for transmission scattering response is developed using a boundary integral formulation, and associated approximate scattering theories are discussed. Numerical results are presented demonstrating application of the model to laminates containing distributed micro-cracking

The effect of crack morphology on ultrasonic response

A numerical study is presented of the influence of crack morphology on ultrasonic pulse-echo response. Crack morphology is described as a planar crack onto which a random normal direction deviation is imposed with a specified tangential correlation length. Pulse-echo responses for ensembles of random crack profiles are computed as a function of profile height, correlation length, crack length, angle of incidence, wave mode type and signal bandwidth. Mean and variance of signal peak amplitude are compiled. Limits of validity of a Kirchhoff scattering approximation are observed through comparison to boundary element method (BEM) predictions

Guided wave signal transport in curved and tapered plates

A numerical study is presented of the influence of plate curvature and taper on the transport of ultrasound guided wave signals for nondestructive evaluation (NDE) and structural health monitoring (SHM) applications. Model formulations for transmission at sharp transitions in plate curvature and thickness taper are summarized. Results are presented showing that transitions in plate curvature and tapered plate thickness have minimal effect on signal transmission efficiency when associated characteristic dimensions are large compared to plate thickness

Porosity Characterization in Fiber-Reinforced Composites by Use of Ultrasonic Backscatter

The use of ultrasonic backscatter to characterize anomalous states in fiber-reinforced composites has received considerable attention in recent years. The ultrasonic backscatter from composites with oriented fiber reinforcement, unlike that from monolithic materials, displays a strong angular dependence. Hence, three independent variables are available over which to analyze the backscatter. These are the azimuthal angle ∅ (the rotation orientation of the composite plate about the perpendicular), the elevation angle θ (the angle between the ultrasonic beam and the perpendicular to the insonified composite plate), and time. Bar-Cohen and Crane [1] considered various ways of exploiting the angular dependence of backscatter to examine anomalies in composite laminates such as fiber misalignment, cracks, and porosity. Several other efforts employing similar approaches have followed [2–4]. This paper addresses specific questions concerning the angular dependence of backscatter and the use of this angular dependence to assess porosity levels. Additionally, problems inherent in the analysis of the temporal behavior of backscatter are discussed, and an approach to the spectral analysis of backscatter for porosity assessment is demonstrated

Support minimized nonlinear acoustic inversion with absolute phase error correction

The predominant factors which prohibit the inversion of acoustic scattering data for the purposes of flaw characterization are 1) limited angular access to the flaw, 2) limited temporal frequency signal bandwidth, and 3) lack of absolute phase information between individual measurements (zero of time problem). An additional complication which impedes the data inversion is the non-linear dependence of the scattering data on the scattering object. This problem must be handled by either linearizing the problem or by applying an iterative procedure which may have questionable convergence properties. An approach to data inversion is presented here which shows potential in overcoming the aforementioned difficulties. This approach compensates for the lack of data by constructing a solution which yields simulated scattering consistent with the measured data, while simultaneously minimizing a functional measure of the support (i.e. volume) of the flaw. Such an approach to limited data inversion has proven effective in limited view X-ray CT applications when reconstructing discontinuous boundary flaws such as cracks and inclusions [1, 2, 3]. The application presented here is by-and-large analogous to the X-ray CT application, except for the additional complication of the lack of absolute phase between measurements. This zero-of-time problem is handled here by treating the absolute phase of each measurement as a variable in the minimization of the flaw support

Effects of crack morphology and closure on ultrasonic response

Progress is reported on the development of tools for the study of the influence of crack morphology and closure on ultrasonic inspection response. An in‐situ phased array sector scan is described for the monitoring of crack responses under load, and results are presented showing the anticipated dependence of signal amplitude on crack closure. Acoustic microscopy mapping of actual crack morphology is outlined. A measurement model is presented for prediction of responses using the measured crack morphology as input. Results are shown demonstrating the reduction in inspection signal amplitude arising from the deviation of measured crack morphology from the ideal planar geometry

Leak detection using structure-borne noise

A method for detection and location of air leaks in a pressure vessel, such as a spacecraft, includes sensing structure-borne ultrasound waveforms associated with turbulence caused by a leak from a plurality of sensors and cross correlating the waveforms to determine existence and location of the leak. Different configurations of sensors and corresponding methods can be used. An apparatus for performing the methods is also provided

Measurement of dynamic full-field internal stresses through surface laser Doppler vibrometry

We present a method for evaluating internal dynamic stresses in a solid vibrating body from measurements of surface motion. The method relies on the same mathematics as boundary element method: A boundary reciprocity integral represents interior motion as a surface integral of boundary motion times the Green’s function. The surface motions are measured with a laser vibrometer rather than simulated, giving a direct measurement of internal motions and internal dynamic stresses. Experimental results on a flexing beam demonstrate that stresses measured in this fashion match those calculated from elementary theory