Keywords

12.1 Introduction

Structural health monitoring (SHM) is the interdisciplinary field of carrying out diagnosis and prognosis processes in different domains, including aerospace, civil, mechanical, and naval structures. By definition, it is the mechanism of enforcing damage revelation techniques and characterization methods for monitoring defect-prone systems affected by external factors [1]. It utilizes the concept of failsafe design, which affirms that any structure can have defects as long as they do not lead to failure to the system. In other words, SHM intends to move toward the performance-based design philosophy [2]. Various state of the art technologies such as acoustic emission, ultrasonic, thermal imaging, etc., are being utilized to monitor structural health. The diagnosis is generally carried out either actively or passively. In the passive system, the measurement of various parameters is carried out on the passive structure. However, in the active system, the measurement is done in real time. Hence, the methodologies focused on the active process are generally non-destructive based in nature.

The main objective of performing SHM is to decline the growth of maintenance needs and hence reduce the overall cost required for prolonged functioning of the system or structure. It also aims to enhance existing design performance and provide feedback that helps improve the future design based on the experience. The techniques for SHM vary from simple human sense like visual inspection to machining operations, which require skill like in the case of non-destructive evaluation (NDE). Based on the premise that damage will alter the characteristic properties of the structure, SHM is classified into two main categories [3]. The first category is the static-based SHM performed when the damage affects the static properties of structure like displacement and rotation. The second category is vibration-based SHM which is done when the damage affects the dynamic properties of the structure like frequency, modes and mode shapes, etc. In each of these categories, SHM is performed in four fundamental and essential steps [4]:

  1. (i)

    Detection: determining whether there is any damage present in the structure,

  2. (ii)

    Localization: localization of the damage (if detected),

  3. (iii)

    Characterization: quantification of damage severity, and

  4. (iv)

    Prognosis: prediction of remaining functioning life of the structure.

To detect any fault, the array of sensors is placed strategically on the structure that collects dynamic structural response either continuously or at regular intervals. By smartly analyzing these measured responses, one can identify damage occurrence. During data acquisition, environmental effects, such as temperature and humidity levels, and measurement noise create uncertainty which causes error in the signal processing. If non-stationary inputs like traffic, wind, and earthquake are avoided and if every point on the structure is observed, it is possible to detect any damage and its location. Though it is computationally expensive, it may even be used to assess the damage type and estimate the structure’s remaining life if used under the supervision of high user expertise. Too many unknowns and modeling assumptions from physical to modal domain like boundary conditions, number of DOFs and material properties, etc., affect the analysis method. SHM can be used for monitoring metals, composites, laminates, and sandwich materials.

12.2 Structural Health Monitoring Using Lamb Waves

Lamb waves are unique ultrasonic waves that travel the controlled path between two free parallel surfaces, such as a thin plate or shell's upper and lower surfaces. For this reason, Lamb waves are often referred to as guided plate waves [5]. It was discovered by Horace Lamb in 1917, taking inspiration from the Rayleigh wave by Lord Rayleigh [6, 7]. The detailed theoretical development of the wave was set up by Mindlin in 1950 [8], supported by the experimental work carried out by Schoch in 1952 and Frederick in 1962 [9, 10]. After its discovery, most further improvements and applications were aimed mainly at the medical field during World War II [11,12,13]. Later in 1961, Worlton introduced Lamb waves as a damage detection means, which may be extended for smart structures [14, 15]. Subsequently, all these initial studies helped establish the basics of Lamb waves as an outstanding NDE technique.

Lamb waves comprise elastic-wave-based propagation, which causes specific scattering of wave and mode reconstruction depending upon structural damages,. A quantifiable assessment of faults and defects can be achieved by processing and analyzing the wave signals dispersed by damage.

Advantages of Lamb waves: some of the advantages are: cost-viability, quick and repeatable, a short-term inspection of huge structures, responsive to smaller defects, no need for transducer movement, using up little energy, capable of detecting surface and internal defects.

Limitations of Lamb waves: some of the limitations include: the need for practical and refined signal analysis techniques because of composite wave signal generation and output, collectively more than one wave modes accessible concurrently, wave propagation in complex structures difficult to simulate, heavy reliance on previous models or standard signals.

12.3 Sensors and Actuators in SHM

Following sensors are generally used for various signal detection in the devices [16]:

  1. 1.

    Fiber optic sensors,

  2. 2.

    Piezoelectric sensors,

  3. 3.

    Electrical resistance,

  4. 4.

    Electromagnetic techniques, and

  5. 5.

    Capacitive methods.

Out of these sensors, piezoelectric sensors are the extensively used active sensors for monitoring the health of various structures, thus have been discussed in detail in the present paper.

Piezoelectric wafer active sensors (PWAS) are the most used transducers for detecting acoustic signals. The specialty of these sensors is that they can be used for both actuation and sensing purposes. These transducers work on the piezoelectric concept and bring together the effects of the electrical and mechanical output. When working as an actuator, these piezoelectric devices convert electrical energy directly into mechanical energy, stimulating waves and vibrations in a structure. These can act as high-frequency vibrations generators in the supervised structure. Similarly, PWAS transducers can further be used for perceiving stress–strain because they directly convert the mechanical stress–strain energy into electrical energy. Since the output voltage and the strain rate are proportional, this type of measurement would be highly useful at high frequencies. In the case of Lamb waves, PWAS transducers act as both transmitting and receiving devices for the Lamb waves passing through any structure. When PWAS transmitters are excited with an electrical signal, it generates Lamb waves in the structure. These Lamb waves travel in the thin-line structure following a guided path and are reflected or diffracted when they encounter its boundaries, any discontinuities, and damages. These reflected or diffracted waves then reach the PWAS receiver, where they are all converted into electric signals. The receiving of reflected signals is performed using either of the two configurations—pitch-catch configuration, where one device transmits the signal, at the same time, another one captures it, or a pulse-echo configuration where the same device is used for transmitting as well as receiving the signal [3].

PWAS transducers are mostly used nowadays because they have few useful advantages over conventional ultrasonic transducers. By adhesive bonding, PWAS is strongly joined to the structure, whereas traditional ultrasonic transducers are joined by gel, water, or air which provides weaker bonding. These modern non-resonant devices can be tuned selectively into multiple guided-wave modes, whereas the conventional transducers are single-ultrasonic-resonance devices. The PWAS are small in size, light in weight, and economical to use, so more number of these transducers can be used simultaneously on structures, which was a limitation in conventional ultrasonic transducers because of being relatively expensive bigger in size. A detailed summary of the research work done in detecting and locating damages in various structures using Lamb waves is provided in a tabular form in Table 12.1.

Table 12.1 Detecting and locating damages in various structures using Lamb waves
Full size table

12.4 Conclusion

The present paper has provided a brief survey of the SHM methods considering Lamb waves and piezoelectric material-based sensors and actuators. The thin structures comprising metallic and composite-based material were focused on revealing the efficacy of Lamb waves-based SHM monitoring for passive and active structures. Thus, the Lamb wave-based SHM technique is valid when the wavelength is five to ten times the element size. In the near future, the diagnostic and prognostic procedure for SHMs can be explored from the viewpoint of different domains, including mechanical, food, or aerospace industries.