posted on 2017-03-27, 03:20authored byBenjamin Steven Vien
This thesis
reports the results of a study of the scattering of the Lamb wave field due to
the presence of a small crack in isotropic structures. The study addressed the
challenge of detecting and quantifying a small defect at a hard-to-inspect
location for structural health monitoring (SHM). The need for improved aircraft
performance and efficiency has made manufacturing design and technology far
more complicated, and consideration of nondestructive inspection is usually not
factored into these designs. Thus, conventional methods are insufficient, as
they do not to account for these advances in design.
A fuel weep hole is a classical configuration in metallic
aircraft structure, which is vulnerable to fatigue cracks in hard-to-inspect
locations. Conventional diagnosis methods, such as the Eddy Current Technique,
are not optimal for damage detection especially in such cases where built-in
sensors are restricted to very limited areas. Previous studies involved using
high-frequency bulk-wave wedge transducer techniques for weep hole inspection,
but the difficulty of reliably detecting and quantifying the small hidden crack
still remains in SHM. A reliable and novel analytical model is needed as a
reference for monitoring and assessing the early development of a small crack
before reaching it reaches a critical size.
This study investigated the scattered Lamb waves on the
defect, which carries information such as the severity, location, and size of
the crack for SHM purposes. The isotropic specimen used in this research is
aluminium plates with different geometry, which depends on the specific
investigation. There is significant interest in the use of Lamb waves for
hidden crack detection and quantification due to their advantageous properties
of rapid wide area inspection with minimal attenuation.
It is also anticipated that the point source equivalence,
which consists of a particular combination of body-force doublets, applies for
Lamb wave scattering. This research has demonstrated that the point source
model can represent the Lamb wave mode scattering by a small crack. This
research first investigated edge cracks on the straight edge of the plate, then
on the hole boundary, and, lastly, on the top boundary of a rectangular slot.
This approach will guide us to the next problem by using the previous study’s
findings to determine the later study’s configuration and to compare its
results. The Finite Element method and experimental method are used for this
research to explore not only the common scattered Lamb waves amplitude but also
the mode, pattern, and wave directivity as a conjunction for small crack
detection and characterisation.
The study of the scattering of fundamental Lamb waves by a
small edge crack showed a quadratic-like relationship between crack length and
scattered wave amplitude and the scattered wave pattern remains independent
from the crack length. Within the small crack length-to-wavelength limit, these
relationships highly correlate to point source model. It is noteworthy that the
dominant scattered Lamb wave modes by an edge crack are the edge-guided and SH0
waves.
The leaky circumferential edge wave is an interesting
scattering phenomenon that can direct and coalesce with SH0 waves to the
geometric shadow zone. The rectangular slot study has experimentally and
computationally demonstrated that leaky symmetric edge-guided waves could
propagate to hard-to-inspect location impinged with a hidden crack. The
scattered waves are then redirected back to the location of excitation.
The findings in this thesis give a fundamental understanding
of the scattering of Lamb waves by crack on boundaries and holes as well as
cracks in hard-to-inspect locations. The novelty of this research, which are
the scattered wave amplitude, the wave pattern, and its directivity are
important measurements to locate and quantitatively evaluate small cracks,
based on the point source model. Under specific conditions, edge-guided waves
can propagate around a curved edge to the blindsight area and redirected back,
as shown in the hole and rectangular slot studies. These findings will assist
in the development of Lamb wave propagation for SHM as a damage diagnosis tool.