Shape memory alloy fastener

A fastener adapted for insertion into an opening having two opposing sides separated by a predetermined thickness, said fastener comprising:

• an elongated shank made of a shape memory alloy with upper and lower ends, adapted for insertion into said opening;
• a head at the upper end of said shank, adapted to provide a predetermined limit for the maximum depth of insertion of said fastener in said opening;
• a reduced segment made of a shape memory alloy extending from the lower end of said shank;
• and
• an annular collar adapted to fit over at least a portion of said reduced segment;
• said reduced segment having a deformed shape with cross-sectional dimensions such that said collar and said reduced segment can be inserted through said opening, and a trained shape with a cross-section substantially greater than said deformed shape, such that heating said reduced segment above a critical transformation temperature results in said reduced segment causing radial expansion and deformation of said collar to form a lower head substantially larger than said opening.

A method of fastening together a plurality of workpieces having an opening extending therethrough, said method comprising the following sequence of steps:

• forming a fastener made of a shape memory alloy into a predetermined trained shape and heat treating said fastener to a temperature above the critical transformation temperature for said shape memory alloy, said trained shape having:
• (a) an elongated shank having an upper end and a lower end adapted for insertion into said opening;
• and
• (b) an expandable portion at the lower end of said shank having a cross-sectional shape larger than said opening;
• cooling said fastener in said trained shape to a temperature below said critical transformation temperature;
• deforming the expandable portion of said fastener into a deformed shape having a cross-sectional shape suitable for insertion into said opening;
• inserting said fastener in its deformed state into said opening;
• and
• heating said fastener to a temperature above said critical transformation temperature, thereby causing said lower portion of said fastener to transform from said deformed shape to an approximation of said trained shape.

A method of fastening together a plurality of workpieces having an opening extending therethrough, said method comprising the following sequence of steps:

• forming a fastener made of a shape memory alloy into a predetermined trained shape and heat treating said fastener to a temperature above the critical transformation temperature for said shape memory alloy, said trained shape having:
• (a) an elongated shank having an upper end and a lower end;
• (b) a lower annular segment extending from the lower end of said shank having a cross-sectional shape larger than said shank;
• and
• (c) an upper annular segment extending from the upper end of said shank having a cross-sectional shape larger than said shank;
• cooling said fastener in said trained shape to a temperature below said critical transformation temperature;
• deforming at least one of said annular segments into a deformed shape having a smaller cross-section than its trained shape;
• inserting said fastener in its deformed state into an elongated annular sleeve adapted to receive said shank and said deformed annular segment;
• deforming said deformed annular segment into a shape having a cross-section sufficiently large to retain said fastener within said sleeve;
• inserting said fastener into said opening;
• and
• heating said fastener to a temperature above said critical transformation temperature, thereby causing said lower annular segment to transform from said deformed shape to an approximation of its trained shape, and thereby causing deformation of at least a portion of said sleeve to a shape larger than said opening.

A method of fastening together a plurality of workpieces having an opening extending therethrough, said method comprising the following sequence of steps:

• forming a fastener made of a shape memory alloy into a predetermined trained shape and heat treating said fastener to a temperature above the critical transformation temperature for said shape memory alloy, said trained shape having:
• (a) an elongated shank having an upper end and a lower end;
• (b) a head at the upper end of said shank, adapted to provide a predetermined limit for the maximum depth of insertion of said fastener in said opening;
• (c) a reduced segment extending from the lower end of said shank having cross-section dimensions smaller than said shank;
• and
• (d) a retaining segment at the lower end of said reduced segment, having a cross-sectional dimensions larger than said reduced segment;
• cooling said fastener in said trained shape to a temperature below said critical transformation temperature;
• deforming said retaining segment and said reduced segment to smaller cross-sectional shapes;
• inserting the retaining segment and the reduced segment of said fastener through an elongated annular sleeve adapted to receive said reduced segment;
• deforming said retaining segment into a cross-sectional shape sufficiently large to retain said sleeve on said reduced segment;
• inserting said fastener into said opening;
• and
• heating said fastener to a temperature above said critical transformation temperature, thereby causing said reduced segment and said retaining segment to transform from said deformed shapes to an approximation of their trained shapes, and thereby causing radial expansion of at least a portion of said sleeve to a shape larger than said opening.