•A novel VBNES-EH is proposed to enhance the vibration suppression and energy harvesting.•The stiffness softening effect of the VBNES-EH can reduce the threshold of the optimal TET.•The VBNES-EH has ...better performance than the FBNES-EH under low and intermediate energy level impulse.•The VBNES-EH with optimized stiffness have good resonance peak suppression ability and broadband energy harvesting.
A variable-potential bistable nonlinear energy sink coupled electromagnetic harvesters (VBNES-EH) is proposed, that can simultaneously achieve high efficiency, broadband vibration suppression and energy harvesting under ultra-low and ultra-wide input energy level excitation. The performance is enhanced by introducing two horizontal energy harvesters (HEHs) to dynamically reduce the potential barrier height of the bistable nonlinear energy sink (BNES) or called bistable energy harvester (BEH) to form lower excitation thresholds of the chaotic and strong modulated responses. Firstly, the dimensionless mathematical model of the VBNES-EH is proposed and verified by the experiment. The experimental results are qualitatively consistent with the simulation results. The transient dynamics and the corresponding energy dissipation rates of five target energy transfer (TET) mechanisms for the weakly damped system are summarized numerically. The variable-potential energy effect and its benefits for the dual functions of vibration suppression and energy harvesting are analyzed. Afterward, the stiffness of the VBNES-EH and the fixed-potential BNES-EH (FBNES-EH) are optimized, and their energy dissipation rates under impact excitation are compared. The results indicate that the former has better performance under low- and intermediate-level impact excitations. Furthermore, the influence of system parameters on the energy harvesting rate is discussed to guide the optimal design of the performance. Finally, the dynamical evolution of the VBNES-EH and the parameter effects on the dual performance of the system under harmonic sweep excitation are investigated. Numerical results demonstrate that the VBNES-EH can achieve high performance under the different amplitudes of the harmonic excitation through stiffness optimization.
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•Negative stiffness online tunable spring based on Maxwell normal stress.•Improved stiffness tunable range and energy efficiency through a novel magnetic circuit design.•High-static-low-dynamic ...isolator constructed by the parallel connection of the negative spring and a linear mass-spring-damper system.•Tunable vibration isolator with reduced starting isolation frequency.
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A vibration isolator based on a tunable negative stiffness mechanism combines the advantages of high-static-low-dynamic stiffness (HSLDS) isolators to expand the isolation frequency band and variable stiffness isolators to suppress resonance. In this paper, a novel tunable negative stiffness spring using Maxwell normal stress (SMNS) is proposed. The stiffness tunable range and energy utilization efficiency are greatly improved due to the newly designed magnetic circuit. Moreover, the electromagnetic negative stiffness device has the advantages of no friction, no backlash, compact structure and easy control. An analytical model of the electromagnetic force is built based on magnetic circuit analysis, and the parameter analysis is performed. An HSLDS isolator is constructed by connecting the SMNS in parallel with a linear isolator. The stiffness and vibration isolation performance are measured. The experimental results show that the SMNS produces an online tunable negative stiffness, which expands the isolation bandwidth and significantly improves the vibration isolation performance.
•LSP induces more prolonged elastic-plastic transition in tensile deformation of Ti-6Al-4 V alloy.•Accurate modeling is obtained using a deformation-mechanism-based constitutive model.•Residual ...stress distinctly weakens the initial yielding due to the premature yielding of substrate.•Grain refinement enhances both the yielding and strain hardening.
Systematical microstructure characterization, mechanical testing and constitutive modeling were carried out to quantify the effects of residual stress and grain refinement on tensile properties of Ti-6Al-4 V alloy treated by laser shock peening (LSP). Microstructure characterization showed that grain size is refined to nanoscale in the outmost surface. Meanwhile, LSP treatment introduced a maximum compressive residual stress of 600 MPa in the surface region. The macroscopic tensile test indicated a more prolonged elastic-plastic transition in the LSP-treated Ti-6Al-4 V alloy than the as-received one. As a result, the 0.2% offset yield strength of the LSP-treated Ti-6Al-4 V alloy was reduced by about 52 MPa, while the flow strength at the subsequent strain hardening stage was enhanced. To quantitatively evaluate the individual influence of gradient microstructure and residual stress on the tensile response, we established a deformation-mechanism-based and size-dependent constitutive model. Implementation of the constitutive model demonstrated that the surface grain refinement enhances both the initial yielding and strain hardening of LSP-treated Ti-6Al-4 V alloy. In contrast, the residual stress has a significant weakening effect on the initial yielding but has little influence on its strain hardening behavior. The established microstructure and deformation mechanism-based model can help guide the LSP processing to further improve the mechanical performance of Ti-6Al-4 V alloy.
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A new beam element is developed to study the thermoelastic behavior of functionally graded beam structures. The element is based on the first-order shear deformation theory and it accounts for ...varying elastic and thermal properties along its thickness. The exact solution of static part of the governing differential equations is used to construct interpolating polynomials for the element formulation. Consequently, the stiffness matrix has super-convergent property and the element is free of shear locking. Both exponential and power-law variations of material property distribution are used to examine different stress variations. Static, free vibration and wave propagation problems are considered to highlight the behavioral difference of functionally graded material beam with pure metal or pure ceramic beams.
The present work addresses the various ingredients required for reliable finite element simulations of cold isostatic pressing (CIP) of metal powders. A plastic constitutive model for finite ...deformation is presented and implemented into an explicit finite element (FE) code. The FE implementation is verified so that numerical errors (both temporal and spatial errors) are kept under control. Thereafter, uniaxial die compaction experiments are performed required for determining the material parameters in the constitutive model. Subsequently they are applied for the simulation of a “complex” CIP process. The experimental observations of the complex CIP process were used to validate the overall method by comparing the FE results (final dimensions and average relative density) to the experimental observations. The numerical results (final dimensions and relative density) are in good agreement with the experimental observations.
Stress and residual stress distributions in bending are important in calculating springback and loading capacity of a sheet-metal bending part. Great differences have been found in springback ...prediction with the same input (benchmark problems) among different researchers. In order to find out the root cause of these differences, stress and residual stress calculation methods in plane strain bending are briefly reviewed or developed. The influence of deformation theory and incremental theory, repeating bending, unbending and re-bending, cyclic material models and springback calculation methods on the stress or residual stress distributions are examined and shown to be large. This emphasizes the importance of careful selection of these variables in a simulation model in addition to other general input variables, such as material properties, tool geometry and friction condition. This fact also helps to explain the great differences among different research results, and presents a challenge to both the programmers and the users of finite element packages.
The paper deals with a framework for anisotropically damaged elastic–plastic solids and the definition of the Eshelby stress tensor in anisotropic continuum damage mechanics. The proposed anisotropic ...elastic–plastic-damage model is based on a generalized macroscopic theory within the context of nonlinear continuum damage mechanics taking into account kinematic description of damage. It employs the consideration of damaged as well as fictitious undamaged configurations related via metric transformations which allow for the interpretation of damage tensors. A macroscopic yield condition is used to adequately describe the plastic flow properties of ductile metals and a damage criterion takes into account isotropic as well as anisotropic effects providing a realistic physical representation of material degradation. Furthermore, the unbalance of pseudo-momentum is established based on the second law of thermodynamics formulated with respect to the damaged configurations. Evaluation of a strain energy function and assuming the existence of pseudo-potentials of plastic as well as of damage dissipation leads to the definition of the Eshelby stress tensor, inhomogeneity force and material dissipation force.
