Nancy L. Johnson

Shape memory alloy cables

Conventional structural cables (or wire ropes) are composed of steel wires helically wound into strands, which, in turn, are wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising properties for a broad range of new applications. Among the many potential advantages of this form are increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and signicant design flexibility. Currently there are no known studies of SMA cables in the literature, so exploratory thermo-mechanical experiments were performed on two commercially available cable designs as part of an ongoing research program to systematically characterize their thermomechanical behavior and demonstrate their potential utility as adaptive or resilient tension elements.

Effect of Bondline Thickness on Mixed-Mode Debonding of Adhesive Joints to Electroprimed Steel Surfaces

Abstract Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for techniques to assess adhesive joint strength, particularly along bondline interfaces where compliant adhesives contact more rigid metallic surfaces. The present study investigates the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel adherend surfaces. Three bondline thicknesses, representative of structural automotive joints, were evaluated for unprimed and primed bondlines. Experimental results for static load versus debond extension were input to finite element analyses for computing debond parameters (strain energy release rates). The debonds always initiated at a through-the-thickness location that had the greatest peel component of strain energy release rate. The total strain energy release rate values correlated well wi...

Nonlinear modeling of magnetorheological energy absorbers?under impact conditions

Magnetorheological energy absorbers (MREAs) provide adaptive vibration and shock mitigation capabilities to accommodate varying payloads, vibration spectra, and shock pulses, as well as other environmental factors. A key performance metric is the dynamic range, which is defined as the ratio of the force at maximum field to the force in the absence of field. The off-state force is typically assumed to increase linearly with speed, but at the higher shaft speeds occurring in impact events, the off-state damping exhibits nonlinear velocity squared damping effects. To improve understanding of MREA behavior under high-speed impact conditions, this study focuses on nonlinear MREA models that can more accurately predict MREA dynamic behavior for nominal impact speeds of up to 6 m s−1. Three models were examined in this study. First, a nonlinear Bingham-plastic (BP) model incorporating Darcy friction and fluid inertia (Unsteady-BP) was formulated where the force is proportional to the velocity. Second, a Bingham-plastic model incorporating minor loss factors and fluid inertia (Unsteady-BPM) to better account for high-speed behavior was formulated. Third, a hydromechanical (HM) analysis was developed to account for fluid compressibility and inertia as well as minor loss factors. These models were validated using drop test data obtained using the drop tower facility at GM R&D Center for nominal drop speeds of up to 6 m s−1.

Shape memory alloy resetable spring lift for pedestrian protection

Pedestrian protection has become an increasingly important aspect of automotive safety with new regulations taking effect around the world. Because it is increasingly difficult to meet these new regulations with traditional passive approaches, active lifts are being explored that increase the crush zone between the hood and rigid under-hood components as a means of mitigating the consequences of an impact with a non-occupant. Active lifts, however, are technically challenging because of the simultaneously high forces, stroke and quick timing resulting in most of the current devices being single use. This paper introduces the SMArt (Shape Memory Alloy ReseTable) Spring Lift, an automatically resetable and fully reusable device, which couples conventional standard compression springs to store the energy required for a hood lift, with Shape Memory Alloys actuators to achieve both an ultra high speed release of the spring and automatic reset of the system for multiple uses. Each of the four SMArt Device subsystems, lift, release, lower and reset/dissipate, are individually described. Two identical complete prototypes were fabricated and mounted at the rear corners of the hood, incorporated within a full-scale vehicle testbed at the SMARTT (Smart Material Advanced Research and Technology Transfer) lab at University of Michigan. Full operational cycle testing of a stationary vehicle in a laboratory setting confirms the ultrafast latch release, controlled lift profile, gravity lower to reposition the hood, and spring recompression via the ratchet engine successfully rearming the device for repeat cycles. While this is only a laboratory demonstration and extensive testing and development would be required for transition to a fielded product, this study does indicate that the SMArt Lift has promise as an alternative approach to pedestrian protection.

Behavioral Model and Experimental Validation for a Spool-Packaged Shape Memory Alloy Actuator

Shape memory alloy (SMA) based actuators have the potential to be lower mass, more compact, and more simplistic than conventional based actuators (electrical, hydraulic, etc); however, one of the key issues that plagues their broad use is packaging since long lengths of wire are often necessary to achieve reasonable actuation strokes. Spooling the wire around pulleys or mandrels is one approach to package the wire more compactly and is useful in customizing the footprint of the actuator to the available application space. There is currently a lack of predictive models for actuator designs with spooled packaging that account for the variation of stress and strain along the wires length and the losses due to friction. A spooling model is a critical step toward the application of this technique to overcome the packaging limitations on SMA actuators. This paper presents the derivation of an analytical predictive model for rotary spooled SMA actuators that accounts for general geometric parameters (mandrel diameter, wire length, wire diameter, and wrap angle), SMA material characteristics, loss parameters (friction), and the external loading profile. An experimental study validated the model with good correlation and provided insight into the effects of load and wrap angle. Based upon the model and experimental results, the main limitation to this approach, binding, is discussed. The analytical model and experimental study presented in this paper provide a foundation to design future actuators and insight into the behavioral impact of this packaging technique.

Active Material Reversible Attachments: Shape Memory Polymer Based

We detail the joint concept generation and embodiment development by HRL and GMR&D of shape memory polymer (SMP) based reversible-on-demand attachments. In this initial study of active material enabled reversible-on-demand attachments, our primary focus was on hook-and-loop type fasteners. The approach followed, in broader context, was to incorporate an active material, defined as a material which changes a fundamental mechanical property upon exposure to an appropriate field, in at least one component of the hook-and-loop assembly, in this way allowing a field activated change in the stiffness (raising/lowering) and/or geometry (straightening of the hook) of the component and thus on-demand release of the attachment This paper describes the fabrication method and properties of one of the two principle classes of embodiments made during the development of the concept. This class of embodiments, which utilized thermally activated shape memory polymer materials, was shown to exhibit pull-off forces similar to conventional non-active hook-and-loop fasteners, and significantly, as desired, was reversible with a reduction in the pull-off force of a factor of ~100. This study was thus successful in demonstrating the feasibility of a thermally activated SMP based reversible-on-demand distributed attachment.

Stabilizing shape memory alloy actuator performance through cyclic shakedown: An empirical study

Shape memory alloy (SMA) wires are used increasingly in place of traditional actuators because of their compactness, high work density, low cost, ruggedness, high force generation, and relatively large strains. One well known issue with SMA wires is degradation in performance as actuation cycles accumulate, with significant reductions observed as soon as only tens or hundreds of cycles; thus, manufacturers typically recommend very conservative limits on the operation regime. This paper introduces an alternative approach of cycling or shaking down SMA wires under controlled conditions prior to installation. This enables the designer to design to the stable post-shakedown specification of the wire to produce actuators with repeatable larger forcing capabilities. This paper presents a preliminary experimental study which explores the functional dependence of shakedown performance on loading and strain history. A methodology is developed by which an SMA wire can be thermally cycled under electrical heating and the performance characterized with a double-exponential empirical model fit which captures the steady state performance of the wire and the rate at which shakedown occurs. Several sets of experiments are conducted to explore the functional dependence of the shakedown performance varying the load applied (29 to 78N), the allowed strain (4 to 7%), and the form of the loading function (linear spring vs. constant). These experimental studies expose important shakedown parameters affecting SMA actuator performance and provide a first step towards creating detailed SMA wire shakedown protocols tailored to the application that will enable the design of higher performance, stable SMA actuators.

Stress Analysis of Adhesively Bonded Electroprimed Steel Lap Shear Joints

Abstract Structural applications for adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for precise numerical modeling of adhesive joint behavior, particularly along bondline interfaces where low surface energy adhesives contact high surface energy metallic oxides. The purpose of the present study is to determine the effect of electrodeposited organic paint primer (ELPO) on the stress and strain distributions within an adhesively bonded single-lap-shear joint. Initial experimental studies have shown that bonding to ELPO-primed steel adherends has enhanced strength and durability characteristics compared to conventional bonds to unprimed steel surfaces. Recent studies based on finite element analysis of varied single-lap-shear joint moduli and thicknesses, and subsequent testing of joints with two different adhesive moduli, have indicated the mechanisms involved in...

Spool-packaging of shape memory alloy actuators: Performance model and experimental validation:

Shape memory alloy (SMA) actuators in the wire form are attractive because of their simplistic architecture and electrical operation, and their manufacturability at high yields and low cost. While SMA actuators are known for their superior work density among smart materials, packaging long lengths of SMA wire needed for moderate to large motions is an ongoing technical challenge. This article investigates spooling as a packaging approach to provide more compact actuator footprints. An analytical, quasi-static model is derived to provide a foundational tool for the analysis and synthesis of spool-packaged SMA wire actuators. The model predicts motion with respect to a generalized architecture, and specifiable geometric, material, and loading parameters. The model prediction accounts for the effects of local friction loss and bending strains, and for a “binding” limitation due to accumulated friction. An experimental validation study demonstrates the model’s ability to predict actuator motion well in terms of form and magnitude with respect to load and packaging geometry. This model provides a basis for a systematic application of spooled-packaging techniques to overcome packaging limitations of SMA, positioning SMA wire actuators as a viable alternative in many applications.

Adaptive SMA actuator priming using resistance feedback

Shape memory alloys (SMAs) are a group of alloys which demonstrate the unique ability of returning back to a previously defined shape or size if subjected to the appropriate thermal conditions. They have been implemented as actuators—where heat is controlled via applied current—in a wide range of applications spanning several fields such as robotics, aeronautics, automotive and medicine. SMA manufacturers specify what they refer to as the safe current which is the maximum current that can be applied to the SMA wire indefinitely without damaging it by overheating. However, this current is typically specified at room temperature under natural convection conditions. The objective of this work is to develop controllers for SMA actuators in automotive applications and this requires predictable and consistent functionality across a wide range of ambient temperatures, typically from − 40 to 85u2009°C. Consequently, applying the safe current in cold ambient temperatures may not actuate the SMA whereas it could potentially over-heat the SMA at high ambient temperatures. In this paper, we use a novel approach involving resistance feedback to achieve more consistent actuation across a range of ambient temperatures and compare experimental results for several different control strategies. The results show that controller designs using an adaptive current to actuate the SMA wire achieved more consistent results across the desired range of ambient temperatures compared to using the fixed safe current. Of these designs, a controller strategy dubbed Minus 4.5% achieved the most consistent actuation results and was a significant improvement over conventional control strategies.