Forming Limit Analyses of Cold Rolled IF Steel Sheet Using Differential Work Hardening Model

AbstractA servo-controlled multiaxial tube expansion testing machine was used to measure the multiaxial plastic deformation behavior of a cold rolled IF steel sheet for a range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to a tubular specimen using an electrical, closed-loop servo-control system for an axial force and an internal pressure. Tubular specimens with an inner diameter of 44.6mm were fabricated from a cold rolled interstitial-free steel sheet with a thickness of 0.7mm by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve and forming limit stress curve of the as-received sheet sample, in addition to the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work. The observed differential work hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al., 2003) as a function of the equivalent plastic strain. The forming limit curve and forming limit stress curve were calculated using the Marciniak-Kuczyński-type approach and the differential work hardening model. The calculated results were in fair agreement with the measurement