PhD thesis: Defect detection in plates using guided waves
Starting from a specific problem, the detection of fatigue cracks at fastener holes in aircraft, a general experimental and theoretical study of the underlying problems was performed, employing guided waves. The model system investigated was the scattering of the first antisymmetric Lamb wave mode A0, a flexural wave, at a circular hole with a notch at an arbitrary angle. With the insight gained from this fundamental research, the developed measurement method was re-applied to the specific problem. In collaboration with the fatigue engineering center of RUAG Aerospace, Emmen a monitoring system for the crack length in tensile specimens was implemented and found to allow the reliable detection of defects. However, the nondestructive testing method is usable for a much broader range of problems. Almost all technical systems contain thin-walled structures like plates and shells, connected with joints, at which stress concentration and damage development can occur. Guided waves allow a fast measurement of large areas of the structures, and therefore a significant cost reduction compared to conventional testing methods can be achieved.
With the chosen measurement
method, the excitation and measurement of flexural waves in plates was performed
with good accuracy and repeatability. Piezoceramic and electromagnetic
acoustical transducers were used as excitation transducers, allowing the excitation of waves
in a broad frequency spectrum with a linear transfer function and sufficient
amplitude. Line excitation was achieved by using custom cut piezoelectric
ceramic plates, resulting in waves with a nearly plane wavefront in the
strip-like tensile specimen. Since
the excitation transducer is fixed to the structure, the repeatability of the
measurements is excellent. This allows an averaging over several measurements,
improving the signal-to-noise ratio and canceling out spurious influences that
can occur in a harsher environment, like an aircraft hangar. The cost of the
piezoceramics is rather low, permitting the integration into the structure for a
permanent on-line monitoring.
In the context of Lamb waves in
homogeneous, isotropic plates, the approximate theories for the description of
flexural waves were reviewed. The scattering at a circular hole in a plate was
calculated analytically and the different approaches using classical plate
theory, Mindlinís theory, and an asymptotic expansion were compared to
experimental results. Excellent agreement between the measurements and the
analytical calculations was obtained. Care has to be taken concerning the
validity of the different approximations, as not only the ratio from wavelength
to plate thickness, but also the ratio of hole radius to plate thickness define
the validity of the approximations.
Application to NDT
In cooperation with the fatigue engineering center of RUAG Aerospace, Emmen the applicability of the measurement method for the detection of fatigue cracks in aluminum specimens was investigated. The substantial geometric relation between hole radius and specimen thickness was selected as in a fighter plane. Fatigue cracks at circular holes were generated by cyclic tensile loading in a servo-hydraulic material testing machine. The scattered field around the damaged holes was measured and found to be well described by the numerical model using finite difference methods. An on-line monitoring of the crack length during the cyclic tensile loading was implemented experimentally. Good correlation between measured and calculated change in signal and the optically measured crack length was found, allowing a sizing of the defect. However, the variation in the measured signal due to noise and external influences was still rather high, making a reliable detection of cracks at an early stage of the damaging process impossible. Higher excitation frequencies and the use of other wave types, e.g., Rayleigh waves, would allow the detection of smaller defects.
Insight on the mechanics of the scattering of flexural waves at defects was gained. From the exact measurement of the complex magnitude of the scattered field, the influence of the defect could be accurately described and modeled theoretically. Further fundamental research would be necessary for an analytical model, allowing a faster calculation and possibly a solution of the inverse problem, i.e., the localization and sizing of the crack from a remote measurement. By the use of higher excitation frequencies and the solution of the resulting experimental obstacles, smaller cracks could be reliably detected. Alternatively, different wave modes might be employed, applying the exact measurement of amplitude and phase variations for an improved resolution of small defects. Building on the knowledge gained from the fundamental research, the nondestructive testing of real aircraft or other technical systems can now be approached.