Selective modes of guided Lamb waves are generated in a laminated aluminum plate for damage detection using a broadband piezoelectric transducer structured with a rigid electrode. Appropriate excitation frequencies and modes for inspection are selected from theoretical and experimental dispersion curves. Dispersion curves are obtained experimentally by short time Fourier transform of the transient signals. Sensitivity of antisymmetric and symmetric modes for delamination detection are investigated. The antisymmetric mode is found to be more reliable for delamination detection. Unlike other studies, in which the attenuation of the propagating waves is related to the extent of the internal damage, in this investigation, the changes in the time-of-flight (TOF) of guided Lamb waves are related to the damage progression. The mode conversion phenomenon of Lamb waves during progressive delamination is investigated. Close matching between the theoretical and experimentally derived dispersion curves and TOF assures the reliability of the results presented here.
In this article, a holistic technique for sensing damage initiation, as well as damage progression in composite plates, is presented combining linear and nonlinear ultrasonic techniques. For this investigation, multiple sets of composite plate specimens made of two different composite materials were fabricated to check if the proposed technique works for different types of specimens. The specimens were damaged by impact loading and then inspected by propagating Lamb waves through them. Different failure mechanisms, such as fiber breaks, matrix cracking, debonding, and delamination, cause composite damage. Two groups of composite specimens that were fabricated and damaged were glass fiber–reinforced polymer composite and basalt fiber–reinforced polymer composite. A chirp signal excited by PZT (lead zirconate titanate) transducer was propagated through undamaged and damaged specimens to investigate the effects of varying degrees of damage on the recorded signals. Both linear and nonlinear ultrasonic parameters were extracted from the recorded signals and analyzed. The change in the linear ultrasonic parameters such as the wave speed and attenuation with damage progression were recorded. A new nonlinear ultrasonic parameter, the sideband peak count or sideband peak count-index, is also introduced and calculated from the recorded signals. It is observed that the nonlinear ultrasonic parameter can monitor the early stage of damage progression better than the linear ultrasonic parameters, while some linear ultrasonic parameters are more effective than the nonlinear ultrasonic parameter for monitoring the advanced stage of damage. Therefore, a combination of linear ultrasonic and nonlinear ultrasonic analyses is ideal for the holistic monitoring of the composite panels from the crack nucleation stage to the structural failure stage.
Guided wave–based techniques are becoming popular for damage detection in pipes, rods, and plates. For monitoring reinforced concrete beams, the longitudinal guided wave is excited and recorded after its transmission through the reinforcing steel bar for estimating its corrosion level. Recorded signal amplitude is affected by the corrosion level. Thus, the corrosion level is estimated from the transmitted wave amplitude. Instead of investigating the amplitude of the transmitted guided waves, the differential time-of-flight of the propagating wave modes is recorded in this article. The differential time-of-flight is obtained from the time–frequency representations of the recorded transient signals and from the high temporal resolution using the cross-correlation technique. It is observed that the corrosion level can be quantified from the change in time-of-flight of the L(0,1) mode. The guided wave modes are experimentally generated, recorded, and compared with the theoretical dispersion curves to identify different modes and select the most efficient mode for quantifying the corrosion level. Unlike the recorded signal strength, the time-of-flight is not influenced by the bonding condition between the sensors and the specimen; therefore, the time-of-flight-based corrosion-monitoring technique is less influenced by the sensor bonding condition. This investigation is necessary because most investigators have studied the effect of corrosion on the recorded signal strength instead of its time-of-flight.
Acoustic bulk waves were excited by local electric field probe in an anisotropic piezo-electric crystal Lithium Niobate (X-cut). A narrow pulse with a width of 25 ns was used for excitation to obtain wide frequency content in the Fourier domain. A wide spectrum ensures metamorphosis of bulk waves into Lamb waves for scan lengths comparable to the involved wavelengths. The low frequency content experiences multiple reflections from the two surfaces of the plate and disperses along the propagation direction. Acoustic bulk wave's evolution and transformation to Lamb waves are illustrated and explained with the aid of the Lamb wave dispersion phenomenon. The holographic images in the Fourier domain exemplify the metamorphosis of waves during propagation following the excitation at an approximate point source.
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