Abstract:
Axial sections of a casing assembly such as that of a rocket are maintained interconnected by latching prongs on which thermally responsive Nitinol rings are positioned. Operational control over the latching prongs is achieved by selection of material properties and dimensions of the Nitinol rings during manufacture thereof.

Description:
The present invention relates in general to the formation of thermally responsive control means for releasable latches interconnecting sections of a casing. 
     BACKGROUND OF THE INVENTION 
     Ring-like elements made of shape memory material such as Nitinol have been commercially used for retention of connector pins under ambient temperatures. Such Nitinol rings have also been experimentally used to release latch pins at elevated temperatures within tubular casings as disclosed for example in U.S. patent application Ser. No. 09/107,314 filed Jun. 30, 1998, the disclosure of which is incorporated herein by reference. It is therefore an important object of the present invention to provide a method of manufacturing such Nitinol rings so as to meet the installational and operational requirements of thermally responsive control of latching means used to maintain sections of casings interconnected. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a wire made of Nitinol material having suitable properties is cut into required lengths corresponding to bent shapes such as the circumferential lengths of rings to be radially positioned between nested portions of a releasable latching arrangement interconnecting sections of a casing such as that of a rocket. The cut sections of the Nitinol wire are bent into their ring shapes after the opposite end portions thereof are annealed and flattened for overlapping thereof and then undergo welding to form joints. Welding of the ring joints is performed by use of an electrical resistance technique with either thin nickel foil sheets disposed between the overlapped wire end portions of the rings or plating/coating thereof with nickel to cause diffusion of melted nickel into the wire end portions at spot weld locations according to one embodiment. Cracking of the rings otherwise induced by the heat generated during the welding processes is thereby minimized and/or avoided. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     A more complete appreciation of the invention and many of its attendant advantages will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein: 
     FIG. 1 is a perspective view of a tubular rocket casing assembly as one example of an installational environment with which the present invention is associated; 
     FIG. 2 is a partial section through the tubular rocket casing assembly shown in FIG. 1, illustrating installation of Nitinol rings therein; 
     FIG. 3 is a partial section view taken substantially through a plane indicated by section line  3 — 3  in FIG. 2; 
     FIG. 4 is a block diagram illustrating the thermally responsive control exercised by the Nitinol rings; 
     FIG. 5 is a block diagram illustrating the method used for manufacture of the Nitinol rings; and 
     FIG. 6 is a partial section view taken substantially through a plane indicated by section line  6 — 6  in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawing in detail, FIGS. 1 and 2 illustrate as one example of an installation associated with the present invention, a rocket casing  10  such as that disclosed in U.S. patent application Ser. No. 09/107,314 aforementioned. The casing  10  includes a main tubular aft section  12  constituting a rocket motor and a forward warhead section  13 . Such casing sections  12  and  13  as shown in FIG. 2 are interconnected through a cylindrical adapter component  16  which has internal threads  14  adjacent one axial end thereof in threaded engagement with the forward section  13 . Also, a plurality of circumferentially spaced prong formations  20  of the adapter component  16  project toward its other axial end in radially spaced underlying relation to a radially outer axial end component  18  of the aft casing section  12 . The forward and aft sections  13  and  12  of the casing  10  when axially assembled as shown in FIG. 2 are held interconnected under control of three Nitinol rings  28  positioned in close axially spaced relation to each other, radially between the prong formations  20  and the outer axial end component  18  of the casing section  12 . Also, a polyethylene shield  30  is disposed in protective overlying relation to the three Nitinol rings  28  as shown in FIGS. 2 and 3. The properties and dimensions of the Nitinol rings  28  are selectively adjusted during manufacture thereof in accordance with the present invention to meet various requirements for separation of the casing sections  12  and  13 , otherwise held interconnected by the Nitinol rings  28  through the adapter component  16  in the installational arrangement as hereinbefore described. 
     As diagrammed in FIG. 4, the Nitinol rings  28  undergo heating  32  to a selected temperature range causing contraction  34  of such rings to thereby induce a separation force to be exerted by the rings on the prongs  20 , in a radially inward direction in the installation shown in FIG. 2, sufficient to displace latch projections  35  on the ends of the prongs  20  out of a groove  37  formed in the axial end component  18  of the casing section  12 . The sections  12  and  13  of the casing  10  are thereby unlatched and separated. In the case of a rocket motor casing assembly, such separation of the nested casing section  12  and adapter component  16  was caused to occur before propellant ignition as a result of a 4% contraction in circumferential length of the Nitinol rings  28  because of heating to a temperature range between 210° F. and 240° F. 
     The dimensional and operational requirements for the Nitinol rings  28  were achieved by manufacture thereof from a cold Titanium-rich alloy wire  36  of 0.028 inch diameter as diagrammed in FIG.  5 . Such wire  36  was elongated approximately 6% in length by stretch  38  and then cut into sections  40  of required lengths dimensionally corresponding to the circumferential lengths of the rings  28  plus the overlapping distance. The end portions of such cut lengths of wire were then annealed and flattened as denoted by  42  in FIG.  5 . The flattened wire ends then underwent removal of surface oxides by 800 grit SiC paper and cleansed with acetone and methanol as denoted by  48 . The flattened and cleansed end portions of each cut length of wire were then overlapped to form ring joints by bending of each cut length of wire into the circular ring shape as denoted by  50  in FIG.  5 . Nickel foils  52  were then placed between the overlapped end portions of the wire while positioned on a holding fixture for welding of the joints so formed by use of an electrical resistance technique  54 , to thereby complete formation of the rings  28 . 
     FIG. 6 shows the welded joint of each ring  28  formed by the aforesaid welding of the flattened overlapped end portions  56  and  58  thereof. Such welding involves placement of a consumable nickel foil  60  between the flattened, overlapped portions  56  and  58  of the wire ends causing melting of such foil at spaced locations of resistance spot welding causing the heating and diffusion of melted foil portions  62  into the wire end portions  56  and  58 . The resistance spot welding technique includes the maintenance of forging pressures on opposing electrodes through which electrical resistance heating and cooling occurs at each weld spot location, until the welding process thereat is completed. Use of such electrical resistance welding minimized solidification cracking of the wire which otherwise occurs because of heating during the welding process for high titanium content Nitinol. Secondary cracking was also avoided by the aforesaid spot welding involving placement of nickel foils  60 , of 0.001 inch thickness or less, between the overlapping end portions  56  and  58  of each ring  28  followed by the spot welding processes as hereinbefore described. 
     Obviously, other modifications and variation of the present invention may be possible in light of the foregoing teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.