Method for manufacturing a wind turbine blade and mandrel tool

A mandrel tool for joining two adjacent lengthwise blade sections of a wind turbine blade, the mandrel tool being configured for being arranged inside the two adjacent lengthwise blade sections, and the mandrel tool comprising a frame, and, as seen in cross-section, at least two mandrel portions connected to the frame, wherein at least one of the mandrel portions is connected to the frame by an actuator configured for retracting and/or extending the mandrel portion towards/away from the frame, wherein an outer surface of the at least two mandrel portions is configured for at least partially supporting a fiber lay-up for a joining portion,

• wherein the at least two mandrel portions include at least one leading edge mandrel portion configured for at least partially supporting the fiber lay-up for the joining portion at a leading edge of the wind turbine blade and/or at least one trailing edge mandrel portion configured for at least partially supporting the fiber lay-up for the joining portion at a trailing edge of the wind turbine blade, and
• wherein the mandrel tool is configured to have a cross-section that varies in a lengthwise direction when the mandrel tool is in an extended state and/or the mandrel tool has an outer surface that matches an inner surface of the two adjacent lengthwise blade sections over the joining portion.