Metamaterial inspired designs have been successfully tested in several applications such as gradient negative index lenses, invisibility cloaking, metasurface sub-wavelength imaging [1-4], and many more. Their entry in structural health monitoring (SHM) of Aerospace, Civil and Mechanical (ACM) engineering [5-7] started attracting engineers because of their strong electromagnetic (EM) response to the geometrical changes in the unit element or assemblies of elements , making it suitable for ‘strain’ monitoring, which is a common issue for most ACM structures.
Metamaterial inspired LSP sensors are highly non-fragile, which are manufactured as bi-material composites, made of metal inscriptions on flexible and ultra-thin Rogers materials \(RT5880) using PCB technique. These are bent and twisted-able; and do not require fragile packaging, unlike FBG sensors and moreover their ‘waveguides that transfer EM waves’ do not need any protection unlike optical fiber cables.
Most of the present designs are mostly based on the generation of \(A) ‘progressive wave’ that travel long distances in the ‘far fields’ \(air medium); and \(B) the measurement of the scattered parameters in the far fields. Thus, EM radiation leakages or reflections are common, which forces experimentation in special anechoic chambers. Thus, they were used mostly for biological or medical applications, but not for ACM applications \.
These EM radiations in the microwave range are harmless, and their applications for ACM engineering structure can be considered without any isolation. Hence, ‘near field’ based LSP sensor using the robotic arm and waveguide technique is adopted in our study published in Springer's Journal of Nondestructive Evaluation .
This technique was evaluated in terms of sensitivity for a couple of ACM engineering specimens as demonstrated in Figs 1-6, that shows its potential applications for SHM.
These LSPs, in contact with the ‘engineering structure under inspection’ generate a diagnosable surface plasmons/ wave as output, when subjected to input EM waves in the near fields. This paper is an extension of our article , presents a procedure to analyse the sensitivity of the output surface wave.
The Springer article primarily presented a novel method of transferring ‘surface waves’ from within the confined boundaries of the LSP sensor to the far-off distance along a guided path. Our several articles in the last few years explored the spoof LSP sensor \[6, 10 -12], first developed a few years ago by Shen and co-researcher group \ which resembles a piezoelectric diaphragm \, but it is ultra thin and bendable unlike piezo. In experiments, the input – EM waves and the output – surface waves were handled by a vector network analyser via a 2D robotic arm. Signals generated by LSP; and, LSP plus waveguide were analyzed for monitoring \(A) metal shell’s surface profile and \(B) separation of two adjacent concrete blocks.
Further, in this article, the sensitivity of the output signals using a unique root mean square deviation (RMSD) index is presented.