Phase Field Modeling of Ferroelectric Domain Wall Interactions With Charge Defects

The overall objective of this work is to develop a theoretical model that can track the evolution of the domain structures in ferroelectric crystals, which are responsible for the non-linear electromechanical behavior of these materials. To this end, a continuum thermodynamics framework is devised, and the theory falls into the class of phase-field or diffuse-interface modeling approaches. Here a set of micro-forces and governing balance laws are postulated and applied within the second law of thermodynamics to identify the appropriate material constitutive relationships. The approach is shown to yield the commonly accepted Ginzburg-Landau equation for the evolution of the polarization order parameter. Within the theory a form for the free energy is postulated that can be applied to fit the general elastic, piezoelectric and dielectric properties of a ferroelectric material near its spontaneously polarized state. Thereafter, a principle of virtual work is specified for the theory and is implemented to devise a finite element formulation. The theory and numerical methods are used to investigate the interactions of 180° and 90° domain walls with an array of charge defects and to determine the electromechanical pinning strength of the array on the walls.

Responses of Hybrid-III 50th and Thor-NT 50th in Sled Tests With Different Seat Belt Pretensioner Configurations

This study quantifies the effectiveness of the various seat belt pretensioner configurations relative to the no pretensioner condition and defines the relative sensitivity of the Hybrid-III 50th and THOR-NT 50th percentile male anthropomorphic test devices to pretensioner effects. The results of this study indicate that pretensioners reduced the chest accelerations and Head Injury Criteria (HIC) of both Hybrid-III and THOR-NT dummies. In addition, the pretensioners reduced the chest forward movement by providing restraint earlier in the event. The dual pretensioners and the retractor pretensioners were more effective than the buckle pretensioner and the no pretensioner conditions. Although the Hybrid-III and THOR-NT were different in construction and sitting depth, the Hybrid-III and THOR-NT's responses to the pretensioner conditions were similar. Test-to-test repeatability was acceptable for both dummies.

Smart Impact Management Devices: Experimental Validation of Crush Response of Rapidly Expanded Aluminum Honeycomb

A major limitation of current dedicated impact energy management structures and passive devices used in the transportation industry is that their starting volume is their maximum volume, i.e. they dissipate energy by crushing or stroking from a larger to a smaller volume. Since they occupy their maximum volumes in their uncrushed-as-installed state they occupy space that is then only functional in an impact and is otherwise wasted. This limitation has led to the proposal of a class of "smart" impact energy management devices, based on unexpanded aluminum honeycomb (HOBE), that initially occupy a small volume and based on sensor input are rapidly expanded to a much larger crushable volume (nominally 75 times greater) just prior to or in response to an impact. Energy management devices based on this technology, should this technology successfully pass through the many steps needed to prove its viability, would thus allow empty space to be left adjacent to them for operational clearances, serviceability and repair functions, etc. which spaces would yet be fully utilized, due to the expansion of the device, for impact energy management. This paper documents the second portion of an experimental exploration of the viability of this technology. The specific goal of the here-in documented portion of the test program was the demonstration that the crush response of the rapidly expanded honeycomb is comparable to that of the standard pre-expanded commercial product.

Validation of a New Aluminum Honeycomb Constitutive Model for Impact Analyses

A new constitutive model for large deformation of aluminum honeycomb has been developed. This model has 6 yield surfaces that are coupled to account for the orthotropic behavior of the cellular honeycomb being crushed on-axis and off-axis. Model parameters have been identified to fit uniaxial and biaxial crush test data for high density (38 lb/ft3) aluminum honeycomb. The honeycomb crush model has been implemented in the transient dynamic Presto finite element code for impact simulations. Simulations of calibration and validation experiments will be shown with model predictions compared with test data. Also, the honeycomb model's predictions will be compared with the older Orthotropic Rate Model predictions.

Chirality Effects on Axial Thermomechanical Properties of Carbon Nanotubes

The nearly one dimensional carbon nanotubes with their novel physical and mechanical properties have received ever increasing attention in recent years for the use in a wide range of applications in which semiconductor nano-structures, nano-devices/sensors, and nano-electro-mechanical systems are to be integrated. However, carbon nanotubes exist in various chirality configurations each of which may perform differently when they are subjected to external mechanical and thermal loads, temperatures changes, and magnetic fields. Therefore, a detailed and fundamental investigation of the effects of chirality angles on thermomechanical performance of carbon nanotubes is needed to explain the behavior of such structures. Here in this work, finite element method (FEM) is employed to numerically investigate the responses of carbon nanotubes to external mechanical loads and temperatures changes. Single-walled carbon nanotubes (SWCNTs) with different chirality configurations, i.e., zigzag, armchair, and chiral are modeled and their effective thermomechanical properties are investigated. Finally, results are discussed and compared with the existing results from literature.

The Effects of Yttrium on the Tensile and Creep Behavior of Thermally Grown Oxide at High Temperature

Recently, It has been revealed that TGO(thermally grown oxide) plays important roles on durability of TBC(thermal barrier coating) systems. In this work, Fecralloy foils were chosen as the substrates which form TGO on the surface at high temperature and the tensile and creep experiments were performed with the thick foils 100 μm at 1200°C. During the experiments the load, displacement and the TGO thickness were monitored in-situ. The effect of Yttrium on the mechanical behavior was investigated using the specimens with two different levels of the concentration. As the results, it was found that Yttrium enhances the strength of TGO as well as that of the substrate at the high temperature.

A Comparison Between Head Impact Response of Hybrid III and THOR-NT Dummy Heads in FMVSS201 Tests

The impact response of the forehead of both the Hybrid III dummy and THOR dummy was designed to the same human surrogate data. Therefore, when the forehead of either dummy is impacted with the same initial conditions, the acceleration response and consequently the head impact criterion HIC should be similar. If the THOR dummy is used in the FMVSS 201 free motion headform tests, then when it strikes the interior trim of the vehicle, as prescribed by the FMVSS 201 procedure, the acceleration response should be similar to that of the Hybrid III, as long as only the forehead engages the vehicle interior. To compare and contrast the response of the two dummy heads under FMVSS 201 testing, a design of experiments (DOE), that is a function of seven variables, is utilized to develop a mathematical model of the Head Impact Response. These independent parameters include five trim manufacturing process variables that relate to the interior that the dummy head hits in 201 testing: mold temperature, melt temperature, packing pressure, hold pressure, and injection speed. Two operational variables were also considered: free motion Headform approach angle and the dummy head drop calibration. An incomplete block design approach is utilized in order to significantly reduce the number of experiments. The DOE approach determines the response in the form of the Head Impact Criterion (HIC) with respect to the seven variables at 99% confidence level. The results describe the response data of both dummy heads. The response data of the dummy heads is described. Results indicate that the Hybrid III dummy head and the THOR dummy head have significantly different response characteristics in terms of magnitude of response, variation to different input conditions, repeatability, HIC values, and acceleration time history.

Improving Energy Absorption and Dissipation of Composites Through Optimized Tayloring

Fibre-reinforced and sandwich composites with laminated faces are the best candidate materials in many engineering fields by the viewpoint of the impact resistance, containment of explosions, protection against projection of fragments, survivability and noise and vibration suppression. Besides, they offer the possibility to be tailored to meet design requirements. A great amount of the incoming energy is absorbed through local failures. The most important energy dissipation mechanisms are the hysteretic damping in the matrix and in the fibers and the frictional damping at the fiber-matrix interface. The dissipation of the incoming energy also partly takes place as a not well understood dissipation at the cracks and delamination sites. As self-evident, the local damage accumulation mechanism on the one hand is helpful from the standpoint of energy absorption, on the other hand it can have detrimental effects. To date sophisticated computational models are available, by which the potential advantages of composites can be fully exploited. A large amount of research work has been oriented to improve the impact resistance, the dissipation of vibrations and to oppose the propagation of delamination. These goals can be obtained with incorporation of viscoelastic layers. Unfortunately this makes quite compliant the laminates and reduce their strength. Studies have been recently published that seeks to comply stiffness and energy dissipation. The existence of fiber orientations that are a good compromise between optimal stiffness and optimal absorption of the incoming energy can be supposed by the results of a number of published studies. In this paper, a variable spatial distribution of plate stiffnesses, as it can be obtained varying the orientation of the reinforcement fibres along the plate and their constituent materials, is defined by an optimization process, so to obtain a wanted specific structural behaviour. The key feature is an optimized strain energy transfer from different deformation modes, such as bending, in-plane and out-of-plane shears. Suited plate stiffness distributions which identically fulfil the thermodynamic and material constraints are found that make stationary the energy contributions and transfer energy between the modes as desired. An application to low velocity impacts and to blast pulse loads is presented. The use of the optimized layers with the same mean properties of the layers they substitute were shown to reduce deflection and the stresses that induce delamination. A new discrete layer element is developed in this study, to accurately account for the local effects. Characteristic feature, it is based on a C° in-plane approximation and a general representation across the thickness which can either represent the kinematics of conventional plate models or the piecewise variation of layerwise models.

A Study of Composite Strengthening Through Application of an Electric Field

This paper studies effects of an electric field on the mechanical response of unidirectional carbon fiber polymer matrix composites. The existing experimental evidence suggests that exposure of a composite material to the electromagnetic field leads to changes in the material's strength and resistance to delamination. We have analyzed the effects promoting this phenomenon: coupling of mechanical and electromagnetic fields and Joule heat effects and develop an experimental setup for impact tests of the composites carrying an electric current. Experimental results of low velocity impact tests on unidirectional carbon fiber polymer composite plates carrying a DC electric current show that electrified composites fail at higher impact load. Moreover, a larger electric field leads to a larger impact load that may be sustained by the composite. Finally, analysis of the Joule heat effects reveals that it is not a primary mechanism for the strengthening phenomenon observed in the experiments.