Method for manufacturing a hollow composite structure, particularly a spar beam for a wind turbine rotor blade, and an associated mandrel

A method for producing a hollow composite structure, comprising:

• placing fiber reinforcement material around a mandrel within a mold;
• maintaining a rigid neutral state of the mandrel within the mold, the mandrel comprising a compressible material in the rigid neutral state with a defined shape corresponding to a desired shape of the composite structure and a rigidity in the rigid neutral state to maintain the defined shape during lay up and curing of the fiber reinforcement material;
• curing the fiber reinforcement material with a resin;
• subsequent to curing, drawing a vacuum on the mandrel to compress the compressible material;
• and
• withdrawing the compressed mandrel through an opening in the composite structure, the opening having a size such that the mandrel could not be withdrawn through the opening in the rigid neutral state of the mandrel.