Stock material or miscellaneous articles – Structurally defined web or sheet – Including variation in thickness
Reexamination Certificate
1999-02-25
2001-12-25
Loney, Donald J. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including variation in thickness
C428S053000, C428S061000, C428S112000, C428S113000, C428S161000, C428S162000, C428S163000
Reexamination Certificate
active
06333092
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
The present invention relates to composite structures and to methods and apparatuses pertaining to same, more particularly wherein the composite structures are to some degree or in some respect characterized by lamination.
Many composite structures include layers which are bonded together. Various applications have given rise to concern about delamination resistance at either or both of primary bond sites and secondary bond sites. The term “delamination resistance” is conventionally understood to encompass “strength” (e.g., through-thickness tensile strength) and/or “toughness” (e.g., Mode I fracture toughness). The terms “through-thickness strength,” “out-of-plane strength” and “interlaminar strength” are synonymous in conventional usage.
Improvement of the delamination resistance of composite laminates has been attempted through a variety of mechanisms. Among the known mechanical methodologies for increasing delamination resistance are the following: (i) the insertion of metal pins, stitches or fibrous rods through the thickness of the composite laminate; and, (ii) the alteration of the style of reinforcement, e.g., through utilization of tufted fabrics to improve adhesion. There are drawbacks associated with these mechanical methodologies, such as cost, degradation of mechanical properties in the plane of the laminate, etc. Another conventional methodology for enhancing delamination resistance involves toughening of brittle resins with particles made of rubber (or another high elongation material); according to these approaches, toughness is generally achieved at the expense of strength.
It is often desirable to improve both strength and toughness, for the ability to do so could delay both crack initiation and crack propagation in composite laminates. Furthermore, any improvements in through-thickness strengths in composite laminates can be viewed as advantageous, since their low strengths in that direction are usually the limiting factor in design of structures with composites. Moreover, through-thickness strength is normally very sensitive to quality; thus, improvements in toughness could minimize the flaw sensitivity of the through-thickness strength. This is significant particularly because through-thickness stresses tend to arise in structural details which are difficult to fabricate at the level of quality of flat panels.
Composite structural details for U.S. Navy marine applications frequently require the use of secondary bonds for fabrication in a shipyard environment. Secondary bond sites are interfaces where there has been lamination over a cured laminate, and they can represent a weak link in composite laminate performance. The typical microstructural appearance of a secondary bond is a discrete, linear resin-rich region between the layers of a composite laminate. This resin-rich region can result in a composite laminate with reduced strengths through-the-thickness of the laminate (i.e., normal to the secondary bond) and reduced resistance to delamination.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a composite structure, and method and apparatus for fabricating same, wherein the composite structure has superior performance in terms of delamination resistance.
It is a further object of the present invention to provide such composite structure, method and apparatus wherein the delamination resistance includes either or both of through-thickness strength and fracture toughness.
Another object of this invention is to provide such composite structure, method and apparatus wherein the improvement of delamination resistance with respect to toughness does not result in the worsening of delamination resistance with respect to strength, or vice versa.
A further object of this invention is to provide such composite structure, method and apparatus wherein the improvement of delamination resistance does not result in the worsening of a mechanical property unrelated to delamination resistance.
Another object of this invention is to provide such composite structure, method and apparatus which are cost-effective.
The present invention features the effectuation of a fractal form of disordered geometry at a composite lamina interface. Fractal geometry is advantageous (vis-a-vis' non-fractally disordered geometry) because it represents a reproducible and simplified mathematical regime for introducing geometric disorder. The disordered interface geometry—and specific characteristics associated therewith—are inventively related to specific mechanical or material properties such as through-thickness strength and fracture toughness. According to this invention, fractal topology is not only related to certain material/mechanical properties, but is also used to selectively enhance particular material/mechanical properties. In particular, fractal interfaces in composite laminates are inventively used as strengthening and/or toughening mechanisms.
In accordance with this invention, a composite structure comprises a first lamina and a second lamina. The first lamina has a first laminal surface which defines a first laminal fractal profile. The second lamina has a second laminal surface which defines a second laminal fractal profile. The second laminal fractal profile is complementary with respect to the first laminal fractal profile. The first laminal surface and the second laminal surface join so as to form an interface which defines an interfacial fractal profile. The interfacial fractal profile is described by the engagement of the first laminal fractal profile and the second laminal fractal profile.
Also in accordance with this invention, a method for making a composite structure comprises: providing a metal mold; resin transfer molding a first lamina; and, resin transfer molding a second lamina. The metal mold has a mold surface which defines a mold fractal profile. The first lamina has a first laminal surface which defines a first laminal fractal profile which is effected by the mold fractal profile. The second lamina has a second laminal surface which defines a second laminal fractal profile which is effected by the first laminal fractal profile.
The present invention admits of embodiments wherein there is secondary bonding of the first lamina and the second lamina, as well as embodiments wherein the first lamina and the second lamina are joined in the absence of secondary bonding. When secondary bonding is implemented, the inventive composite structure comprises a secondary bond layer which at least substantially occupies the fractally profiled interface between the first lamina and the second lamina. The inventive fabrication method can thus include secondarily bonding the second laminal surface with respect to the first laminal surface, in association with the resin transfer molding of the second lamina.
The following papers, hereby incorporated herein by reference, disclose relationships between various forms of microstructural disorder and improvement in various macroscopic properties:
Chen, Z. and Mecholsky, Jr., J. J. September 1993. “Control of Strength and Toughness of Ceramic/Metal Laminates Using Interface Design.”
Journal of Materials Research
8(9):2362-2369;
Tancrez, Jean-Pierre, Pabiot, Jose and Rietsch, Francois. 1996. “Damage and Fracture Mechanisms in Termoplastic-Matrix Composites in Relation to Processing and Structural Parameters.”
Composites Science and Technology
56:725-731;
Zumbrunnen, D. A. 1997. “Microstructures and Physical Properties of Composite Materials Evolved from Chaos.” Proceedings of the
Fourth Experimental Chaos Conference, Aug.
6-8, 1997, Boca Raton, Fla.
Tancrez et al. disclose improved ductility in toughened polymers where small, non-propagating crases formed a stable “micronet” through which a dominant crack would ha
Gipple Karin L.
Karr Dale G.
Salvino Liming W.
Kaiser Howard
Loney Donald J.
The United States of America as represented by the Secretary of
LandOfFree
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