| In small radius bending (with radius-to-thickness ratio R/t < 3) large plastic deformation occurs in both outer surface (in tension) and inner surface (in compression). While outer surface fracture is a widely studied subject in sheet metal forming, the inner surface fracture due to residual tensile stress from recovery of compression has not been reported.;In this thesis, experimental observation on inner surface cracking of 6000 series aluminum alloys is reported, and a new method of through-thickness strain distribution measurement using statistical grain shape change is developed. To understand the mechanism of inner surface cracking computer simulation is performed by FEA with Abaqus standard code and solid element. Complex strain and stress evolution and neutral surface movement during initial small-radius bending and residual stress development after recovery are reported. The characteristics of bending deformation is first described for progressive strain and stress development as a function of punch travel displacement. Then a parametric study is provided on the effect of R/t (R = 0.5, 1, 2, 3 and 4 mm) and material parameters (strain hardening exponent). With decreasing bending radius the resulting circumferential stress and strain are found to be increasing, so is the residual stress. However, the FEA results indicate that the residual stresses at the inner surfaces generally do not exceed the post-yielding flow stress at the end of loading cycle. Based on microstructural study under SEM and Optical Microscopy, the inner cracking is found to closely related to the non-uniform deformation between lattices and grain boundaries that produces surface notches and leads to much higher local stress and stress intensity. |
