Steffen Brinckmann

The sandia fracture challenge: Blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

Crack deflection in multi-layered four-point bending samples

Four-point bending experiments are conceptually the method of choice when investigating the delamination strength of multi-layered components, which are of particular interest for semiconductor applications. However, experimental studies have shown that the crack continues as mode-I crack in most cases while delamination is rarely observed, thus making the four-point bending method useless. This study uses the finite element method with cohesive zones to study crack propagation and the likelihood of turning the initial mode-I crack into a delamination crack in a multi-layered structure. We close with a conclusion which can help to increase the delamination probability and thereby help to determine the delamination strengths of layered structures.

Scale bridging description of coherent phase equilibria in the presence of surfaces and interfaces

We investigate phase separation including elastic coherency effects in the bulk and at surfaces and find a reduction of the solubility limit in the presence of free surfaces. This mechanism favors phase separation near free surfaces even in the absence of external stresses. We apply the theory to hydride formation in nickel, iron, and niobium and obtain a reduction of the solubility limit by up to two orders of magnitude at room temperature in the presence of free surfaces. We develop in particular a scale bridging description of the solubility limit in the low-temperature regime, where the long-ranged elastic effects are expressed through a geometrical solubility modification factor, which expresses the difference to bulk systems. This expression allows to include elastic coherency effects near surfaces, e.g., in ab initio simulations.

Gamma-channel stabilization mechanism in Ni-base superalloys

A mechanism is presented which opposes coalescence of -precipitates in Ni-base superalloys. The mechanism is based on the non-linear behaviour of the elastic energy in -channels, caused by the misfit strain between matrix and precipitate, as a function of the channel width. Variation of the channel width causes a disjoining pressure dependent on the density of misfit dislocations.

Ductile tearing: applicability of a modular approach using cohesive zones and damage mechanics

Understanding macroscale ductile failure is of importance for the design of macroscale components for today’s use. Ductile failure has been investigated for decades and a multitude of simulation tools exist. However, the comparison and evaluation of the applicability of these tools is limited. This study uses an approach which separates material and fracture description into two modules. It uses uniaxial tension experiments to automatically establish the material properties for a nonlinear elastic and nonlinear plastic model. A fracture toughness sample is used to determine the failure properties. Using both modules, the behavior of a third geometry is predicted. Post-simulation experiments validate and falsify the numerical predictions. This study finds that accurate knowledge of the material is essential even in failure dominated components. The importance of shear failure mechanisms in the description of ductile failure is addressed.