KEYWORDS: Sensors, Actuators, Wave propagation, Microsoft Foundation Class Library, Transducers, Structural health monitoring, Wave plates, Ultrasonics, Inspection, Composites
In cylindrical structures such as pipelines and pressure vessels, cracks are most likely to occur along the longitudinal
(axial) direction and they can be fatal to the serviceability of the structures. Unfortunately, the conventional ultrasonic
crack detection techniques, which usually use longitudinal wave, are not very sensitive to this type of cracks. This paper
focuses on the detection and monitoring of axial cracks in cylindrical structures using torsional wave generated by
piezoelectric macro-fiber composite (MFC). The first order torsional wave is a kind of non-dispersive pure shear wave
which propagates at a fixed wave speed. Torsional wave is utilized in this work because, intuitively, it is more sensitive
to axial cracks than the family of longitudinal waves. Numerical simulation has been performed using ANSYS to show
the effectiveness of torsional wave in detecting and monitoring axial cracks. The time of flight (TOF) of the waves is
used to determine the crack position, while the crack propagation is monitored by measuring the variation in the crack
induced disturbances. Experiments have also been conducted to investigate the feasibility of the proposed method. MFC
transducers oriented at 45° against the axis of the specimen are used to generate and receive torsional waves. The
experimental results demonstrated that the crack position can be indentified and its growth can be well monitored with
the presented approach using torsional wave.
Fatigue cracks often initiate at the weld toes of welded steel connections. Usually, these cracks cannot be identified by
the naked eyes. Existing identification methods like dye-penetration test and alternating current potential drop (ACPD)
may be useful for detecting fatigue cracks at the weld toes. To apply these non-destructive evaluation (NDE) techniques,
the potential sites have to be accessible during inspection. Therefore, there is a need to explore other detection and
monitoring techniques for fatigue cracks especially when their locations are inaccessible or cost of access is
uneconomical.
Electro-mechanical Impedance (EMI) and Lamb wave techniques are two fast growing techniques in the Structural
Health Monitoring (SHM) community. These techniques use piezoelectric ceramics (PZT) for actuation and sensing.
Since the monitoring site is only needed to be accessed once for the instrumentation of the transducers, remote
monitoring is made possible. The permanent locations of these transducers also translate to having consistent
measurement for monitoring.
The main focus of this study is to conduct a comparative investigation on the effectiveness and efficiency of the EMI
technique and the Lamb wave technique for successful fatigue crack identification and monitoring of welded steel
connections using piezoelectric transducers. A laboratory-sized non-load carrying fillet weld specimen is used in this
study. The specimen is subjected to cyclic tensile load and data for both techniques are acquired at stipulated intervals. It
can be concluded that the EMI technique is sensitive to the crack initiation phase while the Lamb wave technique
correlates well with the crack propagation phase.
Fatigue is a progressive and localised damage that occurs when a material is subjected to cyclic loading. Historical cases
have shown that undetected fatigue cracks often lead to catastrophic failure, including loss of lives and assets. It is
therefore important to have a robust Structural Health Monitoring (SHM) technique to detect and monitor these cracks.
The Lamb Wave technique for SHM is promising due to its ability to interrogate a large area of the structure from only a
few locations. The feasibility of fatigue crack detection in wide specimens, where the effect of boundary reflections is
not significant in the signal processing and damage quantification process, have been investigated by other researchers7-9.
However, in a narrow structural component, the boundary reflection has a significant role in the sensor signal and the
damage quantifier from available literatures cannot be applied readily.
The main focus of this study is to investigate the feasibility of monitoring fatigue crack growth in a narrow structural
component using the Lamb Wave technique. Experimental study conducted on lab-sized aluminum beam finds that as
crack propagates amplitude of the sensor signal decreases. A damage index is proposed, and a linear relationship
between the damage index and the crack length is identified. With the proposed damage index, a crack length can be
estimated from the acquired sensor signals through a correlation factor.
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