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Damage in dual phase steels under industrial forming conditions.

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2019
Çobanoğlu, Merv
Dynamic strain aging (DSA) is an undesirable mechanism that affects the deformation of dual-phase (DP) steels. In the first part of this thesis, the DSA behavior was studied with tensile tests coupled with digital image correlation (DIC) technique. Strain maps revealed band-type strain localizations under the DSA conditions, which were similar to PLC-type of banding. After the tensile tests, fracture surfaces were investigated by electron microscopy, and the fracture behavior has changed with different test conditions. In the second part, the relation between DSA and damage in DP steels were investigated through tensile tests that were designed to reveal the changes in elastic modulus. The damage parameter was measured and compared under quasi-static and industrial deformation conditions (temperatures: 25 vs. 200, 300 ºC and strain rates: 10-3 vs. 10 s-1) for DP590 and DP800 steels. At a given strain, the damage values can be up to 700% higher at industrial forming conditions, under which dynamic strain aging (DSA) controls the deformation behavior. Measured damage parameters were also confirmed with the void fraction characterization by microscopy, which also provided details on the void shape and distribution with respect to the deformation conditions.