A novel resonator-based metasurface is devised to control reflected underwater waves. Each metasurface unit is constructed with an aluminum plate attached to a lead mass. By tailoring the thickness of the plate, full 2π phase shift of the reflected wave can be achieved. Examples of redirection, focusing, and directional carpet cloaking are demonstrated as applications. The target frequency can be as low as 500 Hz and it can operate in a broadband range. Slight modifications allow the design to perform excellently from tens to thousands of hertz. This metasurface provides a new design paradigm when there is a need to manipulate low frequency underwater waves, such as in submarines for stealth operation or detection-signal manipulation to provide deceptive information.

We investigate the effect of structural periodicity on elastic longitudinal and flexural wave propagation in parity-time ($\mathcal{PT}$) symmetric prototypical elastic metamaterial rods and beams. Through theoretical analysis and numerical demonstration, we reveal the link between the observed band structure of infinite periodic $\mathcal{PT}$ symmetric elastic metamaterials and the unique scattering properties of the corresponding finite periodic $\mathcal{PT}$ symmetric systems. In particular, we study the coherent perfect absorber and laser (CPAL). There is an observed periodic-like occurrence of the CPAL formation when varying the cell number in the finite periodic metamaterial that correlates to the band structure of the infinite periodic system. For this, we provide simple equations to estimate the periodicity and the minimum cell number. Using a piezoelectric-aided elastic metamaterial consisting of piezoelectric patches and active circuits, we propose a feasible configuration to realize tunable flexural wave CPALs in a periodic-structured $\mathcal{PT}$ symmetric metamaterial beam, and demonstrate that the CPAL can be induced in the Bragg's bandgap.

An extremely thin metasurface is proposed for manipulating underwater reflected waves. Each metasurface unit is only 1/61.7 of the target wavelength in thickness and comprises an air cavity capped with a thin vibration plate held in place by rubber spacers on steel supports. The unit-cell design is thin, simple, and can be adjusted to obtain a full 2π phase shift in water waves that are reflected from the metasurface. It also provides this phase shift for a broad frequency range of 20–800 Hz for incident waves. The effectiveness of the design and the resolution of the expected effect is demonstrated for sharp focusing, self-bending, and directional carpet cloaking, which are applications with great potential in energy harvesting, underwater communication, and submarine stealth and antidetection.

One group of models proposed for characterizing the mechanical response of glassy polymers is based on a structure that resembles finite plasticity. In most cases, a constitutive equation for stress is proposed, which depends on the elastic deformation gradient, supplemented by a flow rule for the plastic deformation, which depends on the “over stress.” The over stress is a properly invariant difference between the stress and the back stress (equilibrium stress). The back stress represents conditions under which relaxation events should stop and the material should be able to carry an applied load indefinitely without a need to change the strain. Questions that arise in using these models are whether such equilibrium stresses exist, how can they be evaluated, and what experiments can be used to characterize the flow rule. One challenge in accurately evaluating the locus of equilibrium conditions is the fact that the relaxation process substantially slow down around these points, and, therefore, a method that does not directly require being at the equilibrium is desirable. Focusing on shear, a thermodynamic theory for characterizing the response of glassy polymers, similar to models currently used for this purpose, is developed, and using this model it is shown that one can set up a method to calculate the plastic strain rate. This method is based on evaluating the slope of stress-strain response under conditions of similar elastic and plastic strain, but different strain rates. Since the equilibrium stress occurs when the plastic strain rate goes to zero, the evaluated plastic strain rates allow evaluation of the needed information for developing the flow rule and obtaining the back stress. This method is used to evaluate the plastic strain rate and back stress at room temperature for polycarbonate. The evaluated results match well with results obtained by direct probing of the equilibrium stress, in which one searches for points at which the stress remains constant at a constant strain over long durations. The method proposed looks promising in evaluating the back stress of glassy polymers. The added advantage of this method is that it also provides a map of plastic strain rate and tangent modulus over a large range of loading conditions.