International Center for Applied Mechanics

This paper reviews various techniques to characterize material by interpreting load-displacement data from instrumented indentation tests. Scaling and dimensionless analysis was used to generalize the universal relationships between the characteristics of indentation curves and their material properties. The dimensionless functions were numerically calibrated via extensive finite element analysis. The interpretation of load-displacement curves from the established relationships was thus carried out by either solving higher order functions iteratively or employing neural networks. In this study, the advantages and disadvantages of these techniques are highlighted. Several issues in an instrumented indentation test such as friction, size effect and uniqueness of reverse analysis algorithms are discussed. In this study, a new reverse algorithm via neural network models to extract the mechanical properties by dual Berkovich and spherical indentation tests is introduced. The predicted material properties based on the proposed neural network models agree well with the numerical input data.

The paper reviews recent research and developments on simulated indentation tests at micron and nanometer levels. For indentation at the maximum depth of several microns or hundreds of nanometer, classical continuum plasticity framework incorporating Taylor dislocation model via strain gradient plasticity embedded in the constitutive equation may be adopted to take care of the size effect. As higher-order stress components and higher-order continuity requirements can be made redundant, only C0 finite elements incorporating strain gradient plasticity have to be formulated. This results in the significant ease and convenience in finite element implementation requiring minimal additional computational effort and resources. Alternatively, when the indentation depth is lower at nanometer level, either a large scale molecular dynamics model or a hybrid finite element and molecular dynamics simulation has to be adopted. The article includes certain results from the former approach on nanoindentation based on combination of both Morse potential and embedded-atom model potential.

This paper investigates the snoring mechanism of humans by applying the concept of structural intensity to a three-dimensional (3D) finite element model of a human head, which includes: the upper part of the head, neck, soft palate, hard palate, tongue, nasal cavity and the surrounding walls of the pharynx. Results show that for 20, 40 and 60Hz pressure loads, tissue vibration is mainly in the areas of the soft palate, the tongue and the nasal cavity. For predicting the snoring noise level, a 3D boundary element cavity model of the upper airway in the nasal cavity is generated. The snoring noise level is predicted for a prescribed airflow loading, and its range agrees with published measurements. These models may be further developed to study the various snoring mechanisms for different groups of patients.

One of the unique property of polymeric gel is that the volume and shape of gel can dramatically change even at mild variation of external stimuli. Though a variety of instability patterns of slender and thin film gel structures due to swelling have been observed in various experimental studies, many are not well understood. This paper presents analytical solutions of swelling induced instability of various slender and thin film gel structures. We adopt the well developed constitutive relation of inhomogeneous field theory of a polymeric network in equilibrium with a solvent and mechanical load or constraint with the incremental modulus concept for slender beam and thin film gel structures. The formulas of buckling and winkle conditions and critical stress values are derived for slender beam and thin film gel structures under swelling induced instability using nonlinear buckling theories of beam and thin film structures. For slender beam structure, we construct the stability diagram with the distinct stable and unstable zones. The critical slenderness ratio and corresponding critical stresses are provided for different dimensionless material parameters. For thin film gel structures, we consider the thin film gel on an elastic foundation with different stiffness. The analytical solutions of critical stress and corresponding wrinkle wavelength, as well as buckling condition (or critical chemical potential) are given. These analytical solutions will provide a guideline for gel structure design used in polymeric gels based MEMS and NEMS structures such as sensors and actuators. More importantly, the work provides a theoretical foundation of gel structure buckling and wrinkle, which instability phenomena are different from normal engineering material buckling.

The transmissions of transient energy flow and dynamic transient response of plate structures under low velocity impact are presented. The structural intensity approach is used to study the transient dynamic characteristics of plate structures under low velocity impact. In the dynamic impact response analysis, nine-node degenerated shell elements with assumed shear and membrane strain fields are adopted to model the target and impactor.  The dynamic contact-impact algorithm and the governing equations for both the target and impactor are derived based on the updated Lagrangian approach.  Explicit integration algorithm has been adopted in the time integration process. The novel structural intensity stream line representation is introduced to interpret energy flow paths for transient dynamic response of plates under low velocity impact. The effects of plates with and without structural damping on the energy flow and energy path are discussed. Numerical results, including contact force, deflection histories and transient energy flow vectors as well as structural intensity stream lines, show that the present method and representation are an efficient approach for exploring dynamic response for plate structures subjected to low velocity impact.

In this paper, numerical simulation has been used to predict the interior noise of tracked vehicles. The methodology of predicting interior noise level of tracked vehicles has been proposed. In order to determine interaction forces between running track system and the chassis hull of a tracked vehicle, the rigid multi-body tracked vehicle mode, which includes track moving system, was constructed and simulated using software ADAMS. The finite element (FE) and boundary element (BE) models of chassis hull of a tracked vehicle were created and adopted to perform the vibro-acoustic analysis. Correlation between the FE and BE models and physical measurements was improved to the extent that the models could be used to predict the interior noise of the tracked vehicle. The structural frequency dynamic response was carried out by employing software MSC/NASTRAN to obtain the structural vibration response. The interior noise was predicted by adopting the software, SYSNOISE. The predicted noise levels of a tracked vehicle were compared with physical measurements. From the comparison, it was demonstrated that the predicting interior noise levels of a tracked vehicle were in reasonably good agreement with the test results.

This paper presents passive and active vibro-acoustic noise control methods for attenuating the interior noise level in box structures which can be an analogy of cabins of vehicle and aircraft. The structural intensity (SI) approach is adopted to identify the predominant vibration panels and interior noise sources for box structures. In the study, the finite element method is used to determine the structural vibration and structural intensity in the box surfaces. According to structural intensity vectors plot and structural intensity stream lines presentation, the possible effective control positions where the dampers may be attached and the active control forces may act to reduce vibration and interior noise, are identified. From the study, it can be demonstrated that the structural intensity approach and stream line presentation are possible methods for identifying the vibro-acoustic interior noise source and predominant panels which may be modified to reduce the interior noise level. The structural intensity methodology, passive and active noise control results can be extended to the further study of the vibration and interior noise control of actual cabins of vehicles and aircraft.

Classical molecular dynamics with the AIREBO potential is used to investigate and compare the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing various densities of Stone-Thrower-Wales (STW) and double vacancy defects, within a temperature range of 100-600 K. Our results indicate that the presence of both kinds of defects can decrease the thermal conductivity by more than 80% as defect densities are increased to 10% coverage, with the decrease at high defect densities being significantly higher in zigzag compared with armchair nanoribbons. Variations of thermal conductivity in armchair nanoribbons were similar for both kinds of defects, whereas double vacancies in the zigzag nanoribbons led to more significant decreases in thermal conductivity than STW defects. The same trends are observed across the entire temperature range tested.

We investigate the thermal conductivity of fluorinated graphene using molecular dynamics simulations. We find that fluorination reduces the thermal conductivity of graphene, with the amount of reduction strongly depending on both distribution and coverage of fluorination. The thermal conductivity shows a somewhat U-shaped change with coverage from 0% to 100%. At the same coverage, random fluorination results in a larger reduction in thermal conductivity than patterned fluorination, and the patterned strips oriented perpendicular to heat flux cause a stronger reduction than the strips parallel to heat flux. We also find that fluorination makes the thermal conductivity less sensitive to strain.

The deformation behavior of a pH-sensitive hydrogel micro-fluidic valve system is investigated using inhomogeneous gel deformation theory, in which the fluid-structure interaction (FSI) of the gel solid and fluid flow in the pipe is considered. We use a finite element method with a well adopted hydrogel constitutive equation, which is coded in commercial software, ABAQUS, to simulate the hydrogel valve swelling deformation, while FLUENT is adopted to model the fluid flow in the pipe of the hydrogel valve system. The study demonstrates that FSI significantly affects the gel swelling deformed shapes, fluid flow pressure and velocity patterns. FSI has to be considered in the study on fluid flow regulated by hydrogel microfluidic valve. The study provides a more accurate and adoptable model for future design of new pH-sensitive hydrogel valves, and also gives a useful guideline for further studies on hydrogel fluidic applications.

Proposed constitutive model for metals focusing on strain rate and adiabatic heating effects. Detailed procedure for evaluating material properties of the proposed model. Establishing material model properties for steel and aluminum based on test results. High velocity perforation study of steel and aluminum targets and comparison with test results. Application of the coupled smooth particle hydrodynamics-finite element method.

This paper develops a model that incorporates damage band evolution at three levels:(i) at the mechanism level, the damage mechanisms, such as diffusive void growth and fatigue cracks, determine the damage growth rate;(ii) at an intermediate level, the damage band is modeled as springs connecting undamaged materials, and the spring constants change as damage develops;(iii) at the continuum level, the damage band is modeled as an array of dislocations to satisfy equilibrium.

In a ceramic multilayer actuator, the abrupt end of an internal electrode concentrates the electric field, inducing stresses in the ceramic. Crack nucleation and growth have been observed experimentally, but have not been well modeled due to the complex material behaviors. We write a finite element program to solve this coupled electromechanical problem. The material is taken to be nonlinearly dielectric with electrostrictive strain quadratic in electric displacement.

On a solid surface, an epitaxial monolayer may separate into phases that self-assemble into patterns on the nanoscale. The self-assembly minimizes the combined free energy of mixing, phase boundary, and elasticity. Our recent numerical simulation has revealed an intriguing mesophase transition. The surface stress is a second-rank tensor and can be anisotropic. Depending on the degree of the anisotropy, the lowest energy stripes can be either parallel to, or at an angle from, a principal axis of the surface stress tensor.

Abstract: The objectives of this research program were to:(1) understand the nature of the charge resulting from contact electrification,(2) understand the mechanisms and fundamental limits of tribocharging,(3) develop new materials. Specifically, the goals were to establish what determine the rate of electrostatic charging and discharging in solid electrets, to understand the process by which charge transfer occurs and to explore self-assembly based on electrostatic interactions.