# Review of Structural Health Monitoring Techniques in Pipeline and Wind Turbine Industries
## 2 Health Monitoring Techniques in Pipeline Industry
### In Pipeline Industry
#### 2.1.5 Ultrasonic Technique  
In the ultrasonic technique, we exploit the propagation of ultrasonic waves to obtain information about a mechanical component. For example, we can determine the mechanical properties of a material (the wave velocity is dependent on the material properties) or detect defects in a component (the presence of a defect will modify the characteristics of the propagating wave). Through combinations of various types of wave modes, coverage, transduction methods, transducer motions, and transducer configurations, a large number of possible configurations of detection of damage can be brought to light. The measurements of the voltages, time, and various other factors are listed in the following equations [31; 33; 34; 35; 36]. The main advantages of this technique are the low power consumption of the sensors and the capability to cover large areas using few sensors. The ultrasound technique for SHM consists of a system that is composed of several modules: signal generators that provide waves at specific amplitude and frequency with enough power to drive a series of sensors; several parallel acquisition systems for the signals received at the transducers; a digital control system to synchronize the modules, perform calculations on the signals, and determine the state of the structure; a communications system with a central node, which is critical when multiple structures are monitored; and, obviously, the piezoelectric transducers. All these modules must be embedded in a low volume system and show large capacities: high reliability, signal integrity, accuracy, speed of response, etc. \[\rho\frac{\partial^{2}u}{\partial t^{2}}=C^{2}\frac{\partial^{2}u}{\partial x ^{2}} \tag{13}\]  
\[C_{1}=\sqrt{\frac{\lambda+2u}{\rho}}=\sqrt{\frac{E(1-V)}{(1+V)(1-2V)\rho}} \tag{14}\]  
\[C_{t}=\sqrt{\frac{\mu}{\rho}}=\sqrt{\frac{G}{\rho}}=\sqrt{\frac{E}{2(1+v)\rho}} \tag{15}\]\[C=C^{0}-(dC/dT)\Delta T \tag{16}\] \[\mu=Ae^{i(\{x-\omega t\}}\] (17) \[f=\omega/(2\pi)\] (18) \[\lambda=C/f=(2\pi C)/\omega\] (19) \[K=\omega/C\] (20) \[z=\rho C\] (21) \[R=\frac{Z_{2}-Z_{1}}{Z_{2}+Z_{1}}\] (22) \[C_{1}=\sqrt{\frac{\lambda+2u}{\rho}}=\sqrt{\frac{E(1-V)}{(1+V)(1-2V)\rho}} \tag{14}\]0  
where \(u\) = velocity, \(t\) = time, \(x\) = displacement, \(\rho\) = mass desnity, \(C\) = speed of sound, \(C_{1}\) = speed of sound for longitudinal, \(\lambda\) = [PERSON]'s first parameters, \(\mu\) = wavefront particle displacement, \(E\) = [PERSON]'s modulus, \(V\) = [PERSON]'s ratio, \(C_{t}\) = speed of sound for transverse, \(G\) = shear modulus, \(\Delta T\) = temperature change, \(A\) = maximum particle displacement amplitude, \(\omega\) = angular frequency, \(f\) = frequency, \(K\) = angular wave number, \(z\) = material acoustic impedance, \(R\) = reflection coefficient, \(Z_{2}\) = transmission material acoustic impedance, \(Z_{1}\) = incident material acoustic impedance, and \(t_{r}\) = total round trip time. The waves that need the boundaries for their existence are known as guided waves. When the longitudinal and shear bulk waves undergo constructive interference, ultrasonic guided waves are generated. There are several advantages of using ultrasonic guided waves in structural health monitoring, which include (1) long-range inspection, (2) full exposure of waveguide cross-section, and (3) high sensitivity to small defects compared to overall vibrations [37]. Due to their capability of inspecting large formations from a single probe position, ultrasonic guided waves are considered a great tool for structural health monitoring. Furthermore, it also possesses excellent flaw detection, and it could also establish wave resonance by tuning its frequency and setting up its mode. Different methods comprising magnetostrictive-type sensors, angle beam transducers, and comb-type transducers can be utilized to produce ultrasonic guided waves in a structure.