Search

A technique to in-situ measure the displacements in the vicinity of the crack tip during fatigue crack propagation has been developed. High-resolution images of the crack tip were taken continuously throughout the fatigue load cycles with a scanning electron microscope (SEM), and an image analysis program was used to determine the displacements at different positions with respect to the crack tip. The displacements were then used to determine crack shapes and compliance curves. The measured crack shapes show a general √ r dependence versus the distance to the crack tip. However, close to the crack tip the crack shape is clearly affected by plastic deformation, even in cases when small scale yielding prevails. The compliance curve measurements close to the crack tip can be used to determine the global stress level when the crack surfaces are separated, so that the exact opening and closure stresses can be determined. [ABSTRACT FROM AUTHOR]

A paste comprising fine alumina particles was applied on surfaces of steel plate specimens and a welded joint specimen, and the effects of the paste on restraint and visual detection of fatigue crack growth were experimentally investigated by performing fatigue tests with in situ observations by a harge oupled evice (CCD) microscope and through various kinds of fractographic observations and elemental analyses using an scanning electron microscope (SEM) and an electron probe microanalyser (EPMA). As a result, the crack growth rate in the plate specimen was drastically retarded by the wedge effect of the alumina particles, and a 311% increase in failure life was produced on an average. The restraint of crack opening displacement (COD) value by the wedge materials was estimated by elastic finite element (FE) analyses, and the results were compared with the experimental data. Similar crack growth restraint effects were also observed on the welded joint specimen, producing a 117% increase in failure life. In the fatigue tests of the specimens on which the alumina paste was applied, a remarkable black colour developed in the white alumina paste along the paths of crack propagation, facilitating the visual detection of the cracks. An analysis using an X-ray diffractometer showed that the black matter in the paste consists of fine debris derived from the base metal. [ABSTRACT FROM AUTHOR]

This paper examines fatigue crack paths in coarse-grained magnesium alloys. Visualization of the microstructure by etching and continuous crack monitoring with a microscope provide evidence of the crack-grain interaction at different fatigue crack growth regimes. Near-threshold fatigue crack growth promotes the development of rough surfaces by the activation of single slip mechanisms and crack deflection. TheR-ratio effect on near-threshold results is discussed in the light of the partial crack-closure model. The role of grain boundaries and grain-to-grain crack deflections on fatigue crack growth rates is discussed. [ABSTRACT FROM AUTHOR]