Lutz Zybell

Influence of graphite spherical size on fatigue behaviour and fracture toughness of ductile cast iron EN-GJS-400-18LT

Abstract The employment of ferritic nodular cast iron for components subjected to high stress requires a detailed fracture mechanics evaluation of crack growth under static and cyclic loading. During operation, for instance, in wind power plants, such component parts are subjected to loading of variable amplitude, which influences their lifetime considerably. For the evaluation of crack propagation and the remaining service life in this case, the calculation methods currently well-established in practice cannot be employed for cast iron with nodular graphite, since overloads lead to microstructure-related and material-specific load history effects in terms of crack growth acceleration. In this work, investigations of crack growth under constant and variable amplitude loading as well as static fracture toughness investigations and strain-controlled cyclic deformation experiments are presented.

Ductile crack propagation by plastic collapse of the intervoid ligaments

In the present study ductile crack initiation and propagation is investigated by means of a micro-mechanical model under small-scale yielding conditions. Voids are resolved discretely in the fracture process zone where steep gradients occur during the loading history and are taken into accounted by a homogenized porous plasticity law elsewhere. The size of the region of discrete voids is not set a priori but is determined consistently. The results show that effective crack growth occurs by plastic collapse, i.e. purely geometric softening of the intervoid ligaments without incorporating material separation. Due to this mechanism a limit load exists coinciding with the maximum fracture toughness. In addition, it turns out that the shielding due to the growth of voids around the crack plane has a considerable influence on the computed R-curves compared to models neglecting this effect. Depending on the void arrangement a diffuse softening zone or even crack branching is observed. A comparison with experimental data from literature indicates that plastic collapse and the formation of diffuse zones of void growth are realistic mechanisms of ductile crack propagation.

Investigation of time dependence of dissipation and strain induced crystallization in natural rubber under cyclic and impact loading

Strain induced crystallization (SIC) is one of the special features of natural rubber (NR), responsible for its outstanding mechanical performance. Especially, for the short term behavior of NR, e.g. within a rolling tire, the kinetics of SIC plays an important role. SIC can be observed to some extend by thermography, but it is not directly possible to separate the heating effects accompanying SIC from dissipative heating of the rubber material (LeCam et al. 2015, Spratte et al., 2017). By means of synchrotron x-ray diffraction, SIC can be monitored during deformation including cyclic deformations and impact loading with a time resolution of less than 10 ms (Brüning at al., 2013). Combining this method with thermography enables the separation of processes of structure formation and dissipation. This can serve as a basis for a more realistic modelling of the dynamic and thermomechanical behavior of natural rubber. Within the presentation we report about investigations of cyclic loadings of differently carbon black (CB) filled NR samples with respect to SIC, melting of the crystallites as well as the dissipative heating and convective cooling of the rubber materials.

Fatigue crack growth in nodular cast iron ? Influences of graphite spherical size and variable amplitude loading

Investigations of constant and variable amplitude loading, including a variety of overloads, were carried out experimentally as well as numerically in order to characterize the fatigue crack growth behaviour of nodular cast iron with a ferritic matrix. Under constant cyclic loading the crack growth rate, which depends on the graphite size can be described well by the NASGRO equation. The mean distance between the graphite particles and the shape factor also influence the fracture behaviour. The experimental investigations show that overloads yield to acceleration effects in fatigue crack growth. The calculation of the a-N curves based on different usual life prediction models yields non-conservative results. Therefore, a modified strip yield model is presented which allows the prediction of the crack growth acceleration after overloads in nodular cast iron.

Meinhard Kuna: Physics and Engineering at the Crack Tip—A Retrospective

The present volume appeared on occasion of the 65th birthday of Prof. Meinhard Kuna. This section outlines the biography of Meinhard Kuna including main scientific projects and some personal anecdotes.

Recent trends in fracture and damage mechanics

Part I Historical Perspective Meinhard Kuna: Physics and Engineering at the Crack Tip - A Retrospective Experimental and Numerical Fracture Mechanics - An Individually Dyed History Part II Applications Fracture Mechanics Assessment of Welded Components at Static Loading Application of Fracture Mechanics for the Life Prediction of Critical Rotating Parts for Aero Engines Consideration of Fatigue Crack Growth Aspects in the Design and Assessment of Railway Axles Part III Fracture Testing Assessment of Material Properties by Means of the Small Punch Test Determination of Fracture Mechanics Parameters for Cast Iron Materials under Static, Dynamic and Cyclic Loading On the Development of Experimental Methods for the Determination of Fracture Mechanical Parameters of Ceramics Transition from Flat to Slant Fracture in Ductile Materials Part IV Smart Materials Interaction of Cracks and Domain Structures in Thin Ferroelectric Films Modeling Approaches to Predict Damage Evolution and Life Time of Brittle Ferroelectrics Numerical Analysis of Interface Cracks in Layered Piezoelectric Solids Part V Analytical Mechanics Crack-Tip Fields of a Crack Impinging upon the Yielding/Debonding Slippage in Anisotropic Body On Conservation Laws and Reciprocity in Configurational Mechanics Part VI Local Appro ach to Fracture A Model for Predicting Fracture Toughness and Scatter in Thermally Embrittled Steels Micromechanical-Based Models for Describing Damage of Ferritic Steels Recent Trends in the Development of the Gursons Model