INVESTIGATION OF MICROSTRUCTURAL FEATURES ON DAMAGE ANISOTROPY

In the recent years, dual phase steel sheets have been used extensively in automotive industry in order to reduce the total weight of the vehicle without sacrificing from mechanical properties. The microstructural morphology of industrially relevant ferritic-martensitic dual phase steels consists of altering bands of ferrite and martensite which causes anisotropic and highly localized strain distributions. The effect of banded morphology on active damage mechanisms and damage evolution is studied by tensile tests along rolling and transverse directions. Tensile tests are carried out until preselected points of the stress strain curve and metallographic investigation is executed from the deformed samples. An automated Matlab© code is used in order to quantify the images in terms of void size, distribution. Three different damage mechanisms have been observed; void formation between ferrite-martensite interface, around unwanted inclusions and cracking of martensite in addition voids are heterogeneously distributed through the thickness direction of the samples. Further, strain gradient enhanced rate independent crystal plasticity simulations were conducted on idealized hexagonal microstructures with a pre-existing void under different stress states and grain orientations. It is shown that void evolution as well as stress field are highly dependent on loading conditions and grain orientations.