having now more particular reference to the accompanying drawings and more particularly to fig1 thereof , there is shown an improved closure device built in accordance with a first embodiment of the present invention , which comprises two independent substrates generally shown at 1 and 5 in in fig1 of the drawings , each one of said substrates being provided with suitable means to be adhered or fastened to the surfaces of two corresponding opposite parts to be joined by the device of the present invention . substrate 1 is preferably a substrate provided with at least one but preferably a plurality of openings or perforations 4 which , in the particular instance of the embodiment shown in fig1 are formed between a plurality of weft fibers or cords 2 and a perpendicular plurality of warp fibers or cords 3 , said openings 4 being capable of expansion or retraction , depending on a force applied in a sideward direction to the warp and weft fibers outwardly of the center of the corresponding substrate 1 , the higher said sideward force , the larger will the openings become . substrate 5 , in turn , includes one or more fasteners such as that shown at 6 , said fasteners , in the particular instance of the embodiment of fig1 comprising a rigid stem 7 perpendicularly arranged on the surface of substrate 5 and integrally joined at one of its ends to the surface of said substrate 5 , with the other or free end 8 of said stem 7 being formed so as to provide a secure fastening end when entangled with the fibers or cords of substrate 1 , such as in the shape of a hook ( as illustrated in fig1 ) or as a harpoon head , arrow head , pin head and the like . the substrate 5 may be of an inelastic nature as shown in fig1 to 3 , or it may also be of an elastic nature as illustrated in connection with fig4 and 5 for a purpose that will be more clearly explained hereinbelow . when the substrates 1 and 5 are brought together with the hooks confronted with the fibers , said hooks 8 force the fibers 2 and 3 outwardly of the center of the hook and increase the size of openings 4 , thus permitting their passage therethrough . when the fastening ends of stems 7 fully pass through the openings 4 , the fibers 2 and 3 are released to return to their original position , thus closing the openings 4 , whereby the two substrates are firmly fastened . to release the substrates from each other , the fibers or cords 2 and 3 of substrate 1 are manually pulled apart for broadening the openings 4 , and the hooks 8 are thusly released to remove substrate 5 from substrate 1 . in fig2 and 3 of the drawings , there is shown the manner in which the closure device of the present invention accomplishes its function , and particularly fig2 shows the perforate substrate 1 subjected to coplanar tensile forces in two perpendicular senses , with the purpose of increasing the area of the openings or spans 4 , for permitting , when the two opposite substrates 1 and 5 approach towards each other , the heads 8 of the fasteners 6 to pass through said openings 4 , to thereafter , upon complete passage of the heads 8 through the perforate substrate 1 , release the coplanar forces so that said openings 4 decrease in size , thus firmly fastening the heads 8 and avoiding disengagement thereof , as shown in more detail in fig3 of the drawings . in order to remove one substrate from the other , the reverse process is effected , that is , the substrate 1 is subjected to coplanar tensile perpendicular forces , the heads 8 of the fasteners 6 are removed from the openings 4 , and the forces applied on the substrate 1 are released upon separation of both substrates as described . of course that the forces that will expand the openings 4 , may also be pressure forces , for instance , when the heads of the fasteners 6 have a mechanical resistance sufficient to push by themselves on the edges of openings 4 , thereby expanding the same when passing therethrough , said openings returning thereafter to their original position when the heads of the fasteners have completely passed through said openings . in fig4 and 5 of the drawings , there is shown a fastening procedure which is similar to that already described in connection with fig2 and 3 , but with the variation that the openings or spans 4 of the substrate are subjected to tensile forces along one single direction , for instance , in the particular case of this embodiment , the warp fibers 2 are stretched thus spacing apart the fibers of the weft 3 , for permitting the insertion of the hooks 8 of fasteners 6 through the openings 4 , whereafter the forces applied on the warp fibers 2 are released , to entangle the hooks 8 of the fasteners 6 in the fibers of the weft , which are approached to each other as shown in fig5 . in the particular embodiment of the invention shown in fig4 and 5 , the substrate 5 is shown as an elastic substrate which may at least be stretched in the direction of the arrows shown in fig4 such that , when the fibers 2 are stretched to expand the opening 4 , the substrate 5 will also be stretched in order to increase the distance between the fasteners 6 in the same proportion of the expansion of the openings . when the fibers 2 are released , the substrate 5 is also released to contract and maintain the perpendicularity of the fasteners , in the direction of the arrows shown in fig5 . it will be obvious to any one skilled in the art that substrate 5 may be stretchable in both perpendicular directions or may be non stretchable as shown in fig3 without departing from the spirit of the present invention . fig6 of the drawings shows another preferred embodiment of the invention , which is a variation in which the perforate substrate indicated by means of reference numeral 21 , comprises a plurality of openings 24 , said openings having the characteristic that they may be expanded when the heads 28 of the fasteners 26 are introduced therethrough , said heads being , in the instance of this embodiment , in the shape of an arrow head , whereby said heads 28 expand the openings 24 that , after passage of the heads 27 and considering that the material of the substrate 21 is elastic , decrease in size by contraction thereof , thus firmly fastening the heads 28 and preventing disengagement thereof . in the above described embodiments , when it is desired to remove the substrate 5 or 25 provided with the fasteners , from the perforate substrate 1 or 21 respectively , it is necessary to apply a tensile force on the perforate substrate 1 or 21 , in the direction of the highest elastic capacity therof or in the direction of the highest capacity of any one of its properties such as elasticity , compressibility , etc ., thus performing as some sort of a safety device which prevents disengagement of the substrate provided with the fasteners after the closure operation , and facilitating , however , the disengagement thereof only when the perforate substrate is subjected to the tensile force mentioned above , whereby when the perforate substrate 1 or 21 is subjected to a tensile force for increasing the area of the openings , only a minimum force applied in a perpendicular direction to the surface of the substrates will be necessary to accomplish separation of said substrates . one other desirable characteristic of the material of which the perforate substrate is made , may be the expansibility thereof , which may also be provided artificially on the material in order to expand or reduce the size of the openings , for instance , by varying its temperature , which may be useful for certain particular applications . one other embodiment of the invention is shown in fig7 of the drawings , wherein the closure device in accordance with the invention comprises a substrate 41 provided with openings 44 and also provided with perpendicular fasteners 46 with their respective terminal heads 48 , said fasteners being arranged both on the upper surface 50 and on the lower surface 51 of the perforate substrate 41 , attached to the solid portions 52 surrounding said openings 44 . in this embodiment of the closure built in accordance with the invention , the substrate may be placed over two surfaces or objects which are to be joined , such that it will be supported at the same time on both surfaces or objects , and such that when one of said objects or surfaces is approached to the other , said substrate will be bent over itself , thus joining one portion of said substrate with the remainder thereof . in another application this type of closure device may comprise three perforate substrates , one to be used as an internal substrate and two as external substrates , the internal substrate being built as described and illustrated in fig7 of the drawings , that is , being provided with openings 44 as well as with fasteners 46 on both surfaces thereof , whereas the outer substrates comprise openings 44 as well as fasteners 46 only on one face thereof , preferably on their outer or working surfaces , such that three surfaces or objects may be joined to each other , much in the manner of a sandwiching operation . fig8 of the drawings shows still another embodiment of the substrate provided with the fasteners , which in this particular instance is designated by means of reference numeral 55 , and in which the fasteners 56 comprise a straight stem 57 perpendicular to and joined at its lower end to the substrate 55 , and provided along its length with two or more branches or locking projections 59 in the form of harpoon heads , as well as a hook 58 at its free end , for use in combined action with the openings of anyone of the perforate substrates described in connection with the prior figures for forming a closure device . having now reference to fig9 and 10 of the accompanying drawings , there is shown an additional embodiment of the present invention , similar to the embodiment shown in fig6 except for the fact that in this embodiment the length of the stem 27 shown in fig6 is reduced to obtain a short stem in accordance with the present embodiment , such that said stem has the same length as the thickness of the perforate substrate which is thicker in the case of the present embodiment than in the case of the embodiment of fig6 whereby , as the length of the stem of the fastener coincides with the thickness of the substrate provided with the openings , except for the head of the fastener , said head is locked against the inner surface of said substrate provided with openings . in the embodiment of the invention shown in the above mentioned figures , as it may be clearly seen , the process of fastening is effected by taking advantage of the elastic characteristics of the substrate 65 , to which the fasteners 66 are attached , said fasteners 66 consisting of a short straight stem 67 and an arrow head 68 , the compressible characteristics of the perforate substrate 61 permitting the entrance of the heads 68 of fasteners 66 through the openings 64 , upon expansion thereof due to the triangular form of the arrow heads , and preventing disengagement of the same after said openings return to its original size . finally , in fig1 and 12 of the drawings , showing a further embodiment of the invention , together with the process for fastening the two substrates , the fasteners 76 of the substrate 75 each comprises an elongated stem 77 provided along the length thereof with a plurality of bell shaped projections 78 , the widest portion of said projections looking towards substrate 75 , and the perforate substrate 71 having a thickness coincident with the length of the stem 77 of the fastener 76 , such that the openings 74 are expanded due to the conical shape of the projections 78 provided on said stem , or due to the application of a tensile force along anyone of the directions of substrate 71 , to permit the passage of the stem 77 and its projections 78 , which will be pressed between the walls of the openings 74 when the tensile forces are released , or when the fastener ceases to push inwardly of the openings , as clearly shown in fig1 of the drawings . a variety of combinations may be made with the different embodiments of perforate substrate and fastening substrate of the present invention , and it may be seen that for the first time , a closure device having the remarkable properties of being easily closeable and at the same time of remaining securely locked upon closure thereof and yet easily openable by a very simple operation , has been provided , which essentially comprises two or more substrates , at least one of which is provided with expandable openings and at least one other of which is provided with perpendicular elongated fasteners , constituted by a stem having a head at its free end or provided along the length thereof with a plurality of projections similar to the head or different in shape with respect to the head , wherein the free areas of the openings may vary in size either by the pushing force exercised by the heads of the fasteners of the fastening substrate , or when applying a tensile force to the substrate provided with openings , in one or more directions , with the purpose of permitting the introduction of the heads of the fasteners through said openings , which thereafter are contracted when the heads of the fasteners have completely passed through the openings , or when the tensile force ceases to be applied , such that the disengagement of the fasteners from the openings of the substrate will be prevented , thus generating a permanent joint , until the area of said openings is again increased in order to release the fastening devices by the application of a tensile force in one or more directions of the perforate substrate . on the other hand , the operation of the closure device of the present invention is fast , simple and of practical operation , as it will be evident to anyone skilled in the art , and it must be understood that the illustrative embodiments shown in the accompanying drawings are only of an exemplifying character and do not limit the scope of the invention , inasmuch as the closure device of this invention may be subject to many different variations in the details thereof , for instance , by making the head of the fasteners in a different suitable form , with the only condition that said head must be able to be locked within the openings , and said openings may also have any different suitable shape in order to permit the passage of the fasteners , but not the extraction thereof , except after the application of the above described tensile forces . on the other hand , the material for manufacturing the closure device may be any material having suitable physical characteristics , and may be built by means of a net formed by warp and weft fibers , or may also be built as a solid surface having openings therein , or the stems of the fasteners of the substrate may have a head or said head plus two or more projections along the length thereof , or may be deprived of the head itself and may be only one or more projections along the length of the stem , and the like , all of these variations being intended to fall within the true scope and spirit of the present invention . although certain specific embodiments of the present invention have been shown and described above , it is to be understood that many modifications thereof are possible . the present invention , therefore , is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims .