Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-06-28
2001-01-30
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06178825
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of material testing. More particularly the present invention relates to the testing of the mechanical properties of composite materials, such as honeycomb composites.
BACKGROUND OF THE INVENTION
Honeycomb structures have been used in many structural applications. Honeycombs have been widely used in both aircraft and aerospace industries to reduce weight while maintaining mechanical strength. For example, honeycombs are used in cargo floors in commercial aircraft, are used as skin panels of military aircraft wings, are used in payload shrouds on space launch vehicles and are used as optical structures in spacecraft systems. Honeycomb structures are also used as energy absorbers such as barriers in automobile crash tests. The energy absorption is based on the work done through elastic buckling, plastic yielding, and brittle crushing of the cell walls.
In analytical modeling, such as finite element analysis (FEA), honeycomb structures are generally assumed to be orthotopic. Material properties such Young's moduli, shear moduli, and Poisson's ratio, and inelastic characteristics, yield strength, and the failure strength or strain are needed as input for these models. All the mechanical property information must be accurately determined and provided as input for the computer codes before any reasonable prediction can be obtained using the codes.
At the present time, only limited material property information for honeycomb is available. Such data as is available is primarily simple compression data because compression testing is simple and easy to implement. Very little data is available for shear or tension loading, in particular, at high strain rates. When shear or tension data are available, they are limited to modulus measurements in the elastic range, or to incipient yielding. At higher tension or shear loads, the honeycomb separates at the bonding interface with the face sheet, making it impossible to determine the tension and shear properties of the honeycomb structure itself. Thus it is difficult to address the overall constituent behavior of honeycomb structures. For completely assessing the failure of a honeycomb structure, data from simple tension, simple compression, simple shear, and various combined stress fields are needed. The fact that little of the needed data is available for analytical prediction has established a high need to generate these data experimentally.
Hexcel Corporation has long provided data on honeycomb structures. An exemplar configuration of an aluminum honeycomb, designated as 1/4-5052-0.004-7.9, has a cell size of 0.25 inches, is made from 5052 aluminum, has a wall thickness of 0.004 inches and a density 7.9 lb per cubic foot. The width direction shear strength data of this Hexcel honeycomb ranges from 390 to 440 psi. The tension shear or compression shear is what has been used to determine the shear strength. This data has limitations, in that the data book indicates that the honeycomb is not being subjected to pure shear but to a combination of shear and tension/compression. The tensional compression component varies with core thickness so that thicker cores will have a lower apparent shear strength than thinner cores. This method of testing does not generate valid shear mechanical properties yet this method for testing honeycomb specimens has been used for more than 40 years. Accurate shear failure data are needed in establishing failure criteria. Currently there is no test method existing to conduct a full range of valid shear-strain tests on core material up to failure. These and other disadvantages are solved or reduced using the invention.
SUMMARY OF THE INVENTION
An object of the invention is to provide a test method for determining the mechanical properties of honeycomb materials.
Another object of the invention is to provide a test method for determining the shear properties of honeycomb materials.
The invention is directed to a testing method to determine mechanical shear properties including shear failure strength of a honeycomb specimen. An improved test fixture is used to place a honeycomb specimen under shear without tension or compression loading so as to obtain valid shear mechanical properties. A filler material is injected under partial vacuum pressure into the ends of the cells of a honeycomb specimen, and the ends are then fastened to opposing fixture-fastening means. A load is then applied to one end to place the honeycomb specimen in shear without introducing tension or compression forces upon the honeycomb specimen. The testing method applies to any two opposite faces of a cubic honeycomb specimen. The testing method works on all three principal directions. Using vacuum injection of filler material, the test method is applied to the two opposite faces whose normal is parallel to the honeycomb cell. When conducting the same test in the two other directions, the potting operation is much simpler and no partial vacuum is necessary. The test method produces consistent reliable mechanical property data for the honeycomb specimen. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.
REFERENCES:
patent: 5237860 (1993-08-01), Kato et al.
patent: 5275489 (1994-01-01), Borneman et al.
patent: 5315861 (1994-05-01), Egan et al.
patent: 5913242 (1999-06-01), Stussi
Chang Dick J.
Valenzuela Pierre R.
Welle Richard P.
Noori Max
Reid Derrick Michael
The Aerospace Corporation
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