Constrained Forming of Contoured Composite Hat Stringers

Abstract

Cap wrinkling in a contoured composite hat stringer is reduced by constraining the cap as the hat stringer is being formed from a flat composite charge. The cap is constrained by an inflatable bladder placed in a tool set used to form the composite charge.

What is claimed is: 
     
         1 . Apparatus for making a contoured composite hat stringer, comprising:
 a pair of dies configured to be contoured along a length and defining a die cavity into which a composite charge may be formed into a contoured hat section having a cap;   a punch configured to form the composite charge into the die cavity; and   a bladder located within the die cavity and configured to constrain the cap as the contoured hat section is being formed.   
     
     
         2 . The apparatus of  claim 1 , wherein the bladder is formed of an elastomer. 
     
     
         3 . The apparatus of  claim 1  wherein the bladder has a rigidity sufficient to constrain the cap as the composite charge is being formed. 
     
     
         4 . The apparatus of  claim 1 , wherein the bladder is located between the dies and beneath the composite charge. 
     
     
         5 . The apparatus of  claim 1 , wherein the bladder includes reinforcement in an area of the bladder facing the cap. 
     
     
         6 . The apparatus of  claim 1 , wherein the bladder is sized to completely fill the die cavity when the bladder is inflated. 
     
     
         7 . The apparatus of  claim 1 , further comprising:
 a controller configured to coordinate operation of the punch with inflation of the bladder.   
     
     
         8 . A method of forming a contoured composite hat stringer having reduced wrinkling, comprising:
 forming a composite charge into a contoured hat section having sides and a cap by forcing a composite charge into a die cavity; and   reducing stress in the cap during the forming by constraining the cap as the composite charge is being formed.   
     
     
         9 . The method of  claim 8 , further comprising:
 placing the composite charge on a pair of dies, and   wherein forcing the composite charge into the die cavity includes using a punch to punch the composite charge into the die cavity while constraining the cap.   
     
     
         10 . The method of  claim 9 , further comprising:
 using the dies to apply pressure to the sides of the hat section as the composite charge is being punched into the die cavity.   
     
     
         11 . The method of  claim 9 , further comprising:
 contouring the hat section along a length of the hat section.   
     
     
         12 . The method of  claim 8 , wherein constraining the cap includes:
 inflating a bladder, and   using the bladder to apply a force against the cap.   
     
     
         13 . The method of  claim 12 , wherein inflating the bladder is performed before forcing the composite charge into the die cavity. 
     
     
         14 . The method of  claim 8 , wherein:
 forming the hat section includes contouring regions of the hat section along a length of the hat section, wherein the forming produces stresses in the cap along the regions, and   reducing the stresses in the cap includes sweeping the stresses from the cap in the regions to other areas of the hat section.   
     
     
         15 . A method of forming a contoured composite hat stringer having a hat section including sides and a cap, comprising:
 placing a flat composite charge on a pair of dies defining a die cavity;   forcing the flat composite charge into the die cavity to form a hat section of the stringer having sides and a cap;   contouring the die cavity; and   reducing wrinkling of the cap during forming of the hat section by transferring stress in the cap away from the cap.   
     
     
         16 . The method of  claim 15 , wherein reducing wrinkling of the cap includes constraining the cap. 
     
     
         17 . The method of  claim 15 , wherein transferring stress away from the cap includes applying pressure against the cap from within the die cavity. 
     
     
         18 . The method of  claim 15 , wherein the contouring is performed before forcing the flat composite charge into the die cavity. 
     
     
         19 . The method of  claim 15 , wherein the contouring is performed after forcing the flat composite charge into the die cavity. 
     
     
         20 . The method of  claim 15 , wherein applying pressure against the cap is performed by:
 inflating a bladder within the die cavity, and   using the bladder to apply a force to the cap.