This research will develop an experimental research program to create a new paradigm for Non Destructive Evaluation and Structural Health Monitoring (NDE/SHM) of materials and structures based on Highly Nonlinear Solitary Waves (HNSWs). The proposed research leverages on the tunability provided by highly nonlinear systems to open up a new field of theoretical and experimental investigations aimed at: a) understanding the coupling between a highly nonlinear oscillators and linear structures; b) detecting defects across scales: from the micro- to the macro-scopic level; c) evaluating applied stress in a given system; d) characterizing the mechanical properties of materials tailoring the pulse properties during propagation (inverse approach) aided by numerical modeling, and e) designing new, and therefore patentable, actuators/sensors technology for stress wave generation and detection. In the last two decades researches and applications of elastic stress waves (both in the sonic and ultrasonic range) for NDE/SHM have thrived owing to their capability of assessing the elastic properties of materials and the presence of damage. The recent discovery and development of the highly nonlinear wave theory and its numerical and experimental validation offer a new tool to the NDE/SHM community. The soundness of engineering systems is essential to avoid catastrophic failures that may be accompanied by severe consequences for the environment, can lead to the loss of human life, and produce tonnage of demolition waste. It is therefore of paramount importance to the nation?s sustainability, economy growth and safety that NDE/SHM, to be able to accurately detect defects at early stages or to characterize the mechanical properties of a given structure. With the proposed research we plan to delve in the fundamental understanding of highly nonlinear waves coupling with materials and structures, offering a direct opportunity to transfer the technology in viable commercial applications much improved over the state-of-the-art actuating/sensing technology for NDE/SHM. The work builds on complementary expertise at the University of Pittsburgh and the California Institute of Technology Caltech and will establish a solid research collaboration between the two institutions. The program will train graduate and undergraduate student to a new approach to sustainable engineering solutions.
The outcome of this award was the development of a new acoustic method to test non-destructively various materials and engineering structures. The proposed method relies on the use of unconventional acoustic waves, namely, highly nonlinear solitary waves, generated in granular actuators that we designed, assembled and tested as part of this project. This new method allows characterizing the mechanical properties, stress state, and stability of different engineering structures, with improved performance over conventional methods. In particular, the use of our highly nonlinear actuators enabled us to increase the signal-to-noise ratio, to improve the resolution of conventional testing methods for specific applications (see, for example, Fig. 1), and to obtain better control of the signal propagation. The actuators we assembled and tested consist of chains of particles in contact, assembled in preselected geometries (one of them is shown in Fig. 2). Our work was based on experiments, supported by theoretical approaches and numerical simulations. The results of this work enabled us to: a) understand the behavior of highly nonlinear waves in granular crystals, and their dissipative properties; b) understand how nonlinear waves interact with adjacent systems; c) detect defects in engineering systems; d) characterize the mechanical properties of different materials. The outcome of this work resulted in 18 journal publications, numerous invited and contributed talks to conferences and academic seminars, as well as 12 filed inventions. Broader Impact: The soundness of engineering systems as pipelines, railroads, prestressed concrete and other structural elements is essential to avoid catastrophic failures that may be accompanied by severe consequences for the environment, can lead to the loss of human life, and produce tonnage of demolition waste. It is therefore of paramount importance to the nation’s economy growth and safety that NDE/SHM, able to accurately detect defects at early stages or to characterize the mechanical properties of a given structure, are used. In the last two decades researches and applications of elastic stress waves (both in the sonic and ultrasonic range) for NDE/SHM have thrived owing to their capability of assessing the elastic properties of materials and the presence of damage. The recent discovery and development of the highly nonlinear wave theory and its numerical and experimental validation offer a new and more powerful tool to the NDE/SHM community. The highly nonlinear dynamic regime supports the formation and propagation of highly nonlinear solitary wave with a compact shape and a wide range of unique tunable properties. In particular, the tunability provides a complete control over tailoring: i) the choice of the wave’s width (spatial size) for defects investigation, ii) the composition of the excited train of waves (i.e. number and separation of the waves used for testing); and iii) their amplitude and velocity. Moreover owing to the simple setup devised for the generation of nonlinear waves, the method we developed relaxes the need for power requirements (i.e. function generators); as such, the approach may facilitate the transition from tethered to wireless ultrasonic technology. Intellectual Merit: The work developed as part of this award achieved the following: a) it advanced the fundamental understanding of the propagation of highly nonlinear solitary waves in granular crystals, and at the interface between highly nonlinear materials and linear or weakly nonlinear materials; b) it demonstrated and developed a new NDE/SHM scheme based on nonlinear acosutics; c) it implemented a numerical model capable of predicting the response of the system and discriminate information associated with structural deficiencies from one associated with environmental factors; d) it designed and developed new actuators/sensors for specific applications, ranging from composite plates testing to bone implants (Fig. 1 and Fig. 2).
