Stochastische Simulation der Schädigungsentwicklung in einem martensitischen Stahl

A probabilistic simulation model was developed for damage accumulation in the martensitic steel F82H‐mod under fatigue loading. Empirical relations for crack initiation, crack growth and coalescence of cracks are derived from an observation of experimental crack patterns and inserted in a stochastic simulation model based on a random cell structure. The simulation results are compared with the experimentally observed crack patterns.     

Low activation steels welding with PWHT and coating for ITER test blanket modules and DEMO

EUROFER weldability is investigated in support of the European material properties database and TBM manufacturing. Electron Beam, Hybrid, laser and narrow gap TIG processes have been carried out on the EUROFER-97 steel (thickness up to 40 mm), a reduced activation ferritic-martensitic steel developed in Europe. These welding processes produce similar welding results with high joint coefficients and are well adapted for minimizing residual distortions. The fusion zones are typically composed of martensite laths, with small grain sizes. In the heat-affected zones, martensite grains contain carbide precipitates. High hardness values are measured in all these zones that if not tempered would degrade toughness and creep resistance. PWHT developments have driven to a one-step PWHT (750 °C/3 h), successfully applied to joints restoring good material performances. It will produce less distortion levels than a full austenitization PWHT process, not really applicable to a complex welded structure such as the TBM. Different tungsten coatings have been successfully processed on EUROFER material. It has shown no really effect on the EUROFER base material microstructure.     

IFMIF specifications from the users point of view

This paper summarizes the proposals and findings of the IFMIF Specification Working Group established to update the users requirements and top level specifications for the facility. Special attention is given to the different roadmaps of fusion pathway towards power plants, of materials R&D and of facilities and their interactions. The materials development and validation activities on structural materials, blanket functional materials and non-metallic materials are analyzed and specific objectives and requirements to be implemented in IFMIF are proposed. Emphasis is made in additional potential validation activities that can be developed in IFMIF for ITER TBM qualification as well as for DEMO-oriented mock-up testing.     

Finite deformation plasticity and viscoplasticity laws exhibiting nonlinear hardening rules

  We consider two finite deformation plasticity models, which differ mainly in the evolution equation governing the response of kinematic hardening. Both models reduce to the same constitutive law in the case of small deformations. The aim of the paper is to discuss these models by calculating the predicted responses for some representative loading conditions. The numerical calculations needed are performed by using an efficient time-integration algorithm which has been developed with a view to implementation in the ABAQUS finite element code. Generally, there are some differences between the predicted responses and in particular between the second-order effects predicted by the two models. For some simple deformation processes, e.g. simple shear and simple torsion, the differences concerning second-order effects exhibit some kind of regularities, which are independent of material parameters. Also, even if boundary value problems are considered where global deformations are small, significant differences can exist between the predicted model responses according to the finite deformation and the limiting small deformation theory.     

Crack interaction modelling

The growth characteristics of short fatigue cracks under axial loading were investigated using specimens of the ferritic–martensitic steel F82H-mod. Interest focused on crack propagation due to coalescence, which proved to be the dominant mechanism of crack growth. Crack propagation due to coalescence under a certain loading state is strongly influenced by the microstructure of the material on the one hand and by the interaction of cracks on the other. This study deals with an elasto-plastic fracture mechanics analysis of two interacting cracks neglecting the microstructural influence. Finite element calculations based on a Ramberg–Osgood model for the material properties were performed to quantify the interaction of two cracks in terms of an interaction function Y depending on the material and crack configuration. Finally, a neural network was trained to determine the interaction function for two cracks within the range of interest.     

Weight functions for edge cracks in thin surface layers

A procedure is given for the calculation of the weight function for a surface crack located in a thin surface layer perpendicular to the interface. The procedure is demonstrated for cracks approaching the interface and cracks going beyond the interface.     

Linear micropolar elastic crack-tip fields under mixed mode loading conditions

Micropolar elasticity laws provide a possibility to describe constitutive properties of materials for which internal length scales may become important. They are characterized by the presence of couple stresses and nonsymmetric Cauchy stress tensor. Beyond the classical displacement field, the kinematical variables are augmented by a so-called microrotation field and its gradient, the latter introducing an internal length scale in the theory. For an isotropic, linear micropolar elastic material, the near-tip asymptotic field solutions for mode I and mode II cracks are derived. It is shown that these solutions behave similar to those according to the so-called couple stress theory, which has been investigated by Huang et al. (1997a), or similar to those derived for cellular materials by Chen et al. (1998). In particular, the singular fields have an order of singularity r ^–1/2 and are governed by some amplitude factors, having the meaning of stress intensity factors as in the classical linear elastic theory. The effect of material parameters on the stress intensity factors is studied by applying the finite element method to calculate the values of the stress intensity factors for an edge-cracked specimen of finite width.     

Finite deformation plasticity and viscoplasticity laws exhibiting nonlinear hardening rules

 This paper deals with plasticity and viscoplasticity laws exhibiting nonlinear kinematic hardening as well as nonlinear isotropic hardening rules. In Tsakmakis (1996a, b) a constitutive theory has been formulated within the framework of finite deformations, which is based on the concept of so-called dual variables and associated time derivatives. Within two families of dual variables, two different formulations have been proposed for kinematic hardening, referred to as Models 1 and 2. In particular, rigid plastic deformations without isotropic hardening have been considered. In the present paper, the constitutive theory of Tsakmakis (1996a, b) is appropriately extended to take into account isotropic hardening as well as elastic deformations. Care is taken that the evolution equations governing the hardening response fulfill the intrinsic dissipation inequality in every admissible process. For the case of small elastic strains combined with a simplification concerning kinematic hardening, to be explained in the paper, an efficient, implicit time-integration algorithm is presented. The algorithm is developed with a view to implementation in the ABAQUS Finite Element code. Also, explicit formulas for the consistent tangent modulus are derived. Received 22 September 1999