On multiscale modeling of pearlitic steel

Sammanfattning: By studying pearlitic steels using a scanning electron microscope it can be observed that pearlite is a two-phase material with cementite lamellae embedded in a matrix of ferrite. The cementite is commonly described as being stiff and brittle in comparison to the ferrite which is considered to have a soft and ductile mechanical behavior. The observed macroscopic mechanical behavior of a pearlitic steel is a result of the individual behavior of these two constituents as well as of how they interact. It can also be noted that the cementite lamellae appear in so called colonies within which the cementite lamellae have a privileged orientation. Based on this observation it is natural to assume that the average stress-strain behavior of such a colony is anisotropic. Despite this, the macroscopic behavior of a pearlitic steel (virgin material) is considered to be isotropic which is explained by the fact that the individual orientations of these colonies are randomly distributed such that the local anisotropy is canceled out on average. It is well known, however, that subjecting a pearlitic steel to severe loads will cause the cementite lamellae to align. An example of such type of loading is the contact between a railway wheel and the rail where a large contact force is distributed over a small area. The cyclic loading causes an increasing alignment of the lamellae which results in an evolution of macroscopic anisotropy which in turn is known to also play an important role for other phenomena such as crack initiation and propagation. In this thesis the feasibility of using the concept of MultiScale Modeling (MSM) to model the mechanical behavior of a pearlitic steel is investigated. The proposed model explicitly includes the colonies as well as the cementite lamellae and the ferrite matrix. The drawback of MSM is its increased computational cost compared to traditional macroscopic constitutive modeling. However, a number of benefits of MSM are worth highlighting. To begin with, it offers a possibility to predict the anisotropic mechanisms discussed above (both the initial local subscale anisotropy and the evolving macroscopic anisotropy). It can also help to increase the understanding of the mechanical behavior of the material. Additionally, once the model is calibrated it can be used for virtual material testing in which the subscale features of the material are modified and the resulting impact on the mechanical behavior of the material can be studied. In addition to the modeling efforts presented in this thesis experimental data from tensile testing, micropillar testing and in-situ SEM images (analyzed using digital image correlation) is used to further increase the knowledge about the observed mechanical behavior of pearlite.