Strong bonded joints for cryogenic applications

1. A method for forming a joint in a structure including an inboard skin and an outboard skin, the method comprising:

laying up a cryogenic tank inside the structure, wherein the cryogenic tank is inside the structure and attached to the inboard skin, wherein the cryogenic tank diverges away from the inboard skin further into the structure from a point of contact to the inboard skin;

placing a softening strip into a Y-shaped joint region defined between the inboard skin and the cryogenic tank, starting from about the point of contact, wherein the softening strip is composed of a material that remains flexible at a cryogenic temperature below about 238 degrees Fahrenheit; and

bonding the softening strip to the inboard skin of the structure and to the cryogenic tank to form the joint.

2. The method of claim 1 , wherein the bonding step comprises:

co-curing the structure, the cryogenic tank, and the softening strip.

3. The method of claim 1 , wherein the softening strip comprises:

a three-dimensional preform; and

a plastic matrix impregnated in the three-dimensional preform to form the softening strip.

4. The method of claim 1 , wherein the structure is a spacecraft.

5. The method of claim 1 further comprising:

forming the softening strip as a three-dimensional preform;

infusing a plastic material into the three-dimensional preform to form the softening strip;

attaching a first adhesive layer to a first side of the softening strip; and

attaching a second adhesive layer to a second side of the softening strip.

6. The method of claim 5 , wherein the three-dimensional preform is a three-dimensional fabric.

7. The method of claim 6 , wherein the three-dimensional fabric is composed of at least one of weaved fibers, braided fibers, and stacked fabric layers.

8. The method of claim 7 , wherein the fibers are selected from one of graphite fibers, glass fibers, and aramid fibers.

9. The method of claim 3 , wherein the plastic matrix is selected from at least one of a fluorocarbon and a urethane.

10. The method of claim 1 , wherein the softening strip is wedge shaped.

11. The method of claim 1 further comprising:

forming the softening strip from a material having a coefficient of thermal expansion that about matches adherants used to connect the softening strip to the inboard skin and to the cryogenic tank.

12. The method of claim 1 further comprising:

forming the softening strip from polytetrafluorethylene.

13. The method of claim 1 further comprising:

forming the softening strip from a stack of fabric materials held together using Z-pinning and filling spaces in the stack of materials with a plastic matrix.

14. The method of claim 1 further comprising:

forming the softening strip into a three-dimensional preform by braiding fibers.

15. The method of claim 1 further comprising:

forming the softening strip into a three-dimensional preform by weaving fibers.

16. A method for forming a joint in a spacecraft, the method comprising:

forming a three-dimensional fabric preform;

infusing a plastic material into the three-dimensional fabric preform to form a plastic matrix to form a softening strip, wherein the softening strip is capable of remaining flexible at cryogenic temperatures below about 238 degrees Fahrenheit, wherein the three-dimensional fabric preform is comprised of at least one of weaved fibers, braided fibers, and stacked fabric layers in which the fibers are graphite fibers, and wherein the plastic material is selected from at least one of a fluorocarbon and a urethane;

attaching a first adhesive layer to a first side of the softening strip;

attaching a second adhesive layer to a second side of the softening strip;

laying up a cryogenic tank attached to an inboard skin of the spacecraft;

placing the softening strip into a Y-shaped joint region between the inboard skin of the spacecraft and the cryogenic tank inside the spacecraft, wherein the joint region is located where the cryogenic tank diverges away from the inboard skin further into the spacecraft from a point of contact to the inboard skin, and wherein the softening strip is capable of remaining flexible at a temperature at which a gas has a liquid form; and

co-curing the softening strip to the spacecraft and to the cryogenic tank to form the joint.

17. The method of claim 16 , wherein the softening strip is wedge shaped.