Laminates that exhibit high and negative Poisson’s ratios can be used as solid-state actuators, p... more 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 ... more 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. ...
ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Volume 2, 2011
Morphing aircraft and other shape-changing structures are well suited to McKibben-like flexible c... more 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...
Smart Materials, Adaptive Structures and Intelligent Systems, Volume 1, 2008
Laminates that exhibit high and negative Poisson’s ratios can be used as solid-state actuators, p... more 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...
Volume 2: Mechanics and Behavior of Active Materials; Integrated System Design and Implementation; Bio-Inspired Materials and Systems; Energy Harvesting, 2012
ABSTRACT Cylindrical soft actuators efficiently convert fluid pressure into mechanical energy and... more 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.
It is desirable to obtain materials that have high vibration damping while maintaining other stru... more 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-ture... more 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 abilitie... more 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... more 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 ... more 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 milita... more 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...
Laminates that exhibit high and negative Poisson’s ratios can be used as solid-state actuators, p... more 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 ... more 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. ...
ASME 2011 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Volume 2, 2011
Morphing aircraft and other shape-changing structures are well suited to McKibben-like flexible c... more 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...
Smart Materials, Adaptive Structures and Intelligent Systems, Volume 1, 2008
Laminates that exhibit high and negative Poisson’s ratios can be used as solid-state actuators, p... more 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...
Volume 2: Mechanics and Behavior of Active Materials; Integrated System Design and Implementation; Bio-Inspired Materials and Systems; Energy Harvesting, 2012
ABSTRACT Cylindrical soft actuators efficiently convert fluid pressure into mechanical energy and... more 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.
It is desirable to obtain materials that have high vibration damping while maintaining other stru... more 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-ture... more 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 abilitie... more 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... more 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 ... more 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 milita... more 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...
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Papers by Larry Peel