This Small Business Innovation Research (SBIR) Phase II project will demonstrate an inspection and monitoring sensor system that addresses the problem of structural evaluation of composite components with an innovative nondestructive evaluation (NDE) sensor system. Composites have always been a challenge for inspection due to their multilayer and anisotropic material construction. This challenge is increased when dealing with wind turbine blades due to their enormous size, construction, strength requirements, operational environment, and safety considerations. The Phase II effort will further upgrade and refine the sensor operational capabilities developed in Phase I. The signal to noise ratio and inspection coverage area (sensor footprint) will be further improved. The system capability will be expanded so that a single control unit can operate and receive data from a networked array of sensor. The sensor system will have application during manufacture to verify part quality, for pre- and post-installation inspection to check for shipping or assembly damage and during the component?s service life as a structural health monitor system. These sensors offer the possibility for substantial savings and reduction of downtime as manufacturing defects are discovered at the point of origin, before catastrophic blade failure can occur.
The broader impact/commercial potential of this project will be to facilitate the economical installation and operation of wind energy generators. The U.S. has set a goal of 20% (300GW) of electrical power to be generated from wind by 2030. Based on the typical utility scale turbine (1.5-2.0MW) this translates to having over 500,000 turbine blades in domestic service by 2030. Depending on the wind turbine size, blade costs are $55k to $300k each with a 1.5-2.0MW turbine costing $2-3M to install. The growth of wind energy represents a huge manufacturing challenge to produce, install and maintain the turbine blades. The sensor system developed during this program has the potential to detect defects or damage both early in the supply chain and during the life cycle so that expensive energy capacity downtime or catastrophic tower failures can be avoid. Blade failure is not only a cost issue but also a safety one as well. An accurate method for inspection of complex blade structures can have a major economic impact on the industry. The sensor system being developed also has use in Aerospace and Infrastructure/Bridge applications.
This project developed methodology for using interdigitated transducers (IDTs) to conduct acoustic nondestructive testing of composite structures. Composites are important for their light weight, high strength and corrosion resistance. As such, they are being applied to solve a growing number of infrastructure problems. However composites are also challenging to inspect by acoustic methods because their structure is nonhomogenous, usually anisotropic, and typically include acoustically attenuative materials such as polymers. As a result, it is difficult transmit a probing acoustic signal over long range and challenging to interpret the received signal. This project addressed these challenges by using IDTs to produce surface waves, and in some situations plate waves, that have relatively long propagation range. Figure 1 shows an example of an IDT and its highly directional signal. This project began by developing methods for inspecting the very long composites used to fabrication wind turbine blades. The project succeeded in showing practical methods for detecting and mapping a variety of composite defects including ply misalignment, simulated cracking, impact or crushing damage (Figure 2), with demonstrated detection range of 8 ft. The project explored and compared the use of both linear acoustic properties (e.g. attenuation) and nonlinear acoustic parameters for defect characterization (Figure 3), generally finding nonlinear methods to be more accurate, but requiring greater data averaging and collection time. A few months into Phase II of the project, the developers of the domestic wind energy infrastructure experienced significant dislocations and revisions to future projected markets. As a result, the focus of this project was diversified to a wider range of composite structure types and applications. These included composite panels for deployable shelters, vacuum panels for refrigeration containers, and adhesively bonded composite panels. The inspection methods developed for wind turbine blade composites were essentially transferable to the shelter panels due to similarity in their construction. The inspection method for vacuum panels involved not a search for defects in the composites per se, but rather a determination as to whether the vacuum component in the structure (an open foam core panel surrounded by an evacuated and sealed foil bag) had maintained its vacuum integrity (and therefore its high thermal R-value.) A technique was developed for acoustically detecting changes in acoustic impedance between the vacuum panel and a structural composite panel to which it was bonded. A significant change increase in acoustic impedance was shown to be an indicator of loss of vacuum integrity. This work has the potential to improve refrigeration containers used for transport of medically critical materials as well as for deployable refrigerated shelters used for food storage and temporary mortuaries in disaster relief missions. Finally, a method was developed for using nonlinear acoustic parameter measurements to accurately detect changes in the adhesive bond strength between joined flat composite panels. The details of the method are being held proprietary until suitable intellectual property protection is put in place. This work has the potential to help drastically reduce the cost and weight of aerospace composite structures through the elimination or large numbers of fasteners. Designs and manufacturing capability to take advantage of purely bonded composite structures exists, but fasteners are still used for added safety due to a lack of methods to verify bond quality. If this method proves commercially successful, if may help to realize the goal of fastener reduction.
