Thermoplastic seam welds

Abstract

The present invention is a method for joining thermoplastic composite sandwich panels with thermoplastic welds (fusion bonds) made without autoclave processing of the joint. The preferred joint is a double interleaf staggered joint with supporting titanium doublers providing a tensile strength of at least 12,000 lb/in. The joint is particularly suited for joining sections of a cryogenic tank for spacecraft.

We claim:  
     
         1 . A double interleaf staggered joint for joining thermoplastic fiber-reinforced composites with a thermoplastic seam weld, comprising: 
 (a) at least two fully compacted, laminated face sheets substantially free of volatiles having a plurality of plies of a thermoplastic resin reinforced with fibers, each face sheet having an edge with a plurality of fingers adapted for interleaving into the joint, the face sheets having a right hand and a left hand configuration suitable for forming the joint    (b) a resin film between fingers on each joint mating interface to insure that fusion of the fingers produces a resin-to-resin bond; and    (c) optionally, a metal foil on one surface of the face sheets in the region of the joint to reinforce the joint;    wherein each finger is a plurality of plies of fiber-reinforced resin composite with the plies staggered to reduce areas of stress concentration in the joint.    
     
     
         2 . The joint of  claim 1  wherein each face sheet includes two fingers that divide the plies in the face sheets substantially in half.  
     
     
         3 . The joint of  claim 2  wherein the stagger is at least 0.1 inch/ply.  
     
     
         4 . The joint of  claim 3  wherein the metal foil is titanium.  
     
     
         5 . A process for making a joint in thermoplastic composite sandwich structure without autoclave processing, comprising the steps of: 
 (a) laying up upper and lower, left and right laminated face sheets having a plurality of plies of fiber-reinforced thermoplastic matrix resin;    (b) positioning separator plies in the edges of the face sheets that will form the joint to create an interleaf split of fiber-reinforced laminated composite fingers;    (c) consolidating the face sheets to produce composites substantially free of volatiles or porosity;    (d) bonding the upper and lower face sheets, respectively, to a honeycomb core with a resin rich layer on the surface of the face sheet that contacts the honeycomb core to form left and right honeycomb core sandwich panels;    (e) removing the separator plies;    (f) interleaving fingers of the left and right face sheets of the left and right panels above and below the core to define double staggered interleaf joints;    (g) optionally, positioning a metal foil between each face sheet and the core in the region of the joint to reinforce the joint; and    (h) melting the resin in the joints to form thermoplastic fusion welds.    
     
     
         6 . The method of  claim 5  wherein fingers include staggered plies to reduce areas of stress concentration in the joint.  
     
     
         7 . The method of  claim 5  wherein the separator plies are a nonporous release film.  
     
     
         8 . The method of  claim 5  further comprising the step of: 
 positioning a thermoplastic resin film between each interleaved finger to insure a resin-to-resin bond upon fusion.  
 
     
     
         9 . The method of  claim 5  wherein consolidating the face sheets occurs in an autoclave at an elevated temperature and pressure and wherein the bonding step occurs at a lower temperature and a lower pressure than the consolidating step.  
     
     
         10 . The method of  claim 5  further comprising the step of: 
 curing a foaming adhesive between the honeycomb core of the left and right panels to create a structural joint when forming the fusion welds.  
 
     
     
         11 . The method of  claim 10  wherein the fusion weld step occurs outside an autoclave using an inductively heated press having facing tools for contacting the inside and outside of the panels.  
     
     
         12 . A product obtained by the method of  claim 5 .  
     
     
         13 . A product obtained by the method of  claim 11 .  
     
     
         14 . A process for supporting plies in a welded joint in a thermoplastic honeycomb sandwich structure, comprising the step of: 
 placing a composite encapsulated titanium foil between the laminate plies of the joint and the underlying honeycomb core.    
     
     
         15 . An apparatus for forming a fusion weld in a joint between interleaved fingers of at least two, preconsolidated, fiber-reinforced thermoplastic resin matrix composite sandwich panels, comprising: 
 (a) clamping elements adapted for applying a pressure to the joint, the elements including heated, conformal platens matching the configuration of the panels; and    (b) insulation associated with edges of the platen to contain heat at the joint.    
     
     
         16 . The apparatus of  claim 15  wherein the clamping apparatus includes hydraulic or pneumatic cylinders connected to the platens to move the platens to apply the pressure.  
     
     
         17 . A composite honeycomb sandwich structure, comprising: 
 (a) A honeycomb core having a foaming adhesive butt joint defining a bond line;    (b) upper and lower face sheets adhered to the honeycomb core, each face sheet including a thermoplastic weld substantially along the bond line; and    (c) optionally, a resin-encased metal foil doubler between the face sheet and the honeycomb core to support the face sheet during welding of the weld.