Carbon-fiber-reinforced plastics are widely used in lightweight marine structures due to their high strength and superior fatigue behavior. In this article, we will present an innovative methodology for simultaneous load and structural monitoring of a carbon-fiber-reinforced plastic rudder stock as part of a big commercial vessel. Experimental results are presented here from a quasi-static tensile test in which the load monitoring is performed using embedded strain sensors. Structural monitoring is based on high-frequency electromechanical impedance spectroscopy combined with dedicated signal processing and surface-mounted piezoelectric transducers. We have achieved the following results: (1) the demonstration of a hybrid monitoring system including load and structural monitoring, (2) successful embedding of strain gauges during composite manufacturing of the carbon-fiber-reinforced plastic rudder stock, (3) development of instrumentation hardware for multichannel electromechanical impedance measurements, and (4) successful damage detection by means of electromechanical impedance spectroscopy in thick carbon-fiber-reinforced plastic rudder stock samples exploiting strain data.
In more and more applications within maritime structures, multimaterial mixes are chosen to build efficient structures with advantages in weight and performance. Due to their high strength and especially their superior fatigue behavior, carbon-fiber-reinforced plastics (CFRPs) can be an appropriate material for highly loaded parts. This article considers a rudder stock in CFRP for a big commercial vessel, as shown in Figure 1, where the rudder blade needs to be connected to the stock by press fit. This press fit causes high stresses in the radial direction of the stock, which is perpendicular to the fiber direction of a wet filament wounded CFRP tube. As a consequence, the CFRP part of the rudder stock must be connected to metallic parts, which are able to couple the rudder blade to the stock. This leads to a high stress concentration between metallic and CFRP parts, so that these areas are often most critical (Piggott, 1989; Wang et al., 2017). In CFRP components with very high wall thicknesses, the stress state is often more complex compared to thin-walled CFRP parts and it is difficult to predict the fracture mechanics. This means that a continuous load and structural monitoring for such thick CFRP components is strongly needed.
Several examples can be found in the literature where a load monitoring with embedded sensors is combined with structural health monitoring (SHM) of the same structure (Bosse and Lechleiter, 2016; Ling and Mahadevan, 2012; Yuan et al., 2006). Previous works on ship structures have mainly focussed either on load monitoring or structural monitoring. The most widely used principles for load monitoring are based on resistive or optical (fiber Bragg grating) strain gauges (Torkildsen et al., 2005). Moreover, accelerometers have been used by Phelps and Morris (2013) to detect global resonant behavior (whipping). Such loadmonitoring approaches enable the estimation of the structures’ remaining life time and provide immediate feedback to the operator to allow real-time changes in maneuvering to minimize overstressing (Phelps and Morris, 2013).
However, several SHM techniques for ship structures have been reported in the literature. Okasha et al. (2010) show the integration of SHM in life-cycle performance assessment of ship structures under uncertainty. Model-based SHM of naval ship hulls is demonstrated in Stull et al. (2011). A wireless SHM system for submarine structures is presented in Nugroho et al. (2016). Damage identification in submerged shell structures by means of a differential evolution algorithm has been performed in Reed et al. (2013). A multipurpose monitoring system for icebreakers is presented in Zhirnov et al. (2016).
A promising SHM approach is given by the electromechanical impedance (EMI) method that was first described in Liang et al. (1994). The EMI method is based on the fact that the electrical impedance of a piezoelectric patch is linked to the mechanical impedance of the structure it is bonded to. In practice, the inverse of the complex impedance, the admittance Y, is often used. EMI has been successfully used for the detection of artificially induced delamination in composite materials (Ostachowicz et al., 2017). Structural damage in a grouted connection can also be detected by the EMI approach (Moll, 2018). A review on recent developments of the EMI method in different application domains is presented in Wandowski et al. (2017). To the best of the authors’ knowledge, there is only one publication that employs the EMI method on ships by focussing on aluminum truss structures (Grisso, 2013).
It is well known that EMI spectra depend on external factors such as temperature (Wandowski et al., 2017), load of the structure (Annamdas et al., 2007; Taylor et al., 2013), and the thickness/stiffness of the adhesive (Islam and Huang, 2014; Tinoco and RosasBastidas, 2016; Tinoco and Serpa, 2011). The compensation of these effects is crucial to avoid false-positives in damage detection. Transducer failures by breakage or debonding as well as environmental factors have a higher impact on the imaginary part of the admittance than on the real part (Giurgiutiu and Zagrai, 2000).
The novel contributions of this article are given by the following items:
1. Demonstration of a hybrid monitoring system combining load and structural monitoring. Therefore, thick CFRP samples of a rudder stock have been manufactured and studied in a quasi-static tensile test.
2. Successful embedding of strain gauges during composite manufacturing of the CFRP rudder stock.
3. Development of instrumentation hardware for multichannel EMI measurements. In contrast to available devices, the presented measurement hardware supports high frequencies (up to 1 MHz) and high currents (up to 2A) so that highly attenuated materials such as the proposed CFRP rudder stock can be inspected.
4. Discussion of the successful combination of load and structural monitoring for damage detection by means of EMI spectroscopy in thick CFRP rudder stock samples. The dependency of the EMI signatures on the load cases can be eliminated by exploiting the load information from embedded strain gauges.
The remainder of this article is organized in the following way. Section ‘‘Measurement techniques for load and structural monitoring’’ presents the measurement techniques for load monitoring and structural monitoring. Subsequently, section ‘‘Experimental setup’’ describes the experimental setup of the quasi-static tensile test including a description of the rudder stock samples. Results for the hybrid monitoring concept are presented in section ‘‘Experimental results’’ followed by a discussion in section ‘‘Discussion.’’ Finally, conclusions are drawn in section ‘‘Conclusion.’’
Measurement techniques for load and structural monitoring
Load monitoring with embedded strain gauges
The load monitoring is realized by embedding the strain gauge LI66-10/350 by HBM directly among composite layers. The sensor has a total size of 22 mm 3 10 mm. For the sake of integration, a CFRP tube is manufactured in two different steps. In the former, the laminate is wound and cured up to the thickness where the sensors are then placed. On the emerging surface, the sensors are located in the central part of the final specimen (see Figure 7). An epoxy glue (Hardman Double Bubble Epoxy), chemically very similar to the resin, was used to fix the strain gauges for the wet filament winding of the CFRP. The thickness of the adhesive film is few microns. The sensors have connection pins facing outwards (see Figure 8), making it possible to wind the second part of the CFRP laminate without losing access to the sensor. After that, the sensor is positioned on the laminate plane, where the interesting strain occurs, and the laminate of the second part is wound and cured up to the final thickness. Figure 2 shows the glued sensors on the left and the laminate before final curing step on the right.