PhD thesis: Defect detection in plates using guided waves
Measurements were made using two
kinds of specimens [Experiments]. On large and
thin aluminum plates, the influence of a notch on the scattered field was
studied. For a single hole in the plate, notches were introduced at different
angles and the change in amplitude was measured for different relations of
wavelength (excitation frequency), plate thickness, hole radius, and notch
length. The comparison of these measurements to the numerical calculations and
the numerical study of the defect detectability is described in the previous
section [Theory]. To simulate the multiple scattering
at a line of fasteners in an airplane fuselage, the scattered field around three
holes and the detectability of a notch at one of the holes is investigated.
Broadband excitation and measurements at only one line or a single point are
studied for fast and efficient monitoring measurements.
Applying the measurement method,
several experimental constraints have to be considered. As the tensile specimen
has a plate-strip like geometry, only guided waves propagating along its length
can be employed, to avoid multiple reflections. A thin piezoelectric plate is
used as the excitation transducer to excite a wave with a rather plane
wavefront. However, an amplitude modulation over the width of the specimen was
measured and can not be avoided. The angle between wave propagation and fatigue
crack orientation is always 90°, as the crack grows vertically to the applied
For the second kind of measurements the laser interferometer is affixed to the servo-hydraulic testing machine. The amplitude at one point of the specimen close to the hole is monitored during the cyclic tensile loading, achieving an automated on-line measurement of the fatigue crack growth. During the experiments, the cyclic tensile loading was halted periodically and the crack length measured optically using a microscope. The measured amplitudes are normalized with the amplitude measured at zero crack length, and plotted against the optically measured crack lengths for an excitation frequency of 40 kHz, shown above (black). A significant increase in amplitude, larger than the variation at zero crack length, can be seen for a crack length of 2.5 mm, and therefore a crack of this length can be certainly detected. The amplitude rises further, when the crack increases in length. Comparing the measured monitoring curve to FDM calculations for notches of varying length (red), good agreement is found. The increase in amplitude for small crack lengths is over-estimated as the FDM calculation assumes through-thickness notches, while the cracks in the specimen are still quarter-elliptical. The on-line monitoring measurements allow a certain detection of cracks ca. 2.5 mm in length. The use of higher excitation frequencies shows a better detectability of shorter cracks, as predicted above. However, the use of a higher frequency also introduces experimental problems, like worse repeatability between the measurements at different specimens. For the detection of smaller cracks in the tensile specimen, possibly with a fastener, a further study might investigate the experimental suitability of higher guided wave modes or Rayleigh waves. The application of the described method to large real-life structures like an aircraft fuselage with rows of fasteners, rivets and holes will provide a challenging task.