Composite structures having integrated stiffeners with smooth runouts and method of making the same

1. A method of making a stiffened composite structure, comprising:

fabricating a first fiber preform, wherein fabricating the first fiber preform includes filling a flexible tubular shell with continuous unidirectional fibers;

placing the first fiber preform in a mold recess having the geometry of the stiffener;

placing a second fiber preform over the mold recess covering the first preform;

co-infusing the first and second fiber preforms with a polymer resin; and

curing the resin-infused preforms.

2. The method of claim 1 , wherein:

fabricating the first fiber preform further includes braiding fibers into the flexible tubular shell, and

placing the first fiber preform in the mold recess includes conforming the tubular shell to the geometry of the recess.

3. The method of claim 2 , wherein braiding fibers into a flexible tubular shell includes producing a substantially continuous length of the tubular shell, and the method further comprises:

cutting a section of the continuous length of tubular shell, and wherein placing the first preform in the mold recess is performed by placing the cut section of the shell in the recess.

4. The method of claim 1 , wherein fabricating the first fiber preform includes:

simultaneously braiding together and forming to shape a fiber shell and a fiber core.

5. The method of claim 1 , wherein fabricating the first preform includes:

making a continuous shell having a reinforced core by braiding fibers around a group of continuous reinforcing fibers, and

cutting the continuous shell to desired lengths.

6. The method of claim 1 , wherein fabricating the first preform includes:

placing a first fabric ply over the mold recess,

forming the first ply into the interior of the mold recess,

filling the formed first ply with continuous reinforcing fibers,

overlaying a second fabric ply on the formed fiber filled first ply.

7. A method of making a unitized and stiffened composite aircraft skin, comprising:

fabricating a mold having a recess therein corresponding the geometry of the stiffener, including forming a runout in the recess;

fabricating a first fiber preform by braiding a fabric into a flexible tube, and filling the tube with continuous reinforcement fibers;

placing the first fiber preform in the mold recess, including conforming the first preform to the shape of the runout;

placing a second fiber preform on the mold overlying the first fiber preform;

co-infusing the first and second fiber preforms with a polymer resin; and

curing the resin-infused preforms.