Abstract:
A reusable frangible joint includes a first part having a hemispherical convex surface, and a second part defined by a reversely identical mating hemispherical concave surface. The convex and concave surfaces of the joint are secured and held together in compression until a predetermined applied load causes the joint to fail. The joint includes at least one replaceable fastener that rigidly secures and holds the respective concave and convex surfaces together. The fastener, designed to be the only part of the joint configured to fail, is sheared apart under the predetermined load, which results in separation of the concave and convex parts from one another irrespective of from which direction or to which part the impact load is applied. The failed fastener can then be replaced, wherein the frangible joint becomes immediately reusable, as having incurred no other damage by the impact load.

Description:
GOVERNMENT LICENSE RIGHTS 
       [0001]    This disclosure was developed and/or modified with government support under a program awarded by the United States Department of Defense, entitled “Spectral Bat Phase 3 Frangible Antenna Array”. Thus, the United States government has license rights to any patents obtained hereunder. 
     
    
     FIELD 
       [0002]    The present disclosure relates generally to frangible joints, and more specifically to such joints designed to fail and break apart under predetermined impact loads having any hemispherical orientation. 
       BACKGROUND 
       [0003]    Modern unmanned aircraft are useful in various military reconnaissance and surveillance activities. In many cases, such so-called “unmanned aerial vehicles” (UAVs) are preferred over manned aircraft for reasons of safety and cost, particularly when used in high risk combat missions. 
         [0004]    UAVs are typically catapulted into flight by launch mechanisms for completion of variously defined missions. Upon completion of a mission, a UAV may be physically captured by means of a UAV retrieval system utilizing a vertical pole and rope line, which ensnares and/or snags the UAV at the end of a mission flight. Antennas attached to the UAV may be particularly vulnerable to damage during snagging of the UAV by a rope line during its capture and retrieval. 
         [0005]    Any resultant capture damage to antennas may be minimized or alleviated by the use of frangible booms which hold the antenna apart and away from the physical body of the UAV; the booms are used to avoid signal interference and other issues. One particular issue with frangible booms, however, has involved expense of part replacement. As such, improved frangible booms are needed to reduce both part replacement time and cost. 
       SUMMARY 
       [0006]    In a first form of the disclosed embodiment, a reusable frangible joint includes a first part having hemispherical convex surface, and a second part having a reversely identical mating hemispherical concave surface. The concave surface is held in compression against the convex surface. At least one fastener secures the respective concave and convex surfaces rigidly together. A predetermined impact load applied against either of the first or second parts is configured to shear the fastener, and to separate the first and second parts from one another. 
         [0007]    In a second form of the disclosed embodiment, a system of using a reusable frangible joint having a fracture pin on an unmanned aerial vehicle retrievable via a vertical pole and rope line includes securing an elongated boom containing a frangible joint to a fuselage of an unmanned aerial vehicle. The frangible joint has proximal and distal portions with respect to the fuselage of the unmanned aerial vehicle. The proximal and distal portions are secured together by a fracture pin. An antenna is attached to the distal portion of the frangible joint, and the unmanned aerial vehicle is launched, flown, and subsequently captured while in flight by means of a vertical pole and rope line. If the rope line impacts the distal portion of the frangible joint with a predetermined impact load, the fracture pin of the frangible joint is designed to break apart; i.e. to fail. The failed fracture pin is then removed, and a new fracture pin is installed within the frangible joint to provide for immediate reuse of the frangible joint. 
         [0008]    In a third form of the disclosed embodiment, a method of making a reusable frangible joint having two adjacent parts includes the steps of forming mating hemispherical parts, wherein a first part is concave and a second part is convex; securing the concave and convex surfaces in compression against each other with at least one fastener; applying a predetermined impact load applied against either of the first or second parts, wherein the load is configured to shear the fastener and to separate the first and second parts of the frangible joint from one another. 
         [0009]    All features, aspects, functions, and advantages disclosed herein may be achieved independently in various embodiments or may be combined in yet other embodiments, the details of which may be better appreciated with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic perspective view of an unmanned aerial vehicle that includes an omnidirectional frangible joint, as may be constructed in accordance with the present disclosure. 
           [0011]      FIG. 2  is a schematic perspective view of same unmanned aerial vehicle of  FIG. 1 , but shown being snagged or captured by a UAV recovery system. 
           [0012]      FIG. 3  is a frontal view of an antenna boom which contains the omnidirectional frangible joint, with the boom shown unsecured from the unmanned aerial vehicle of  FIGS. 1 and 2 , and depicting an orientation of the antenna boom as would be viewed along lines  3 - 3  of  FIG. 1 . 
           [0013]      FIG. 4  is cross-sectional view of the antenna boom, shown along lines  4 - 4  of  FIG. 3 . 
           [0014]      FIG. 5  is a perspective phantom-style “holographic” view of the frangible joint as contained in the detailed views of the antenna boom depicted in  FIGS. 3 and 4 . 
           [0015]      FIG. 6  is a perspective view of a fastener of the frangible joint of the present disclosure. 
           [0016]      FIG. 7  is a flowchart of a system that utilizes the omnidirectional frangible joint, as may be constructed in accordance with the present disclosure. 
           [0017]      FIG. 8  is a flowchart of a method that utilizes the omnidirectional frangible joint, as may be constructed in accordance with the present disclosure. 
       
    
    
       [0018]    It should be understood that referenced drawings are not necessarily to scale, and that disclosed embodiments are illustrated only schematically. Aspects of the disclosed embodiments may be combined with or substituted by one another, and within various systems and environments that are neither shown nor described herein. As such, it should be understood that the following detailed description is merely exemplary, and not intended to be limiting in either application or use. 
       DETAILED DESCRIPTION 
       [0019]    The following detailed description is intended to provide both apparatus and methods for carrying out the disclosure. Actual scope of the disclosure is as defined by the appended claims. 
         [0020]    In  FIG. 1 , an unmanned aerial vehicle (UAV), such as a drone, includes a fuselage  12 , left and right wings  14 ,  16 , and a wheel-less under-carriage  18 , to the extent that this particular UAV is designed to be launched into the air by means of a catapult mechanism (not shown). The UAV further includes a tail  22  that includes typical elevator flight controls. In the particular embodiment displayed, the tail  22  is secured to the wings  14 ,  16  by means of right and left tail support booms  24 ,  26 . 
         [0021]    Orthogonal XYZ coordinates are shown offset from the body  12  of the UAV  10  in  FIG. 1 . The coordinates reflect the three-dimensional space in which the UAV travels; the X-X direction is representative of the forward motion of the UAV  10 , while side-to-side motion is reflected by the Y-Y axis. The Z-Z axis reflects the vertical or up-and-down movements of the UAV  10 . It will be appreciated that the fuselage  12  extends along the X-X axis, while the wings  14 ,  16  extend along the Y-Y axis, as examples, only. As indicated in  FIG. 1 , the fuselage  12  of the UAV  10  is configured for primary flight movement along the X-X axis. 
         [0022]    Continuing reference to  FIG. 1 , an elongated antenna boom  30  is secured to the fuselage  12  of the UAV  10 . The boom  30  extends substantially along the Y-Y axis, and thus away from the fuselage. As will be appreciated by those skilled in the art, it is desirable that all associated antennas of the UAV are well spaced from the fuselage to avoid signal interference. For additional support, the antenna boom  30  is also shown secured to the left tail boom  26 , which extends parallel to the X-X axis of the fuselage  12 . 
         [0023]    Referring now also to  FIG. 2 , the UAV  10  is depicted approaching a UAV recovery system  20 , which includes a mast-like vertical pole  36  having a rope line  38  for snagging the UAV  10 . In the depiction, the UAV capture point  28  is at the right wing  14 , although other capture/snag points likely include the antenna boom  30 . 
         [0024]    Referring now to  FIG. 3 , a frontal view of the antenna boom  30  is displayed. The antenna boom  30  has an elongated body  32 , and includes a clamp system  34  for its earlier noted support on the left tail support boom  26 . Wiring or cabling  40  is also shown; those skilled in the art will appreciate that the wiring  40  may potentially support antenna signals for both intelligence communications and flight control. 
         [0025]    Referring now also to  FIG. 4 , it will be appreciated that the antenna boom  30  is hollow, and that the wiring or cabling  40  extends fully through the hollow interior thereof to individual, i.e. first, second, and third, antenna arrays  44 ,  46 , and  48 , as shown. For purposes of protecting the relatively delicate antenna structures from harsh impacts, such as might occur during capture of the UAV  10 , a frangible joint  50  is included within the structure of the antenna boom  30 , as now described. 
         [0026]      FIG. 5  displays interior structures of the frangible joint  50 , which include convex and concave hemispherical structural members  52 ,  54 , each specifically identified by their mating surfaces  52 ,  54 . The concave and convex portions comprise reversely identical mating parts of the joint  50 , and define at least a theoretical ball and socket joint, in which the concave surface comprises at least a portion of a socket and the convex surface comprises a portion of a ball. However, in the described embodiment, the theoretical socket does not completely encompass the theoretical ball. 
         [0027]    The surfaces  52 ,  54  define a contact interface  56  of the frangible joint members  52 ,  54 . The interface  56  is retained under compression by at least one or more threaded fasteners  60 ,  62  (although actual threads are shown only in the case of fastener  62 ) to assure a rigid and secure, although frangible, joint  50 . The fasters extend fully through each of the structural members  52 ,  54  by way of bushings  66  fixed within each of the structural members  52 ,  54  to support the fastener  60 . (For purposes of revealing detail of the fasteners  60 ,  62 , the bushing associated with the fastener  62  was been removed.) Bushings  66  are fixed within both members  52  and  54  for permitting reuse of all parts except for broken fasteners  60 ,  62 . As described and shown herein, if two fasteners are utilized, the two fasteners  60 ,  62  are ideally positioned orthogonally with respect to one another to form a relatively stronger connection within a plane, e.g. as defined by the X-X and Y-Y axes ( FIG. 1 ). 
         [0028]    Continuing reference to  FIG. 5 , the fasteners  60 ,  62  are designed to shear under a predetermined load such as created by an impact of a rope line against the outboard part of the frangible joint  50  connected to a distal portion  80  ( FIG. 3 ) of the antenna boom  30 . Although the two parts are designed to become separated, the cabling and/or wiring  40  is not intended to be damaged by a defined system  100  (described below). As such, upon failure of the frangible joint  50 , the failed fasteners  60 ,  62  may be replaced with new frangible fasteners  60 ,  62  to create an immediately reusable frangible joint  50 . 
         [0029]    The antenna boom  30  may be comprised of carbon fibers and/or lightweight plastics, while the bushings rigidly secured within the frangible joint  50  portion of the boom  30  may be formed of steel. To the extent that the frangible fasteners  60 ,  62  may be formed of a softer material, such as aluminum, the steel provides a stronger material against which to cause the aluminum fastener  60  to fail under a predetermined shear or combined shear and tensile impact load. 
         [0030]    Continuing reference to  FIG. 5 , it will be appreciated that in order to accommodate passage of the wiring or cabling  40  the frangible joint will include aligned apertures  74 ,  76  within the convex and concave hemispherical structural members  52 ,  54 , respectively. The aligned apertures thus form a part of a continuous cable port passageway  70  as shown in both  FIGS. 3 and 4 . 
         [0031]    Referring now to  FIG. 6 , a more detailed representation of a aluminum frangible fastener  60  is depicted (albeit threads are not shown). The fastener includes a head  82 , and a threaded shank portion  84 . The shank portion includes a reduced diameter section  86 , which may have a diameter reduction sized for achieving desired shear failure performance at a predetermined impact load. As such, the failure can be designed to occur omnidirectionally; i.e. irrespective of direction of an impact load with respect to the frangible joint  50 . 
         [0032]    Referring now to  FIG. 7 , a system  100  includes using a reusable frangible joint  50 , having a fracture pin  60  on an unmanned aerial vehicle  10 , retrievable via the pole  36  ( FIG. 2 ) and its associated rope line  38  (Box  102 ). The system  100  further includes securing an elongated boom  30  containing a frangible joint  50  to a fuselage  12  of an unmanned aerial vehicle  10 , the frangible joint  50  having proximal and distal portions  78 ,  80  ( FIG. 3 ) with respect to the fuselage  12  of the unmanned aerial vehicle  10  (Box  104 ). The system  100  further includes securing the proximal and distal portions together by a fracture pin  60  (Box  106 ), and attaching an antenna  44  to the distal portion  80  of the frangible joint  50  (Box  108 ). The unmanned aerial vehicle  10  is then launched, flown, and subsequently captured while in flight by means of the rope line  38 , such that the rope line impacts the distal portion of the frangible joint with an impact load sufficient to break the fracture pin  60  (Box  110 ). Finally, the fracture pin  60  is removed, and a new fracture pin  60  is installed within the frangible joint  50  to provide for immediate reuse of the frangible joint  50  (Box  112 ). 
         [0033]    Referring now to  FIG. 8 , a method  200  of making a reusable frangible joint  50  includes forming two adjacent first and second hemispherical parts  52 ,  54 , the two parts defining adjacent concave and convex surfaces (Box  202 ), and securing the concave and convex surfaces in compression against each other with at least one fastener  60  (Box  204 ). A final step is applying a predetermined impact load against either of the first or second parts, wherein the load is configured to shear the fastener  60 , and to separate the first and second parts  52 ,  54  of the frangible joint  50  from one another (Box  206 ). 
         [0034]    In summary, this disclosure provides a light weight and low cost frangible joint  50  that allows for predetermined breakaway under impact loads applicable in multiple directions, up to and including a complete hemisphere. The frangible joint  50  also allows for the passage of electrical cabling if and as required for an application, as well as the ability to tune fastener design by number, position and geometry as necessary to achieve desired controlled failure characteristics. 
         [0035]    The disclosed frangible joint  50 , the system  100  of using, and the described method  200  of for making, may each have other variations and alternative constructions neither described nor suggested herein. For example, although described only in terms of a few materials, the antenna boom  30 , the bushings  66  and the fasteners  60 ,  62  may be formed of materials other than the ones described. Moreover, other configurations and/or other components may be utilized in potentially in other environments, neither mentioned nor suggested herein. Further, the respective convex and concave hemispherical mating surfaces  52 ,  54  of the frangible joint  50  may contain features not specifically described herein, but yet cause the frangible joint  50  to react to an impact load in the same way. The fasteners  60 ,  62  may have alternative designs that may vary with respect to type, quantity, and or orientation not described herein. Finally, although the disclosure generally presents structures in only the shapes and sizes depicted, numerous variations of any of the disclosed structures may be envisioned for use in manufacture of alternative embodiments that operate and function in the manner as claimed below.