Fracture modeling and experimental validation of Compacted Graphite Iron (CGI) specimens loaded under quasi-static conditions at room temperature are considered. Continuum damage mechanics coupled to plasticity is adopted to describe the evolution of damage. The damage production is based on a recently developed rate dependent damage evolution law, where the damage–plasticity coupling is modeled based on a damage driving energy that involves both stored energy and plasticity contributions. To describe ductile fracture accounting for stress triaxiality on the damage initiation, the inelastic contribution to the damage driving energy is controlled by the Johnson-Cook failure criterion. Three different damage models are defined based on elastic/inelastic damage driving energies. The damage models are validated against experiments on a set of notched specimens made of CGI with different notch geometries, where the global force-displacement curves and corresponding strain fields are obtained using digital image correlation technology. It is shown from the testing and the simulations that plastic strains generally need to be accounted for in order to properly describe the different failure processes of the CGI specimens. In addition, the ductile damage model is shown to more accurately predict the experimental force-displacement response as compared to the more simplistic stress drop, element deletion technique.

When subjecting cast irons to mechanical loading the deformation and damage mechanisms occur on a microstructural level and are dependent on the inherent microstructure. A deeper understanding of the relation between the different microstructural constituents and the macroscopic mechanical behaviour would be beneficial in material development efforts and for the ability to design and cast components with tailored properties. Traditionally, microscopy examinations on sectioned cast iron samples have been used when analysing the microstructure in cast irons. Since all microstructural heterogeneity is in three-dimensions (3D), methods that provide a three-dimensional characterisation are essential for a deeper understanding of, both the microstructural features as well as the deformation and damage of cast irons. Therefore, different cast iron grades have been studied using synchrotron X-ray tomography and 3D x-ray diffraction (3DXRD) at ESRF in Grenoble, France. The samples were stepwise loaded and unloaded in-situ at in the tomography/3DXRD set-up to study the deformation with regard to microstructural constituents and the microstructural evolution in 3D. Based on the 3D tomography image sequences, digital volume correlation (DVC) was used for full strain field analysis and for the analysis of damage and deformation mechanisms In addition, 3DXRD data were analysed to provide details on the lattice parameters and lattice strain of individual ferrite grains. This work shows the possibilities of such synchrotron experiments for advanced study of the mechanical behaviour of cast iron.

The paper describes work done using synchrotron light to investigate the microstructure and how it behaves in 3D when a load is applied. Two different cast iron materials with different matrix structures and graphite morphologies were investigated; SiMo51, which is basically a spheroidal iron (SGI) alloyed with Si and Mo, and a lamellar graphite iron (LGI). The tensile test specimens were loaded in steps, at which x-ray tomography as well as 3DXRD measurements were made to characterize the microstructure. The result shows how the crack propagates and which path it takes through the materials. DVC was applied to analyze the strain fields. This work also shows how useful synchrotron experiments can be in the study of the mechanical behavior of cast iron.

This paper presents an experimental methodology for the characterization of thermomechanical displacement and friction properties in a free-floating press-pack structure, and evaluation of the tensile stress on the semiconductor die through simulation of different mechanical and thermal loading conditions. The press-pack structure consists of a single silver-metallized (1 mu m) silicon carbide die (400 mu m) in contact with rhodium-coated (0.4 mu m) molybdenum square plates. The thermomechanical displacements in the press-pack structure have been obtained using the digital image correlation technique, and the mean random error has been +/- 0.1 mu m, which is approximately 10 ppm of the measured length (10.5 mm). The developed experimental method has led to an analytical estimation of friction coefficients on the interfaces' silicon carbide-molybdenum and molybdenum-copper. The results demonstrate that the thin silver layer behaves as a solid film lubricant. A 2-D finite-element model representing the experimental setup has been implemented. The difference in displacement between measurement and simulation is less than 8%. Furthermore, the coinfluence of the design parameters on the thermomechanical performance of the stacked structure has been analyzed through simulations. Finally, design guidelines to reduce the tensile stress on the silicon carbide die have been proposed regarding free-floating press-pack power electronics packaging.

ONKALO POSE Experiment - Laboratory Determination of Density, Porosity and Mechanical Anisotropy of Gneiss and Granite

Additive manufacturing, or 3D-printing as it is often called, build parts in a layer-by-layer fashion. A common concern, regardless of the specific additive manufacturing technique used, is the risk of inadequate fusion between the adjacent layers which in turn may cause inferior mechanical properties. In this work, the local strain properties of titanium parts produced by Electron Beam Melting (EBM®) were studied in order to gain information about the quality of fusion of the stock powder material used in the process. By using Digital Image Correlation (DIC) the strain fields in the individual layers were analyzed, as well as the global strain behavior of the bulk material. The results show that fully solid titanium parts manufactured by EBM are homogenous and do not experience local deformation behavior, neither on local nor on a global level.

In a previous study, the thermomechanical fatigue resistance of four compacted graphite irons (CGIs) and one grey cast iron was investigated. The molybdenum content of the four CGIs varied between 0 and 1.01 wt-%. It was observed that during thermal cycling, the maximum value of the compressive stress continuously decreased while the value of the maximum tensile stress continuously increased. The continuous decrease in compressive stresses showed that stress relaxation occurs at elevated temperatures during thermal cycling. The goal of the present investigation was to investigate the phenomenon of stress relaxation at elevated temperatures. The tests were performed at 350 and 600 degrees C respectively. The results of the stress relaxation tests performed at 600 degrees C showed the same trend observed at thermomechanical fatigue testing. The tests showed that additions of molybdenum improved the fatigue resistance of CGI by lowering the stress relaxation rate.

The Metalock method is a mechanical joining technique most commonly employed in cracked castings of iron, aluminum and steel. It is based on inserting custom-made “keys”, which are meant to take up tensile and shear stresses, perpendicular to the crack. The marine diesel engine designer MAN Diesel & Turbo, uses the Metalock method for crack patching in large cast iron components, such as cylinder frames, in their engines. The service life of these components can thereby be significantly extended, reducing both replacement costs and environmental impact. The purpose of this thesis was to investigate the mechanical behavior of the Metalock method and study the material properties of the components included in a Metalock joint. The material properties were determined by performing thorough material testing, and the mechanical behavior of a Metalock joint was studied by full scale testing, using a non-contact deformation measurement technique known as digital image correlation, of cast iron specimens joined together by the Metalock method. Furhermore, finite element simulations were performed, and verified by experimental results, in order to study the mechanical behavior in detail and to carry out a parametric study on some of the components included in the joint. Results from material testing show that the material of the keys are made of an iron–nickel alloy and have a very low or even negative coefficient of thermal expansion at ordinary temperatures. Differences between initial tests and simulations indicate that the installation procedure of the joint introduce residual strains which affect the behavior of a joint subjected to mechanical loading. Experimental tests show that the distance from an edge to the first key as well as the distance between keys in a joint are important parameters which affect the load carrying capacity and the location of crack initiation. In addition, numerical simulations show that an increase of the key length yields less impact on the surrounding cast iron material, if a joint is subjected to tensile loading.

On characterization and modelling of mechanical properties of materials for cast components

Digital image correlation techniques (DIC) have been used in this study to reveal how different phases (graphite, ferrite and pearlite) of compacted graphite cast irons (CGI) accommodate strains at loading. A DIC-software was used to analyse sets of successively acquired images. The images were acquired with a light optical microscope revealing the microstructure of polished and etched CGI materials at different load levels. Five CGI materials, having percentages of pearlite in the range 35 to 90 area%, were included in the study. Apart from the different matrix constituents (ferrite and pearlite) the nodularity was altered and varied from 5% to 65%. It is concluded from this study that the different phases in CGI are affected by the global strain and load to a greater or less extent. The graphite phase appears to accommodate high strains but, due to fracture of the graphite particles at relatively low strains, the resulting high strain values show the opening up of graphite cavities. The ferrite phase is subjected to a strain concentrating effect of the graphite phase and with a low nodularity, i.e., with graphite particles that are stretched out and interconnected, the effect becomes more pronounced resulting in high strains. The pearlitic phase is the strongest of the constituents within the microstructure and the lowest local strains are observed in this phase. The study shows that DIC strain-field images reveal the microstructural strain level distribution for the CGI materials. The future outlook is to use these sets of strain-field images for verification of micro mechanical finite element analysis on a microstructural scale of CGI materials.

The matrix structure formation of cast irons is strongly affected by the casting process where different alloying elements and cooling conditions are methods used to achieve the desired structure and performance of the material. In the presented study, six pearlitic grey cast irons have been analysed regarding how the pearlitic structure formation might be controlled. Different amounts of copper and chromium were added, ranging from 0.07 to 1.11 wt% and 0.08 to 0.60 wt%, respectively. Three different section sizes (Ø20, Ø45 and Ø85 mm) and three different cooling conditions through the eutectoid transformation were used to control the matrix structure formation. The three different cooling conditions were achieved by shake-out at 950°C and cooling in air or furnace, or by keeping the casting in the mould. The present paper focuses on the pearlite appearance, since it strongly affects the mechanical properties. The analysis shows that the refining effect of Cr is much stronger than that of Cu. Comparing the low alloyed base melt with the ones alloyed with Cu and Cr, it is seen that additions of 0.75 wt% Cu refines the pearlite by approximately 10%. Keeping this Cu level constant and adding Cr, it is observed that an addition of ~0.6 wt% refines the pearlite by another 20%. The most potent refining effect of Cr is achieved by additions up to 0.35 wt%. Keeping the Cr constant at 0.35% and changing the Cu content (0.35 to 1.10 wt%), almost no variation is observed in the overall interlamellar spacing. The eutectoid cooling rate most strongly affects the interlamellar spacing down to cooling rates of about -0.75 °C/s. At higher (i.e. lower value) cooling rates the interlamellar spacing is fairly constant. In addition to studying the interlamellar spacing, the graphite structure has also been analysed and evaluated concerning effects from the different casting variables.

The use of aluminium in lightweight structures and corrosive environments is continuously increasing. Welding is often used to assemble different aluminium components and it is of great importance to take into account the deteriorating effect the welding operation on the aluminium’s strength when designing these structures. In the present study, the strain-fields of welded aluminium tensile specimens have been analysed by digital image correlation (DIC) techniques, micro hardness measurements and finite element (FE) modelling. The stress-strain curve of the weld material, the heat affected zone (HAZ) and the base material have been determined by tensile tests. The material properties and the extension of the HAZ were also correlated to the micro hardness. The experimental data has been used as an input to the material models of the FE analysis. The study shows that using FE-analysis in combination with strain-field determination by DIC is very powerful. The resulting strain-fields are easily compared and the FE-analysis is verified in a straightforward way. Furthermore, the results of the study suggest that micro hardness measurements could be used to derive the strength of the material affected by the welding operation.

During the last years the use of digital image correlation techniques (DIC) has become wide spread within different areas of research. One area in which these techniques are used is in the analysis of deformation of engineering materials. By the analysis of a set of successive images taken during a tensile test DIC makes it possible to determine how the deformation is localized. The observed local strains are often several times higher than the global strain measured by standard strain gauges. In this study, a set of compacted graphite cast irons (CGI) with different ratios of pearlite to ferrite have been examined by the use of DIC. In contrast to the normal use of DIC, where a pattern is sprayed on the tensile test sample as a reference for the determination of deformation taking place between successive images, the materials natural microstructural pattern has been used in this study. The use of the natural microstructural pattern makes it possible to study how the macroscopic deformation is accommodated within the different phases in the CGI studied. It is shown that the graphite phase accommodates a large portion of the strain and that the soft ferrite is strained more than the stronger, less ductile pearlite. The local strain of the observed area might be up to ten times higher than the global strain measured. The use of DIC improves the understanding of the deformation behaviour of compacted graphite cast irons and will be a useful tool when validating future finite element analyses of the micro-mechanical properties of cast irons.

In this experimental study, six pearlitic grey cast irons with different Cu and Cr content, different section thicknesses and different eutectoid cooling rates have been examined. The eutectoid cooling rate was approximated by casting simulation analysis. The purpose of the experiments was to study the effect of the matrix structure on the overall mechanical properties. An emphasis is put on the pearlite interlamellar spacing because this controls the resulting mechanical properties to a large extent. By keeping the graphite structure constant, the effect of the matrix structure was able to be studied. This was achieved by shake-out at temperatures above the eutectoid transformation range followed by subsequent cooling in air, mould or in a furnace. The pearlite interlamellar spacing ranged from 90 to 330 nm for the matrices studied. Comparing the strength of the fine structured and coarse structured materials, the tensile and yield strength was reduced by almost 50%. Regarding the elastic deformation, a weak increase in the tangent modulus with increasing alloying content was observed. It was also observed that lower cooling rate decreased the tangent modulus. The tangent modulus ranged between 70 and 110 GPa. Analysing the plastic deformation of the materials, in terms of strain hardening exponent, n, and strength coefficient, K, a strong dependence on the pearlite coarseness was observed. It was concluded that the effect of graphite particle length on tensile strength was negligible and the major improvement on the strength was due to refinement of the pearlite.

Castings are produced by a manufacturing method which gives the components properties that are dependant on design, metallurgy and casting method. The wall thickness influences the resulting coarseness and type of microstructure and the material will have properties dependant on local metallurgical and thermal history. The local properties in a cast iron component can vary essentially in the casting volume, which makes it difficult to optimize the castings with good accuracy. Often simulation of stress/strain of cast products use constant material properties throughout the castings. If the microstructure is determined or predicted at a given point, it is possible to calculate the local material properties and its deformation behavior.