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J. Appl. Mech. 2017;85(1):011001-011001-14. doi:10.1115/1.4038216.

Shale is a typical layered and anisotropic material whose properties are characterized primarily by locally oriented anisotropic clay minerals and naturally formed bedding planes. The debonding of the bedding planes will greatly influence the shale fracking to form a large-scale highly permeable fracture network, named stimulated reservoir volume (SRV). In this paper, both theoretical and numerical models are developed to quantitatively predict the growth of debonding zone in layered shale under fracking, and the good agreement is obtained between the theoretical and numerical prediction results. Two dimensionless parameters are proposed to characterize the conditions of tensile and shear debonding in bedding planes. It is found that debonding is mainly caused by the shear failure of bedding planes in the actual reservoir. Then the theoretical model is applied to design the perforation cluster spacing to optimize SRV, which is important in fracking. If the spacing is too small, there will be overlapping areas of SRV and the fracking efficiency is low. If the spacing is too large, there will be stratum that cannot be stimulated. So another two dimensionless parameters are proposed to evaluate the size and efficiency of stimulating volume at the same time. By maximizing these two parameters, the optimal perforation cluster spacing and SRV can be quantitatively calculated to guide the fracking treatment design. These results are comparable with data from the field engineering.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011002-011002-13. doi:10.1115/1.4038285.

An adaptive vibration isolation system is proposed in this paper to combine the advantages of both linear and nonlinear isolators. Because of the proposed structural piecewise characteristics for different levels of response, the stiffness and damping properties could be designed according to the vibration performances. The adaptive stiffness and damping properties are achieved by the joined utilization of symmetrical precompression triangle-like structure (TLS) and column frame with cam. In order to design the control mechanism with optimum structural parameters, nonlinear vibration performances are analyzed by using averaging method and singularity theory. The parameter plane is divided into transition sets, and then the optimization criterions for structural design are provided according to multiple nonlinear vibration performances including frequency band for effective isolation, multisteady state band and resonance peak, etc. The experiment is carried out to verify the theoretical selection of desirable parameters and indicates the advantages and improvement of vibration isolation/suppression brought by the structural property adaptation. This study provides a novel method of achieving structural property adaptation for the improvement of isolation effectiveness, which shows the intelligent realization by passive components.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011003-011003-6. doi:10.1115/1.4038286.

Up to now the theoretical analysis for fracture behaviors of bulk metallic glasses (BMGs) are limited to uniaxial loading. However, materials usually suffer complex stress conditions in engineering applications. Thus, to establish an analysis method that could describe fracture behaviors of BMGs under complex loading is rather important. In this paper, a universal formula for the fracture angle is proposed toward solving this problem. The ellipse criterion is used as an example to show how to predict fracture behaviors of BMGs subjected to complex loading according to this formula. In this case, both the fracture strength and fracture angle are found to be well consistent with experimental data.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011004-011004-17. doi:10.1115/1.4038287.

This work focuses on elastic wave propagation in three-dimensional (3D) low-density lattices and explores their wave directionality and energy flow characteristics. In particular, we examine the dynamic response of Kelvin foam, a simple-and framed-cubic lattice, as well as the octet lattice, spanning this way a range of average nodal connectivities and both stretching-and bending-dominated behavior. Bloch wave analysis on unit periodic cells is employed and frequency diagrams are constructed. Our results show that in the low relative-density regime analyzed here, only the framed-cubic lattice displays a complete bandgap in its frequency diagram. New representations of iso-frequency contours and group-velocity plots are introduced to further analyze dispersive behavior, wave directionality, and the presence of partial bandgaps in each lattice. Significant wave beaming is observed for the simple-cubic and octet lattices in the low frequency regime, while Kelvin foam exhibits a nearly isotropic behavior in low frequencies for the first propagating mode. Results of Bloch wave analysis are verified by explicit numerical simulations on finite size domains under a harmonic perturbation.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011005-011005-7. doi:10.1115/1.4038327.

A majority of dielectric elastomers (DE) developed so far have more or less viscoelastic properties. Understanding the dynamic behaviors of DE is crucial for devices where inertial effects cannot be neglected. Through construction of a dissipation function, we applied the Lagrange's method and theory of nonequilibrium thermodynamics of DE and formulated a physics-based approach for dynamics of viscoelastic DE. We revisited the nonlinear oscillation of DE balloons and proposed a combined shooting and arc-length continuation method to solve the highly nonlinear equations. Both stable and unstable periodic solutions, along with bifurcation and jump phenomenon, were captured successfully when the excitation frequency was tuned over a wide range of variation. The calculated frequency–amplitude curve indicates existence of both harmonic and superharmonic resonances, soft-spring behavior, and hysteresis. The underlying physics and nonlinear dynamics of viscoelastic DE would aid the design and control of DE enabled soft machines.

Commentary by Dr. Valentin Fuster

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