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Research Papers

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.
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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
J. Appl. Mech. 2017;85(1):011006-011006-8. doi:10.1115/1.4038328.

In this paper, an exclusive testing rig was built to experimentally investigate the friction and slip at elevator traction interface under different traction conditions. The experimental results indicated that slipping occurs at both ends of contact arc first and then expands to the middle region gradually until the full slip along the sheave occurs. In addition, the full slip occurs earlier under lower rope pretension. Meanwhile, by setting similar boundary and loading conditions as in the experiments, the finite element analysis was performed. The simulation results agree with the experiments very well but reveal more details about traction behavior.

Topics: Friction , Pulleys , Ropes , Traction
Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011007-011007-10. doi:10.1115/1.4038427.

Wrinkles can be often observed in dielectric elastomer (DE) films when they are subjected to electrical voltage and mechanical forces. In the applications of DEs, wrinkle formation is often regarded as an indication of system failure. However, in some scenarios, wrinkling in DE does not necessarily result in material failure and can be even controllable. Although tremendous efforts have been made to analyze and calculate a variety of deformation modes in DE structures and devices, a model which is capable of analyzing wrinkling phenomena including the critical electromechanical conditions for the onset of wrinkles and wrinkle morphology in DE structures is currently unavailable. In this paper, we experimentally demonstrate controllable wrinkling in annular DE films with the central part being mechanically constrained. By changing the ratio between the inner radius and outer radius of the annular films, wrinkles with different wavelength can be induced in the films when externally applied voltage exceeds a critical value. To analyze wrinkling phenomena in DE films, we formulate a linear plate theory of DE films subjected to electromechanical loadings. Using the model, we successfully predict the wavelength of the voltage-induced wrinkles in annular DE films. The model developed in this paper can be used to design voltage-induced wrinkling in DE structures for different engineering applications.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011008-011008-7. doi:10.1115/1.4038426.

To make sure the safety, durability, and serviceability of structures in-service, health monitoring systems (HMS) are widely used in management of civil infrastructures in recent years. Compared with traditional force sensors, lead zirconium titanate (PZT) sensor performs better in smart sensing in HMS with advantages of high sensitivity, self-powering and fast response to highly dynamic load. Here, we propose to utilize PZT sensor arrays to identify the position and magnitude of external loads that are applied on a simply supported beam. An identification method is proposed based on experimental tests and theoretical electromechanical analyses, which is proved effective by comparing the identified parameters with the actually applied loading conditions and signals recorded by commercial force sensors. Experimental observations also reveal that PZT sensors respond faster to loading process than commercial force sensor, which makes it qualified in identification of transient loading such as impact processing in loading history. Results also demonstrate the applicability of the method to identify multiple concentrated load and the average moving speed of the applied load. The current method may provide a useful tool for identifying load conditions on various beam structures.

Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011009-011009-7. doi:10.1115/1.4038470.

In the analysis of origami structures, the deformation of shells usually couples with the rotation of creases, which leads to the difficulty of solving high-order differential equations. In this study, first the deformation of creased shell is solved analytically. Then, an approximation method named virtual crease method (VCM) is employed, where virtual creases are used to approximate the deformation of shells, and then a complex structure can be simplified into rigid shells connected by real and virtual creases. Then, VCM is used to analyze the large deflection of shells as well as the bistable states of origami structures, such as single creased shell and cell of Miura-Ori. Compared with experiment results, the deformed states given by VCM are quite accurate. Therefore, this generalized method may have potential applications in the analysis of origami structures.

Topics: Deformation , Shells
Commentary by Dr. Valentin Fuster
J. Appl. Mech. 2017;85(1):011010-011010-8. doi:10.1115/1.4038506.

A phase-field damage model for orthotropic materials is proposed and used to simulate delamination of orthotropic laminated composites. Using the deviatoric and hydrostatic tensile components of the stress tensor for elastic orthotropic materials, a degraded elastic free energy that can accommodate damage is derived. The governing equations follow from the principle of virtual power and the resulting damage model, by its construction, conforms with the physical relevant condition of no matter interpenetration along the crack faces. The model also dispenses with the traction separation law, an extraneous hypothesis conventionally brought in to model the interlaminar zones. The model is assessed through numerical simulations on delaminations in mode I, mode II, and another such problem with multiple initial notches. The present method is able to reproduce nearly all the features of the experimental load displacement curves, allowing only for small deviations in the softening regime. Numerical results also show forth a superior performance of the proposed method over existing approaches based on a cohesive law.

Commentary by Dr. Valentin Fuster

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