Maraging steels are a class of precipitation hardened steels wherein different micro-mechanisms of deformation such as planar slip, interaction with coherent/incoherent precipitates, and reverted austenite affecct the overall mechanical behavior of the material. High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time using in-situ characterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements.
EXTENDED ABSTRACT

The fracture behavior of the stainless-steel SS-304L is assessed by loading microtubes of 2.38 mm diameter under combined axial force and internal pressure, using a custom apparatus. The force/pressure ratio is controlled in the experiments, to generate different biaxial stress paths that are proportional or nearly proportional. The results from the experiments are used to calibrate the non-quadratic anisotropic yield function Yld2004-3D. Then, finite element (FE) models of the microtubes are created after incorporating the anisotropic material modeling framework, and compared with the experiments to establish their fidelity. The FE models are then used to probe the fracture behavior under the proportional loading. The failure modes of the microtubes are different depending on the stress state being axial- or hoop-stress-dominant. It is found that the structural instabilities that precede necking are different and appear at different levels of strain. The strains at the onset of fracture, as determined by probing the FE model, reveal significant fracture anisotropy, that can be possibly also attributed to the specimen geometry, beyond the material processing.
EXTENDED ABSTRACT

In this work, a comprehensive experimental campaign is conducted to investigate the effect of pre-strain on the mechanical properties of X140 steel used in high performance threaded connections. Mechanical tests are used to characterize the plastic and fracture behavior of the material. Smooth tensile (ST), notched tensile (NT), plane strain (PE) and shear tests (STC) were performed. Cyclic tension-compression tests are used to characterize kinematic hardening. Initially qualified as isotropic, this material showed an anisotropic behavior after undergoing a pre-strain expansion as its plastic flow becomes loading direction dependent. This pre-strain effect is well reproduced using a phenomenological modeling combining isotropic and kinematic hardening contributions with a Hosford’s criterion.
EXTENDED ABSTRACT

The purpose of this study is to predict the effect of loading path on ductility of two-phase steels based on micro-structural damage analyses. A micro-structural damage model that consists of 3D micro-structural FE-model and ductile damage model is proposed. Isotropic / kinematic hardening model is introduced for considering the mechanical behavior of Bauschinger effect. The effective damage concept for considering micro-scopic behavior of Bauschinger effect which is dislocation behavior in loading path change is introduced into the damage model. Two types of ferrite-pearlite two-phase steels with different volume fraction of pearlite, and ferrite and pearlite single-phase steels are used. Tensile tests using micro-tensile specimen extracted orthogonal to pre-strained direction from tensile pre-strained round-bar specimens are conducted. Ductility is increased due to loading path change, and the effect is greater in the case of higher volume fraction of pearlite. The mechanism of the effect is analyzed by numerical simulation based on the proposed micro-structural damage model. It is presented that the improvement of ductility by loading path change is caused by micro-structural heterogeneity, delay of necking due to mechanical behavior of Bauschinger effect, and non-effective plastic strain for damage evolution due to micro-scopic behavior of Bauschinger effect.
EXTENDED ABSTRACT

Improved vehicle fuel efficiency and driving safety requirements have promoted the development of Advanced High Strength Steels (AHSS) in the last few decades. The mechanical performance of AHSS is commonly characterized by the product of the ultimate tensile stress and total elongation. However, tensile elongation is not suitable for predicting the performance of a material under complex forming operations. This work aims to investigate the effect of the steel microstructure on bending performance and to make a parallel between strain partitioning and damage nucleation in tension and bending.
EXTENDED ABSTRACT

Considerable research has been invested in developing thin sheet Advanced High Strength Steels (AHSSs) and to metastabilize phases at ambient temperatures; however, little has been done to determine the extent to which the transformation from austenite to martensite (TRIP), can suppress/delay damage. The damage processes that lead to fracture in AHSSs are complex and understanding them requires a careful assessment of the strain partitioning amongst the phases, the evolution of microstructure with strain and how damage accumulates in the form of voids and microcracks. This can only be accomplished by applying a range of methodologies tracked as deformation proceeds, including micro-Digital Image Correlation (µDIC), Electron Backscattered diffraction (EBSD), X-ray microtomography (µXCT) and synchrotron-sourced High Energy X-ray diffraction (HEXRD). Such experiments have also been applied to notched specimen to further understand the response of AHSSs at different states of stress. Data will be presented on a range of ultrahigh strength AHSSs with and without TRIP-assistance (dual phase (DP), quench & partition (Q&P), and Medium-Mn steels). The data suggests that grain refinement, TRIP and decreased mechanical heterogeneity amongst phases can be used to suppress damage. It remains a challenge to quantify these effects separately, opening new avenues for experimental and modeling investigations.
EXTENDED ABSTRACT