A ircraft and rotorcraft structures are being redesigned with new lower weight materials; however, older metallic structures continue to be used. These older structures need monitoring and characterization of damage for con-

tinued operation. The use of ultrasonic guided waves is one nondestructive technique that can be used to interrogate such structures. Typically, large cracks or geometric variations can be found by examining signal amplitude,

but smaller damage that can suddenly grow in size and cause serious damage cannot be detected.

Guided Lamb wave nondestructive evaluation is a tool used in structural health monitoring of aircraft, civil, and mechanical infrastructures. The goal is to determine how wave velocity and mode shape are influenced by changes in geometry and/or boundary conditions caused by structural damage or degradation. These waves propagate in thin plates and plate-like structures, and are formed by the interference of multiple reflections and mode conversions of longitudinal and shear waves at the plate surfaces. Guided waves are typically generated and detected using conventional piezoelectric transducers, oriented either flat or at an angle with respect to the surface. Two types of waves propagate a symmetric wave and an antisymmetric wave, each with multiple modes and at different frequency and speeds that complicate data analysis. To avoid analysis complications, fundamental symmetric (S0) and antisymmetric (A0) modes at low-frequency thickness values are typically used for inspections by limiting source transducer bandwidth. Processing and analysis of guided wave signals are typically performed in the time and frequency domain. Other popular analysis methods include integrated time-frequency domain and wavelet analysis. In some cases, baseline signals are subtracted from inspection signals to enhance results.

A finite-difference model was used to simulate wave propagation in a thin aluminum plate with a notch. For this modeling and simulation, commercial software was used based on a published algorithm that solves a 2D (plane strain) acoustic wave equation. In the model, the ultrasonic source and receiver were each 10 mm in diameter and in planar contact with the sample surface positioned 100 mm apart. The aluminum plate was 2.0 mm thick and 600 mm wide. A wide width was used to prevent interference of edge reflections. Aluminum properties of the material used in the model were longitudinal velocity of 6420 m/s, transverse velocity of 3040 m/s, and density of 2700 kg/m3. The simulated damage (the notch) had a width of 6.0 mm and depth from the sample back surface that ranged from 0.0 to 1.0 mm in 0.1-mm steps. Notch depth translated to a depth-to-thickness ratio of 0% to 50%. To vary the location of the notch, the source and receiver transducers were repositioned but always kept 100 mm apart. For example, the notch was centered at 0.0 along the axis, the source was positioned at -40 mm, and the receiver positioned at 60 mm.

Two different transmitter or source signals were used. A sine Gaussian pulse and sine exponential pulse were available as functions in the software. The sine Gaussian pulse spectrum has a full-width-half-height (FWHH) bandwidth of ~0.32 MHz and ranges from 0.0 to 1.0 MHz. The sine exponential pulse spectrum has a FWHH bandwidth of ~0.43 MHz and frequency range from 0.0 to ~2.0 MHz, with low amplitude frequency components above 1 MHz.

Time, frequency, and time-frequency analysis methods of ultrasonic guided waves for detecting a small notch were present. Ultrasonic-guided wave signals were numerically generated using a finite-difference model. In this model, damage was simulated in a rectangular notch of varying depth and position between a source and receiver transducer. The signals displayed the classical symmetric and antisymmetric modes. Analysis of these signals as a function of notch depth showed a flat response or no change in signal amplitude, spectral magnitude, and average power up to a damage depth of ~0.5 mm (or 25% of the sample thickness). For greater damage or notch depths, larger changes were observed. This result showed that gross damage could be detected, but not shallow damage less than 25% of material thickness.

This work was done by Robert F. Anastasi of the Army Research Laboratory. ARL-0127

This Brief includes a Technical Support Package (TSP).
Ultrasonic Guided Wave Analysis for Detecting and Classifying Damage in a Thin Metallic Plate

(reference ARL-0127) is currently available for download from the TSP library.

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This article first appeared in the August, 2011 issue of Defense Tech Briefs Magazine.

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