In the first phase of the hydroform intensive body structure (HIBS) studies, simple hydroform assemblies were substituted for conventional stamped body structures to understand cost and mass implications. In Phase II, the hydroform designs took advantage of the latest material offerings, such as boron and variable-thickness tubes, and one-sided welding in the body shop as options for additional mass savings. Both studies demonstrated solid improvements over traditional stamped designs.
In HIBS Phase III, the intent is to take a closer look at how hydroformed tubes achieve their structural efficiency over stampings. Recent applications of hydroformed tubes in new vehicles will reinforce the hypothesis that hydroform tubes are an excellent choice for primary vehicle structures
Tubes Lengthen to Allow Proper
Load Transfer to Floor Structure
View Looking Up From Bottom
No grade changes were made to the tubes since DP980 was recommended for HIBS I
Any further grade increase, brings questions about material elongation in impact
t = 1.1 mm
t = 1.3 mm
Baseline gage of the tubes were 1.3 mm (HIBS I)
Straighten areas reduced to 1.1mm
Bends maintained at 1.3mm
Targeted for opportunity to take out weight and cost.
In order to maintain continuity from the front structure to the dash/underbody, the rails of the front structure were lengthened and assumed to be laser welded into the floor structure. In addition, smaller stampings were added to provide an attachment for the front cradle.
Higher material grades allow tube thickness to be reduced and optimized
t = .70 mm
t = .75/.80 mm
t = .90 mm
Tube Material Grade Boron was chosen to increase material strength and save weight. Boron can be heat treated to achieve its material properties through several different methods. In this analysis, it was assumed to be processed similar to a stamping process where the hydroformed tube would be heated and then cooled in a holding fixture.
Tube Gage Optimization
Similar to the front end structure, the b-plr. and roof rail tubes were segmented and evaluated for optimum gage. Both the roof rail and b-plr. tubes were split in 3 areas.
Roof Crush Data
Roof Crush Example:
Parallels of constant value
Cost vs. Mass Parallels of constant value: US$9.00/kg.
Tube Material Grade
In the HIBS I rear structure design, the material of the tubes was primarily DP800. Because the tube gage was already low, the material was not increased to DP1000. To take as much weight out as possible, in HIBS II, the material grade was increased to DP 1000 to understand its effects on weight.
Tube Gage Optimization
the hydroformed rear
rail was broken into 2
areas. This rail is
already highly optimized
there was some weight
reduction by reducing
the gage of the front
portion of the rail.
Stampings have been the primary way to manufacture Body-in-White (BIW) parts since the early days of the automobile. Stamping allows the design flexibility to create simple or complex parts using a well-understood technology with an established manufacturing base. Using stampings, a BIW engineer must provide for a number of design criteria; including attachment features such as holes and slots, formations to clear adjacent components and manufacturing feasibility for the specified material. Balancing these criteria can sometimes compromise the most important function of the stamping; its structural capability.
In addition, with increasing impact requirements and safety regulations, these structures now demand higher performance.
BIW engineers are very familiar with designing for stamped parts, so it is common practice to balance the packaging needs with new structural requirements by simply increasing the gage and grade of the stampings.
Also, since stampings have been the mainstay of BIW construction, most body shops are set up to accommodate them. Well established spot welding lines are in place to bring these stampings together to form the body.
So, if an engineer can alter gage and grade to meet changes in performance, utilize a welding method that has been in place for decades, why go to any other method to manufacture these parts? Two words; structural efficiency.
Thicker and stronger stamped designs may meet the new structure and packaging needs, but typically at a mass penalty.
In this study we will take a look at why hydroform tubes are structurally more efficient than stampings, elaborate on the findings from HIBS I and lastly look at real world applications that recognize the efficiency of hydroform tubes.
Structural efficiency is a key concern when dealing with compression-dominated structures. Geometry is a critical element for the load carrying capability of these structures. This is not the case for tension dominated structure. These compression forces are usually a result of bending, where one side of the section is in tension and the other side is in compression.
There are many structures in a BIW that are subject to bending, but of note are the roof rail and B-pillar. The roof rail will see high bending stresses in load cases such as roof crush, side pole impact and small offset. The B-pillar will see high bending stresses in load cases such as roof crush and side impact.
Today’s typical B-Pillar construction involves three parts; the body side inner, B-Pillar reinforcement and body side outer as
In some instances other local reinforcements may be added, but let’s stick to a three-piece construction.
In this view, the long fore-aft walls are the walls that are of concern when it comes to structural performance (buckling). Under roof crush loads, the body side inner is in compression. In a side impact, the B-Pillar reinforcement and body side outer are in compression.
The cost and availability of high tensile strength steel has improved significantly over the past 15 years. What once were mild steel reinforcements in the A and B-Pillars are now routinely made out of boron steel
The typical steel “banana chart” compares various steel grades. When reviewing the history of pillar material choices, there is a gap where advanced high-strength materials suddenly jumped to ultra high-strength steel such as boron. Part of this is due to new side-impact crash management needs where the high strength of boron steel helps meets the performance targets. Hot-formed steels also permit good shape and manageable spring back. While it is possible lesser grades could have met performance targets, the lack of forming properties and high spring back of HSLAs, and DP materials or the high prices of CP and TRIP materials made boron an attractive choice.
In creating automotive structures, larger section sizes are desired to provide a stiffer body, yet wall thickness is typically decreased to offset the increase in mass. There is a temptation to throw as strong a material as possible into the design to compensate for this strength compromised structure. In compression-dominated members, any local buckling of the surface will reduce load-carrying capability. They are further compromised by the myriad holes and features needed for integration reasons, shown here :
By calculating the critical buckling stress for the section, we can better refine our choice of material parameters. Shown below is the equation to determine the critical buckling stress of a simple section. Please note that in this equation, geometry is the main factor with no influence of material strength.
First, the edge boundary conditions and the aspect ratio (a/b) of the compression surface(s) are needed to determine the buckling coefficient kc.
Using the buckling coefficient chart
we observe that as the aspect ratio (a/b) approaches 2, the buckling coefficient (kc) can be considered constant for a defined edge boundary condition.
Depending on the section, the edge boundary conditions can fall between fully constrained (Curve A) and simply supported (Curve C). For this example we will assume a balance between simple supported (kc = 4) and fixed (kc = 7), so kc will be 5.5
Assuming Poisson’s Ratio for steel is 0.3 and Young’s modulus of elasticity is 29.6×106 psi, the critical buckling stress is:
By plotting the critical stress against the t/b ratio we can then narrow down our material parameters. For a given t/b ratio, there is a critical stress above which
the material is no longer effective in resisting local buckling. Increasing the material grade will get you very little performance as the structure is “geometry-limited”.
Any improvements will need to be focused on section shape before reconsidering material grade.
Another way to interpret is to consider the typical yield strengths of common steel grades. For a t/b ratio that falls to the right of the critical stress curve, a material upgrade could improve performance. However, for those ratios that fall to the left of the curve, the shape of the section (compression side) will drive performance.
A t/b ratio of 0.02 is a likely ratio in automotive structures (such as a 1.2mm thick surface (t) with a width of 60mm (b)). Using Figure 6, for 0.02, the critical buckling stress is approximately 420MPa. A BIW engineer could specify up to a DP780 material and be confident the material is working efficiently. Anything more than that will fall to the left of the critical stress curve and provide little strength benefit.
To understand how critical geometry is in compression, we constructed a very simple model of a beam and evaluated it in both column strength (straight compression) and in bending (combination tension and compression). For the large open section (t = 2.0 mm, b = 6.0 in.) t/b = .013. This section would be good for about 180 MPa compressive stress before buckling. Now if we take that same over-all section size and split it in two (assume 2 hydroform tubes side by side) we have in essence increased the t/b ratio by a factor of 2 to t/b = .026 (assuming a gage of 2.0 mm). This would give you an allowable buckling stress of 650 MPa. This is 3.6 times higher than the base structure. Now what can be done is down gaging to get equivalent performance between the two sections. Down gaging to 1.4 mm ended up giving equivalent performance but provided a weight reduction of approximately 15%. This shows how improving the section geometry can allow for weight reduction.
Results (designs with similar performance):
Certainly one can reference the Critical Buckling chart
and read off a certain t/b ratio to determine if a structure is geometry-limited. Real-world examples cannot be categorized this easily. As can be seen from the surfaces there are holes, formations etc. which complicate the choice of a simple t/b ratio. Vari-Form has done extensive CAE analysis on both production intent sheet metal and hydroform structures.
Shown here are the results of a roof crush was performed on a structure and iterated for three material grades. Once the material strength approaches 1000 MPa (tensile strength) on the roof rail reinforcement, there is little performance difference in roof crush if the material properties are upgraded to UHSS grades. Yet, if you reduce the material strength, you see a proportional drop in load carrying capability that is directly proportional to the drop in yield strength.
So if you go back to the Critical Buckling stress curve, we believe that most of the structure is going to act in the t/b range of ≤.025. At this point, material > DP 980 strength is not fully utilized because the structure buckles before the full yield strength is reached.
Our experience over many similar roof studies show that materials beyond 1000MPa provide little roof crush improvements unless the geometry supports it.
While boron has good formability on its side, much of the potential strength of the material is never utilized. BIW engineers are well advised to explore technologies that allow geometric optimization and more material flexibility. That is why hydroformed tube structures are an excellent choice to maintain performance and be competitive in weight.
At EuroCarBody 2014 in Germany, Mazda presented similar findings:
Mazda demonstrates that, as thickness decreases in a structure, the actual percentage of “useable” yield strength decreases. Another way to look at it is if you increase the yield strength of the material by 2x, don’t expect to get a 2x gain in load carrying capability.
If you go back to our roof crush analysis (Figure 7), this is exactly what you see. Ford released similar findings at the 2011 Great Designs in Steel Conference in Michigan.
So do these analytical studies translate into real world vehicles? The answer is yes. Again we take a look at some information from a previous Ford presentation on the Fusion/Mondeo. The hydroform B-pillar and its components improves upon hot-stamped boron designs.
Stiffness is important when evaluating things like NVH. In Ford’s case, the tubular design is not as stiff. However, for A and B-pillars, the load carrying capability is more important. Roof crush, pole impact and side impact are the loads that these structures are designed to manage efficiently. This change from tailor welded boron stampings to hydroform tubes resulted in a mass savings of 4 kg per vehicle. Ford also commented on significant cost savings with using the hydroform tubes, but did not quantify those results.
As we showed in HIBS I, weight reduction could be achieved in the body side structure by going to hydroform tubes that create compact sections that are not geometry-limited. This was validated by creating simple beam models for compact sections. HIBS II showed that there was more weight that could be taken out of the structure by exploiting the latest material grades, but not nearly what the initial change to a compact hydroform tube provided. In HIBS III, we talk about the theory behind the efficiency of the hydroform tubes, and use OEM data to justify these claims.
As can be seen from this paper and the ones before it, there are real opportunities to reduce the mass of a vehicle by creating better compression dominated structures. Ever higher material strength is only valuable if the geometry of the section can support it. Hydroform tubes are a good way to execute this geometry and can be readily integrated into the vehicle.