Abstract:
A bumper beam includes an aluminum tubular carrier that is wrapped with a plurality of carbon fiber fabric layers. The carbon fiber fabric is pre-impregnated with a two-part epoxy resin system. The layers of carbon fiber fabric are applied in an alternating series with the carbon fibers in adjacent layers extending perpendicularly relative to the carbon fibers in adjacent layers.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to lightweight vehicle bumpers that have superior strength to weight properties. 
       BACKGROUND 
       [0002]    There is a substantial need to reduce vehicle weight to meet stringent fuel economy standards. Vehicle bumpers are relatively large parts that are relatively heavy due to the need to protect the vehicle and passengers during a collision. 
         [0003]    This disclosure is directed to providing a solution to the above problem and other problems as summarized below. 
       SUMMARY 
       [0004]    According to one aspect of this disclosure, a bumper beam is provided for a vehicle that comprises a metal carrier, and a plurality of layers of carbon fiber fabric. The fabric is wrapped about the carrier and has carbon fibers extending in one direction. The fabric is pre-impregnated with a resin and a curing agent. The fabric is wrapped about the carrier with the fabric being arranged so that carbon fibers in alternating layers are oriented perpendicular to adjacent layers. 
         [0005]    According to a further aspect of this disclosure, the carbon fibers in a first set of layers are oriented to extend in the longitudinal direction that is aligned with the length of the bumper beam. The carbon fibers in a second set of layers are oriented to extend in the transverse direction that is aligned with the vertical direction on a front wall and a back wall of the bumper and aligned with a longitudinal vehicle direction on a top wall and a bottom wall of the bumper beam. 
         [0006]    According to other aspects of this disclosure, the metal carrier may be a rectangular cross-section aluminum tube. The aluminum tube may have a front wall, a rear wall, a top wall, and a bottom wall. A bracket for a crash can is secured to the rear wall of the tube and over the layers of fabric. The bracket may include an upper clamp and a lower clamp that clamp the layers of fabric to the top wall and the bottom wall, respectively. 
         [0007]    The resin and curing agent are two parts of an epoxy bonding system. At least fifteen layers of carbon fiber fabric may be wrapped about the bumper beam. In another embodiment, the bumper beam may have between sixteen and twenty layers of carbon fiber fabric are wrapped about the bumper beam. 
         [0008]    According to other aspect of this disclosure as it relates to a method of making a bumper beam, a method is disclosed for making a bumper beam. The method comprises selecting a metal carrier; and wrapping the carrier with a plurality of layers of carbon fiber fabric. The fabric may be coated with a resin. The layers include a first set of layers that include carbon fibers extending parallel to the length of the bumper beam and a second set of layers that include carbon fibers extending around the width and height of the bumper beam. 
         [0009]    According to other optional aspects of this disclosure, the method may further comprise assembling the carrier with the plurality of layers of carbon fiber fabric to a crash can bracket. The aluminum tube may have a front wall, a rear wall, a top wall, and a bottom wall and wherein a bracket for a crash can is secured to the rear wall of the tube and over the layers of fabric. The bracket may include an upper clamp and a lower clamp that clamp the layers of fabric to the top wall and the bottom wall of the tube, respectively. 
         [0010]    The resin and curing agent may be two parts of an epoxy bonding system. The method may include wrapping at least fifteen layers of carbon fiber fabric about the bumper beam. Alternatively, the bumper beam may have between sixteen and twenty layers of carbon fiber fabric are wrapped about the bumper beam. The method may further comprise a step of stacking a plurality of layers of the fabric with the fibers in adjacent layers being oriented perpendicularly relative to the adjacent layers that is performed before the step of wrapping the carrier. 
         [0011]    The above aspects of this disclosure and other aspects will be described in greater detail below with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a fragmentary perspective view of part of a front end of a vehicle including a bumper beam made according to one aspect of this disclosure, crush cans, crush can brackets, and front rails. 
           [0013]      FIG. 2  is a fragmentary perspective view of a bumper beam showing the carbon fiber reinforcement layers partially removed. 
           [0014]      FIG. 3  is a schematic representation of a test fixture used to test a bumper bar. 
           [0015]      FIG. 4  is a graph comparing the force of resistance over time for a boron bumper bar, and aluminum bumper bar, an aluminum bumper bar having 20 layers of carbon fiber fabric and aluminum bar having 16 layers of carbon fiber fabric. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A detailed description of the illustrated embodiments of the present invention is provided below. The disclosed embodiments are examples of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale. Some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed in this application are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art how to practice the invention. 
         [0017]    Referring to  FIGS. 1 and 2 , a bumper beam  10  for a vehicle includes a carrier  12  that is wrapped with a carbon fiber fabric  14 . The carbon fiber fabric  14  includes a plurality of carbon fibers  16  (shown in  FIG. 2 ) that are oriented in one direction. It should be understood that when the carbon fibers  16  are stated to extend in one direction, that the majority of the carbon fibers  16  in the carbon fiber fabric  14  extend in that one direction while some of the fibers may be oriented in another direction. The bumper beam  10  has a length L, a height H and a width W, that are indicated in  FIGS. 1 and 2 . 
         [0018]    The carrier  12  includes a front wall  18  and a rear wall  20 . A top wall  22  and bottom wall  24  extend between the front wall  18  and rear wall  20 . The carrier  12  is attached by a bracket  26  that is assembled to a crash can  28 . An upper clamping member  30  and a lower clamping member  32  are provided by the bracket  26  to retain the carrier  12  in position in front of the crash can  28 . 
         [0019]    The carbon fiber fabric  14  is pre-impregnated with a resin. The resin is preferably included as part of a two-part resin system such as an epoxy resin that includes a curing agent. Alternatively, other resin systems may be used to pre-impregnate the carbon fiber fabric  14 , such as a resin that sets upon the application of heat. 
         [0020]    With specific reference to  FIG. 2 , the bumper beam  10  is shown wrapped with a plurality of layers of carbon fiber fabric  14 . The carbon fibers  16  in the outer layer extend in the direction L and the next layer includes carbon fibers  16  that are oriented in the height direction H. The carbon fiber fabric  14  is wrapped over the top wall  22  and bottom wall  24  with the carbon fibers extending in the width direction W. The number of layers of carbon fiber fabric  14  wrapped on the carrier  12  is determined by the desired strength of the bumper beam  10 . At least fifteen layers of carbon fiber fabric should be wrapped around an extruded aluminum tube having a wall thickness of 1 mm to obtain the required strength. 
         [0021]    In one example, sixteen plies of carbon fiber fabric  14  are wrapped about a carrier  12  that has aluminum walls having a thickness of 1 mm. The combined weight of the sixteen plies of carbon fiber fabric and the aluminum carrier is 2.541 kg. 
         [0022]    As another example, twenty layers of carbon fiber fabric  14  are wrapped around a carrier  12  having aluminum walls with a thickness of 1 mm. The carrier  12  and twenty layers of carbon fiber fabric have a weight of 2.715 kg. 
         [0023]    Referring to  FIG. 3 , a test fixture  34  that is used to test the strength of a bumper beam by evaluating the force absorbed over time by the bumper beam  10 . The test fixture  34  includes a pair of backing posts  36  that are spaced apart a predetermined distance and engage the rear wall  20  of the bumper beam  10 . An impact post  38  is driven into the bumper beam  10  at a central location relative to the backing post  36  in a direction shown by the arrow in  FIG. 3 . The impact post  38  engages the front wall  18  of the carrier  12  to apply a measured force to test the strength of the bumper beam  10 . 
         [0024]    Referring to  FIG. 4 , a chart is provided that compares load test results for carbon fiber fabric wrapped bumper beams to bumper beams made of boron having a wall thickness of 1.46 mm and a weight of 4.123 kg and an aluminum bumper beam having a wall thickness of 3.5 mm and a weight of 3.421 kg. All of the bumper beam test results presented are considered to be acceptable in that they met the load bearing specifications for a vehicle bumper. The first example above of an aluminum carrier having aluminum walls with a thickness of 1 mm wrapped with sixteen plies of carbon fiber fabric had greater initial load carrying stiffness than either the aluminum or boron bumper beam. The overall strength of the sixteen ply wrapped bumper beam was comparable to the boron bumper beam and the extruded aluminum bumper beam and was within approximately 3 KN over the last half of the test. 
         [0025]    The bumper beam  10  having an aluminum carrier with 1 mm thick walls that is wrapped with 20 plies of carbon fiber fabric  14  is shown by the graph to have slightly greater initial impact strength of more than 20 KN at approximately seven ms. The strength of the beam throughout the latter half of the test is nearly identical to the boron bumper beam and the aluminum bumper beam that were tested. The carbon fiber wrapped bumper beams show a 20 to 38% lighter weight compared to traditional boron steel and extruded aluminum bumper beams. The carbon fiber fabric wrapped bumper having twenty wraps achieves a weight savings of 1.4 kg, or 34% compared to a boron bumper beam. The carbon fiber fabric wrapped aluminum carrier having sixteen wraps achieves a weight savings of 1.582 kg, or a 38% weight savings compared to a boron bumper beam. 
         [0026]    The weight savings compared to an extruded aluminum bumper is approximately a 20% weight savings compared to an aluminum carrier wrapped with 20 plies of carbon fiber fabric. 
         [0027]    When compared to a carrier wrapped with 16 plies of carbon fiber fabric, the weight savings exceeded 25% compared to the extruded aluminum bumper. 
         [0028]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the disclosed apparatus and method. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure as claimed. The features of various implementing embodiments may be combined to form further embodiments of the disclosed concepts.