Pre-tensioning device for fastening elements and method for pre-tensioning a fastening element

A pre-tensioning device for use with a fastening element having a hole extending longitudinally therethrough, said device comprising;

• a shape memory metal cartridge adapted to fit inside and rest on a bottom of said hole;
• a restrictor adapted to fit inside said hole above said cartridge and attached to an end of the fastening element where the hole is located, whereby when said cartridge is heated to its transformation temperature it expands against said bottom of said hole and said restrictor to effect an elongation of said fastening element.

A pre-tensioning device for fastening elements which are to be subjected to a pre-tensioning expansion and are used to fasten together at least two pieces, said device comprising:

• a pre-tensioning unit connected at its first end to a first end of a fastening element and at its second end to be supported by a rigid structure, in relation to which the pre-tensioning expansion of the fastening element is to take place;
• a tightening member, which is separate from the pre-tensioning unit and attached to the fastening element to be movable independently of the pre-tensioning unit relative to a piece to be fastened and limiting the movement of the fastening element in the direction of expansion, said tightening member being arranged to tighten the fastening element by movement relative to said piece to contact with a piece to be joined by means of the fastening element;
• the pre-tensioning unit including between the rigid structure and said first end of the fastening element a shape-memory metal element, expandable through a phase change to stretch said fastening element and allow the tightening member to be moved to the tightening position, closer to the piece to be joined by means of the fastening element, before a final tightening of the fastening element.

A method for pre-tensioning a fastening element through a pre-tensioning expansion, comprising the steps of:

• attaching one end of the fastening element to a pre-tensioning unit at a first region of the pre-tensioning unit, the pre-tensioning unit being supported at its second region by a rigid structure in relation to which the pre-tensioning expansion takes place, wherein the pre-tensioning unit contains between the rigid structure and said one end of the fastening element a shape-memory metal having a certain transformation temperature where it expands through a phase change;
• heating said shape-memory metal to said transformation temperature to expand it and to cause a pre-tensioning effect by stretching the fastening element so that a tightening member attached to the fastening element in tightening relationship therewith and mobile relative to a piece limiting the movement of the fastening element in the direction of expansion, is tightened by moving closer to a piece to be fastened by means of the fastening element to said piece limiting the movement of the fastening element, whereafter the fastening element through contact of the tightening member with the piece to be fastened is tightened finally by removing the pre-tensioning effect of the shape-memory metal.