The invention relates in general to honeycomb assemblies formed of corrugated strips. In particular, the invention concerns honeycomb structures wherein the corrugated strips form I-beams within the open cells of the honeycomb structures and wherein the cells extend at an angle to perpendicular between the face plates.
Honeycomb material has come into increasing use in fields of application where both strength and light weight are important. Honeycomb structures commonly include a sandwich of two face sheets between which a cellular web is fixed to form a unitary structure. Typically, the web is fixed to the face sheets by welding, brazing, or some other similar process. The cellular web is composed of hexagonal or otherwise shaped cells formed by joining similarly configured corrugated strips. The strips are laterally displaced with respect to one another to form webs, as with natural honeycomb. Various materials are used for both the face sheets and the corrugated strips depending upon the application, including steel, aluminum, stainless steel, titanium, and various alloys, both common and exotic.
U.S. Pat. No. 4,632,862, which is commonly assigned, describes a dramatically improved honeycomb structure. That patent pertains to an I-beam honeycomb structure in which conventional honeycomb, having corrugated strips arranged to define, for example, six sided cells, is further formed by centrally depressing the major flat surfaces of the corrugated strips to form parallel sub-flat surfaces. The sub-flat surfaces are connected to the major flat surfaces by side walls which are perpendicular to both the major flats and the sub flats. Joining two or more such corrugated strips with alternate strips inverted, by bonding sub-flats to sub-flats and major flats to major flats, produces a honeycomb web which incorporates I-beams extending end-to-end through the web. The I-beams provide a tremendous degree of strength not found in conventional honeycomb.
One application in which honeycomb structures are frequently used is in the construction of aircraft components. For example, it is known that honeycomb core material assembled with a solid face sheet on one side and a perforated face sheet on the other side is useful for sound attenuation. When the perforated face sheet is exposed to engine exhaust, the honeycomb structure absorbs a portion of the sound being generated by the exhaust. One theory proposes that such a structure's ability to absorb sound is related to the relationship between the frequency of the sound and the depth of the cells of the honeycomb structure.
Varying the depth of the honeycomb cells requires varying the thickness of the entire structure. Accordingly, designing honeycomb structure for attenuation of sound can conflict with structural requirements.
An acoustic honeycomb structure is described in U.S. Pat. No. 3,821,999 in which cells are arranged between two face plates and are tilted relative to perpendicular to the plates. That patent proposes that the use of slanted cavities provides maximum absorption of high intensity pure tone and shock waves, in contrast to other structures which absorb low amplitude sound. It also states that angling the cells relative to the flow of jet engine exhaust enables the absorption of single frequency, slightly irregular sawtooth shock patterns such as are generated in the immediate vicinity of supersonic blade tips of a turbofan engine. In this manner, the patent states, low-frequency harmonic components of the shock pattern are not allowed to develop, which components would require for absorption an acoustic structure operable over a broad bandwith.
A problem with this known angled honeycomb structure, however, is that the angling of the honeycomb cells increases the structure's anticlastic nature. Accordingly, the structure pictured in the patent is in reality unsuitable for many applications because the material cannot be adequately shaped into the configurations typically required of sound attenuating structures.
Another proposed honeycomb structure for attenuating sound over a broad range of frequencies, includes core material which defines cells which taper outwardly at the solid face sheet. This solution has not been entirely satisfactory, however, and alternatives to known honeycomb structures are still being sought.
It is an object of the invention, therefore, to provide a honeycomb structure having increased strength and configuration flexibility. It is another object of the invention to provide a honeycomb structure in which cell depth can be varied independently of face sheet separation distance. Still another object of the invention is to provide a honeycomb structure having highly synclastic qualities.