Abstract:
A fixture for testing a tubular member simultaneously in bending and torsion is disclosed. The fixture includes first and second attachment elements that are configured to couple to the tubular member, and a load shoe that is configured to fit within the tubular member. The load shoe includes a body having a contact surface configured to contact a portion of an internal surface of the tubular member and at least one load reaction strut coupled to the body. The load reaction strut is configured to pass through a hole in the tubular member and accept a first force applied in a first direction that is perpendicular to an axis of symmetry of the tubular member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     BACKGROUND 
     1. Field 
     The present disclosure generally relates to testing methods and, in particular, testing a tubular member simultaneously in bending and torsion. 
     2. Description of the Related Art 
     Thin-walled tubular structural members are sometimes used in applications where the member is subjected to combined bending and torsional loads. For example, tubular members may be used as part of the structure of a wing or tail of an aircraft. It is often desirable to test the performance of these structural members to determine their true capabilities and thereby increase the confidence in the design. 
     Current methods of performing bending and torsional tests of thin-walled tubular members present a number of difficulties. Bending tests are commonly performed by the application of a lateral load to the midpoint of the tubular member. For very thin-walled tubes, performing a bending test alone can be problematic due to the tendency of any test fixture restraint to crush or damage the tube wall. In addition, one failure mode of a bending test is a buckling failure in the exact region where the lateral load is applied, raising the potential that the test fixture will interfere with the failure mode. In these cases, an edgewise compression test is sometimes used as a compromise approach, but this type of test may not capture the true bending and buckling stability capability of the tubular member. 
     If a torsional load is applied at the same time as the lateral load, the portion of the fixture that is applying the lateral load may induce an inadvertent torsional restraint in the test. The local stresses created by the application of the side load and the inadvertent torsional restraint at the midpoint may result in a failure at the midpoint due to stress conditions that are not representative of the true condition in service. 
     To avoid the problems and uncertainties related to performing a combined bending and torsion test of a thin-walled tubular member, bending and torsion tests are often performed separately and combined by analytical means, which introduces uncertainties that may preclude achieving minimum weight structure. 
     SUMMARY 
     There is a need to simultaneously perform accurate and realistic bending and torsion tests of thin-walled tubular members. The disclosed system and method provide a means of accomplishing this testing without the drawbacks of current methods and equipment. 
     The disclosed system includes a load shoe that fits within the tubular member under test. The load shoe conforms to the lower portion of the interior surface of the tubular member to evenly apply the lateral load. Application of the load in this area avoids interaction with the buckling behavior of the upper portion of the tubular member that is in compression. The load shoe incorporates struts that protrude through small holes in the lower wall of the tubular member in regions that will not influence the test results. Loads applied in pure tension to these struts results in a net lateral load on the tubular member. The struts are attached to the load shoe through a rotating shaft such that torsional loads are transmitted down the length of the tubular member without inducing a reaction load on the struts. Use of this load shoe enables a tubular member to be realistically tested simultaneously in bending and torsion, enabling the designers to achieve a minimum weight structure while maintaining the necessary safety margins. 
     In certain embodiments, a fixture for testing a tubular member is disclosed. The fixture includes first and second attachment elements that are configured to couple to the tubular member, and a load shoe configured to fit within the tubular member. The load shoe includes a body having a contact surface configured to contact a portion of an internal surface of the tubular member and at least one load reaction strut coupled to the body. The load reaction strut is configured to pass through a hole in the tubular member and accept a first force applied in a first direction that is perpendicular to an axis of symmetry of the tubular member. 
     In certain embodiments, a load shoe for applying a lateral load to a tubular member is disclosed. The load shoe includes a body having a contact surface configured to contact a portion of an internal surface of the tubular member and at least one load reaction strut coupled to the body. The load reaction strut configured to pass through a hole in the tubular member and accept a first force applied in a first direction that is perpendicular to an axis of symmetry of the tubular member. 
     In certain embodiments, a method of apply a lateral load to a tubular member is disclosed. The method includes the steps of placing a load shoe comprising a contact surface configured to contact a portion of an internal surface of the tubular member and at least one load reaction strut configured to pass through a hole in the tubular member within the tubular member such that the at least one load reaction strut extends through a hole in the tubular member, and applying the lateral load to the at least one load reaction strut. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide further understanding and are incorporated in and constitute a part of this specification, illustrate disclosed embodiments and together with the description serve to explain the principles of the disclosed embodiments. In the drawings: 
         FIG. 1  is a free-body diagram illustrating exemplary load tests to be performed on a thin-walled tubular member. 
         FIG. 2  is a cross-section of the tubular member of  FIG. 1 . 
         FIGS. 3A and 3B  illustrate an exemplary test fixture according to certain aspects of this disclosure. 
         FIG. 4A  is a perspective view of a load shoe and tubular test element according to certain aspects of this disclosure. 
         FIGS. 4B and 4C  are cross-sections of the load shoe and tubular test element of  FIG. 4A  according to certain aspects of this disclosure. 
         FIG. 5A  is a perspective view of a tubular test element under test using the load shoe of  FIG. 4A  according to certain aspects of this disclosure. 
         FIG. 5B  is a cross-section of the load shoe and tubular test element of  FIG. 5A  according to certain aspects of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Testing of extremely light-weight, thin-walled, tubular structural members subjected to combined bending and torsional loads presents several challenges regarding the design of fixtures required to react the applied loads and restrain the test article. These challenges include: (1) how to provide restraint to the tubular test section to react the applied vertical loads and not crush the test section, (2) how to provide this vertical restraint and not interfere with the buckling behavior of the upper compression surface of the tube, (3) how to provide vertical restraint and still allow the torsional moment to be transmitted freely through the length of the test section. The disclosed system and method provide at least an improvement in meeting these challenges. 
     In the following detailed description, numerous specific details are set forth to provide a full understanding of the present disclosure. It will be apparent, however, to one ordinarily skilled in the art that embodiments of the present disclosure may be practiced without some of the specific details. In other instances, well-known structures and techniques have not been shown in detail so as not to obscure the disclosure. 
     The method and system disclosed herein are presented in terms of a thin-walled tubular structural member. It will be obvious to those of ordinary skill in the art that this same configuration and method can be utilized in a variety of applications wherein there is a desire to apply a lateral load to a hollow element. Nothing in this disclosure should be interpreted, unless specifically stated as such, to limit the application of any method or system disclosed herein to a testing of thin-walled tubular members. 
       FIG. 1  is a free-body diagram illustrating exemplary load tests to be performed on a thin-walled tubular member  10 . In this example, a bending load is created by the application of force F 1  at the point  17  at the midpoint of the tubular member  10  with corresponding forces F 2  and F 3  applied in the opposite direction at the ends of the tubular member  10 . A bending load is also applied by the application of moments M 1  and M 2  in the plane  22  of  FIG. 2 . A torsional load is applied by the application of moments M 3  and M 4  in opposite directions at the ends of the tubular member  10 . In this example, a tension load is also applied by loads L 1  and L 2  in opposite directions at the ends of the tubular member  10 . If the loads F 1 , F 2  are negative (i.e., the load vectors pointed in the opposite directions), a compressive load would created in the tubular member  10 . 
       FIG. 2  is a cross-section of the tubular member  10  of  FIG. 1 . The tubular member  10  has an interior surface  12  and an exterior surface  14 . There is a centerline  20  along the length of the tubular member  10 , coming out of the page as seen in  FIG. 2 . A first plane  22  is defined as including the centerline  20 , with a second plane  24  also including the centerline  20  and perpendicular to the first plane  22 . The tubular member  10  also is characterized by an inner diameter D and a wall thickness T. In the embodiment shown in  FIG. 1 , point  17  is coincident with plane  22 . 
       FIGS. 3A and 3B  illustrate an exemplary test fixture  40  according to certain aspects of this disclosure.  FIG. 3A  illustrates a tubular test element  42  of the tubular member  10  connected to load application tubes  44  at each end. The load application tubes are rigid in comparison to the tubular test element  42  so that deformation occurs primarily in the tubular test element  42 . In certain embodiments, the load application tubes  44  are solid rods. In certain applications, the load application tubes  44  are larger in diameter than the tubular test element  42 . Application of shear loads F 2  and F 3  create moments at the ends of tubular test element  42 . A load F 1  is applied at the middle of test element  42 . Torques M 3  and M 4  are applied to the ends of the load application tubes  44  to create a torque load in the tubular test element  42 . The test fixture truss  46  provides a rigid reference for the actuators that apply the loads and torques to the tubular test element  42 . 
       FIG. 3B  is an enlarged view of the tubular test element  42  of  FIG. 3A . The forces and moments present at the interface between the load application tubes  44  and the tubular test element  42  (i.e., the ends of the tubular test element  42 ) are shown in  FIG. 3B . The forces F 2  and F 3  applied to the outer ends of the load application tubes  44  have created moments M 1  and M 2  at the ends of the tubular test element  42 , as previously discussed with respect to  FIG. 1 , as well as the forces F 2  and F 3  being transferred through the load application tubes  44  to the ends of the tubular test element  42 . Moments M 3  and M 4  have also been transferred to the ends of tubular test element  42 . 
       FIG. 4A  is a perspective view of a load shoe  30  and tubular test element  42  according to certain aspects of this disclosure. In this embodiment, the load shoe  30  has a hollow body  32  with a contact face  37  (not visible in  FIG. 4A ). A compliant layer  38 , such as a rubber pad, covers the surface  37  of the body  32 . A pin  34  is rotatably coupled to the body  32  such that the pin  34  is located at the centerline  20  of the tubular test element  42  when the load shoe  30  is installed within the tubular test element  42 . In this embodiment, two load reaction struts  36  are fixedly coupled to the pin  34  and extend out through holes  33  in the body  32  of the load shoe  30  as well as through holes  16  in the tubular test element  42 . The holes are sized such that the expected deformation of the tubular test element  42  will not cause the struts  36 , which will remain fixed in position and orientation relative to the test fixture truss  46 , to contact either the body  32  or the tubular test element  42 . The interaction between the load shoe  30  and the tubular test element  42  are discussed in greater detail with respect to  FIGS. 4B-4C  and  5 A- 5 B. 
       FIGS. 4B and 4C  are cross-sections of the load shoe  30  and tubular test element  42  of  FIG. 4A  according to certain aspects of this disclosure.  FIG. 4B  is a cross-section taken along section line A-A through the center of load shoe  30  in  FIG. 4A . In  FIG. 4B , it can be seen that the pin  34  is on the centerline  20 . This ensures that rotation of the tubular test element  42  that may be induced by the applied torques of moment M 3  and M 4  do not displace the point of application of the loads applied through the struts  36 . The external surface  35  of the compliant layer  38  can be seen to be in contact with the interior surface  12  of the tubular test element  42 . 
       FIG. 4C  is a cross-section taken on section line B-B through the center of load shoe  30  in  FIG. 4A .  FIG. 4C  illustrates how the profile of the contact face  37  of the body  32  and therefore the profile of the external surface  35  of the compliant layer  38  are matched to the inside diameter D of the tubular test element  42 . As such, a different load shoe  30  is required for each different inner diameter D of tubular test elements  42 . It can be seen how the struts  36  pas through the holes  33  and  16  in the body  32  and tubular test element  42 , respectively. The holes  16  in the tubular test element  42  shown in  FIG. 4C  are not in areas of the wall of tubular test element  42  that are highly stressed or expected to be the points of failure, as this test load combination is usually expected to fail in buckling at point  17 . As the lateral load Fl is now applied through the struts  36 , as is discussed in greater detail with respect to  FIG. 5B , rather than at the surface at point  17 , the load application method does not distort the buckling behavior of tubular test element  42  at point  17 . As no test load is applied directly at point  17 , the stresses at point  17  are solely the result of the applied moments M 1  and M 2 , forces F 1 , F 2 , and F 3 , and the torque created by moment M 3  and M 4 . 
     If one of the load application tubes  44  is restrained from rotating at one outer end, with the other load application tube  44  free to rotate, application of moments M 3  and M 4  may cause the tubular test element  42  to rotate with respect to a test fixture such as the truss  46 . The orientation of the load show  30  will remain fixed with respect to the test fixture. The holes  16  in the wall of tubular test element  42  need to be large enough to ensure that this rotation of the tubular test element  42  does not cause the edges of the holes  16  to contact the struts  36 . As the rotation of tubular test element  42  is expected to be small, especially if the tubular test element is a composite structure, the size of the holes  16  will remain small enough to not detrimentally affect the stress patterns in the tubular test element  42 . 
     The profile seen in  FIG. 4C  of the body  32  can be seen to contact the interior surface  12  of the tubular test element  42  over the lower portion, i.e. the  180  degree region below plane  24 . This lower portion of tubular test element  42  is expected to be in tension in this test arrangement and is not expected to be a region of failure. Therefore, application of a lateral load through the compliant layer  28  over the lower portion of tubular test element  42  is expected to have little, if any, effect on the performance or failure mode of the tubular test element  42 . In certain embodiments, the body  32  and compliant layer  38  contact less than a 180 degree portion of the interior surface  12 . 
       FIG. 5A  is a perspective view of a tubular test element  42  under test using the load shoe  30  of  FIG. 4A  according to certain aspects of this disclosure. Load extension tubes  44  are connected at their inner ends to the ends of tubular test element  42 . In this embodiment, a torque in tubular test element  42  is applied by the combination of moments M 3  and M 4  applied to the outer ends of the load extension tubes  44 . A downward lateral force at the middle of tubular test element  42  is created through the combination of forces F 4  and F 5  that are respectively applied in line with the two struts  36 A and  36 B, producing a net force Fl as shown in  FIG. 5B . Lateral loads F 2  and F 3 , are applied at the outer ends of the load extension tubes, wherein the vector sum of loads F 2  and F 3  is equal in magnitude and opposite in direction to the net load F 1 . 
       FIG. 5B  is a cross-section of the load shoe  30  and tubular test element  42  of  FIG. 5A  according to certain aspects of this disclosure. It can be seen how the loads F 4  and F 5  are applied in line with the struts  36 A and  36 B, resulting in a net force F 1  applied through the centerline  20 . It can be seen that the compliant layer  38  contacts the interior surface  12  over the lower 180 degree portion of the tubular test element  42  at point  17 . As the net load F 1  is applied downward, in this embodiment, the contact force between the compliant layer  38  and the interior surface  12  at point  19 , i.e. at the neutral axis of tubular test element  42  when bent in the manner illustrated in  FIG. 5A , is approximately zero. The maximum tensile stress created in the wall of tubular test element  42  may occur, in some test configurations, at point  18  where the load shoe does not apply a point load and has little or no effect on the stress or failure at this point. 
     The concepts disclosed herein provide a method of simultaneously applying a torque and a bending moment to a tubular element that reduces the distortion induced by test fixture on the stresses in the expected failure areas. The load shoe disclosed herein provides a method of applying a lateral load to the portion of the tubular test element that is in tension. The load shoe also applies the lateral load over an extended area rather than at a point, avoiding local damage to the tubular element under test. The use of a load shoe of the type disclosed herein allows realistic testing of combined bending and torsion loads that may result in a lower component weight while maintaining adequate performance and safety margins. 
     The previous description is provided to enable a person of ordinary skill in the art to practice the various aspects described herein. While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the terms “a set” and “some” refer to one or more. Pronouns in the masculine (e.g., his) include the feminine and neuter gender (e.g., her and its) and vice versa. Headings and subheadings, if any, are used for convenience only and do not limit the invention. 
     It is understood that the specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged. Some of the steps may be performed simultaneously. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented. 
     Terms such as “top,” “bottom,” “front,” “rear” and the like as used in this disclosure should be understood as referring to an arbitrary frame of reference, rather than to the ordinary gravitational frame of reference. Thus, a top surface, a bottom surface, a front surface, and a rear surface may extend upwardly, downwardly, diagonally, or horizontally in a gravitational frame of reference. 
     A phrase such as an “aspect” does not imply that such aspect is essential to the subject technology or that such aspect applies to all configurations of the subject technology. A disclosure relating to an aspect may apply to all configurations, or one or more configurations. A phrase such as an aspect may refer to one or more aspects and vice versa. A phrase such as an “embodiment” does not imply that such embodiment is essential to the subject technology or that such embodiment applies to all configurations of the subject technology. A disclosure relating to an embodiment may apply to all embodiments, or one or more embodiments. A phrase such an embodiment may refer to one or more embodiments and vice versa. 
     The word “exemplary” is used herein to mean “serving as an example or illustration.” Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. 
     All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” Furthermore, to the extent that the term “include,” “have,” or the like is used in the description or the claims, such term is intended to be inclusive in a manner similar to the term “comprise” as “comprise” is interpreted when employed as a transitional word in a claim.