Larry Peel

Laminates that exhibit high and negative Poisson’s ratios can be used as solid-state actuators, passive and active vibration dampers, and for morphing aircraft structures. Recently, fiber-reinforced elastomer (FRE) laminates have been fabricated that exhibit extreme (high and negative) Poisson’s ratios [1]. The current research explores twisted fiber bundle elastomeric laminates (both single and double helix) which are being investigated using experimentation, linear and non-linear finite element analysis (FEA). Twisted fiber bundles can be made from carbon fibers, fiberglass, etc, but for simplicity the current work uses twisted cotton string. It is observed that uniaxial fiber-reinforced elastomer laminates, where the fibers are twisted as shown in Figure 1, exhibit stress stiffening. Negative Poisson’s ratios may be produced if the fiber bundles have a double helical path as simulated by a series of laminated tubes. Future auxetic FRE laminates may be developed that do experience extreme shear.

Fasteners are used in every industry, and in virtually every component. The ideal fastener would be relatively easy to insert or push into a hole, but take much more force to remove or pull out of the hole. Such fasteners would not require nuts screwed onto bolts, or other retaining rings. The current work attempts to develop a simple fastener with a low Insertion Force / Removal Force ratio by exploiting auxetic behavior. Auxetic materials or negative Poisson’s Ratio materials have properties that are counter-intuitive. A rubber band, for example, becomes thinner in width when pulled lengthwise Auxetic materials, however, will expand in width, when pulled along their length, or when compressed along their length, will also contract in width. Hence, normal materials have positive Poisson’s Ratios while auxetic materials are revealed to have negative Poisson’s Ratio. By exploiting the theory behind negative Poisson’s Ratios, a suitably designed fastener can exhibit auxetic behavior. ...

Morphing aircraft and other shape-changing structures are well suited to McKibben-like flexible composite actuators. These actuators, made from fiber-reinforced elastomeric composites, are extremely efficient in converting potential energy (pressurized air) into mechanical energy. Such actuators are promising for use in micro air vehicles, prosthetics and robotics because they offer excellent force-to-weight ratios and behave similar to biological muscle. Use of an incompressible pressurizing fluid instead of compressible air may also offer higher actuator stiffness, better control, and compatibility with existing actuation systems. Using incompressible fluids also allows the actuator to serve as a variable stiffness element which can be modulated by opening and closing valves that constrain or allow fluid flow. The effect of an incompressible fluid (water) on the performance of Rubber Muscle Actuators (RMA), with varying diameters, lengths and segment lengths, was experimentally in...

Laminates that exhibit high and negative Poisson’s ratios can be used as solid-state actuators, passive and active vibration dampers, and for morphing aircraft structures. Recently, fiber-reinforced elastomer (FRE) laminates have been fabricated that exhibit extreme (high and negative) Poisson’s ratios [1]. The current research explores twisted fiber bundle elastomeric laminates (both single and double helix) which are being investigated using experimentation, linear and non-linear finite element analysis (FEA). Twisted fiber bundles can be made from carbon fibers, fiberglass, etc, but for simplicity the current work uses twisted cotton string. It is observed that uniaxial fiber-reinforced elastomer laminates, where the fibers are twisted as shown in Figure 1, exhibit stress stiffening. Negative Poisson’s ratios may be produced if the fiber bundles have a double helical path as simulated by a series of laminated tubes. Future auxetic FRE laminates may be developed that do experience...

There is great interest in making shape-changing aircraft structures that are more biomimetic. Cylindrical McKibben-like flexible actuators efficiently convert fluid pressure into mechanical energy and thus offer excellent force-to-weight ratios while behaving similar to biological muscle. McKibben-like Rubber Muscle Actuators (RMAs) were embedded into elastomer panels. The effect of actuator spacing on the performance of these shape-changing panels was investigated. The work included nonlinear finite element analysis, fabrication, and testing of panels where four RMAs were spaced side-by-side, 1/2, 1, and 1.3 RMA diameters apart.Nonlinear “Laminated Plate” and “Rod & Plate” finite element models of individual RMAs were created from existing RMA dimensions. After adjusting for an initial “activation pressure,” the models produced realistic RMA forces. The laminated plate models used less computer resources, but only produced small amounts of actuator contraction (actuator strain). The more resource-intensive Rod & Plate models better replicated fiber/braid re-orientation and produced axial strains up to 60% of test values. Three types of embedded RMA panel FEA models; a “2D Cross-Section,” a “Full 3D Panel” (with either Laminated Plate or Rod & Plate RMAs) and a “3D Unit Cell” (also with either Laminated Plate or Rod & Plate RMAs). The Full 3D Rod & Plate model gave the most accurate strains and forces, but required unsustainable levels of computing resources. The 2D cross-section model predicted optimal RMA spacing to be at 1 diameter. All other FEA models show optimal panel performance between 1/2 and 1 diameter spacing.Panels with embedded RMAs were fabricated and tested with air or water pressure. Panel force as a function of pressure and as a function of contraction (strain) was obtained.Overall, FEA and test results for panels indicate that optimal performance occurs when the RMAs are spaced between 1/2 to 1 diameter apart. Actuator force as a function of spacing is fairly flat in this region, indicating that minor design or manufacturing differences may not significantly affect performance. However, the total amount of axial contraction decreases significantly at greater than optimal spacing. Useful design, simulation, and test methodologies for embedded RMA panels have been demonstrated.Copyright © 2012 by ASME

ABSTRACT Cylindrical soft actuators efficiently convert fluid pressure into mechanical energy and thus offer excellent force-to-weight ratios while behaving similar to biological muscle. McKibben-like rubber muscle actuators (RMAs) were embedded into neat elastomer and act as shape-changing panels. The effect of actuator spacing and modeling methods on the performance of these panels was investigated. Simulations from nonlinear finite element models were compared with results from test panels containing four RMAs that were spaced 0, 1/2, 1, and 1.3 RMA diameters apart.

ABSTRACT Elastomer-matrix composites show promise for high Poisson's ratio and negative Poisson's ratio (auxetic) applications due to high orthotropy. There are approximately five orders of magnitude between the axial stiffness of high modulus graphite fibres and the stiffness of low durometer elastomers. Although the maximum Poisson's ratio for isotropic elastomers is 0.5, it is easily shown that inplane Poisson's ratios twice unity can be obtained with a graphite/epoxy angle-ply laminate at 25°. Chou and others have predicted inplane Poisson's ratios greater than 7 for certain cord-rubber combinations. Peel previously predicted inplane Poisson's ratios higher than 32 and less than –60. Preliminary experimental results have produced inplane Poisson's ratios as high as 14. Certain combinations of un-balanced highly orthotropic laminates may also produce inplane negative Poisson's ratios. Inplane Poisson's ratios, experimentally obtained from two laminate configurations with approximately the same axial stiffness are compared. A symmetric, balanced laminate produced an inplane Poisson's ratio of 3.7; while an unbalanced laminate with an equivalent axial stiffness produced an average inplane Poisson's ratio of –1.5. Certain high and negative Poisson's ratio elastomer-matrix laminates appear to have quasi anti-symmetric relationships about 0°, although laminate designers may consider dual angle-ply laminates to be the more likely counterpart to the unbalanced dual angle auxetic laminates. Unbalanced symmetric laminates experience significantly more inplane shear deformation when axially loaded than their balanced counterparts. The high shear may be desirable for damping applications, but less desirable otherwise. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is desirable to obtain materials that have high vibration damping while maintaining other structural properties. Aerospace quality composite materials typically have greater strength and stiffness to weight ratios than most metals, while providing increased damping. However, their passive damping capacities are not enough for some applications. Fiber reinforced elastomers (FRE) or elastomer composites are receiving attention because of the ability to tailor their vibration damping, strength, fracture toughness, and elongation to the range needed. The current work considers a series of aluminum panels that are laminated with "skins" of graphite cloth, fiberglass cloth, and with chopped fiberglass mat. The reinforcements are impregnated with a series of polyurethane elastomers that range from flexible to rigid in hardness. For a given type of fiberglass, the lower durometer elastomer produced the highest loss factor. For a given elastomer stiffness, the stiffest reinforce...

Accurately predicting the response of fiber-reinforced elastomer or flexible composite struc-tures can be improved by the addition of material, geometric and fiber-rotation nonlinear models to classical laminated plate theory. Material nonlinearity is included in the form of nonlinear orthotropic material properties as functions of extensional strain. Nonlinear properties were obtained from the experimental results of fiber-reinforced elastomeric (FRE) angle-ply specimens at 0°, 45°, and 90 ° discussed in Reference [xx]. Axial stiffness and Poisson’s ratio are considered constant. Geometric nonlinearity is removed from the transverse and shear stiffnesses. A six-coefficient Ogden model was chosen to represent the nonlinear stiffnesses. Geometric nonlinear-ity is included through the addition of nonlinear extensional terms from the Lagrangian strain ten-sor. The nonlinear strain-displacement relations and the nonlinear material models were added to the code of a pre-existing composit...

Fiber-reinforced elastomer composites are receiving increased attention because of their abilities to undergo large elastic deformation, reduce vibration, and absorb high shock loads. Additional applications may be possible because of the highly orthotropic nature of fiber-reinforced elastomers (elastomer composites). It is well known that the maximum Poisson’s ratio for isotropic materials is 0.5 as found in elastomeric incompressible materials. However, it is easily shown that Poisson’s ratios twice unity can be obtained with a graphite/epoxy angle-ply laminate at about 25°. Chou predicted Poisson’s ratios as high as 7 for certain elastomer composite combinations. Peel used four experimentally obtained material combinations to show Poisson’s ratios as high as 32. However the very nature of high Poisson’s ratios caused specimen gripping and testing problems. In the current work, preliminary experimental results have produced Poisson’s ratios as high as 14, and results from new conf...

The present work deals with modes and mechanisms of failure in compression of angle-ply laminates. Experimental results were obtained from 42 angle-ply IM7/8551-7a specimens with a lay-up of ((plus or minus theta)/(plus or minus theta)) sub 6s where theta, the off-axis angle, ranged from 0 degrees to 90 degrees. The results showed four failure modes, these modes being a function of off-axis angle. Failure modes include fiber compression, inplane transverse tension, inplane shear, and inplane transverse compression. Excessive interlaminar shear strain was also considered as an important mode of failure. At low off-axis angles, experimentally observed values were considerably lower than published strengths. It was determined that laminate imperfections in the form of layer waviness could be a major factor in reducing compression strength. Previously developed linear buckling and geometrically nonlinear theories were used, with modifications and enhancements, to examine the influence o...

High hardness ceramics are commonly used in lightweight armor systems to defeat the intrusion of high-speed armor piercing (AP) projectiles. However, bare ceramic tiles are intrinsically brittle, and fragments of various sizes can be generated when subjected to impact, which can cause secondary impact to the wearer and surroundings. In a typical armor design, the ceramic tile is usually covered with a compliant thin sheet to mitigate fragments splattering. In this study, the effect of a Kevlar-29 composite cover layer on a bilayer ceramic/composite armor system is investigated through a combined approach of experimental testing and finite element (FE) simulation. In the experiments, 7.62 mm APM2 projectiles were used to impact single Kevlar-29 composite layer covered ceramic/composite armor systems at three different velocities: 884 m/s, 1070 m/s and 1164 m/s. The restraining effect of the cover layer on the ceramic fragments was clearly observed. A 3D FE model was further developed...

There is a continuing need for better fracture and impact-resistant materials in civil and military applications. This work characterizes the impact resistance and fracture toughness of several fiber-reinforced polyurethane composites. A series of Charpy impact tests were conducted to measure the impact strength and specific impact strength for five combinations of fiber reinforced elastomer composites, and epoxy matrix composites. The elastomers covered a range of durometers (hardness). Impact strength and specific impact strength of the composites were also compared to results from aluminum and steel specimens. For the same fiber type, all elastomer composites had greater impact strength than the epoxy composite. When specific impact energies are considered, the intermediate and rigid elastomer composite specimens have greater impact resistance than the baseline metal specimens and compared favorably with hot rolled 4140 steel. The fiberglass and intermediate hardness elastomer sp...

In this study, 3D printed magnetorheological (MR) elastomer has been characterized through a force vibration testing. The 3D printed MR elastomer is a composite consisting three different materials, magnetic particles and two different elastomers. The MR elastomers were printed layer-by-layer by encapsulating MR fluid within the polymeric elastomer and then allowed to cure at room temperature. The 3D printing allowed to print various patterns of magnetic particles within the elastomeric matrix. In the presence of an external magnetic field, both elastic and damping properties of the 3D printed MR elastomers were changed. Natural frequency, stiffness, damping ratio, damping coefficient, and shear modulus were increased with increasing magnetic field. For the single degree-of-freedoms system, shear mode MR elastomers suppressed the transmitted vibration amplitude up to 31.4% when the magnetic field was 550 mT. The results showed that the 3D printed MR elastomer could be used as a tuna...

Heightened interest in flexible (elastomeric) composite applications such as bio-mechanical devices, flexible underwater vehicles, and inflatable space structures highlight the need of improved fabrication techniques for fiber-reinforced elastomeric materials (FRE). Previous methods have generally been limited to a fairly low percentage of fibers in an elastomeric matrix, or used calendering manufacturing methods that are not generally suitable for non-tire fiber-rein-forced elastomeric composites applications. Other researchers have noted problems with fiber-elastomer adhesion. The current work demonstrates a method for making small batches of high quality fiber-reinforced elastomer pre-preg. Strengths of the current work include excellent fiber adhesion, medium to high fiber volume fractions, highly parallel fibers, use of traditional advanced composites fabrication methodologies, and reproducible ply thicknesses. The method combines standard techniques of filament winding, wet la...

The present work deals with modes and mechanisms of failure in compression of angle-ply laminates. Experimental results were obtained from 42 angle-ply IM7/8551-7a specimens with a lay-up of ((plus or minus theta)/(plus or minus theta)) sub 6s where theta, the off-axis angle, ranged from 0 degrees to 90 degrees. The results showed four failure modes, these modes being a function of off-axis angle. Failure modes include fiber compression, inplane transverse tension, inplane shear, and inplane transverse compression. Excessive interlaminar shear strain was also considered as an important mode of failure. At low off-axis angles, experimentally observed values were considerably lower than published strengths. It was determined that laminate imperfections in the form of layer waviness could be a major factor in reducing compression strength. Previously developed linear buckling and geometrically nonlinear theories were used, with modifications and enhancements, to examine the influence o...

ABSTRACT Artificial or “bionic” limbs have been the subject of considerable research, TV shows, and dreams by children. The “Six Million Dollar Man” show was about a man who received artificial limbs after his own were lost in an accident. To get students interested in practical engineering, the current work showcases a simple artificial arm that produces greater force than a typical man, demonstrates the capability of Rubber Muscle Actuators (RMA), and provides a portable “arm wrestling platform” for student recruitment efforts. The actuators for “Kingsville Arm One & Two” are McKibben-like actuators made from fiber-reinforced elastomeric composites. These actuators offer excellent strength-to-weight ratios and contract similar to a human muscle. RMAs produce greater force and have less “blow-outs” than typical McKibben actuators because of optimized braid angles and ends that transfer loads through the braid fibers. Kingsville Arm One (KA1) was developed in just two weeks. It consisted of carbon/fiberglass/epoxy composite tubular bones, a metal clevis “elbow” and four RMAs. With considerable effort, a very large student was able to overcome the force generated in an “arm wrestling” contest. KA1’s actuators had end attachments that transferred loads well and enabled flexibility, but easily tore and had air leaks. Kingsville Arm Two (KA2) had new “bones” and RMAs. Although slightly smaller diameters, the KA2 RMAs produced comparable forces to the KA1 RMAs and had molded end attachments. The rigid ends did not allow as much rotation as expected and necessitated using just 2 RMAs. With only two RMAs, KA2 produced approximately the same “arm strength” as KA1. Future work will focus on flexible but durable RMA molded ends, life-like skins and a realistic “hand.”

Elastomer-matrix composites show promise for high Poisson's ratio and negative Poisson's ratio (auxetic) applications due to high orthotropy. There are approximately five orders of magnitude between the axial stiffness of high modulus graphite fibres and the stiffness of low durometer elastomers. Although the maximum Poisson's ratio for isotropic elastomers is 0.5, it is easily shown that inplane Poisson's ratios twice unity can be obtained with a graphite/epoxy angle-ply laminate at 25°. Chou and others have predicted inplane Poisson's ratios greater than 7 for certain cord-rubber combinations. Peel previously predicted inplane Poisson's ratios higher than 32 and less than –60. Preliminary experimental results have produced inplane Poisson's ratios as high as 14. Certain combinations of un-balanced highly orthotropic laminates may also produce inplane negative Poisson's ratios. Inplane Poisson's ratios, experimentally obtained from two laminate configurations with approximately the same axial stiffness are compared. A symmetric, balanced laminate produced an inplane Poisson's ratio of 3.7; while an unbalanced laminate with an equivalent axial stiffness produced an average inplane Poisson's ratio of –1.5. Certain high and negative Poisson's ratio elastomer-matrix laminates appear to have quasi anti-symmetric relationships about 0°, although laminate designers may consider dual angle-ply laminates to be the more likely counterpart to the unbalanced dual angle auxetic laminates. Unbalanced symmetric laminates experience significantly more inplane shear deformation when axially loaded than their balanced counterparts. The high shear may be desirable for damping applications, but less desirable otherwise. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Compact actuation that is integrated into a structure's material system has the potential to provide rapid structural reconfiguration while reducing weight. The effect of scale (diameter, overall length and segment length) on the performance of cylindrical fiber-reinforced McKibben-like Rubber Muscle Actuators (RMA) was investigated. An "activation" pressure was observed for all actuators at a value that depended upon the actuation construction. Upon pressurization past the activation threshold, the overall force, stroke, and work capacity increased with increasing actuation length and diameter. The actuation force per unit RMA cross-sectional area was predicted, and experimentally observed, to be roughly constant after activation. By segmenting a longer actuator, a larger contraction and lower actuation force could be achieved. Though actuation forces decreased as actuator diameter and length decreased, the force per unit actuator volume was shown to increase with decreasing diameter including a roughly 4-fold increase in force/volume between the 0.5" and 0.05" actuators. However, due to the small amount of total contraction for the smaller diameter actuators, the relative work per actuation volume was decreased by roughly 35% in comparing those same actuators. Thus, small diameter RMAs have great potential to provide needed linear actuation force within adaptive material systems.