PPI Graded Claim Drafting Assignment October 1998
Truss reinforced foam-core sandwich structure.


Instructions


General Instructions

1. Turn in your typed or printed double-spaced claims to the Registrar before 5PM, Wednesday, October 28, 1998.
2. Do not include your name. Use only your student testing number. Get a new number, if you have turned in claim assignments with your original number.
3. Do not seek assistance from any TA.


Claim Drafting Instruction

There are nine figures. Figures 1-3 are prior art. Figs. 4-6 illustrate your client's truss-reinforced foam-core apparatus. Figs. 7-9 illustrate the method of construction. Draft two (2) claims differing scope in independent format that recite the structure shown in Figs. 4-6. Draft three (3) apparatus claims of varied scope in dependent format Draft one (1) method claim in independent format that recites the method shown in Figs. 7-9.

You may depart from the language of the specification below, if: (1) it is necessary; and (2) you explain and define the terms you use in a separate section following your claims.

A. Introduction.

This invention relates to truss-reinforced sandwich structures, and in particular to a truss- reinforced foam-core sandwich structure and method of making same.


B. The -prior art,

Prior art foam core sandwich structure 10, Fig. 1, includes face sheets 12 and 14 and foam core 16 there between. Such sandwich structures are used in the aerospace industry where weight is a significant concern. Prior art foam-core sandwich structures often have low compressive strength due to the compactability of the foam material. Foam-core sandwich structures can also suffer from low shear strength.

Structure 20, Fig. 2 illustrate one attempt to address this problem by separating face sheets 22 and 24 with columns 25. Unfortunately, columns 25 tend to buckle upon the application of pressure to face sheets 22 and 24.

Another response to the limitations of prior art sandwich structures was the addition of honeycomb cores to the sandwich structures. Prior art honeycomb core sandwich structure 30, Fig. 3, includes honeycomb core 36 secured to face sheets 32 and 34 by adhesive layers 38 and 39, respectively. Honeycomb core structures, however, suffer from susceptibility to moisture intrusion resulting from their open-cell construction. Even non-visible face sheet damage can create a path for moisture intrusion. When moisture condenses on the outside of an aircraft wing skin fabricated with honeycomb core panels, the pressure differential between the sandwich interior and the atmosphere during descent can force moisture into the honeycomb core. The moisture then becomes trapped and causes corrosion. Repeated freeze/thaw cycles are also known to cause corrosion of aluminum honeycomb cores.

Additional problems occur with the prior art sandwich structures from impact damage from dropped tools, hail, and the like. A honeycomb core may be crushed at the point of impact, even when the face sheet is not visibly damaged. Such core damage may result in face sheet buckling and delamination under stress. Visual inspection of an aircraft wing may not show the underlying core damage.

Therefore, honeycomb core structures, although efficient because of their high strength to weight ratio, are not suitable for all applications.

C. The subject invention,

In accordance with this invention, truss-reinforced foam-core sandwich structure 30, Fig. 4, includes face sheets 32 and 34 and foam core 36 there between. Foam core 36 includes a plurality of truss members 38 disposed in foam core 36 and extending at least between face sheets 32 and 34.

The foam core 36 supports the truss members 38 and prevents them from buckling. In turn, truss members 38 increase the compressive strength of the foam core because they form columns in the core, which are mechanically strong and resist compression. Unlike prior art honeycomb core structure 30, Fig. 3, truss reinforced foam core sandwich structure 30, Fig. 4, is not susceptible to moisture absorption, particularly when closed-cell foam is used as the material for foam core 36.

Therefore, the benefits of prior art foam-core (alone) structures and truss-core (alone) sandwich structure are realized in the subject invention without the problems associated with honeycomb core sandwich structures. Face sheets 32 and 34 may be made of layers of dry fiber matting or impregnated material, collectively called resin

reinforced composite laminate, as shown in the prior art. Truss members 38 may be dry fiberglass fibers having a typical diameter of 1/10 inch, titanium and other wire and fiber material. Multifilamentary bundles preimpregnated with resin and then cured may also be used.

The material of foam core 36 may be thermosetting or thermoplastic foam, or felt, or fibrous insulation. "Foam" as used herein is given broad interpretation and means any cellular material capable of receiving and supporting truss members 38. Suitable foam products are available from the E.I. DePont & de Nemours Co. Wilmington, Delaware under the trademarks Rohacell TM, Dinvinylcell TM, and Klegecel TM. In one embodiment, foam core 40, Fig. 5, includes several layers, namely layer 42 of a high-density foam (e.g. "Rohacell 5111 TM) and layers 44 and 46 of a lower-density foam (e.g. "Rohacell 3111 TM). Also in this embodiment, ends of truss members 38 extend into face sheets 32 and 34. The truss members lock the face sheets to the foam core.

In another embodiment, truss members 52 and 54, Fig. 6, are set at angles as x-braced chords to increase the shear strength of the structure. Also, in the area of a fastener such as bolt 56, many perpendicular truss members 58 may be inserted in the foam core to support the face sheets in the area of bolt 56.

The use of angled truss members 52 and 54 of the present invention is based on truss theory where the load is transferred and shared between contiguous truss members. The bending loads are reacted by the face sheets and the core reacts to the shear loads.

The core and truss structure react in parallel to applied shear in proportion to the relative shear stiffiness. The total shear modulus G-total for the structure is given by:
Gtotal = Gfoam + (2S/h)(EA)sin(T)cos(T)
where (T) = Theta






EI is the truss member bending stiffness, k is the foundation modulus of the foam core, and m is the integer number of half wavelengths in the buckled mode shape.

Optimization of the concept is illustrated by an example:

TABLE I
Rohacell
Divinycell
Klegecel Polimex
Grade
51 WF
71 WF
HT 50
HT 70
TR-55
TR-75
Density (lb/ft^3)
3.23 4,68
3.1 4.4
3.5 4.7
Tensile strength (psi)
L32 39
160 260
170 308
Compressive strength (psi)
116 246
100 160
108 154
Flexural strength (psi)
232 420
115 230
136 271
Shear strength (psi)
116 188
73 116
98 135
Modulus of elasticity (psi)
10.875 15.225
--- ---
8.285 13.635
Shear modulus
2.755 4.205
2.465 3.625
2.944 4.162
Thermal stability IV.
up to 360* F
up to 260* F.
250* F.


Fiber spacing of eight truss members per inch was used for fiberglass truss members each 0.030 in. diameter.

In one example, a truss reinforced foam core structure in accordance with this invention is manufactured as follows. Fiber tows 140, Fig. 7 are manually placed in or automatically inserted into foam core 142 either perpendicularly as shown or at one or more of various angles. Face sheets 144 and 146 are then placed on each side of core 142, Fig. 8. The application of pressure, Fig. 9, compacts the foam and drives tows 140 at least partially into face sheets 144, 146.

In another example, Rohacell TM closed cell foam was used and is typically compressed at temperatures from 3000 to 3500 F with pressures of approximately 50 to 90 psi. A 40 to 50 percent thickness reduction is common. The foam thickness indicated was 1.3 in., which allows for a 45 percent thickness compaction. The choice of foam is dependent on the resin system being used and on the compressive strength requirements of the foam. The face sheets were pre-impregnated material and the fiber tows 140 were pre-impregnated with resin. The resin impregnated tows - previously inserted into the foam core - pierced the face sheets and formed truss members in the foam core. The truss members extending in and between each face sheet locked the face sheets to foam core 142. The resulting structure can be used as a portion of an aircraft fuselage or in sporting equipment such as skis. As delineated above, the foam core supports the truss members against buckling and the truss members improve the compressive strength of the foam core and in addition lock the face sheets to the foam core to prevent delamination.