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

PCT No. PCT/FI94/00539 Sec. 371 Date Aug. 1, 1996 Sec. 102(e) Date Aug. 1, 1996 PCT Filed Nov. 30, 1994 PCT Pub. No. WO95/15442 PCT Pub. Date Jun. 8, 1995A pre-tensioning device is used with a fastening element for fastening pieces together, the fastening element having a tightening element movable longitudinally along a first region thereof, which fastening element is to be subjected to a pre-tensioning expansion. The device comprises a locking piece fixedly attachable to the first region of the fastening element and extending over the tightening element; the tightening element being movable with respect to a piece to be fastened independently of the locking piece; and an annular sleeve made of a shape memory metal and positioned between the locking piece and the piece which is to be fastened to another piece, whereby when the annular sleeve is heated it undergoes an expansion due to a phase transformation and forces the locking piece upwards such that the fastening element is stretched and the tightening element can be tightened against the piece to be fastened.