FATIGUE-RESISTANCE SHEET SLITTING METHOD AND RESULTING SHEET

1 - 32 . (canceled)

33 . A sheet of material formed for bending along a bend line comprising:

a sheet of material having a plurality of bend-inducing structures configured and positioned to produce bending along the bend line, at least one bend-inducing structure having arcuate return portions extending from opposite ends of the bend-inducing structure;

wherein the arcuate return portions return along the bend-inducing structure toward the other return portion; and

wherein the arcuate return portions curve back toward the bend line at distal ends of the return portions.

34 . The sheet of material as defined in claim 33 , wherein the bend-inducing structures are one of slits, grooves and steps.

35 . The sheet of material as defined in claim 34 , wherein the bend-inducing structures are configured and positioned to produce edge-to-face engagement of the sheet of material on opposite sides of the bend-inducing structures during bending.

36 . The sheet of material as defined in claim 33 , wherein the return portions are each configured to significantly reduce stress concentrations resulting from loads oriented in a direction transverse to the bend line.

37 . The sheet of material as defined in claim 36 , wherein the arcuate return portions have a radius of curvature section over a majority of their length of at least about 2 times the thickness dimension of the sheet of material.

38 . The sheet of material as defined in claim 36 , wherein at least one of the bend-inducing structures is an arc having a convex side facing and extending along the bend line, and each of the opposite ends of the arc transition to the respective return portion along an arc having a radius of curvature of between about 0.1 to about 1.0 times the thickness dimension of the sheet of material.

39 . The sheet of material as defined in claim 36 , wherein the transverse dimension of the bend-inducing structure from the bend line is less than about 20 percent of the overall length dimension of the bend-inducing structure.

40 . The sheet of material as defined in claim 33 , wherein the radii of curvature of the return portions are at least about 5 times the radii of curvature of the opposite ends of the bend-inducing structure.

41 . The sheet of material as defined in claim 33 , wherein the opposite ends have a first radius of curvature less than the thickness of the sheet of material, and the arcuate return portions have a second radius of curvature several times greater than the thickness of the sheet of material.

42 . A sheet of material as defined in claim 41 , wherein the first radius of curvature is about 0.124 to about 0.468 times the thickness of the sheet of material.

43 . A sheet of material as defined in claim 42 , wherein the second radius of curvature is about 3.161 to about 4.320 times the thickness of the sheet of material.

44 . The sheet of material as defined in claim 33 , wherein the arcuate return portions have chords oriented substantially parallel to the bend line, each of the return portions having a chord length along the bend line at least about 20 percent of the length of the bend-inducing structure along the bend line.

45 . The sheet of material as defined in claim 33 , wherein the sheet of material is bent along the bend line into a three-dimensional structure suitable for loading transversely to the bend line.

46 . A method of increasing the fatigue resistance of a structure formed by bending a sheet of material along a bend line having a plurality of bend-inducing structures, the method comprising:

forming the sheet of material with the plurality of bend-inducing structures to produce bending of the sheet of material along the bend line;

bending the sheet of material along the bend line to produce a three-dimensional bent structure; and

during the forming step, forming at least one of the bend-inducing structures with opposite ends curving away from the bend line and arcuate return portions;

wherein the arcuate return portions curve away from the opposite ends and back along the slit toward the other return portion; and

wherein the return portions curve back toward the bend line at distal ends of the return portions.

47 . The method as defined in claim 46 , wherein the step of forming the bend-inducing structures with arcuate return portions is accomplished by forming the arcuate return portions to have a radius of curvature between about 2 and about 4 times the thickness dimension of the sheet of material.

48 . The method as defined in claim 46 , wherein during the forming step, forming the bend-inducing structures as one of slits, grooves and steps in the sheet of material.

49 . The method as defined in claim 48 , wherein during the forming step, forming the bend-inducing structures with a configuration producing edge-to-face engagement of the sheet of material on opposite sides of the bend-inducing structures during bending.

50 . The method as defined in claim 46 , and the step of: after the forming step, bending the sheet of material into a three-dimensional structure.

51 . The method as defined in claim 46 , wherein during the forming step, forming the arcuate return portions with a length dimension along the bend line and radius of curvature sufficient to substantially increase the resistance to fatigue failure when the structure undergoes transverse loading.

52 . The method as defined in claim 46 , wherein during the forming step, forming the arcuate return portions with chords oriented substantially parallel to the bend line, each of the return portions having a chord length along the bend line at least about 20 percent of the length of the bend-inducing structure along the bend line.