# Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring
## 9 Ultrasonic Method
### Introduction and Principle of the Application
#### 9.1.5 Fibre Reinforced Materials  
[PERSON] et al. [114] also used an ultrasonic guided waves system to extract information on the detection of disband-type defects of glass fibre reinforced polymer material. [PERSON] et al. [129] have also used a 2-D axis symmetry finite element model (FEM) to estimate ultrasonic signal response from symmetrical defects. [PERSON] et al. [130] have proposed a non-contact laser ultrasonic method to estimate five engineering constants of carbon fibre reinforced composites based on measurement of velocities of transverse and longitudinal waves. [PERSON] and [PERSON] [131] employed an ultrasonic testing method to assess fibre metal laminates (FML). They adopted a novel approach by performing quantitative and qualitative transmission NDT tests on the FML under dynamic loads. The results were then compared with the conventional pulse-echo ultrasonic approach. They observed that the novel transmission phased array method was more effective and efficient in assessing the condition of the FML in terms of detection of defects, quality and quantity, notwithstanding the type and surface condition of the laminates.  
The ultrasonic technique is able to identify internal flaws in an element over large volumes. Another advantage is that there is a hand-held type, whose application is relatively easy [113]. Moreover, the automation of the technique ensures the dependability and repeatability of the NDT methods [132].  
The main drawback in the ultrasonic non-destructive method is that there are no suitable methods to process the noise from the ultrasonic signals in order to estimate defects on a structure effectively [133]. In addition, there is normally an influence of conditions of coupling. It is also very difficult to interpret results, especially for irregular and heterogeneous elements, such as timber. It is extremely difficult to acquire information on NDT parameters because of certain complicated factors such as microstructure features, transmission, mode of conversion, stress and diffraction [118; 120]. However, such complexity can be overcome by using several NDT methods depending on various physical approaches [120]. There are also difficulties in identifying signals from the opposite side of tested elements or structures emerging from secondary reflections from composite structures, which might lead to errors in the analysis of results [131].  
This technology was also used in sensing structural health. An ultrasonic sensing network was developed by [PERSON] et al. [134] to evaluate corrosion of steel reinforcement. The ultrasonic waves are defined as sound waves with a frequency that exceeds the human hearing limit [105; 106; 107]. This technology was also used in sensing structural health. An ultrasonic sensing network was developed by [PERSON] et al. [134] to evaluate corrosion of steel reinforcement. Guided ultrasonic waves were utilized to monitor reinforced mortar specimens under accelerated uniform and localized corrosion. Ultrasonic guided waves were utilized by [PERSON] and [PERSON] [124] to monitor the progression of corrosion in steel reinforcement in chloride and oxide environment.