Leon M. Headings

Nonlinear model for Galfenol cantilevered unimorphs considering full magnetoelastic coupling

This article presents a fully coupled, nonlinear model for the dynamic response of Galfenol-driven unimorph actuators in a cantilever configuration. The hysteretic magnetomechanical behavior of Galfenol is modeled using a discrete energy-averaged model, and the structural behavior of the unimorph is modeled using the finite element method. The weak form equations that describe bending of the unimorph are obtained using the principle of virtual work. Since the local strain and stress are nonlinearly coupled with both the vertical and horizontal displacements, a nonlinear solver is developed to approximate the coupled finite element equations. The nonlinear solver is verified against the analytical solution and experimental data for the case of a passive beam. The analytical solution is obtained using beam theory for free and harmonic responses. The analytical model and experimental data verify that the nonlinear solver correctly quantifies the first natural frequency of the composite beam. The numerical simulations match the analytical solutions for both free and harmonic responses. Finally, the dynamic response of the nonlinear magnetoelastic model is investigated and experimentally validated from 0.1 to 500 Hz, the range in which the model is accurate without the need for adjustable parameters.

Stress Averaging in PVDF Sensors For In-Plane Sinusoidal and Impact-Induced Stresses

Polyvinylidene flouride (PVDF) sensors are shown to exhibit very high stress sensitivity and high speed operation, with nanosecond response times. These characteristics allow PVDF sensors to measure in-plane stress waves in a structure. The voltage output of these sensors can be related to the average stress acting on the sensor. This paper presents an analysis on the effects of stress averaging for both in-plane sinusoidal stress waves and in-plane impact-induced stresses. Analytical models are developed that show the relationship between measured PVDF output voltage and sensor parameters, structure material, and input stress conditions. Parametric studies are conducted, which show that the error introduced by stress averaging is minimal for most typical cases; however, the effects of stress averaging become more significant as the sensor length increases, density of the structures material increases, and magnitude of the input stress increases. For sinusoidal stress inputs, the error increases as the sensor length approaches the wavelength of the stress in the structure.

Opportunities for Thermoelectric Energy Conversion in Hybrid Vehicles

Much analysis has been performed on the application of thermoelectrics in automobiles, but the low efficiency of the materials has so far limited their use. As a result, little has been done in the physical design of how to most efficiently utilize thermoelectrics in a vehicles energy system. However, much progress has been and continues to be made in the field of thermoelectric materials. Developments in the areas of nanostructured materials have produced materials with double the efficiency of current commercially available materials. This, coupled with a growing need for the reduced consumption of fossil fuels and production of greenhouse gases, has generated renewed interest in the application of thermoelectrics in automotive systems. Hybrid-electric vehicle (HEV) designs have provided significant improvements in fuel efficiency and continue to evolve. This modified energy management strategy introduces new components and energy distributions which force traditional designs to be reconsidered. For example, the temperature and quantity of thermal energy transferred through the exhaust and radiator are lowered. Also, the IC engine may not be run continuously, creating difficulties in maintaining temperature in the catalytic converter, powering belt-driven accessories, and regulating cabin temperature. This contributes to an increased demand for electrical energy. Finally, the power electronics are typically liquid cooled (order of 60-65 °C) and the high voltage battery packs must be kept cool (typically below 45 °C) to maximize their life. A detailed computer model which captures the details of the energy transfers in HEVs, including thermal loads will be used to assess the unique thermal requirements of hybrid vehicles under average engine loads. Based on these requirements, specific thermal energy management strategies will be proposed. These modified systems will be added to the computer model in order to evaluate their potential using currently available thermoelectrics materials. Finally, the preferred thermal energy management system will be selected as the basis for future design optimization.Copyright

Self-Folding Laminated Composites for Smart Origami Structures

Origami-folding principles can be used with laminated composites to produce lightweight structures that are capable of drastic changes in shape. This paper presents a smart composite that can actively change its crease pattern to fold itself into different rigid shapes and provide a large range of motion. The composite uses a smart material with variable modulus sandwiched between two fiber-reinforced elastomeric skins, one of which is prestressed. Change in modulus of the sandwiched core layer allows prestress in the elastomeric skin to actuate the fold. Unfolding the structure to a flat shape can be accomplished through either embedded or external actuation. Passive composite panels were fabricated for model development and validation. An analytical model was developed based on classical laminate plate theory to study the influence of core modulus, core thickness, and elastomeric skin prestress on the equilibrium curvature of the composite structure. Selected smart materials that provide a change in modulus when stimulated are discussed as candidates for the core layer of the self-folding composite.Copyright

High Sensitivity Polyvinylidene Fluoride Microphone Based on Area Ratio Amplification and Minimal Capacitance

This paper presents an inexpensive high-sensitivity microphone based on polyvinylidene fluoride (PVDF) film. High sensitivity is achieved through pressure amplification created by the area ratio between the rigid surface exposed to acoustic waves and a crosshair-shaped PVDF film, in combination with the reduced capacitance created by a similarly shaped top electrode. The crosshair shape is obtained through simple chemical etching from commercial PVDF film. Finite-element simulations including static structure analysis, modal analysis, and harmonic response are performed to design the microphone. Peak coalescence of the first three adjacent natural frequencies caused by the structure damping ratio is observed. Static and dynamic stress analyses ensure that the design meets the mechanical constraints imposed by PVDF. A plane wave tube experiment and signal conditioning electronics are developed. Measurements include benchmarking against a commercial microphone and show that the PVDF microphone exhibits a linear response up to a sound pressure level of 140 dB and overall fluctuations of less than ±4 dB over the frequency range of 10 to 20000 Hz. The sensitivity of the microphone alone, without a conditioning circuit, is measured as 27.8 μV/Pa, which is 3.01 times the sensitivity of commercial PVDF film operating in 3-3 mode. This sensitivity gain is close to the physical area ratio of 3.2. We experimentally characterize the directivity of the sensor and measure a decay of -10.5 dB at ±90° from the microphones axis.

High Temperature Thermoelectric Auxiliary Power Unit for Automotive Applications

While the thermoelectric effects have been known for over 100 years, their traditionally low conversion efficiency for power generation has limited their use to highly specialized applications. With the rapid advancement of thermoelectric materials in recent years, their inherent reliability and power density is being augmented by improvements in efficiency. Recent increases in the figure of merit of materials suitable for operation around 500 °C make them candidates for waste heat recovery, as well as primary power using combustion heaters. The characteristic scalability of thermoelectric generators makes them best suited for low power applications where alternative generators become impractical. However, with the development of thermoelectric device technology in parallel with materials advancements, it may become viable to design thermoelectric generators for auxiliary power in automotive applications. The research presented here represents the initial stages of the development of a thermoelectric power unit (TEPU). While thermoelectric generator technology can be applied to any fuel, this research targets the use of diesel fuel which is readily available for both military and consumer applications and is more easily and safely transported than many alternatives. The use of diesel fuel for a TEPU is enabled by the use of an atomizer technology developed at The Ohio State University Center for Automotive Research. A baseline prototype incorporating this novel diesel fuel atomizer/combustor with conventional thermoelectric materials and heat exchange designs has been constructed and tested. Preliminary data highlights the viability of diesel fuel for thermoelectric power generation as well as the areas which demand further development. This prototype will serve as the baseline for evaluating future designs incorporating advanced materials and novel system designs.Copyright

Seam Welding of Aluminum Sheet Using Ultrasonic Additive Manufacturing System

Ultrasonic welding was investigated as a method of joining 0.076 in. (1.93 mm) thick aluminum 6061 flat sheet material. Joints were produced with ultrasonic additive manufacturing (UAM) equipment in a modified application of the ultrasonic welding process. Through joint design development, successful welds were achieved with a scarf joint configuration. Using a design of experiments (DOE) approach, weld parameters including weld amplitude, scarf angle, and weld speed were optimized for mechanical strength. Lower angles and higher amplitudes were found to provide the highest strengths within the levels tested. Finite-element studies indicate that 5 deg and 10 deg angles produce an increased relative motion of the workpieces as compared to 15 deg, 20 deg, and 25 deg angles, likely leading to increased strength. Successful joints showed no indication of voids under optical microscopy. As-welded joints produce tensile strengths of 221 MPa, while heat treated joints produce tensile strengths of 310 MPa, comparable to heat treated bulk material. High-temperature tensile testing was conducted at 210 C, with samples exhibiting strengths of 184.1 MPa, similar to bulk material. Room temperature fatigue testing resulted in cyclic failures at approximately 190,000 cycles on average, approaching that of bulk material. [DOI: 10.1115/1.4034007]

Speed of Sound Measurement in Solids Using Polyvinylidene Fluoride (PVDF) Sensors

Piezoelectric film sensors such as polyvinylidene flouride (PVDF) generate an electrical voltage in response to an applied mechanical stress with a remarkably high sensitivity. They provide very fast response times and do not require extensive signal conditioning. This paper presents a straightforward method of measuring the speed of sound in solid materials and structures using commercial PVDF sensors.PVDF sensors are most commonly used to measure stresses applied in the sensors’ thickness direction. However, this requires that the sensors be located in the load path, which may result in damage to the sensor or affect the response of the system. In this paper, two PVDF sensors are bonded to the side of a structure and a small impact is applied to one end. The sensors are used to measure the time for the impact-induced plane stress wave to travel between the sensors. The observed speed of the propagating stress wave is shown to be in good agreement with the theoretical speed of sound for the material and finite element calculations. In addition, the finite element simulations confirm the validity of the plane wave assumption for non-ideal and non-uniform impact inputs.Copyright

Analysis of Shape Memory Polymer-Alloy Composites: Modeling and Parametric Studies

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) are interesting due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be used to create materials or systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite fixity and interfacial stresses. Analytical models show that certain SMPs can achieve sufficient shape fixing. COMSOL Multi-Physics simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP-SMA two-way actuation system.Copyright

Near DC force measurement using PVDF sensors

There is a need for high-performance force sensors capable of operating at frequencies near DC while producing a minimal mass penalty. Example application areas include steering wheel sensors, powertrain torque sensors, robotic arms, and minimally invasive surgery. The beta crystallographic phase polyvinylidene fluoride (PVDF) films are suitable for this purpose owing to their large piezoelectric constant. Unlike conventional capacitive sensors, beta crystallographic phase PVDF films exhibit a broad linear range and can potentially be designed to operate without complex electronics or signal processing. A fundamental challenge that prevents the implementation of PVDF in certain high-performance applications is their inability to measure static signals, which results from their first-order electrical impedance. Charge readout algorithms have been implemented which address this issue only partially, as they often require integration of the output signal to obtain the applied force profile, resulting in signal drift and signal processing complexities. In this paper, we propose a straightforward real time drift compensation strategy that is applicable to high output impedance PVDF films. This strategy makes it possible to utilize long sample times with a minimal loss of accuracy; our measurements show that the static output remains within 5% of the original value during half-hour measurements. The sensitivity and full-scale range are shown to be determined by the feedback capacitance of the charge amplifier. A linear model of the PVDF sensor system is developed and validated against experimental measurements, along with benchmark tests against a commercial load cell.