A fiber composite material containing at least one layer of reinforcing fibers embedded in a polymer matrix, with the polymer matrix being formed by a curable resin and including at least one first region having a first curing property of the resin and at least one second region having a second curing property of the resin, wherein the at least one first curing property is different from the at least one second curing property, a method of manufacturing a composite structure, and a composite structure manufactured in the method.

Aspects of the disclosure relate to manufacturing a balloon envelope for use in a stratospheric balloon system. For instance, a stream of polyethylene mixture is extruded through an extruder in order to orient molecules of polymer chains of polyethylene and to provide an oriented film. The oriented film is passed through an electron beam and thereby crosslinking the polymer chains to provide a cross-linked film. The cross-linked film is heat sealed to form the balloon envelope.

The invention pertains to a method for integrating a fitting fitted between two wings of a composite profile comprising the steps of: obtaining a preform of a fitting impregnated with a thermosetting resin, comprising two flanges and a rib; pre-curing the laminate preform of the fitting to obtain a partial polymerization of the preform; laying up a laminate preform of the profile so that fibers impregnated with a thermosetting resin included in two of the wings of the laminate preform of the profile are laid up over the two flanges of the preform of the partially polymerized fitting; and completely polymerizing an assembly comprising the laminate preform of the profile and the laminate preform of the partially polymerized fitting while maintaining into contact the flanges of the laminate preform of the fitting with the wings of the laminate preform of the profile.

Aspects of the disclosure relate to manufacturing a balloon envelope for use in a stratospheric balloon system using an improved double-bubble blown-film extrusion process with water quenching and electron beam processing. A stream of polyethylene mixture is extruded through an extruder to orient molecules of polymer chains and provide an oriented film. The oriented film is passed through an electron beam, crosslinking the polymer chains to produce a cross-linked film with enhanced mechanical properties. The cross-linked film is heat sealed to form the balloon envelope comprising multiple gores. The resulting balloon envelope exhibits superior characteristics including crystallinity greater than 50%, strength to weight ratio greater than 1 MPa/(dtg/cm.sup.3), thickness less than 1.5 mil, thermal emissivity less than 0.03 at 193K, and optical clarity greater than 95%. The manufacturing process may include water quenching to enhance optical clarity and dual electron beam treatment for uniform crosslinking throughout the film thickness.