Materials Science Forum Vol. 482

Abstract: The behavior of short fatigue cracks is a matter of importance not only because much of the fatigue lifetime is spent in propagating these cracks, but also because the boundary between propagation and non-propagation separates the safe from the potentially unsafe fatigue regimes. The method of analysis is based upon the following equation:

Abstract: Fracture mechanics based test and evaluation techniques are used to gain insight into the phenomenon of stress corrosion cracking (SCC) and to develop guidance for avoiding or controlling SCC. Complementary to well known constant load and constant deflection test methods experiments that are based on rising load or rising displacement situations and are specified in the new ISO standard 7539 – Part 9 may be applied to achieve these goals. These are particularly suitable to study cases of SCC and hydrogen embrittlement of high strength steels, aluminium and titanium alloys and to characterise the susceptibility of these materials to environmentally assisted cracking. In addition, the data generated in such R-curve tests can be used to model the degradation of the material caused by the uptake of atomic hydrogen from the environment. This is shown for the case of a high strength structural steel (FeE 690T) where in fracture mechanics SCC tests on pre-cracked C(T) specimens a correlation between the rate of change in plastic deformation and the crack extension rate due to hydrogen embrittlement was established. The influence of plastic strain on the hydrogen diffusion was additionally studied by electrochemical permeation experiments. By modelling this diffusion based on the assumption that trapping of the hydrogen atoms takes place at trap sites which are generated by the plastic deformation, a good agreement was achieved between experimentally obtained data and model predictions.

Abstract: The ideal (theoretical) strength was originally defined as the stress or strain at which perfect crystal lattice became mechanically unstable with respect to arbitrary homogeneous infinitesimal deformation. This has been intensely investigated because the ultimate strength without defects is a fundamental mechanical characteristic of materials. In the analyses, the instability criteria have been studied on the basis of elastic constants. Recent developments in computational technology make it possible to analyze the ideal strength on the basis of quantum mechanics. On the other hand, it is well known that the mechanical strength of components is dependent not only on (1) material (atom species), but also on (2) loading condition and (3) structure. Because most studies on the strength in terms of atomic mechanics have focused on the factor (1) (materials), analysis has mainly been conducted on simple crystal consisting of perfect lattices (e.g. fcc and bcc) under simple loading conditions (e.g. tension), though some have explored the properties of bulk materials with defects (e.g. vacancy and grain boundary). Small atomic components (nano-structured components) such as nano-films, nano-wires (tubes) and nano-dots (clusters) possess their own beautiful, defect-free structures, namely ideal structure. Thus, they show characteristic high strength. Moreover, utilizing the structure at the nanometer or micron level is a key technology in the development of electronic devices and elements of micro (nano) electro-mechanical systems (MEMS/NEMS). Therefore, it is important to understand the mechanical properties not only for the sake of scientific interest, but also for engineering usefulness such as design of fabrication/assembly processes and reliability in service. In the other words, the effects of structure (factor (3); e.g. film/wire/dot) have to be understood as the basic properties of atomic components. Thus, the definition of ideal strength should be expanded to include the strength at instability of components with ideal structures under various external loads (factor (2)), which provides fundamental knowledge of nano-structured materials. In this paper, we review works on the strength of ideal nano-structured components in terms of factor (3), mainly under tension.

Abstract: The state of the art of ab-initio calculations of the theoretical strength (TS) of materials is summarized and a database of selected theoretical and experimental results presented. Differences between theoretical and experimental TS values are discussed by assessing the stability conditions.

Abstract: The geometric phase technique (GPA) for measuring the distortion of crystalline lattices from high-resolution electron microscopy (HRTEM) images will be described. The method is based on the calculation of the “local” Fourier components of the HRTEM image by filtering in Fourier space. The method will be illustrated with a study of an edge dislocation in silicon where displacements have been measured to an accuracy of 3 pm at nanometre resolution as compared with anisotropic elastic theory calculations. The different components of the strain tensor will be mapped out in the vicinity of the dislocation core and compared with theory. The accuracy is of the order of 0.5% for strain and 0.1° for rigid-body rotations. Using bulk elastic constants for silicon, the stress field is determined to 0.5 GPa at nanometre spatial resolution. Accuracy and the spatial resolution of the technique will be discussed.

Abstract: The possibilities of atomic force microscopy in studying surface features and early fatigue damage in materials are reviewed. Examples of a true relief arising on the surface of cyclically strained materials as recorded by atomic force microscopy are given. Characteristic features of the surface relief are well-defined persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions can be obtained by observation of metal surface and replicas and in combination with high resolution scanning electron microscopy. The growth of extrusions during fatigue life is reported for austenitic and ferritic stainless steels. The experimental data are compared with models of the localized cyclic plastic straining and predictions of the fatigue crack nucleation models.

Abstract: In elastic plastic solids, approaching the sub micron scale, critical experiments indicated significant differences in the mechanical response. Thus, mainly in small volume behavior a length scale issue is introduced with implications on the basic understanding of deformation and fracture processes. The current study is centered on the mechanical response of silicon particles in the range of 20-50 nm on sapphire substrate. Monotonic and cyclic mechanical tests have been performed by contact mechanics methodology at ambient temperature. Mechanical information and visualization assisted by scanning probe microscope-based nano indentation alluded to a model founded on dislocation dynamic effects. This facilitated developments regarding the length scale subject in the light of fatigue concepts and structural integrity aspects.

Abstract: This paper discusses micropstructural aspects of brittleness fracture of polycrystalline materials caused by intergranular fracture. Structure-dependent intergranular brittle fracture in bicrystals and polycrystals are discussed and predicted theoretically. Experimental evidence for the structure-dependent intergranular fracture is shown and some general features are discussed to demonstrate the relationship between grain boundary structure/character, grain boundary energy and intergranular fracture strength. Theoretical prediction of the fracture toughness based on the strongest-link theory is introduced for polycrystals with different grain boundary microstructures, primarily defined by the grain boundary character distribution, grain boundary connectivity. Finally recent achievements of successful control of intergranular brittleness by grain boundary engineering based on the strongest-link theory are introduced for different materials.