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A sparse-array structural health monitoring (SHM) system based on guided waves was applied to the door of a commercial shipping container. The door comprised a corrugated steel panel approximately 2.4 m by 2.4 m surrounded by a box beam frame and testing was performed in a nonlaboratory environment. A 3-D finite element (FE) model of the corrugations was used to predict transmission coefficients for the A0 and S0 modes across the corrugations as a function of incidence angle. The S0 mode transmission across the corrugations was substantially stronger, and this mode was used in the main test series. A sparse array with 9 transducers was attached to the structure, and signals from the undamaged structure were recorded at periodic intervals over a 3-week period, and the resulting signal database was used for temperature compensation of subsequent signals. Defects in the form of holes whose diameter was increased incrementally from 1 to 10 mm were introduced at 2 different points of the structure, and signals were taken for each condition. Direct analysis of subtracted signals allowed understanding of the defect detection capability of the system. Comparison of signals transmitted between different transducer pairs before and after damage was used to give an initial indication of defect detectability. Signals from all combinations of transducers were then used in imaging algorithms, and good localization of holes with a 5-mm diameter or above was possible within the sparse array, which covered half of the area of the structure.

Sparse-array structural health monitoring systems based on guided waves have been proposed by many authors, current signals being compared with a baseline obtained when the structure was known to be defect free. An image of the structure in the form of a ‘C-scan’ map showing likely defect locations can be produced by combining information from different sensor pairs in the array. It is generally recognized that temperature compensation is essential for the method to work and various compensation methods have been proposed with good results. However, artifacts are commonly seen in the images, making reliable defect location difficult. This is because, as well as the first reflection from the defect that maps to the correct defect location in the image, shadowing effects occur later in the signal and these combine to produce artifacts. This effect can be reduced by appropriate gating of signals, although at a cost in area coverage. If images are formed with multiple different gate locations, the artifact positions and intensities change, but the defect always produces a strong indication. Therefore by combining multiple images, the artifacts can be suppressed and the defect is located more reliably. This strategy has been successfully demonstrated for defects both within the transducer array and close to an edge of both a simple plate and a shipping container door.

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The reliability of structural health monitoring (SHM) systems based on guided ultrasonic waves is improved if pure modes are generated by the transducers. A piezoelectric- based transducer generating high purity A0 mode guided waves at low frequencies (around 20 kHz) was developed. The through-thickness resonance of a piezoelectric element was lowered to the frequency region of interest by use of backing masses and low-stiffness front layers. Soft front layers also reduced significantly the transmission of in-plane displacements caused by Poisson's ratio effects in the piezoelectric element to the structure. Parametric studies were undertaken by varying the backing mass length, the transducer diameter, and the thickness of the front layer. The thickness of the plates on which the transducers were operated was found to be a critical issue and these effects were evaluated. Results obtained by finite-element analysis were validated by experimental measurements and showed that signals with A0/S0 energy ratios substantially above 40 dB can be obtained.

Friction welding techniques are widely used in several industrial sectors. A non-destructive inspection is mandatory in postprocessing for repair quality evaluation. The present study examines through conventional ultrasonic technique Friction Hydro Pillar Processing (FHPP) repairs on ASTM A36 low carbon steel plates. The repairs were made using four different axial forces, 200, 250, 300 and 350 kN with constant rotational speed of 1 000 rpm. The features of the echogram signals obtained made possible to identify and distinguish microcracks, lack of bound, clustered inclusions and isolated inclusions, forming three critical regions within the weld. It was seen that high processing forces increases probability to generate microcracks from discontinuities. These ultrasonic results were related and validated with micrographic analysis and it was possible to make a relationship between echograms features and micrographs to differentiate the discontinuities founded within the repairs. This information could be used as a guideline for operating procedure to locate discontinuities.

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