Abstract:
A method includes designing a part. The part includes at least one internal structure. The internal structure is designed to provide strain mitigation, energy dissipation, or impact resistance for the part during an emergency condition. The part is built by a layer-by-layer additive manufacturing process. While building the part, the internal structure is connected to the part.

Description:
BACKGROUND 
     This invention relates generally to the field of additive manufacturing. In particular, the present disclosure relates to internal structures of additive manufactured articles. 
     Additive manufacturing is an established but growing technology. In its broadest definition, additive manufacturing is any layerwise construction of articles from thin layers of feed material. Additive manufacturing may involve applying liquid, layer, or particle material to a workstage, then sintering, curing, melting, and/or cutting to create a layer. The process is repeated up to several thousand times to construct the desired finished component or article. 
     Critical joints in aircraft engines, airframes, automobiles, and other structures must be designed to carry both limit and ultimate loads under static and dynamic loading conditions. In certain emergency conditions the load on critical joints can spike in such a manner and to such a degree that catastrophic failure may occur. 
     SUMMARY 
     A method includes designing a part that includes at least one internal structure. The internal structure is designed to provide strain mitigation, energy dissipation, or impact resistance for the part during an emergency condition. The part is built by a layer-by-layer additive manufacturing process. While building the part, the internal structure is connected to the part. 
     An apparatus includes a part designed for and built by a layer-by-layer additive manufacturing process. The part includes an internal structure integrally formed to the part. The internal structure is designed to provide strain mitigation, energy dissipation, or impact resistance for the part during an emergency condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a strut end fitting. 
         FIG. 2  is a perspective cross-sectional view of a first bolt bushing. 
         FIG. 3  is a perspective cross-sectional view of a second bolt bushing. 
         FIG. 4  is a perspective cross-sectional view of a third bolt bushing. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional view of strut end fitting  10 . Strut end fitting  10  includes strut end  12  and joint  14 . Joint  14  is connected to strut end  12  at a first end of joint  14 . Aperture  16  is located in a second end of joint  14 , the second end of joint  14  being opposite from the first end of joint  14 . Joint  14  includes first enhanced impact resistance structure  18  and second enhanced impact resistance structure  20 . Each of strut end  12 , joint  14 , first enhanced impact resistance structure  18 , and second enhanced impact resistance structure  20  are integrally formed together during an additive manufacturing process. The additive manufacturing process may include powder based, selective laser sintering, or free-form additive manufacturing. Strut end fitting  10  can be made of metallic, polymer, composite, or other materials. 
     Strut end fitting  10  also includes sensor  22 . Sensor  22  is located within first enhanced impact resistance structure  18 . Sensor  22  is added to strut end fitting  10  either during or after the additive manufacturing process used to build strut end fitting  10 . Sensor  22  is designed to monitor stress experienced by strut end fitting  10 . During an emergency event or other stressful conditions, sensor  22  indicates when strut end fitting  10  has suffered plastic and/or elastic deformation. Sensor  22  may include a strain gauge, frangible conducting element, capacitive proximity sensor, or other sensors. 
     Under normal use conditions, strut end fitting  10  experiences force  24 . Force  24  represents a common load direction experienced by end strut fitting  10 . Factors such as the direction and amount of force  24  dictate the specific geometry and material selection for enhanced impact resistance structures  18  and  20 . 
     For instance, enhanced impact resistance structure  18  of strut end fitting  10  includes a matrix shaped geometry that contains larger sized voids than the matrix shape geometric structure of enhanced impact resistance structure  20 . The smaller sized voids of enhanced impact resistance structure  20  provide a decreased amount of elastic deformation. During an event, enhanced impact resistance structure  20  deforms less than enhanced impact resistance structure  18 , thus providing a more structurally strong impact resistance region than enhanced impact resistance structure  18 . Enhanced impact resistance structure  18 , with its larger sized voids, allows for more plastic and elastic deformation of strut end fitting  10  during an event than does enhanced impact resistance structure  20 . The increased distances between the vertices of enhanced impact resistance structure  18  allows for enhanced impact resistance structure  18  to absorb more force and to plastically or elastically deform before a failure of the material. In this embodiment, the geometry of enhanced impact resistance structure  20  is designed to allow for less plastic or elastic deformation because enhanced impact resistance structure  20  is co-axial with the core of strut end fitting  10 . During a failure event, it is important that a portion of strut end fitting  10  maintains a region that does not plastically or elastically deform to a large degree. The smaller voids of enhanced impact resistance structure  20  allow for a smaller degree of deformation of strut end fitting  10 , therefore providing strut end fitting  10  structural strength in the instance of a failure event. 
     Additionally, the internal structure of enhanced impact resistance structures  18  and  20  may include a pocket, rib, strut, blade, truss matrix, honeycomb, gradient honeycomb, cellular element, shear core, or spring element. 
     Enhanced impact resistance structures  18  and  20  offers tailored radial stiffness and enhanced protection against catastrophic failure from extreme transient loads during an emergency condition in critical joints for aircraft engines, airframes, automobiles, and other structures. The structures and material suites in regions of critical structural joints incorporate additive manufacturing design features to mitigate strain rate, dissipate energy, and generally extend the magnitude of elastic and plastic deformation the joint can tolerate before catastrophic failure. Factors such as spring rate, material ductility, energy absorption capacity, allowable deformation, and operating temperature will also dictate specific geometry and material selection for joint  14  and enhanced impact resistance structures  18  and  20 . Examples of materials to be used for strut end fitting  10  may include metallic, polymer, or composite materials. 
     Failure events of part assemblies including strut end fitting  10  can include events with mild load spikes and/or severe load spikes. An example of a mild load spike can include an event involving an automobile. Failure events suffered by automobiles often cause mild load spikes in part assemblies causing elastic deformation of parts. Strut end fitting  10  with enhanced impact resistance structures  18  and  20  would allow an automotive part to experience a mild load spike while only elastically deforming. Without enhanced impact resistance structures  18  and  20 , an automobile part would likely suffer plastic deformation resulting in catastrophic failure of the part. Examples of a severe load spike may include an airplane crash, clear air turbulence, blade out event, disk rupture, ballistic impact, and other events. In an airplane crash or ballistic event, damage to an airplane often results in plastic deformation of the part. In these events, enhanced impact resistance structures  18  and  20  allow an airplane part to experience a severe load spike while only plastically deforming. Without enhanced impact resistance structures  18  and  20 , an airplane part would likely suffer splintering, shattering, or destruction resulting in catastrophic failure of the part. 
     Additive manufacturing processes allow complex geometries of enhanced impact resistance structures to be achieved which may not be economically feasible with non-additive manufacturing processes. Additive manufacturing processes eliminate the need to employ commonly expensive non-additive manufacturing processes of forming enhanced impact resistance structures after the build of the part. Additionally, non-additive manufacturing processes used to create complex geometries can become very expensive. Enhanced impact resistance structures integrally formed within the part, made possible by additive manufacturing processes, enable fewer raw materials to be used therefore decreasing the weight of the part, while providing a crumple zone allowing for elastic and plastic deformation of the part. The decreased amount of raw materials also provides a lower-cost alternative to parts with a solid structure. 
       FIG. 2  is a perspective cross-sectional view of bolt bushing  26 . Bolt bushing  26  includes external structure  28  and enhanced impact resistance structure  30 . External structure  28  defines an outer wall of bolt bushing  26 . Enhanced impact resistance structure  30  is located within external structure  28 . Enhanced impact resistance structure  30  is integrally formed to external structure  28  through an additive manufacturing process. 
     External structure  28  circumferentially surrounds bolt bushing  26  and includes a relatively constant thickness for the entire portion of external structure  28 . The thickness of external structure  28  can be designed to allow for a desired degree of deformation depending on the environment in which bolt bushing  26  will be used. For example, a smaller thickness of external structure could allow for a greater range of plastic or elastic deformation of external structure  28 , whereas a thicker external structure  28  would allow for a lesser range of plastic or elastic deformation of external structure  28 . 
     Enhanced impact resistance structure  30  allows for plastic and/or elastic deformation of bolt bushing  26  during failure events during in-use conditions in automobiles, aircraft, or other vehicles. The structure of enhanced impact resistance structure  30  includes a curved rib structure in the shape of a waveform that extends along an inside circumference of external structure  28 . The nodes of enhanced impact resistance structure  30  connect to the inside circumference of external structure  28 . The distance between and frequency of successive nodes of enhanced impact resistance structure  30  is designed based upon desired plastic or elastic deformation response capability of bolt bushing  26 . For instance a small amount of nodes in enhanced impact resistance structure  30  increases the distance between successive nodes. This would allow for more plastic or elastic deformation to be experienced by external structure  28  due to there being more spacing between the contact points of the successive nodes and external structure  28 . In an additional instance, a large amount of nodes in enhanced impact resistance structure  30  decreases the distance between successive nodes. The decreased distance between successive nodes would allow for less plastic or elastic deformation to be experienced by external structure  28  due to there being less spacing between the contact points of the successive nodes and external structure  28 . 
       FIG. 3  is a perspective cross-sectional view of bolt bushing  32 . Bolt bushing  32  includes external structure  34 , enhanced impact resistance structure  36 , spring elements  38 . External structure  34  defines an outer wall of bolt bushing  32 . Enhanced impact resistance structure  36  is located within external structure  34 . Enhanced impact resistance structure  36  is integrally formed to external structure  34  through an additive manufacturing process. Spring elements  38  are located within bolt bushing  32  and are integrally formed with external structure  34  and enhanced impact resistance structure  36  through an additive manufacturing process. 
     External structure  34  circumferentially surrounds bolt bushing  32  and includes a relatively constant thickness for the entire portion of external structure  34 . The thickness of external structure  34  can be designed to allow for a desired degree of deformation depending which environment in which bolt bushing  32  will be used. For example, a smaller thickness of external structure could allow for a greater range of plastic or elastic deformation of external structure  34 , whereas a thicker external structure  34  would allow for a lesser range of plastic or elastic deformation of external structure  34 . 
     Enhanced impact resistance structure  36  includes a gradient honeycomb formation. The gradient honeycomb formation of enhanced impact resistance structure  36  includes a large number of contact points with external structure  34  which provide a large degree of structural support for bolt bushing  32  and to external structure  34 . Another feature of the gradient honeycomb formation of enhanced impact resistance structure  36  involves decreasing the size of the voids within the gradient honeycomb formation of enhanced impact resistance structure  36  as the radial distance from external structure  34  increases. This gradient transition of the gradient honeycomb formation of enhanced impact resistance structure  36  allows for more plastic or elastic deformation nearer external structure  34  due to the larger size of the honeycomb voids. Closer towards the center of bolt bushing  32 , the voids of the gradient honeycomb formation of enhanced impact resistance structure  36  are smaller allowing for less plastic or elastic deformation nearer the center of bolt bushing  32  due to the smaller size of the honeycomb voids. 
     In addition to enhanced impact resistance structure  36 , spring elements  38  provides an additional energy absorbing feature to bolt bushing  32 . Spring elements  38  are oval shaped to help absorb forces experienced by bolt bushing  32  during a failure event and allow for a higher degree of elastic deformation of bolt bushing  32  before a catastrophic failure occurs. Spring elements  38  help to absorb radial, normal, and other forces placed upon external structure  34  during events. 
     Other embodiments of bolt bushing  32  may include a pocket, rib, strut, blade, honeycomb, truss matrix, cellular element, or shear core in place of or in addition to enhanced impact resistance structure  36  and spring elements  38 . 
     Enhanced impact resistance structure  36  and spring elements  38  allow for plastic and/or elastic deformation of bolt bushing  32  during failure events during in-use conditions in automobiles, aircraft, or other vehicles. 
       FIG. 4  is a perspective cross-sectional view of bolt bushing  40 . Bolt bushing  40  includes external structure  42  and enhanced impact resistance structure  44 . External structure  42  defines an outer wall of bolt bushing  40 . Enhanced impact resistance structure  40  is located within external structure  42 . Enhanced impact resistance structure  40  is integrally formed to external structure  42  through an additive manufacturing process. 
     External structure  42  circumferentially surrounds bolt bushing  40  and includes a relatively constant thickness for the entire portion of external structure  42 . The thickness of external structure  42  can be designed to allow for a desired degree of deformation depending which environment in which bolt bushing  40  will be used. For example, a smaller thickness of external structure could allow for a greater range of plastic or elastic deformation of external structure  42 , whereas a thicker external structure  42  would allow for a lesser range of plastic or elastic deformation of external structure  42 . 
     Enhanced impact resistance structure  44  includes a gradient honeycomb formation. The gradient honeycomb formation of enhanced impact resistance structure  44  includes a large number of contact points with external structure  42  which provide a large degree of structural support for bolt bushing  40  and to external structure  42 . Another feature of the gradient honeycomb formation of enhanced impact resistance structure  44  involves decreasing the size of the voids within the gradient honeycomb formation of enhanced impact resistance structure  44  as the radial distance from external structure  42  increases. This gradient transition of the gradient honeycomb formation of enhanced impact resistance structure  44  allows for more plastic or elastic deformation nearer external structure  42  due to the larger size of the honeycomb voids. Closer towards the center of bolt bushing  40 , the voids of the gradient honeycomb formation of enhanced impact resistance structure  44  are smaller allowing for less plastic or elastic deformation nearer the center of bolt bushing  40  due to the smaller size of the honeycomb voids. 
     Other embodiments of bolt bushing  40  may include a pocket, rib, strut, blade, honeycomb, truss matrix, cellular element, or shear core in place of or in addition to enhanced impact resistance structure  44 . 
     Enhanced impact resistance structure  42  allows for plastic and/or elastic deformation of bolt bushing  40  during failure events during in-use conditions in automobiles, aircraft, or other vehicles. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.