Page 44 - MetalForming Magazine March 2022
P. 44

 Metal Matters
ness requires changes to either the thickness or part geometry, or both.
Part design and geometry also influ- ence stiffness. Just as putting a crease in a sheet of paper changes it from flimsy to rigid, automakers use char- acter lines to increase stiffness in flat Class A panels. Beads or stiffening ribs in body-structure members increase stiffness, but the sheet metal must have sufficient formability to accommodate the additional geometry in the stamped part. This becomes more challenging in components stamped from lower- modulus materials having lower forma- bility than steel.
Thickness (t) has an outsized effect, as stiffness scales with t3. With alu- minum having one-third the modulus of steel, manufacturers negate the stiff- ness effect by designing aluminum parts 45-percent thicker than similarly designed steel parts. Some parts will still see weight savings even at this increased thickness because aluminum is one-third the density of steel.
Application-Dependent Considerations
Manufactured parts have different functions and constraints. Strength factors into the design of certain parts, while stiffness is the dominating con- sideration in others. All stampings per- manently deform the incoming flat stock, indicating that the stresses exceed the material yield strength, with parts designed to ensure that the applied stresses fall below the tensile strength. These are strength consider- ations.
Automotive-body-structure stiffness influences handling and noise, vibra- tion and harshness. Design changes, such as switching from a C-channel frame to a closed section, improve stiff- ness, as does the reduction of spacing between cross members and the use of panels with deeper draw depths or greater curvature. In these cases, designers must consider the charac- teristics and forming capabilities of the selected sheet material.
Parts used for crash-energy man- agement in automotive applications typically are stamped from the high- est-strength materials available that still are formable into the targeted shape. In contrast, when forming skin panels, while strength is important since it influences dent resistance, stiffness is the primary consideration. Here, automakers typically specify the thinnest sheet that meets the stiff- ness criteria for the skin-panel design.
Body panels stamped from alu- minum often include contours and fea- ture lines to increase stiffness, to coun- teract the reduced material stiffness as compared to higher-elastic-modulus steel grades. MF
Note: Schaeffler will speak about higher-strength steels at the PMA/Met- alForming magazine Metal Stamping and Lubrication Technology Conference, March 29-30, 2022, in Cincinnati, OH.
  42 MetalForming/March 2022
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