The extraordinary sensitivity of nonlinear ultrasonic waves to the early stages of material degradation makes them excellent candidates for nondestructive material characterization. However, distinguishing weak material nonlinearity from instrumentation nonlinearity remains problematic for second harmonic generation approaches. A solution to this problem is to mix waves having different frequencies and to let their mutual interaction generate sum and difference harmonics at frequencies far from those of the instrumentation. Mixing of bulk waves and surface waves has been researched for some time, but mixing of guided waves has not yet been investigated in depth. A unique aspect of guided waves is their dispersive nature, which means we need to assure that a wave can propagate at the sum or difference frequency. A wave vector analysis is conducted that enables selection of primary waves traveling in any direction that generate phase matched secondary waves. We have tabulated many sets of primary waves and phase matched sum and difference harmonics. An example wave mode triplet of two counter-propagating collinear shear horizontal waves that interact to generate a symmetric Lamb wave at the sum frequency is simulated using finite element analysis and then laboratory experiments are conducted. The finite element simulation eliminates issues associated with instrumentation nonlinearities and signal-to-noise ratio. A straightforward subtraction method is used in the experiments to identify the material nonlinearity induced mutual interaction and show that the generated Lamb wave propagates on its own and is large enough to measure. Since the Lamb wave has different polarity than the shear horizontal waves the material nonlinearity is clearly identifiable. Thus, the mutual interactions of shear horizontal waves in plates could enable volumetric characterization of material in remote regions from transducers mounted on just one side of the plate.

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Two-dimensional (2D) molybdenum disulfide (MoS 2 ) has attracted significant attention because of its outstanding properties, suitable for application in several critical technologies like; solar cells, photocatalysis, lithium-ion batteries, nanoelectronics, and electrocatalysis. Similar to graphene and other 2D materials, the physical and chemical properties of MoS 2 can be tuned by the chemical functionalization and defects. In this investigation, our objective is to explore the mechanical properties of single-layer MoS 2 functionalized by the hydrogen atoms. We moreover analyze the effects of different types of defects on the mechanical response of MoS 2 at the room temperature. To investigate these systems, we conducted reactive molecular dynamics simulations using the ReaxFF forcefield. We demonstrate that an increase in the hydrogen adatoms or defects contents significantly affects the critical mechanical characteristics of MoS 2 ; elastic modulus, tensile strength, stretchability and failure behavior. Our reactive molecular dynamics results provide useful information concerning the mechanical response of hydrogenated and defective MoS 2 and the design of nanodevices.

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The sensitivity of ultrasonic wave interactions to material and geometric nonlinearities makes them very useful for nondestructive characterization. The ability of guided waves to interrogate inaccessible material domains, be emitted and received from a single surface, and penetrate long distances provides capabilities that bulk waves do not. Furthermore, mutual interactions between waves propagating in collinear or non-collinear directions provide excellent flexibility as to which types of waves are used, as well as their frequencies and interaction angles. While the interaction of bulk waves is well established, the mutual interaction of guided waves traveling in arbitrary directions in a plate is not and requires a general vector-based formulation. Herein, by vector-based calculations, the internal resonance criteria are formulated and evaluated for waves propagating in arbitrary directions in a plate. From the analysis, it is found that non-collinear guided wave interactions transfer power to secondary guided wave modes that is impossible for collinear interactions, which is completely analogous to bulk waves. For the case of tone burst-pulsed wave packets at nonzero interaction angles, the wave interaction zone has a finite size, and its size is dictated by many factors, including, for example, the group velocities of the waves, interaction angle, pulse duration, and dispersion. An analytical model is introduced for finite-sized interaction zones and used to demonstrate the effect of group velocity mismatch on the generation of secondary waves. In addition, finite element simulations are compared to the analytical model and provide additional insight into secondary wave generation and propagation.