Ductile damage mechanism under shear-dominated loading: In-situ tomography experiments on dual phase steel and localization analysis

Abstract

The nucleation, evolution and coalescence of voids is the well-established mechanism leading to ductile fracture under tension-dominated loading conditions. From a theoretical point of view, the same mechanism also applies to sheardominated loading conditions. Here, an attempt is made to provide for the first time tomographic evidence of damage nucleation and evolution under shear-dominated loading in a modern engineering material. Monotonic experiments are performed on a flat double gage section smiley-shear specimen on the laminography stage of a synchrotron X-ray line. Based on fifteen scans of the entire gage section at a voxel resolution of ~1µm 3 , the mesostructural evolution inside a ferrite-bainite steel (FB600) is imaged in 3D up to the instant of specimen fracture. It is found that the asreceived material includes a volume fraction of about 0.015% CaO particles. Upon mechanical loading at stress triaxialities evolving from 0 to 0.3, the ductile matrix detaches from these ~5µm diameter second phase particles, creating a prolate void space whose principal axis is aligned with the principal direction of the applied macroscopic field of deformation. The void space continues to grow while developing micro-crack like features. A large deformation analysis is performed on a representative volume element of the particle-matrix mesostructure replicating the experimental observations of void growth in an approximate manner. Furthermore, the simulation results suggest that a porosity as low as 0.05% is already sufficient to cause the ductile failure under shear-dominated loading through the formation of a band of localized plastic deformation at the mesoscale. The combined experimentalnumerical results demonstrate that the well-known ductile damage mechanism of void nucleation, growth and coalescence is also responsible for ductile failure at low stress triaxialities.

A micromechanics-based non-local damage to crack transition framework for porous elastoplastic solids

“…Therefore, even if a proper calibration of the GTN model can crudely reproduce the coalescence stage, a different model is more suitable to describe and to predict this physical process (Brocks et al, 1995;Benzerga and Leblond, 2010;Besson, 2010). In this context, different coalescence modes compete depending on the loading conditions and the micromechanical structure as recently studied by Pineau et al (2016); Cortese et al (2016); Liu et al (2016); Brünig et al (2018); Roth et al (2018). The most investigated mode is by internal necking: the onset of coalescence is reached when plastic flow localises inside the intervoid ligament oriented normal to the main loading direction.…”

Modelling of localized ductile fracture with volumetric locking‐free tetrahedral elements

“…Both conventional and localizing gradient enhanced models are calibrated to match the hardening and softening behavior obtained experimentally. As reported in Roth et al, 49 unstable crack growth occurs when the loading goes beyond loading point 6 (which is too fast to be recorded by equipment), following the notation in Roth et al 49 Referring to Figure 23, this rapid softening behavior is captured by localizing gradient model. In contrast, a steady and smooth softening response is obtained after point 6 with the conventional gradient model.…”

“…Focussing on ductile fracture, the fracture process has been observed to initiate from diffused void nucleation and growth, followed by void coalescence, and eventually lead to an experimentally localized crack growth and fracture. [47][48][49] Since the nonlocal term at the macro-level characterizes the interactions between active micro-processes, the length scale parameter in a gradient enhancement should decrease with damage. A more intuitive formulation with a decreasing length scale (c) was also considered in Geers et al, 45 where in Equation (32) decreases with increasing damage.…”

“…In this section, we compare and contrast the predictions obtained with the conventional and localizing gradient enhancement, in capturing the development of a macroscopic crack. To this end, we refer to the experiment by Roth et al, 49 with a so-called smiley plate in shear-dominated fracture. This test is designed and conducted on ferrite-bainite steel (FB600).…”

“…In Roth et al, 49 an anisotropic plasticity model with a linear combination of power and exponential hardening law 53 is adopted. In this study, the isotropic plastic-damage model in Equation (9) with power hardening law is considered for simplicity.…”