Search

Plastic deformation characteristics and limit strains are simulated for textured face‐centered cubic polycrystalline sheets using a generalized Taylor‐type crystal plasticity model. The r‐values are predicted to be 1.04, 7.74 and 0.17 for the random, {111}〈uvw〉 and {001}〈uvw〉 textures, respectively. The {111}〈uvw〉 texture gives limit strains as large as the random texture, whereas the {001}〈uvw〉 texture yields limit strains evidently higher than the other two even though its r‐value is extremely low. Thus, the r‐value cannot act as an indicator to the stretchability of sheet metals. For the {001}〈uvw〉 texture, a superior strain‐hardening ability under plane‐strain stretching mode is found to be responsible for the increase in the limit strains under plane‐strain and equi‐biaxial stretching modes. We conclude that the enhancement of the strain‐hardening ability for plane strain mode is one of the key factors for high stretchability sheets.

Forming limit strains of 270 MPa grade cold rolled steel sheet have been precisely measured under linear and combined strain paths using a newly developed, double‐action punch‐stretching testing apparatus. In the combined strain paths, the test material is subjected to equibiaxial tension followed by plane‐strain tension. It is found that the forming limit strains are path‐dependent and that those measured for linear strain paths are higher than those for the combined strain paths. Furthermore, in order to check the path‐independency of the forming limit stresses [Arrieux, R., Bedrin, C. and Bovin, M.: Proc. 12th IDDRG Congress, (1982), 61–71], forming limit stresses of the test material have been calculated for the linear and combined strain paths using the numerical method proposed by Stoughton [Stoughton, T. B.: Int. J. Mech. Sci., 42 (2000), 1–27]. The forming limit stresses calculated from the combined strain paths have almost coincided with those determined for linear strain paths. The FLSC concept is therefore valuable for predicting the failure of metal sheets in plane stress states, particularly in multistage forming.

The effects of changing strain paths on forming limit stresses of sheet metals are investigated using the Marciniak‐Kuczynski model. Forming limits are analyzed for proportional loading and two types of combined loadings: combined loading which includes unloading between the first and second loadings and that which includes an abrupt strain path change without unloading between the first and second loadings. The forming limit stress curves in stress space calculated for the combined loading with unloading are in good agreement with those calculated for the proportional loading, while the forming limit curves in strain space are strongly dependent on the strain paths. The forming limit stresses calculated for combined loading with an abrupt strain path change, however, do not coincide with those calculated for proportional loading. The strain path dependence of the forming limit stresses is discussed in detail.