Paul W. Alexander

Part orientation and build cost determination in layered manufacturing

As more choices of materials and build processes become available in layered manufacturing (LM), it is increasingly important to identify fundamental problems that underlie the entire field. Determination of best build orientation and minimizing build cost of a part are two such issues that must be considered in any LM process. By decoupling the solution to these problems from a specific LM technology, not only can the solution be applied to a variety of processes, but more realistic cost comparisons of parts built on different machines become possible.

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.

Layered manufacturing of surfaces with open contours using localized wall thickening

Sheet surfaces are commonly used in design to represent thin-walled surfaces. A method of manufacturing these surfaces via layered manufacturing (LM) using local wall thickening is presented. By considering the accuracy of only one side of the surface important, the need for support structured can be removed by thickened regions on the unimportant side. This preserves the accuracy of the accurate side while reducing the total material used and build time during manufacture. A method of selecting an orientation to maximize the accuracy of the important side while minimizing the amount of wall thickening is presented. Also, algorithms for locally thickening the surface and generating slices for manufacturing on a LM machine are developed. Finally, several examples are manufactured using the proposed method to compare material and build time savings over conventional support methods.

Modeling and Study of the Quasi-Static Behavior of Piezoceramic Telescopic Actuation Architectures

Piezoelectric stacks are often used in smart structures applications that demand large forces. However, there are numerous applications that require slightly more displacement than is available from stacks, and the performance requirements allow room for some force to be sacrificed to obtain this displacement. A new type of piezoelectric actuation architecture, referred to as telescopic, was designed to meet the need for moderate displacement amplification (up to 20 times) while still producing large forces. This architecture internally leverages the piezoelectric strain by a series of cascading shells uniquely connected by end caps such that the shells “telescope” out when activated. This paper presents an analytical model to predict the force-deflection behavior of this actuator with compliant end caps. To aid in evaluating this new architecture, the losses due to architectural features such as end caps, gap size, number of shells, and constant thickness/area are presented along with a comparison to the current state of the art.

Fabrication and experimental characterization of d31 telescopic piezoelectric actuators

A popular and useful piezoelectric actuator is the stack. Unfortunately with this type of actuation architecture the long lengths normally required to obtain necessary displacements can pose packaging and buckling problems. To overcome these limitations, a new architecture for piezoelectric actuators has been developed called telescopic. The basic design consists of concentric shells interconnected by end-caps which alternate in placement between the two axial ends of the shells. This leads to a linear displacement amplification at the cost of force; yet the force remains at the same magnitude as a stack and significantly higher than bender type architectures. This paper describes the fabrication and experimental characterization of three different telescopic prototypes. The actuator prototypes discussed in this paper mark a definitive step forward in fabrication techniques for complex piezoceramic structures. Materials Systems, Inc. has adapted injection molding for the fabrication of net shape piezoceramic actuators. Injection molding provides several advantages over conventional fabrication techniques, including: high production rate, uniform part dimensions, uniform piezoelectric properties, and reduced fabrication and assembly costs. Acrylate polymerization, developed at the University of Michigan, is similar to gelcasting, but uses a nonaqueous slurry which facilitates the production of large, tall, complex components such as the telescopic actuator, and is ideal for the rapid manufacture of unique or small batch structures. To demonstrate these fabrication processes a five tube telescopic actuator was injection molded along with a very tall three tube actuator that was cast using the acrylate polymerization method. As a benchmark, a third actuator was built from off-the-shelf tubes that were joined with aluminum end-caps. Each prototypes free deflection behavior was experimentally characterized and the results of the testing are presented within this paper.

Piezoceramic Telescopic Actuator Quasi-Static Experimental Characterization

Piezoceramic telescopic actuators capitalize upon an internally-leveraged amplification technique consisting of interconnected concentric, cascaded cylinders that telescope out when activated. The building-block nature of the telescopic design makes it an efficient, densely packed actuator that yields high work output for a given volume. As with any actuator, once fabricated, there are factors that lead to losses in performance that are not captured in most predictive models. To identify these and gain insight into the overall device behavior, this paper presents an experimental investigation of the quasi-static force-deflection performance. For a broader perspective, three unique telescopic prototypes were fabricated and experimentally tested, each manufactured using different techniques, materials, and geometries. As expected, discrepancies between the experimental and modeled behavior were observed. Therefore, to accurately predict the observed behavior of this architecture, a full three-dimensional numerical model was constructed for each prototype and was used to revise a previously derived analytical model accounting for the complex actuator behavior observed in the experiments and improving the characterization of the loss mechanisms in the telescopic actuation architecture.

The fabrication and material characterization of PZT based Functionally Graded Piezoceramics

Functionally Graded Piezoceramics (FGP) increase actuator lifetime and provide complex deformations; however, to reap these benefits sophisticated grading and fabrication techniques beyond the conventional layered bonding techniques are required. This paper introduces the Dual Electro/Piezo Property (DEPP) gradient technique via MicroFabrication through CoeXtrusion (MFCX). The Dual Electro/Piezo Property (DEPP) grading technique pairs a high displacement lead zirconate titanate (PZT) piezoceramic with a high permittivity barium titanate (BT) dielectric. These compatible materials act synergistically to form dramatic gradients in permittivity across the structure, concentrating the electric field in the more piezoelectrically active region leading to electrically-efficient, large-displacement actuators; with the benefit of increased reliability stemming from the continuous gradients and monolithic nature of the ceramic. The DEPP variation was first evaluated independently of the MFCX process through fabrication and experimental characterization of a powder pressed bimorph. While simple one-dimensionally graded FGPs can be realized by this process, MFCX is needed for any complex, multidimensional gradient. The MFCX process was adapted for DEPP grading and demonstrated by creating a more complex linearly-graded FGP. Both the bimorph and linearly graded specimens had good material quality and generated high displacements correlating well with published FGP theory; with the linear gradient reducing internal stress levels, extending actuator lifetime. This paper presents a general FGP methodology that couples grading and fabrication to generate high yield, low cost monolithic actuators with complicated one-dimensional gradients. Extension of this research will pave the way for more complicated gradients yielding such deformation capabilities as warping, twisting, rippling, and dimpling.

The design tradeoffs of linear functionally graded piezoceramic actuators

It is common practice to reduce the voltage level within piezoelectric actuators by utilizing multiple layers, typically bonded together. Unfortunately, this has a tendency to result in device failure due to delamination. For example, with benders the typical lifetime is 105 to 106 cycles, limiting its use in practical applications. This poses an interesting design tradeoff: the stroke is increased due to sharper gradients between material layers; however, the higher gradients lead to high stress concentrations at those interfaces. One approach to reducing these stresses is to grade the material properties through a monolithic piece of piezoceramic so that no interfaces or bonding elements exist, but this comes at the cost of stroke. This paper explores the design tradeoff inherent to monolithic functionally graded piezoelectrics. An analytical free-displacement model for a monolithic piezoceramic beam with a generic gradient is derived. Key to this is the inclusion of the complex electric field distribution which rises from the non-homogeneous material properties. This model is used along with finite element models to examine the effect of continuous linear and stepwise material gradients on the displacement performance as well as the stress levels. The study shows that using monolithic functionally graded piezocermics can significantly reduce the stresses with only a minor impact on the device stroke.© 2003 ASME

Fabrication and experimental characterization of d 31 telescopic piezoelectric actuators

Stacks are a popular form of piezoelectric actuation. Unfortunately with this type of actuation architecture the long lengths normally required to obtain necessary displacements can pose packaging and buckling problems. To overcome these limitations, a new architecture for piezoelectric actuators has been developed called telescopic. The basic design consists of concentric shells interconnected by end-caps which alternate in placement between the two axial ends of the shells. This leads to a linear displacement amplification at the cost of force; yet the force remains at the same magnitude as a stack and significantly higher than bender type architectures. This paper describes the fabrication and experimental characterization of three different telescopic prototypes. The actuator prototypes discussed in this paper mark a definitive step forward in fabrication techniques for complex piezoceramic structures. Materials Systems Inc. has developed an injection molding process that produced a functional, five-tube, monolithic telescopic actuator. A three-tube, monolithic actuator was fabricated using a novel polymerization technique developed at the University of Michigan. As a benchmark, a third actuator was built from off-the-shelf tubes that were joined with aluminum end-caps. Each prototypes free deflection behavior was experimentally characterized and the results of the testing are presented within this paper.

Experimental characterization and model validation of the quasi-static performance of piezoceramic telescopic actuators

The piezoelectric telescopic actuation architecture capitalizes upon an internally leveraged amplification technique to produce large actuation forces with amplified displacements. This building-block type actuator consists of interconnected concentric, cascaded cylinders with end cap joints that allow for a telescopic type motion. The internal amplification scheme and building-block nature of the telescopic design allow for efficient, densely packed actuators that yield a high work output for a given volume. This paper presents an experimental investigation of the quasi-static force-deflection performance of three unique telescopic prototypes, each manufactured by different means, from various materials, and in distinct geometries. To accurately predict the observed behavior of this architecture, a full three-dimensional numerical model was constructed for each prototype and was used to revise a previously derived analytical model. These models were refined to include extra compliance factors to account for observed actuation losses, focusing primarily on the bonding layer effects. The revised models captured more accurately the complex actuator behavior observed in the experiments and characterized better the loss mechanisms in the telescopic actuation architecture.