Abstract: A plate-like washer of generally annular shape and a method of using the washer for pre-tensioning. The washer is adapted to be placed between a fastening element, used to attach two pieces to form a coupling, and a piece to be fastened. The washer is manufactured from a shape-memory metal whose upper transformation temperature is higher than the highest use and assembly temperature of the coupling. The thickness of the washer is adapted in view of the length and pre-tensioning need of the fastening element, as well as the expansion of the washer at the upper transformation temperature. The washer is actively heated to its upper transformation temperature and undergoes a phase transformation which expands the washer and pre-tensions the coupling to the desired degree.

Abstract: A 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.