Simplified approach for ductile fracture mechanics SSTT and its application to Eurofer97

The determination of fracture-mechanical properties is often very challenging, because the available standards like ASTM E1820 need specific size-requirements for the specimen dimensions to obtain valid fracture toughness. Especially in the ductile regime, where the presence of plasticity around the crack tip is affected by the multiaxial stress state and its triaxiality, the size-requirements are frequently not met. The fulfilment of the size-requirements needs the testing of big specimens, which is often not possible. If we now think of specimens, which are irradiated in test modules for future fusion reactors, their size cannot be as big as required, because the available volume for irradiation is restricted. This fact highlights the need of Small Specimens Test Techniques (SSTT) for the determination of fracture-mechanical properties in the ductile regime. The presented work focuses on an approach for the determination of fracture-mechanical properties in the ductile regime including stable crack growth and crack-resistance behavior. The authors have developed the initial approach some years ago and within this work the approach was simplified as much as possible. The basic idea of the approach is, that the crack growth can be simulated using Finite Element Method combined with a cohesive zone model. The cohesive zone model is a two parametric model, namely the cohesive stress σ$_{c}$ and the cohesive energy Γ$_{c}$, which are identified on small specimens only. The new simplified approach was now validated on ferritic-martensitic steel Eurofer97 at room temperature. In the past, the approach used complicated features like a CCD camera system and has now been simplified in a way that no CCD camera system is required. The main part of the approach is the identification of cohesive zone parameters (cohesive stress σ$_{c}$ and energy Γ$_{c}$) on small specimens. The cohesive stress σ$_{c}$ can be determined on notched round tensile specimens with different notch root radius to account for different stress states or stress triaxialities in the specimen. With dedicated Finite Element modelling a local fracture stress dependent on stress triaxiality can be identified. The cohesive energy Γ$_{c}$ can be carried out by simulating the small fracture-mechanical specimen using the Finite Element Method combined with the cohesive zone model and parameter fitting to experimental results. The cohesive energy Γ$_{c}$ is treated to be identified, if the simulated crack-resistance curve describes the experimental behavior. After identification of these parameters, a big fracture-mechanical specimen can be simulated using the cohesive zone parameters already determined on small specimens. Finally, the crack-resistance curve of a big specimen can be predicted and a valid fracture toughness can be identified if the size-requirements of the big specimens are met. In case the requirements are not fulfilled, a bigger specimen geometry can be simulated until all size criteria are met. With this method, the testing of big specimens can be avoided. For the future there is a Round Robin exercise planned including defined test matrices to demonstrate the general applicability of the approach

Effective and back stresses evolution upon cycling a high-entropy alloy

We report on the effective and back stresses evolution of a CoCrFeMnNi high-entropy alloy (HEA) by partitioning its cyclic hysteresis loops. It was found that the cyclic stress response of the HEA predominantly originates from the back stress evolution. Back stress also increases significantly with increasing strain amplitude and reducing grain size. However, the change of effective stress is rather insignificant with altering cycle number, strain amplitude and grain size. This indicates that the effective stress is determined mainly by the lattice friction. Further comparisons to an austenitic steel and a medium-entropy alloy identified the origins of their peculiar cyclic strength. The effective stress and back stress upon cycling a HEA are assessed, both of which are higher than a conventional FCC steel, contributing to the HEA’s higher cyclic strength

Microstructure-specific Hardening of Ferritic-Martensitic Steels pre and post 15 dpa Neutron Irradiation at 330°C: A Dislocation Dynamics Study

In this work, we used Dislocation Dynamics (DD) simulations to investigate the role of the hierarchical defects microstructure of ferritic-martensitic steel Eurofer97 in determining its hardening behavior. A Representative Volume Element (RVE) for DD simulation is identified based on the typical martensitic lath size. Material properties for DD simulations in b.c.c Eurofer97 are determined, including the dislocation mobility parameters. The dependence of material parameters on temperature is fitted to experimental yield strength measurements carried out at room temperature and 330 °C, respectively. Voids and precipitates observed in the microstructure, such as M23C6 and Tantalum-rich MX, are considered in our DD simulations as inclusions with realistic size distribution and volume density, while 〈1 1 1〉 -and 〈1 0 0〉 -type irradiation loops are included directly in the DD simulations. The lath structure, together with its typical precipitates arrangement and the different crystallographic orientation of the martensitic blocks can also be captured in the simulations. DD simulations are used to extract microstructure-specific hardening parameters, which can be used to simulate the properties of Eurofer97 at the engineering scale

Review and critical assessment of dislocation loop analyses on EUROFER97

The understanding of microstructural defects behavior after neutron irradiation is crucial for assessing the applicability of reduced activation ferritic/martensitic (RAFM) steel EUROFER 97 in future fusion reactors. Formation and evolution of dislocation loops is believed to play the major role in material's hardening under neutron irradiation. In this work, transmission electron microscopy (TEM) data on dislocation loop size distribution is provided after different irradiation campaigns to determine the role of neutron dose on the dislocation loop evolution. A comparison of investigations on dislocation loop behavior and appearance yield considerable differences. For a conclusive interpretation, this work reviews available data, and possible reasons for the observed differences are discussed. Recommendation for future TEM investigation are given. Keywords: TEM, Microstructural defects, Neutron irradiation, Fusion, Reduced activation ferritic/martensitic RAFM steel

Thermomechanical design rules for photovoltaic modules

We present a set of thermomechanical design rules to support and accelerate future (PV) module developments. The design rules are derived from a comprehensive parameter sensitivity study of different PV module layers and material properties by finite element method simulations. We develop a three dimensional finite element method (FEM) model, which models the PV module geometry in detail from busbar and ribbons up to the frame including the adhesive. The FEM simulation covers soldering, lamination, and mechanical load at various temperatures. The FEM model is validated by mechanical load tests on three 60-cell PV modules. Here, for the first time, stress within a solar cell is measured directly using stress sensors integrated in solar cells (SenSoCells®). The results show good accordance with the simulations. The parameter sensitivity study reveals that there are two critical interactions within a PV module: (1) between ribbon and solar cell and (2) between front/back cover and interconnected solar cells. Here, the encapsulant plays a crucial role in how the single layers interact with each other. Therefore, its mechanical properties are essential, and four design rules are derived regarding the encapsulant. Also four design rules concern front and back sides, and three address the solar cells. Finally, two design rules each deal with module size and frame, respectively. Altogether we derive a set of 15 thermomechanical design rules. As a rule of thumb of how well a bill of material will work from a thermomechanical point of view, we introduce the concept of specific thermal expansion stiffness ${\hat{E}}_{\alpha }=E\cdotp \alpha \cdotp {A}_{\mathrm{j}}\cdotp h$ as the product of Young\u27s modulus E, coefficient of thermal expansion $\alpha$, joint area A$_{j}$, and materials height h. The difference between two materials is a measure of how much thermal strain one material can induce in another. A strong difference means that the material with the larger value will induce thermal strain in the other