Seminar Schedule – Summer 2019
Thursday, August 1, 2019
Time: 3:30pm – 4:30pm
Place: ASE 2.134
Micromechanical Simulation of Brittle and Ductile Fracture Processes in Metals to Predict Their Fracture Toughness
Meinhard Kuna, TU Bergakademie Freiberg, Germany
Abstract
The paper summarizes micromechanical modelling techniques to simulate the actual failure processes occurring at a crack tip in metallic materials. The aim of such simulations is to establish a relationship between microstructural properties of the material and its macroscopic crack growth resistance curves. For this purpose, a small scale yielding boundary layer concept is used, i.e. an increasing K-factor field is imposed as external load. By means of extensive finite element models, the three-dimensional microstructural details of the fracture process zone are discretized and analysed. Arrays of spherical voids or rigid inclusions are arranged around the crack tip to simulate ductile failure. In the ligaments between the forming voids and along the interface to the inclusions, cohesive elements are placed, which allow to capture cleavage. The deformation behavior of the material in-side the process zone is treated by J2-plasticity, whereas far away damage is included by the Gurson-Tvergaard-Needleman model.
This modelling strategy is applied to nodular cast iron (NCI) [1, 2] and ferritic steel [3] in the brittle, ductile and brittle-ductile transition region. The microstructure of NCI is characterized by spherical graphite particles of about 10 vol%, which can be simulated as voids [1]. In steel, carbides are represented by initial inclusions. When simulating ductile behavior, the plastic collapse between intervoid ligaments in front of the crack was identified as the dominating mechanism. With decreasing temperature, ferritic materials show a transition to brittle fracture. This is modelled by increasing the yield stress, whereas the cohesive strength is assumed to be temperature independent. With increasing ratio between yield stress and cohesive strength, the brittle failure of the cohesive elements becomes the dominating effect. Thus, a smooth transition from dimple fracture to cleavage can be adjusted in the simulations.
The simulation results are compared with experimental crack growth resistance curves from literature. Both for NCI and ferritic steel a good quantitative agreement of the J-∆a curves in the whole temperature range could be obtained with a minimum number of constitutive parameters. Moreover, the influence of microstructural parameters could be well predicted.
Biography
Meinhard Kuna studied physics at the Technical University in Magdeburg, Germany. He graduated as PhD in 1977 at the institute for Solid State Physics at the Academy of Science in Halle, dealing with the development of finite element methods in fracture mechanics. Later, he has been working as the head of research departments at Fraunhofer Institute for Mechanics of Materials in Freiburg and at Materials Testing Institute at University of Stuttgart.
Since 1997, Prof. Kuna is professor of Applied Mechanics and Solid Mechanics at the Technische Universität Bergakademie Freiberg in Germany. His research is devoted to fundamental and applied problems in fracture mechanics, damage mechanics and fatigue crack growth. In particular, his group is developing numerical methods (FEM, BEM, DEM) and software-tools for analyzing safety and lifetime of structural components. Another topic is the constitutive modelling of material behavior on the continuum level and on the micromechanical scale, with special focus on failure and damage processes. Of special interest are brittle and ductile failure mechanisms in metals and ceramics, and multi-field problems in smart materials. The experimental work is focused on miniaturized materials testing.
Prof. Kuna published more than 400 articles in international journals or conference proceedings. He was honored by the TEXTY award for his book about Finite Elements in Fracture Mechanics in 2013. He is engaged in academic societies like German Fracture Group, German Research Foundation and European Structural Integrity Society. Since 2015, Prof. Kuna is an Editor-in-Chief of the international journal Engineering Fracture Mechanics.
For further information, please contact Dr. K. Ravi-Chandar at ravi@utexas.edu or (512) 471-4213.