METALFORMING / MAY 2002

www.metalformingmagazine.com


TechUpdate:
Bistable-Domes Can Control Springback, Create Shape Memory


Bistable domes, formed in thin high strength sheetmetal, create a very thin stiffened structure that bends incrementally and holds its shape, according to Paul Ericson, an independent researcher who has earned patents for applications resulting from these forms.

Flex-actuated bistable domes offer a simple way to control flexural forces in thin, strong, flexible materials, and can be used to control springback and create shape memory, claims Ericson. They may help reduce stamping and tooling requirements and present opportunities for stronger, lighter construction that could increase fuel efficiency for the transportation industry, for example. They also may aid in prototyping and provide a method for computer modeling of shapes.

A flex-actuated bistable dome is a very low profile indentation formed so that when material around a dome is bent, the dome will buckle and snap toward the outside of the curvature. During bending, overlapping bistable domes share and neutralize flexural forces by switching sides (dimpling in the opposite direction) in numbers proportional to curvature.

Bistability only can be created if certain dome material, dimensional and formational requirements are met. Bistable domes generally are not raised much higher than the material they are formed in, and the material must be hard enough so that it doesn’t absorb the flexural forces that switch the domes from one side of the sheet to the other, according to Ericson.

Sensitivity to flexure can be con-trolled by changing material character-istics, relative dimensions and overlap percentage. Within a range a dome can be equally bistable—equally sensitive to flexure from either side—or have a biased sensitivity for one side.

When a bistable dome structure constructed of equally bistable domes is flat-tened, approximately half of the domes will orient to one side of the sheet and half to the other. When all domes are oriented to one side or the other, the structure has reached its minimum radius limits. In between these two states the domes will switch in patterns that reflect the changing curvature of the sheet.

An array of closely packed overlapping bistable domes form a honeycomb pattern. The common perimeters form hexagons, and such sheet appears as a flattened version of Buckminster Fuller’s geodesic dome structures. The similarity does not end there, however. The domes interlock laterally as well as back and forth through the central plane, or neutral flex axis, and offer structural advantages similar to Fuller’s structures, especially in stability and stiffness.

Arrays of closely spaced, overlapping bistable domes can provide control of flexural forces that cause springback when shaping thin high strength materials. For example, 0.1mm thick 300 series spring tempered stainless steel sheet can be preshaped with closely spaced overlapping bistable domes to form a stiff truss-like structure that can be shaped and reshaped with little or no springback. Wherever the sheet is bent, domes adjust orientations incrementally, snapping toward the outside of the curvature to neutralize flexural forces and lock in the new shape.

Bistable dome structures also may be used to create localized flexural forces that produce shape memory. This can be done by managing dome bistability characteristics in patterns and zones. Here, the dome structures force the sheet into a preferred shape.

Domes can be formed while the sheet is in a flattened state. It may be possible, according to Ericson, to manage memory strength so that the domes remain locked in this flattened state until a necessary triggering force is applied.

Overlapping bistable dome arrays can noticeably stiffen the material they are formed in. When bent, the dome experiencing the greatest concentration of forces—or the weakest dome in the area—will switch first. As the bending motion continues, more domes will switch toward the outside of the curvature. Locking domes into one state or another stiffens the structure further.

Orientation patterns, structures and shapes may be locked in mechanically or with external coatings, during or after formation, for permanent stiffening. Coatings with more flexibility may be used to control stiffness/flexibility and indentation rebound in the overall dome structure or particular zones by controlling the ability of individual domes to switch out of their preferred orientations.

For more information on applications, contact Paul Ericson at ericsonp @earthlink.net, or visit his website, www.bistabledome.org.