Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor – Automatic and/or material-triggered control
Reexamination Certificate
2000-03-08
2002-10-22
Yao, Sam Chuan (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Surface bonding means and/or assembly means therefor
Automatic and/or material-triggered control
C156S361000, C156S440000
Reexamination Certificate
active
06467521
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The control of vibrations in composite structures is an important area of research in aerospace, automotive and other industries. For example, spacecraft vibrations initiated by attitude adjusting thrusters, motors and thermally induced stresses inhibit accurate aiming of antennas and other equipment carried by the craft. Such vibrations can cause severe damage to the craft and its associated equipment. Fatigue failure of structural components can occur at stresses well below static load limits.
Composite materials have been used to construct a wide variety of structural elements, including tubes, enclosures, beams, plates and irregular shapes. Objects as diverse as rocket motor housings and sporting goods, notably skis, archery arrows, vaulting poles and tennis rackets have been structured from composite materials. While composite constructions have offered many significant advantages, such as excellent strength and stiffness properties, together with light weight, the poor vibration damping properties of such constructions have been of concern.
The invention relates to composite material structures having increased damping with little or no sacrifice of structural stiffness or strength.
The invention also relates to the methods and apparatus for manufacturing the aforementioned composite material structures.
Another aspect of the invention is directed toward the fabrication of a wavy fiber pregreg (fibers preimpregnated with epoxy resin). Such prepregs not only have an aesthetic appeal but also may be fabricated with selected variable volume fractions to accommodate a variety of applications.
2. Description of Related Art
The following terms used herein will be understood to have their ordinary dictionary meaning as follows:
Fiber: a thread or a structure or object resembling a thread. a slender and greatly elongated natural or synthetic filament. (This definition includes metal fibers).
Matrix: material in which something is enclosed or embedded.
Viscoelastic: having appreciable and conjoint viscous and elastic properties.
Lamina(e) a thin plate . . . : LAYER
Composite: made up of distinct parts.
It may be further helpful to note the definitions of G
0
and G
1
geometric continuity. “If two curve segments join together, the curve has G
0
geometric continuity. If the directions (but not necessarily the magnitudes) of the two segments' tangent vectors are equal at a join point, the curve has G
1
geometric continuity. G
1
continuity means that the geometric slopes of the segments are equal at the join point.” Foley et al.
Computer Graphics Principles and Practice
, Addison-Wesley, 1996, p. 480.
The article “Understanding Vibration Measurements,” George F. Lang, Sound and Vibration, March, 1976, p. 26, presents a generally accepted mathematical treatment of the vibration of mechanical structures under environmental loading. This treatment applies generally to composite structures, and is incorporated by reference for purposes of this disclosure for its explanation of the amplification factor Q and its relationship to the viscous damping factor &zgr;. The “loss ratio” referred to in this disclosure is twice the viscous damping factor as defined by Lang.
Conventional methods used to control the often destructive levels of vibration take many forms, from simple passive treatments to extensive redesign of structures. One of the simplest and often most effective passive damping treatments involves the use of thermo-visco-elastic (TVE) materials. TVE materials such as 3M's Scotchdamp series (ISD-112 is one example), exhibit dissipative qualities which make them useful in a number of passive damping treatments. Some of the first uses of TVE materials to increase structural damping involved the use of surface patches of aluminum foil and viscoelastic adhesives. These conventional approaches to surface damping treatments are called constrained or embedded-layer damping, and produce modest gains in damping over undamped structures.
One of the more common passive damping methods, “constrained layer damping” or CLD is discussed in the article “Damping of Flexural Waves by a Constrained Viscoelastic Layer,” Kerwin, Journal of the Acoustical Society of America, 1959, Vol. 31, Issue 7, pp. 952-962. According to Kerwin, CLD is achieved by bonding a thin layer of metal sheet, usually aluminum, to an existing structure with a viscoelastic adhesive. According to this technique, damping material, typically a viscoelastic material, is applied to the surface of a composite structure, such as an airplane wing. The damping material is sandwiched between the composite surface and a rigid layer, such as a thin aluminum sheet. This approach has generally been remedial in character and is accomplished at the sacrifice of other considerations, such as weight, aesthetics and ideal surface configuration. Shear strains are developed in the viscoelastic material when the original structure bends or extends. Damping occurs when the deformation of the viscoelastic adhesive creates internal heat in the viscoelastic material and dissipates energy.
Compared to an undamped structure, this approach is modestly successful but its effectiveness decreases markedly as the ratio of the thickness of the base structure to the thickness of the viscoelastic material increases. Thus, surface treatments alone cannot provide significant levels of damping to structural members where greater strength and stiffness are important. In the article “Use of Strain Energy Based Finite Element Techniques in the Analysis of Various Aspects of Damping of Composite Materials and Structures,” Hwang, et al., Journal of Composite Materials, 1992, Vol. 26, Issue 17, pp. 2585-2605, this problem was reported, and it showed that the advantage of aluminum foil viscoelastic constrained layer damping was eclipsed by the inherent damping in conventional composites when the required thickness of the structure exceeded about three tenths of an inch. The authors determined that a ±45° graphite/epoxy composite provided approximately uniform damping of about 1.5% in thick sections.
It is known that laminated beams composed of alternating layers of elastic and viscoelastic materials can dissipate vibratory energy while maintaining a degree of structural integrity. The article “Composite Damping of Vibrating Sandwich Beams,” DiTaranto, et al, Jour. of Engineering for Industry, November, 1967, p. 633, presents a theoretical description of such structures.
The feasibility of co-curing embedded layers of damping materials in a composite structure has been demonstrated. See, for example, Rotz, Olcott, Barrett, “Co-cured Damping Layers in Composite Structures,”
Proceedings
23rd
International SAMPE Technical Conference
, Vol. 23, pp. 373-387, 1991. Vibration control can thus be designed into a structure prior to its actual construction. Composite tubes have been constructed from a pair of concentric composite stiffness layers, the annular space between them being occupied by a damping layer. It is known that when fiber-reinforced materials are loaded along any axis not parallel or perpendicular to the fibers, shear deformations are induced. A tube with plies oriented “off-axis” (with respect to the central axis of the tube) will twist when loaded axially. With the plies of the two stiffness layers oriented at opposite but similar angles with respect to the tube axis, intense shear deformation is induced in the. damping layer when the tube is loaded axially. The stiffness of the tube remains high because the load passes only through the stiffness layers, and the damping layer adds little weight to the structure. Unfortunately, the most significant shear displacements occur at the free ends of the tube. Constraining the rotational deformations of the stiffness plies at either end of the tube eliminates any shear deformations in the damping layer at that end, thereby reducing the damping effect of the system. Complicated end fixtures are thus required to allow the requisite free end displa
Brigham Young University
Foley & Lardner
Yao Sam Chuan
LandOfFree
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