David W. Jensen

The Response of Fiber-Reinforced Elastomers under Simple Tension

The mechanical behavior and basic response mechanisms of fiberreinforced elastomers (flexible composites) can be significantly different from those of typical advanced “stiff” composites. This paper presents experimental results of elastomer (rubber) matrices, dry and impregnated fibers, and four sets of fiber-reinforced elastomeric composite, summarizes the corresponding initial and nonlinear orthotropic constitutive properties, and sheds light on fundamental response mechanisms. Silicone and urethane rubber were combined with fiberglass and cotton reinforcing fibers. Balanced angle-ply laminates of each material system were fabricated with off-axis angles ranging from 0° to 90° in 15° increments. Dog-boned test specimens, 76 mm (3 in) long, were fabricated with fiber volume fractions ranging from 12% to 62% using a previously documented non-calendering fabrication method [1–3]. The average extensional stiffness of individual twisted cotton fibers increased 74% to 128% when impregnated with an elastomer. Fiber-reinforced elastomer laminate stiffness and nonlinearity can vary significantly with fiber angle. The nonlinear stiffening or softening trends of the silicone and urethane rubbers are reflected in their respective fiber-reinforced elastomers. Longitudinal stiffness at low off-axis angles is a function of the reinforcement stiffness. At high off-axis angles, longitudinal stiffness and strength are functions of fiber type, fiber volume fraction and elastomer.

On the structural efficiency of three-dimensional isogrid designs

This investigation explored the structural efficiency of a three-dimensional structural configuration which is based in part on concepts which are similar to those used to derive other efficient structural designs, such as the twodimensional isogrid and three-dimensional space truss. The three-dimensional isogrid truss system described in this paper has multiple symmetries based on repeating units which spiral around the major axis of the system. In this study, a numerical analysis was performed to investigate the structural efficiency of a three-dimensional filament wound, graphite/epoxy isogrid truss system which is subjected to various combinations of tensile, compressive, torsional, and/or bending loads. The results of this analysis clearly indicate the significant potential of three-dimensional advanced composite isogrid structures. A potential weight savings of up to 33 times has been demonstrated for a simple application, while simultaneously achieving increased stiffness. INTRODUCTION The drive to eliminate unnecessary weight in structures transcends from the world of aerospace to the arena of civil infrastructure. The introduction of advanced composite materials coupled with the development of novel structural configurations has enabled much more efficient load transfer in a variety of both simple and complex structures. In particular, composite isogrids developed by government sponsored research yielded highly efficient two-dimensional designs which place the material where it provides the greatest benefit for carrying in-plane loads [1]. In these designs, the skin works in tandem with the integral diagonal stiffeners to carry both the axial and in-plane shear loads in a pseudo-isotropic fashion, with sufficient bending rigidity to prevent local buckling. Unfortunately, although composites can be easily manufactured into virtually any shape or form, the cost of producing high-quality isogrids is still too high to be practical for many two-dimensional applications. This preliminary investigation has explored the structural efficiency of a three-dimensional structural configuration which is based in part on concepts which are similar to those used to derive other efficient structural designs, such as the twodimensional isogrid and space trusses. Like a typical simple truss system, the three-dimensional isogrid design incorporates primarily axial force members which are oriented at angular intervals (such as 30 and 60 degrees) to form stable triangular cells. The three-dimensional isogrid system, however, has multiple symmetries based on repeating units which spiral around the major axis of the system. DESCRIPTION OF SYSTEM The basic three-dimensional isogrid system (see Figure 1) consists of six equally-spaced diagonal members spiraling clockwise about the longitudinal axis, another six equally-spaced diagonal members spiraling counter-clockwise about the longitudinal axis, and another six equally-spaced longitudinal members passing through the intersections of the counter-rotating pseudo-helixes (the basic structure is superimposed over a cylinder Associate Professor, Associate Fellow, AIAA Graduate Student, Student Member AIAA Copyright

Hydrodynamic Drag Measurements of Advanced Composite IsoTruss® Lattice Structures

Hydrodynamic drag testing was conducted on eleven different configurations of advanced composite IsoTruss® lattice structures. The specimen test matrix included 6-node and 8-node configurations, single and double-grid geometries, thick and thin member sizes, smooth and rough surface finishes, a helical-only structure, and a smaller outer diameter test specimen. Orientation drag tests measured the hydrodynamic drag force of the IsoTruss test specimens at five different orientations with Reynolds number ranging from 7,000 to 80,000. Height variation drag tests were performed at four different immersed height levels, which corresponded to a Froude number ranging from 0.40 to 0.90. The IsoTruss specimens exhibited an average lower drag coefficient based on the projected cylindrical area than the smooth circular cylinder data throughout the Reynolds number and Froude number ranges. The drag coefficient based on solid member area collapsed to a second-order polynomial when presented as a function of the Froude number, with a correlation coefficient of 0.99. Nomenclature Ac = total projected area (height*diameter) defined by outer-most dimensions of IsoTruss structure Am = projected area of IsoTruss members only CD = drag coefficient based on total cylindrical projected area φ = solidity ratio

Enhanced damping of hat-stiffened panels using continuous wave fiber composites

A unique composite material called Continuous Wave Fiber Composite (CWFC) or wavy composite has shown great promise in improving damping properties of composite structures. In wavy composites, the fiber is oriented in a continuous sine wave which produces a varying fiber angle. This new material has exhibited high levels of damping when two layers, with wave patterns 180 degrees out of phase, surround a layer of viscoelastic material. This research investigated the acoustic transmission loss and flexural damping of hat-stiffened panels produced with graphite/epoxy wavy composite material. The 22- panel test matrix included sixteen exploratory panels used to determine the most highly damped design, four optimized panels based on the best exploratory design, and two control panels including one panel without CWFC and another without VEM or CWFC. The panels were tested to quantify the acoustic transmission loss and flexural damping under free-free boundary conditions. Hat-stiffened panels produced with graphite/epoxy wavy composites provide 17% higher damping than constrained layer damping and slightly higher transmission loss over panels made with conventional unidirectional materials.

Influence of Shear Strains on the Phase of Light Transmitted Through Single-Mode Fiber-Optic Strain Sensors

Since the well-known demonstration of a fiber-optic strain gage by Butter and Hocker in 1978, significant refinements have been made in the area of fiber optic sensing, enabling the measurement of many different physical quantities, including strain, displacement, linear and circular acceleration, temperature, degree of cure in plastics, chemical compositions, pressure, acoustic waves, and fluid flow rates. Both analytical and experimental efforts have contributed to our current understanding of the relationship between the elongation of a host medium and phase changes in the light passing through an optical fiber. This paper describes research which partially fills in the remaining gap by quantifying the influence of shear strains on the phase change of light passing through an embedded optical fiber. In this experiment, optical fibers were embedded in 18-inch long by 2.25-inch diameter composite tubes. Three tubes were fabricated with axial fibers and one with a helical fiber, using a hand layup fabrication technique. These tubes were also instrumented with two strain gage rosettes. The tubes were subjected to pure torsional loads while the surface strains and the fiber-optic phase changes were measured. A modified all-fiber Mach-Zehnder interferometer with active homodyne feedback was used to determine the phase changes in the optical fibers due to the applied strains. The phase changes were also predicted using fundamental concepts of structural mechanics and existing phase-strain models.