Tubular tapered crushable structures and manufacturing methods

0001This 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

0002The present invention relates to crushable structures configured for energy absorption and energy management such as during a vehicle crash.

0003Vehicle 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.

0004Different 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.

0005The 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.

0006Tubular 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.

0007It 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

0008In 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.

0009In 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.

0010In 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.

0011These 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<figref idref="DRAWINGS">FIG. 1</figref> 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<figref idref="DRAWINGS">FIG. 2</figref> is a perspective view of a tapered die for forming the raw tubing component.

0014<figref idref="DRAWINGS">FIG. 3</figref> is a perspective view of a straight section guide tube for use with the tapered die.

0015<figref idref="DRAWINGS">FIG. 4</figref> is a perspective view of a push collar for pushing the round tubing component into the tapered die.

0016<figref idref="DRAWINGS">FIGS. 5</figref><i>a </i>and <b>5</b><i>b </i>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 <figref idref="DRAWINGS">FIG. 5</figref><i>a</i>; and <figref idref="DRAWINGS">FIGS. 5</figref><i>c </i>and <b>5</b><i>d </i>are end views of <figref idref="DRAWINGS">FIGS. 51 and 5</figref><i>b. </i>

0017<figref idref="DRAWINGS">FIG. 6</figref> is a perspective view of a mandrel set, and <figref idref="DRAWINGS">FIGS. 6</figref><i>a </i>and <b>6</b><i>b </i>are perspective views of the outer mandrels and inner mandrel, respectively.

0018<figref idref="DRAWINGS">FIG. 7</figref> is a perspective view of a compression box usable with the mandrels of <figref idref="DRAWINGS">FIGS. 6</figref><i>a </i>and <b>6</b><i>b </i>for the double-tapered rectangular tube component of <figref idref="DRAWINGS">FIG. 5</figref><i>b. </i>

0019<figref idref="DRAWINGS">FIG. 8</figref> is a perspective view of the finished double-tapered rectangular part with crush initiators.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0020The 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.

0021The 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.

0022The 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.

0023In the original round tubular component <b>20</b> (also called “round tubing” herein) (<figref idref="DRAWINGS">FIG. 1</figref>), 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 <b>20</b>B. The partially finished double-tapered rectangular tube <b>20</b>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 <figref idref="DRAWINGS">FIG. 1</figref>.) 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.

0024The round DOM tubing <b>20</b> is forced into a tapered die <b>25</b> (<figref idref="DRAWINGS">FIG. 2</figref>). The die is made from hardened steel and can be produced on a lathe. The die <b>25</b> is made in sections <b>26</b> and <b>27</b> to provide ease of handling and also to provide flexibility in changing taper angle and taper depth. A straight section <b>28</b> of the die <b>25</b> can be used to guide and support the round tubing <b>20</b> into the tapered end of the main die <b>25</b> if there are concerns associated with column bucking of the round tubing <b>20</b> as it is forced into the tapered main die <b>25</b> (<figref idref="DRAWINGS">FIG. 3</figref>). For this particular example, a straight section <b>28</b> to guide and support the round tubing <b>20</b> was not necessary and hence was not used for the DOM tubing.

0025A special push collar <b>29</b> (<figref idref="DRAWINGS">FIG. 4</figref>) was developed that fit inside the round tubing <b>20</b> to transfer push loads to the outside edge of the tubing <b>20</b> as the tubing <b>20</b> was forced into the tapered die <b>25</b>. The round tube <b>20</b> was forced into the tapered die <b>25</b> (<figref idref="DRAWINGS">FIG. 2</figref>) through a distance that coincided to its desired length. At the end point of insertion into the die <b>25</b>, the circumference of the smaller tapered end of partially-finished round tube <b>20</b>A was slightly undersized when compared to the final perimeter of the small end of the tapered rectangular shape in the finished part <b>21</b> (<figref idref="DRAWINGS">FIG. 5</figref>). The now tapered round tube <b>20</b>A is removed from the die <b>25</b> by applying an upward force to the tapered end, forcing the tube <b>20</b>A in a reverse direction back through the top of the die <b>25</b>. It is noted that the described die <b>25</b> used to taper the round tube <b>20</b>A is a piece of prototype tooling and a different die configuration might be more suitable for high volume production.

0026The tapering process may cause a length of the original tubes <b>20</b> 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 <b>20</b>. In the case of this example, the tube <b>20</b>A grew approximately 0.25 inches. The amount of length growth for the tube <b>20</b>A is dependent on the material type, material thickness and the amount of taper that is imparted on the raw tube <b>20</b>. There can be a slight increase in the thickness of the round tube <b>20</b>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 <figref idref="DRAWINGS">FIG. 5</figref>, diameter “a.”) Elongation of the round tube <b>20</b>A during tapering actually minimizes the amount of thickness change at the point where the maximum taper occurs on the tube.

0027For 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.

0028The tapered round tube <b>20</b>A is now ready for reshaping. The tapered round tube <b>20</b>A is now ready to be reshaped to a double-tapered rectangle <b>20</b>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 <b>30</b> was used to reshape the round tube <b>20</b>A (<figref idref="DRAWINGS">FIG. 6</figref>). The outer two pieces <b>31</b> and <b>32</b> of the mandrel <b>30</b> are shaped to represent the shorter sides of the rectangle (<figref idref="DRAWINGS">FIG. 6</figref><i>a</i>). These mandrels <b>31</b> and <b>32</b> include the corner radii of the finished rectangular shape. The third part <b>33</b> of the mandrel <b>30</b> is the center section (<figref idref="DRAWINGS">FIG. 6</figref><i>b</i>). The two mandrels <b>31</b> and <b>32</b> are keyed and fit together with the center section <b>33</b> of the mandrel <b>30</b>. The center section <b>33</b> of the mandrel <b>30</b> is tapered, so as the center section <b>33</b> is moved down between the two mandrels <b>31</b> and <b>32</b>, the mandrels <b>31</b> and <b>32</b> spread apart to create a tapered rectangular mandrel <b>30</b>. <figref idref="DRAWINGS">FIG. 6</figref> shows a constant angle taper to the center section <b>33</b>, but in actuality the center section <b>33</b> and/or mandrels <b>31</b> and <b>32</b> can be made of sections that are tapered and/or sections that are non-tapered.

0029The three-piece mandrel <b>30</b> often can not be used by itself to reshape the tapered round tube <b>20</b>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 <b>35</b> (<figref idref="DRAWINGS">FIG. 7</figref>) was developed to help the material flow during the reshaping operation that uses the internal three piece mandrels <b>31</b>-<b>33</b>. The compression box <b>35</b> 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 <b>35</b> 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 <b>36</b> of the compression box <b>35</b> is also one of the longer sides of the rectangle. This non-fixed face <b>36</b> of the compression box <b>35</b> is adjusted inward and against the tapered round tube <b>20</b>A while the mandrels <b>31</b>-<b>33</b> are forced down the length of the tapered round tube <b>20</b>A. The ability to adjust the non-fixed face <b>36</b> of the compression box <b>35</b> assists in the movement of material in a way that facilitates reshaping the round shape of the tube <b>20</b>A to a rectangular shape of the finished part <b>21</b> without thinning and undesirable weakening.

0030The compression box <b>35</b> 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 <b>35</b> illustrates only one adjustable movable surface. However, it is contemplated and envisioned that multiple sides of the compression box <b>35</b> 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 <b>35</b> 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's surfaces and ends.

0031In a production mode, it is envisioned that the compression box <b>35</b> can be adjusted with hydraulics, pneumatics, and/or servos. It is envisioned that adjustment of the non-fixed face <b>36</b> of the compression box <b>35</b> can be adjusted in synchronization with the position of the mandrels <b>31</b>-<b>32</b> 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.

0032The tapered shape of the rectangle in the finished part <b>21</b> 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 <b>40</b> (<figref idref="DRAWINGS">FIG. 8</figref>) are typically added to the crushable parts. The type, placement, and number of crush initiators <b>40</b> required usually will require a development effort to identify the most optimized design. The crush initiators <b>40</b> can be added to the part preferably after the final shape has been formed. For this example, the crush initiators <b>40</b> would be added to the double-tapered rectangular shape.

0033In a production mode, the crush initiators <b>40</b> can be added using any type of stamping method, hydraulic, pneumatic, etc. Internal support will more than likely be required when the crush initiators <b>40</b> are stamped into the part. It is envisioned that the crush initiators <b>40</b> can be added to the part when the internal reshaping mandrels are positioned in the part. The internal outer mandrels <b>31</b>, <b>32</b> would need relief at each of the locations where the initiators <b>40</b> are to be placed. The central mandrel <b>33</b> could be backed out of the part which would allow the two outside mandrels <b>31</b>, <b>32</b> to come free from the just-stamped-in crush initiators <b>40</b>. In a walking-beam-type production process, the crush initiators <b>40</b> 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.

0034The 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.

0035It 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.