U. Amjad

Detection and quantification of pipe damage from change in time of flight and phase

The use of ultrasonic guided waves for damage detection in pipes is continuously increasing. Generally longitudinal (axial symmetric) modes are excited and detected by PZT (Lead Zirconate Titanate) transducers in transmission mode for this purpose. In most studies the change in the received signal strength with the extent of damage has been investigated while in this study the change in the phase and the time-of-flight (TOF) of the propagating wave modes with the damage size is investigated. The cross-correlation technique is used to record the small changes in the TOF as the damage size varies in steel pipes. Dispersion curves are calculated to carefully identify the propagating wave modes. Differential TOF is recorded and compared for different propagating wave modes. Feature extraction techniques are used for extracting phase and time-frequency information. The main advantage of this approach is that unlike the recorded signal strength the TOF and the phase are not affected by the bonding condition between the transducer and the pipe. Therefore, if the pipe is not damaged but the transducer-pipe bonding is deteriorated then although the received signal strength is altered the TOF and phase remain same avoiding the false positive alarms of damage.

Detection and quantification of diameter reduction due to corrosion in reinforcing steel bars

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.

Determination of the stress dependence of the velocity of Lamb waves in aluminum plates

Mode selective excitation and detection together with chirped excitation and digital pulse compression is employed to study the variation of the time-of-flight (TOF) of Lamb waves. The acoustoelastic coefficients for the variation of TOF depend not only on the actual modes but also on the actual frequency or center frequency of the observed propagating wave. The modes are selected from dispersion relations obtained by appropriate modeling. To allow a continuous and monochromatic variation of the load, specially designed apparatus has been designed minimizing the disturbing influence of static and dynamic friction, generally a demerit in the standard force exerting hydraulic equipment. The possibility of delicate maneuverability of force application with minimal deviation with selective excitation and detection of Lamb wave modes lead us to the observation of positive and negative acoustoelastic coefficients and dependencies of these coefficients on applied stress with respect to central frequency are discussed.

Mode selective excitation and detection of Lamb waves

Transducer arrangements including specially designed electronic drive and detection circuitry are presented, suitable to distinguish between the orthogonal symmetric and anti-symmetric Lamb wave modes. Whereas transducers mounted on both surfaces have already been introduced for this task, novel schemes based on transducers mounted single sided can also be exemplified in combination with advanced electronic schemes providing alignment with respect to the orthogonality of the separated modes. Detail of the developed scheme is exemplified together with experimental results which are compared to the expectations based on established modeling.

Zero order mode selective excitation and highly resolved observations of lamb waves

In acoustics, Lamb waves are extremely useful for damage detection in sheet materials and tubular products. Among variety of techniques for the generation of lamb waves, lamb waves are detected using contact piezo electric transducers mounted on an aluminum sample. For this task, two pairs of transducers were introduced where each pair mounted on both surfaces of the sample. Mode selective excitation technique is used and two pre-amplifier circuits served as a part of the signal detection. For this application and more precisely with low frequency excitation signal, orthogonal zero order modes are observed in a high resolved fashion in aluminum sample of 1mm thickness. Principles of the developed method together with instrumental details are discussed.

Stress detection with guided acoustic ultrasonic waves by non-linear elastic and geometric effects

The detection of stress in bolts based on acoustic bulk waves of longitudinal and transversal polarization is well introduced and respective detection schemes are commercially available. Whereas the time-of-flight of bulk waves observed for detection varies under stress due to non-linear elastic properties, 1- or 2-dimensionally guided waves can in addition and for suitable modes even be dominantly influenced by geometric effects. Even though geometric effects are well known and used for example to tune string instruments, little if any attention has so far been given to similar effects for Lamb waves and other guided modes. The basic effects including anomalous stress dependencies if compared to bulk waves are presented and discussed including a comparison to expectations based on analytical modeling. Novel detection schemes including developments suitable for in-flight detections of stress in structural components of aircrafts are demonstrated.

Monitoring ageing of alkali resistant glass fiber reinforced cement (GRC) using guided ultrasonic waves

Glass fiber reinforced cement (GRC) is a Portland cement based composite with alkali resistant (AR) glass fibers. The main drawback of this material is the ageing of the reinforcing fibers with time and especially in presence of humidity in the environment. Until now only destructive methods have been used to evaluate the durability of GRC. In this study ultrasonic guided wave inspection of plate shaped specimens has been carried out. The results obtained here show that acoustic signatures are capable of discerning ageing in GRC. Therefore, the ultrasonic guided wave based inspection technique is a promising method for the nondestructive evaluation of the durability of the GRC.

Lattice dynamics approach to determine the dependence of the time-of-flight of transversal polarized acoustic waves on external stress

Based on the lattice dynamics approach the dependence of the time-of-flight (TOF) on stress has been modeled for transversal polarized acoustic waves. The relevant dispersion relation is derived from the appropriate mass-spring model together with the dependencies on the restoring forces including the effect of externally applied stress. The lattice dynamics approach can also be interpreted as a discrete and strictly periodic lumped circuit. In that case the modeling represents a finite element approach. In both cases the properties relevant for wavelengths large with respect to the periodic structure can be derived from the respective limit relating also to low frequencies. The model representing a linear chain with stiffness to shear and additional stiffness introduced by extensional stress is presented and compared to existing models, which so far represent each only one of the effects treated here in combination. For a string this effect is well known from musical instruments. The counteracting effects are discussed and compared to experimental results.

Metamorphosis of bulk waves to Lamb waves in anisotropic piezoelectric crystals

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 waves 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.

Generalized representations and universal aspects of Lamb wave dispersion relations

Due to the dependence on a limited amount of parameters, the dispersion relations of Lamb waves can be presented in a generalized way. This is exemplified for the different established typical representations. Special attention is given to the representation of the momentum on energy, which is well suited to discuss basic features since energy as well as momentum is the properties which are strictly conserved in loss free homogeneous materials. Representations involving the phase and group velocity are discussed. Features related to level crossing of interacting modes and relations to basic mechanical properties especially relevant to Lamb waves are included in the presentation and discussion.