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 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.
KEYWORDS: Wave propagation, Microsoft Foundation Class Library, Actuators, Numerical simulations, Ultrasonics, Sensors, Composites, Structural health monitoring, Nondestructive evaluation, Dispersion
In cylindrical structures such as pipelines, cracks are more likely to occur along the longitudinal (axial) direction and
they are usually fatal to the serviceability of the structures. Unfortunately, the conventional ultrasonic crack detection
methods are not very sensitive to this type of cracks. This paper focuses on using piezoelectric macro-fiber composite
(MFC) to generate torsional wave for health monitoring of cylindrical structures. Numerical simulations are performed
using ANSYS. Nodal release method is used to model the crack. Experimental verifications are also presented. Four
pieces of MFC oriented at 45° against the axis of the specimen are used to generate both longitudinal wave and torsional
wave. The numerical results and the experimental results show that the axial-direction crack propagation in cylindrical
structures can be well monitored using the presented wave propagation approach.
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