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 PhD
thesis: Defect detection in plates using guided waves
NDT
Application
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.
In cooperation with the fatigue engineering center of RUAG Aerospace, Emmen,
Switzerland the applicability of the method for real fatigue grown cracks was
verified using tensile specimens. Fatigue cracks at a hole in a tensile specimen
were introduced by cyclic tensile loading in a servo-hydraulic testing machine
(see below). Tensile specimens with a length of 500 mm and a reduced cross
section around the hole were manufactured by RUAG Aerospace from Al-7075 PL-T6
sheets. The hole radius (3.25 mm) is about the same size as the specimen
thickness (3.17 mm), to achieve the same ratio between hole radius and sheet
thickness as used in the planking of fighter jet fuselage. The geometry of the tensile
specimen is shown below, with a blow-up of the geometry of the fatigue cracks at
the fastener hole. The cyclic tensile loading with a maximum tensile stress of
up to 100 N/mm2 results in crack initiation and growth at the
fastener hole. Starting as a quarter-elliptical crack at the side of the hole,
the crack slowly increases in length during the testing.
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
stress.
Two kinds of measurements are performed. The scattered field for different crack
lengths is measured and compared to numerical calculations of
the influence of a through-thickness notch on the scattered field. A qualitative
agreement can be seen and the important changes in the scattered field are
accurately described. However, the quantitative agreement is not as good as for
the case of a notch in a thin plate. This is on one side due to the modeling of
the crack as a notch, disregarding the sharp tip with the stress concentration.
On the other side, experimental deviations like the amplitude modulation over
the width of the specimen and the off-center position and direction of the crack
are not considered for the FDM calculations. To incorporate these effects, a
much finer grid for the finite difference calculations around the notch would be
necessary.
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.
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