Abstract:
A method includes steps of providing round tubing, providing a compression box and wedging dies, and reshaping the round tubing into a single or double-tapered rectangular tube including using the compression box to control an outside shape, while using the wedging dies to force material of the tubing outwardly toward the compression box. This arrangement minimizes material thinning. A tubular crushable structure is produced that is designed for longitudinal impact-energy-absorbing capability. The crushable structure includes a single or double-tapered rectangular tube made of material having a tensile strength of at least 40 KSI. In a narrower form the tensile strength is at least 80 KSI, though it can be 100 KSI or higher.

Description:
[0001]    This application claims benefit under 35 U.S.C. § 119(e) of provisional application Ser. No. 60/863,488, filed Oct. 30, 2006, entitled TUBULAR TAPERED CRUSHABLE STRUCTURES AND MANUFACTURING METHODS. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to crushable structures configured for energy absorption and energy management such as during a vehicle crash. 
         [0003]    Vehicle components are designed to reduce property damage and provide safety to the occupants of an impacted vehicle through energy management. This is typically accomplished by designing vehicle components for predictable and repeatable deformation. In low-speed impacts, components such as bumpers and bumper brackets are designed to absorb significant amounts of energy when impacted via deformation of these components. For higher-speed impacts, the vehicle chassis is designed to absorb energy by deforming. Side impacts also use deformable components such as sills, rocker panels, pillars and door impact beams. One main difference between the side impact components and those components located on the front or the rear of the vehicle is in how they are designed to absorb energy via deformation. The side impact components absorb energy via deformation associated with side-bending-type shape change of the components. Frontal and rear components such as bumper brackets and chassis components are designed to crush in an accordion fashion in a direction parallel to the impacting force. In frontal and rear impacts, the collision is either between a moving vehicle and a fixed object (wall, barrier, pole, tree, etc.) or between two moving vehicles. The impact energies are typically high due to speeds and crash dynamics. Chassis components must be able to deform in a predictable and repeatable manner to provide safety to the occupants and reduce property damage. 
         [0004]    Different types of component failure will produce different response curves and varying degrees of efficiency in terms of how the energy is absorbed. Impact energy absorption is calculated by multiplying a force of impact resistance times the impact stroke of a component. A component having a high efficiency of energy absorption is generally described as a component that absorbs a desired maximum amount of energy continuously over a desired maximum stroke distance. A tubular structure that bends over when impacted in a near axial direction has absorbed energy, but has not done so in a very efficient manner. A more efficient response would be had if the tube folded on itself in an accordion fashion. The accordion-type deformation provides the greatest amount of energy absorption within the provided package space. The final deformed piece represents the smallest packaging space of stacked material. The described innovation defined in this write-up is a crushable tubular structure that when impacted in a near axial direction, will collapse on itself in an accordion fashion. This innovative design can be scaled for small applications such as a bumper bracket or for larger applications such as a chassis component. 
         [0005]    The use of tubular structures for both chassis components and/or bumper brackets is nothing new. These types of tubular structures have been used on many various components throughout the vehicle. Most applications with this type of tubular structures coincide with protection from axial and near axial impacts. There are various manufacturing processes that are capable of producing tubular structures that when impacted in a near axial direction, will collapse on itself in an accordion fashion. The complexity and inherent cost associated with the manufacturing processes tend to increase as the energy management efficiency of the design increases. Manufacturing processes capable of producing tubular structural components and ranked by cost from high to low include hydroformed, clamshell designs fabricated from two stampings spot-welded together, deep-drawn stamping, simple expansion using internal mandrels, and simple rollformed tubular designs with crush initiators. 
         [0006]    Tubular components can be formed by hydroforming processes into complex shapes having non-uniform cross sections that vary along their length, where the non-uniform cross sections are tailored for particular needs and properties, such as for energy absorption. For example, vehicle frames often include hydroformed components. However, hydroforming processes are expensive, messy (since they involve placing a fluid within a tube and then pressurizing the fluid), and tend to require relatively long cycle times. Further, they become generally not satisfactory when higher strength materials are used, such as High-Strength-Low-Alloy (HSLA) materials, and/or Advanced-Ultra-High-Strength Steel (AUHSS) materials, since these materials are difficult to form, have low stretchability and poor formability, and tend to wear out tooling quickly. 
         [0007]    It is desirable to provide a crushable structure that can be made from high-strength steels, yet with reasonable cost and that will crush during an impact with excellent repeatable and predictable results. Thus, a component, and apparatus and method of manufacturing same having the aforementioned advantages and solving the aforementioned problems is desired. 
       SUMMARY OF THE PRESENT INVENTION 
       [0008]    In one aspect of the present invention, a method of forming an axially crushable structure suitable for energy absorption during an axial impact includes providing a section of tubing, providing a compression box and wedging dies, and positioning the tubing in the compression box and positioning the wedging dies at least partially in the tubing. At least a portion of the tubing is reshaped into a tapered polygonal tubular shape with a non-circular cross section, including using the compression box to control an outside shape while using the wedging dies to force material of the tubing outwardly into engagement with the compression box. 
         [0009]    In another aspect of the present invention, a tubular crushable structure is designed for longitudinal impact-energy-absorbing capability. The crushable structure includes a polygonal tube having a tapered portion and a second non-tapered portion aligned with the tapered portion. The tube is made of material having a tensile strength of at least 40 KSI and having a substantially constant wall thickness along its entire length. 
         [0010]    In another aspect of the present invention, a tubular crushable structure is designed for longitudinal impact-energy-absorbing capability. The crushable structure includes a polygonal tube having a tapered polygonal portion and a non-tapered polygonal portion and having a substantially constant wall thickness along its entire length. 
         [0011]    These and other aspects, objects, and features of the present invention will be understood and appreciated by those skilled in the art upon studying the following specification, claims, and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a raw tubing component with constant section and a finished tubular double-tapered rectangular tube component useful as a bumper crush tower. 
           [0013]      FIG. 2  is a perspective view of a tapered die for forming the raw tubing component. 
           [0014]      FIG. 3  is a perspective view of a straight section guide tube for use with the tapered die. 
           [0015]      FIG. 4  is a perspective view of a push collar for pushing the round tubing component into the tapered die. 
           [0016]      FIGS. 5   a  and  5   b  are perspective views of a double tapered round tube formed from the raw tubing component, and a double-tapered rectangular tube component made from the tube of  FIG. 5   a ; and  FIGS. 5   c  and  5   d  are end views of  FIGS. 51 and 5   b.    
           [0017]      FIG. 6  is a perspective view of a mandrel set, and  FIGS. 6   a  and  6   b  are perspective views of the outer mandrels and inner mandrel, respectively. 
           [0018]      FIG. 7  is a perspective view of a compression box usable with the mandrels of  FIGS. 6   a  and  6   b  for the double-tapered rectangular tube component of  FIG. 5   b.    
           [0019]      FIG. 8  is a perspective view of the finished double-tapered rectangular part with crush initiators. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    The present concept combines standard low-cost manufacturing processes to produce a tube of high strength material which, upon near axial impact, produces a lower-weight part having a force/deflection response similar to that produced by the more expensive hydroformed process. The proposed inventive concepts are based on the ability to reform round tubing into a double-tapered rectangular component. Crush initiators are strategically imparted to the double-tapered rectangular component during the manufacturing process. The write-up contained here within will concentrate on the double-taper rectangular design, but it should be noted that the concept and manufacturing process can be used on any sided polygonal-shaped tubular component. It should become obvious to anyone skilled in the trades that the manufacturing processes defined within this write-up overcomes common material limitation associated with reforming a straight constant geometry shape into a double-tapered geometry of a different shape. 
         [0021]    The proposed inventive concepts take advantage of the benefits of and overcome the formability limitations associated with the higher physical properties of such materials as structural steel, High-Strength-Low-Alloy (HSLA) steel and Advanced Ultra-High-Strength Steel (AUHSS). In the present text, when we refer to various steels, we define structural steel as material having a tensile strength of at least about 40 KSI or higher, High-Strength-Low-Alloy (HSLA) steel as material having a tensile strength of at least about 80 KSI or higher, and Advanced Ultra-High-Strength Steel (AUHSS) as material having a tensile strength of at least about 100 KSI or higher. The higher physical properties associated with these materials provide greater energy absorption during deformation and allow for down-gauging of thickness to achieve similar performance to thicker gauge lower grade materials. The ability to down-gauge thickness and maintain performance represents a reduction in part cost and potentially a reduction in piece price. A significant drawback to using materials with higher physical properties is that materials with higher physical properties also have reduced formability as the physical properties get higher. As the yield and tensile strength increase, the elongation and in turn the formability of the material decrease. The presented inventive concepts overcome the formability limitation associated with using higher physical property materials and provide the opportunity to reduce material gauge to achieve similar performance to more formable materials. 
         [0022]    The following process will describe the steps necessary to overcome formability issues associated with using higher grade materials and to produce a double-tapered rectangular shaped tube from a reshaped round tube. By the term “double-tapered,” we mean a tube with a first tapered portion and a different second portion (which can be tapered or non-tapered). For illustration purposes, a round Drawn-Over Mandrel (DOM) commercially available tube will be reformed to create a double-tapered rectangular tube. The DOM tube has higher physical properties than those associated with an Electrically Resistance Welded (ERW) tube due to the additional work hardening associated with the DOM process. The DOM material used for this example had the following physical properties; Yield Strength=67,021 psi, Tensile Strength=83,775 psi, and a 0.2% Elongation=12.65%. DOM tubing with an outside diameter of 4.75 inches was used and the length of the tubing was approximately 24 inches. These physical properties are in line with structural steels and HSLA steels. 
         [0023]    In the original round tubular component  20  (also called “round tubing” herein) ( FIG. 1 ), the outside diameter of the DOM tubing was sized such that the circumference of the tube is slightly undersized when compared to the perimeter of the large end of the partially finished double-tapered rectangular tube  20 B. The partially finished double-tapered rectangular tube  20 B has a double-tapered rectangular shape, including a first rectangular portion with a first taper (or no taper), and a second rectangular portion with a different second taper. (See  FIG. 1 .) Sizing of the circular tube outside diameter in this way will allow for some minor expansion to achieve the required perimeter of the large end of the double-tapered rectangular. The reforming and expansion process will be defined in detail in later paragraphs. The amount of expansion to go from round to rectangular should be kept to a minimum to reduce the stress on the material. Keeping expansion to a minimum is important considering the reduced formability of the higher grade of materials that are desirable for these types of deformable energy management components. 
         [0024]    The round DOM tubing  20  is forced into a tapered die  25  ( FIG. 2 ). The die is made from hardened steel and can be produced on a lathe. The die  25  is made in sections  26  and  27  to provide ease of handling and also to provide flexibility in changing taper angle and taper depth. A straight section  28  of the die  25  can be used to guide and support the round tubing  20  into the tapered end of the main die  25  if there are concerns associated with column bucking of the round tubing  20  as it is forced into the tapered main die  25  ( FIG. 3 ). For this particular example, a straight section  28  to guide and support the round tubing  20  was not necessary and hence was not used for the DOM tubing. 
         [0025]    A special push collar  29  ( FIG. 4 ) was developed that fit inside the round tubing  20  to transfer push loads to the outside edge of the tubing  20  as the tubing  20  was forced into the tapered die  25 . The round tube  20  was forced into the tapered die  25  ( FIG. 2 ) through a distance that coincided to its desired length. At the end point of insertion into the die  25 , the circumference of the smaller tapered end of partially-finished round tube  20 A was slightly undersized when compared to the final perimeter of the small end of the tapered rectangular shape in the finished part  21  ( FIG. 5 ). The now tapered round tube  20 A is removed from the die  25  by applying an upward force to the tapered end, forcing the tube  20 A in a reverse direction back through the top of the die  25 . It is noted that the described die  25  used to taper the round tube  20 A is a piece of prototype tooling and a different die configuration might be more suitable for high volume production. 
         [0026]    The tapering process may cause a length of the original tubes  20  to grow a small amount depending on the amount of the taper. Notably, a perimeter change causes material in these hard-to-form materials to move primarily in a length direction of the tube  20 . In the case of this example, the tube  20 A grew approximately 0.25 inches. The amount of length growth for the tube  20 A is dependent on the material type, material thickness and the amount of taper that is imparted on the raw tube  20 . There can be a slight increase in the thickness of the round tube  20 A, however this thickness change is not considered significant. If there is some thickness increase, the increase of thickness is most evident at the end of the round tube that experiences the greatest amount of taper. (See  FIG. 5 , diameter “a.”) Elongation of the round tube  20 A during tapering actually minimizes the amount of thickness change at the point where the maximum taper occurs on the tube. 
         [0027]    For the example presented here, material thickness at the tapered end increased only by approximately 0.009 inches. This compares to an average material thickness in the present example of about 0.132 inches, such that the thickness change is less than 7%. It should also be noted that for the materials proposed for this concept, the variation in material thickness for as received coil stock in the present example is typically +/−0.005 inches, or about 4%. Therefore, a material thickness change of only 7% was not considered significant in the present example. For the present discussion, a material thickness change of about 7% or less along a length of a tube is considered to be a substantially constant wall thickness along the entire length of the tapered tube. 
         [0028]    The tapered round tube  20 A is now ready for reshaping. The tapered round tube  20 A is now ready to be reshaped to a double-tapered rectangle  20 B. The reshaping process is accomplished with a combination of pure reshaping and some minor expansion. Expansion will be kept to a minimum to maintain the integrity of the wall thickness of the tube. A three-piece mandrel  30  was used to reshape the round tube  20 A ( FIG. 6 ). The outer two pieces  31  and  32  of the mandrel  30  are shaped to represent the shorter sides of the rectangle ( FIG. 6   a ). These mandrels  31  and  32  include the corner radii of the finished rectangular shape. The third part  33  of the mandrel  30  is the center section ( FIG. 6   b ). The two mandrels  31  and  32  are keyed and fit together with the center section  33  of the mandrel  30 . The center section  33  of the mandrel  30  is tapered, so as the center section  33  is moved down between the two mandrels  31  and  32 , the mandrels  31  and  32  spread apart to create a tapered rectangular mandrel  30 .  FIG. 6  shows a constant angle taper to the center section  33 , but in actuality the center section  33  and/or mandrels  31  and  32  can be made of sections that are tapered and/or sections that are non-tapered. 
         [0029]    The three-piece mandrel  30  often can not be used by itself to reshape the tapered round tube  20 A to a double-tapered rectangular because of forming limitations of the desired materials. The mandrel action required to change shape from round to rectangular potentially results in significant material thinning just off the radii of the rectangular final part. The thinning may happen when the reshaping method does not allow the material to flow from one shape to another. To reshape using the internal mandrels and at the same time minimize thinning of the material, an additional fixture is desirable. A compression box  35  ( FIG. 7 ) was developed to help the material flow during the reshaping operation that uses the internal three piece mandrels  31 - 33 . The compression box  35  is a tapered box where three sides of the box represent the finished shape of the double-tapered rectangle. The three finished sides are the two short sides of the rectangle and one of the long sides. The compression box  35  does not mimic the radii of the finished shape but instead only mimics the overall position of the walls of the tapered rectangle. The non-fixed face  36  of the compression box  35  is also one of the longer sides of the rectangle. This non-fixed face  36  of the compression box  35  is adjusted inward and against the tapered round tube  20 A while the mandrels  31 - 33  are forced down the length of the tapered round tube  20 A. The ability to adjust the non-fixed face  36  of the compression box  35  assists in the movement of material in a way that facilitates reshaping the round shape of the tube  20 A to a rectangular shape of the finished part  21  without thinning and undesirable weakening. 
         [0030]    The compression box  35  reduces the amount of expansion that is required to reshape the part and in turn reduces the amount of material thinning. The desire to perform a reduced amount of expansion is necessary to help size the ends of the tapered rectangle and at the same time force the repeatability of end geometries. It is noted that the detailed design of illustrated compression box  35  illustrates only one adjustable movable surface. However, it is contemplated and envisioned that multiple sides of the compression box  35  can be made to move or adjust. It is contemplated that those skilled in the art will understand how to do so once they understand the present concept. The use of multiple moving surfaces of the compression box  35  would assist in the movement of material and this may be required on the reshaping of more complex polygonal shapes. The additional movable surfaces might also be necessary to increase tolerances on geometric sizing of the finished shape&#39;s surfaces and ends. 
         [0031]    In a production mode, it is envisioned that the compression box  35  can be adjusted with hydraulics, pneumatics, and/or servos. It is envisioned that adjustment of the non-fixed face  36  of the compression box  35  can be adjusted in synchronization with the position of the mandrels  31 - 32  as they move down the length of the round tube. This type of control would be based a closed loop control system where the location of one aspect of the process is used to control another aspect of the process. 
         [0032]    The tapered shape of the rectangle in the finished part  21  helps to promote an accordion style of collapse when the tube is impacted in a near axial direction. The repeatability of this type of crush is questionable due to slight variations in the load direction and the location of deformation along the length of the tube. To improve the repeatability of the crushing action, crush initiators  40  ( FIG. 8 ) are typically added to the crushable parts. The type, placement, and number of crush initiators  40  required usually will require a development effort to identify the most optimized design. The crush initiators  40  can be added to the part preferably after the final shape has been formed. For this example, the crush initiators  40  would be added to the double-tapered rectangular shape. 
         [0033]    In a production mode, the crush initiators  40  can be added using any type of stamping method, hydraulic, pneumatic, etc. Internal support will more than likely be required when the crush initiators  40  are stamped into the part. It is envisioned that the crush initiators  40  can be added to the part when the internal reshaping mandrels are positioned in the part. The internal outer mandrels  31 ,  32  would need relief at each of the locations where the initiators  40  are to be placed. The central mandrel  33  could be backed out of the part which would allow the two outside mandrels  31 ,  32  to come free from the just-stamped-in crush initiators  40 . In a walking-beam-type production process, the crush initiators  40  could be added to the part in a stand-alone station. It should also be noted that holes, slots, etc. . . . have been commonly used in the past as crush initiators. The manufacturing process associated with adding holes or slots is similar to the dart type of crush initiator. Both types of crush initiators will require some type of support within the tube, i.e., mandrel, die steels, etc. 
         [0034]    The advantages of the present inventive concept include at least the following. The part can be double-tapered, which is a type of design that has proven itself to be very robust for collapsing in an accordion fashion when loaded in a near axial direction. The manufacturing “build” concept does not require a high degree of formability in the material, which allows for the use of higher grade steels. The present inventive concept expands acceptable raw steels that will work for this application, including structural steels (with tensile strength of at least 40 KSI), High-Strength-Low-Alloy (HSLA) steels (with tensile strength of at least 80 KSI) and Advanced-Ultra-High-Strength Steels (AUHSS) (with tensile strength of at least 100 KSI or more). These acceptable material grades are considerably higher than those that are acceptable for other manufacturing processes such as hydroforming and expansion. The manufacturing steps required are not unique but instead the uniqueness of this concept lies in how these manufacturing processes are combined to produce the end product. Proper material selection can result in a lighter-weight part through down-gauging material thickness and taking advantage of the higher grade materials. This can also result in a reduction of piece price. 
         [0035]    It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.