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Iron−silicon steels have excellent magnetic and electrical properties, especially, with increased silicon and aluminum contents. However, increasing alloying with Si and Al reduces the workability of the material, and spontaneous cleavage fracture may occur during production. The main influence factors on the occurrence of cleavage fracture of Fe−Si−Al alloys are investigated in this paper. Fracture mechanics tests are performed at various temperatures and loading rates. The effects of specimen orientation and strain aging are studied. The tests result surprisingly high fracture toughness values. However, cleavage fracture regions are found in the pre-fatigue regions of the specimens, i.e. at much lower loads. In order to resolve this effect, the strain rates during the various fatigue- and fracture experiments are estimated by an analytical model. Additional fatigue experiments are performed at various frequencies where the stress intensity range is increased until cleavage fracture is detected. Similar experiments are conducted at increasing temperatures. The analyses reveal a significant transition behavior of the stress intensity at the onset of cleavage fracture, where the effects of decreasing temperatures and increasing strain rates are equivalent. Measures for avoiding cleavage fracture can be deduced from the results of this investigation.

The formability of newly developed steel grades is a key parameter for their successful implementation in the car body. In the present work, the influence of the microstructure on formability during air bending is studied. First- and third-generation advanced high strength steels with tensile strengths above 1000 MPa are investigated, which exhibit a broad range of microstructural constituents (ferritic bainite, bainite, martensite, and retained austenite). Interrupted bending experiments are performed on two grades of dual phase and complex phase steels, respectively, in order to reveal the microscopic deformation and failure processes. The formability in hole expansion tests is determined and used as a measure for the resistance of the materials against crack formation. The macroscopic deformation behavior of the materials is taken into account by measuring the differential strain hardening exponent and performing finite element simulations of the air bending process. Advantages and disadvantages of various microstructural features are worked out, and it is explained why the first-generation dual phase steel exhibits the lowest and the third-generation complex phase steel the best formability during air bending.

The fracture behavior of two metal matrix composites (MMCs) at different aging conditions is investigated both at the global and local levels. The study is focused on the processes of void initiation near the crack tip by either particle fracture or particle/matrix decohesion, and on fracture initiation. The global fracture properties are determined by conventional fracture mechanics tests. The local conditions for void and fracture initiation are determined using fractographic analyses and subsequent analytical computations, based on the HRR-field equations, mean-field theory, and a simple analytical model from literature. The correlation between the global and local fracture properties is studied. The applicability of existing models to predict the fracture toughness of MMCs is discussed. The results of the analyses suggest that the maximum principal stresses in the particles at the moment of void initiation are not constant, but exhibit a dependency on the composite yield strength.

A two-dimensional finite element analysis is performed to model the behavior of fatigue cracks in bimaterial specimens made of diffusion-bonded ARMCO-iron and SAE 4340 steel with an interface perpendicular to the crack plane. The numerical analyses are based on experiments conducted by Pippan et al. [Mater. Sci. Eng. A 283 (2000) 225–233]. The concept of configurational forces is used to evaluate by post-processing the crack-tip shielding and anti-shielding effects that occur due to the differences in yield stress, hardening and coefficients of thermal expansion of ARMCO-iron and SAE 4340 steel. For given values of applied stress intensity range Δ K app and applied load ratio R app , the near-tip J -integral J tip is calculated at the maximum and minimum load. To allow for crack closure, the effective near-tip stress intensity range Δ K eff,tip is evaluated from the near-tip stress intensity range Δ K tip and the near-tip load ratio R tip . Calibration curves for homogeneous ARMCO-iron and SAE 4340 steel are used to calculate the crack growth rate da / dN from the values of Δ K eff,tip . The distance between interface and crack tip L is varied to simulate the behavior of a growing crack. The computed curves da / dN versus L curves are in good agreement with the experimental results.

A procedure is proposed to determine, for second-phase particles near a crack tip, the maximum particle stresses at the moment of void initiation by either particle fracture or particle/matrix interface separation. A digital image analysis system is applied to perform a quantitative analysis of corresponding fracture surface regions from stereo image pairs taken in the scanning electron microscope. The fracture surface analysis is used to measure, for individual particles, the crack tip opening displacement at the moment of void initiation and the particle location with respect to the crack tip. From these data, the stress tensor at the moment of void initiation is calculated from the Hutchinson–Rice–Rosengren (HRR) field theory. The corresponding average local stresses within the particle are evaluated by a non-linear Mori–Tanaka-type approach. These stresses are compared to estimates according to the models by Argon et al. [A.S. Argon, J. Im, R. Safoglu, Metall. Trans. 6 (1975) 825] and Beremin [F.M. Beremin, Metall. Trans. 12 (1981) 723]. The procedure is demonstrated on an Al6061–10% Al2O3 metal matrix composite.

An experimental procedure has been developed to systematically investigate the local deformation behavior of polycrystalline materials at the micrometer scale. The procedure consists of a combination of the measurement of the local in-plane strains and the local crystal orientation during an in-situ deformation test in the scanning electron microscope. The local in-plane strains are determined from micrographs that are taken before and after a considered deformation step. By means of a digital image analysis system, homologue points are found on the two micrographs. The homologue points form a deformation field that can be derived to determine the local strain fields. The local crystal orientation is measured by orientation image microscopy analyses, again taken before and after the deformation step. From these measurements, the local rotation of the crystal lattice during the deformation step and the Miller indices of the rotational axis are determined. The combination of the data of the local crystal orientation and the local in-plane strains is used to estimate, with a simple kinematic model, the slip systems that are locally active at arbitrary positions within a grain. A polycrystalline copper specimen was loaded in two deformation steps to 7.4 and 14% global tensile strain. A region of ≈250×200 μm was analyzed. The experiment shows that very strong heterogeneities in the in-plane strain and in the local lattice rotation exist within the single grains. At some grain boundaries, very high lattice rotations are observed. Even far from the boundary, the activation of slip systems can be quite different in different regions of a grain. On the specimen surface, more than three independent slip systems must be activated in some grain boundary regions to accommodate the influence of the neighboring grain. In most regions of the grain interior, three slip systems seem to be enough. With the developed procedure, we have generated a tool for a very comprehensive experimental characterization of crystal plasticity in polycrystals.

An experimental procedure has been developed to systematically investigate the local deformation behavior of polycrystalline materials at the micrometer scale. The procedure consists of a combination of the measurement of the local in-plane strains and the local crystal orientation during an in-situ deformation test in the scanning electron microscope. The local in-plane strains are determined from micrographs that are taken before and after a considered deformation step. By means of a digital image analysis system, homologue points are found on the two micrographs. The homologue points form a deformation field that can be derived to determine the local strain fields. The local crystal orientation is measured by orientation image microscopy analyses, again taken before and after the deformation step. From these measurements, the local rotation of the crystal lattice during the deformation step and the Miller indices of the rotational axis are determined. The combination of the data of the local crystal orientation and the local in-plane strains is used to estimate, with a simple kinematic model, the slip systems that are locally active at arbitrary positions within a grain. A polycrystalline copper specimen was loaded in two deformation steps to 7.4 and 14% global tensile strain. A region of ≈250×200 μm was analyzed. The experiment shows that very strong heterogeneities in the in-plane strain and in the local lattice rotation exist within the single grains. At some grain boundaries, very high lattice rotations are observed. Even far from the boundary, the activation of slip systems can be quite different in different regions of a grain. On the specimen surface, more than three independent slip systems must be activated in some grain boundary regions to accommodate the influence of the neighboring grain. In most regions of the grain interior, three slip systems seem to be enough. With the developed procedure, we have generated a tool for a very comprehensive experimental characterization of crystal plasticity in polycrystals.

The aim of this study is to provide a tool, which is capable of studying, at a sub-grain level, the formation and evolution of strain inhomogeneities at the surface of polycrystalline materials. The capabilities of the tool are demonstrated on an oxygen free high conductivity copper (OFHC) specimen which is plastically deformed in an in-situ tensile test in the scanning electron microscope (SEM). At different deformation stages, SEM micrographs are captured which are processed by a new system for local deformation analysis. The key part of the system is a matching procedure for the determination of homologue points in two deformation micrographs. The homologue points form a deformation field, which can be numerically derived to evaluate the local in-plane strains. Due to the high matching accuracy and the high density of the homologue points, it is possible to determine the in-plane strain fields with an accuracy and lateral resolution that has not been possible so far. The system allows us to conduct local deformation analyses in a wide range of magnifications. As an example, a region of about 180×130 μm is analyzed on the copper specimen in deformation steps of about 1% global strain. The in-plane components of the local strain field are characterized by incremental or cumulative strain maps. The exact distribution of the local strains near grain and twin boundaries is obtained by means of local displacement and strain profiles.