Page 51 - MetalForming September 2012

variables—perhaps excessive heat in the summer—changed the background strain to 15 percent, causing the gradient strain to jump proportionately to 60 per- cent (Curve 3). Since elastic stresses are proportional to the flow stresses, the stamping changed shape. During the late evening, second-shift workers noted that background strains had dropped to 5 percent, with a gradient strain of 20 per- cent (Curve 1). While the background strains varied by only 10 percent during the run, they experienced a significant 40-percent change. The stamping dimen- sions fell above and below normal buyoff values (Fig. 1B to C).
Making matters worse, the constancy-
of-volume rule requires that the
increased stretch in the gradient be bal-
anced by local thinning. Increased thinning at the gradient site acts like a hinge, magnifying or restricting the springback problems as the gradient grows or shrinks. In this case, an ultrasonic thickness gauge proves useful to detect gradi- ents in stampings without the application of grids or other sMurface-strain measurements.
The sheetmetal minimizes or maximizes gradients, but
does not generate them. The workhard- ening exponent (n-value) of the stress- strain curve shown in Fig. 1 is what con- trols gradient growth. A higher stress at the gradient site initiates greater strain than in the surrounding material. For an n-value of zero, no workhardening allows the maximum gradient to form. A low n-value allows workhardening and makes the gradient site stronger as it forms. This restricts the growth of gra- dient formation. Conversely, a high n- value can completely prevent a gradient from forming, forcing strain to redis- tribute elsewhere in the stamping.
For traditional high-strength low- alloy steels, n-value is proportional to yield strength. Increasing yield strength by 45 ksi could lower the n-value by
0.10. A change in n-value of only 0.02 can have a major effect on strain distribution. The increase in yield strength for automotive weight reduction helped drive the development of advanced high strength steels (AHSS). The primary goal of AHSS alloys such as dual-phase and TRIP (transformation- induced plasticity) steels is to increase n-values without changing yield strength. MF
Fig. 3—Die source of gradients: A) Two surfaces not joining tangentially, shown at circle; B) Die sections with gaps or mis- aligned in height or in front.
The Science of Forming
A
B
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