Materials Structure & Micromechanics of Fracture V

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Authors: Yoshiyuki Kondo, H. Eda, Masanobu Kubota
Abstract: Although fatigue limit diagram is defined in principle for constant stress amplitude condition, it is often considered that fatigue failure would not occur even in varying loading if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small-notched specimen and fretting fatigue that fatigue failure occurred in some special case of variable amplitude loading condition even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two-step and repeated two-step stress patterns in which the first step stress was with zero mean stress and the second step stress had a high mean stress. A non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of non-propagating crack and mean stress change.
Authors: Petr Lukáš, Ludvík Kunz, Milan Svoboda
Abstract: Fatigue behaviour of ultrafine-grained copper of purity 99.9 % produced by ECAP technique was studied in a broad region of stress amplitudes. Fatigue strength is by a factor of about 2 higher than that of conventional-grain-size copper in the broad region of fatigue lives from 6x103 to 2x1010 cycles. The grain structure is stable and undergoes only very marginal changes during cycling. Fatigue slip markings on specimen surface follow the trace of the shear plane of the last ECAP pass. Fatigue notch sensitivity is also higher than that of conventional-grain-size copper, but not dramatically. The cyclic stress-strain curve of studied copper is temperature insensitive, while its S-N curve is temperature dependent.
Authors: Michal Kotoul, Tomáš Profant, Oldřich Ševeček, Martin Krejcir
Abstract: Matched asymptotic procedure is used to analyze crack crossing a sharp interface between dissimilar elastic anisotropic materials. The link to the configurational forces approach is suggested.
Authors: David Taylor
Abstract: The work described below investigates, for the first time, the link between microstructural parameters such as grain size and the length constant L which is known as the critical distance. L is frequently used in the prediction of failure processes such as brittle fracture and fatigue, initiated at stress concentration features such as notches. Values of L were calculated using data from the literature on the effect of short cracks and notches in steels and ceramics. In some cases, simple relationships could be established between L and the grain size, or other microstructural features. Sometimes L was found to be much larger than anything in the microstructure and appeared to be related to the size of the damage zone at failure.
Authors: Yannick Champion, Sophie Nowak, Sandrine Guérin, Cecilie Duhamel
Abstract: Copper nanostructure and aluminum-alumina nanocomposite are studied using the stress relaxation technique to determine the activation volume involved in the micro-mechanism of the deformation. These materials exhibiting near-perfect elasto-plastic deformation show similar behavior in the steady state flow domain. Difference is observed when relaxations are carried out in the work-hardening domain where dislocations variation occur.
Authors: Jozef Janovec, Jaroslav Pokluda, Pavel Lejček
Abstract: Chemical and structural changes at the grain boundaries were investigated to quantify their influence on fracture behaviour of austenitic stainless steels and model ferritic Fe-Si-P alloys. The balance between the size and the area density of intergranular particles was found to be one of the most decisive factors influencing sensitivity of the steels to intergranular fracture. The precise dependence of the energy of intergranular fracture on the phosphorus grain boundary concentration was also determined.
Authors: W. R. Tyson
Abstract: Characterization of fracture toughness is discussed in relation to specification of steels for northern pipelines. The state of the art and research trends in measurement of CTOD for girth welds and CTOA for linepipe steel are described.
Authors: Carlos H. Cáceres, A.H. Blake
Abstract: The Kocks-Mecking method of analysis is applied to solid solutions of up to 2.6 at.% Zn to separate the contributions to the alloys’ strain hardening rate from dislocations storage, solute in solution, and twinning, for temperatures between -50 °C (273 K) and 200 °C (473 K). Athermal storage of dislocations seems to account for the largest share of the strain hardening rate for both the pure metal and the solid solutions at or below room temperature. Solute in solution does not increase the strain hardening rate over that of pure Mg, although it delays the onset of dynamic recovery, especially for the higher alloys, presumably due to short range order. Twinning remains a very important deformation mechanism for the pure metal and the dilute alloys up to 200 °C.
Authors: F. Walther, Dietmar Eifler
Abstract: Mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed for the microstructure-related characterization of the fatigue behavior and for the fatigue life calculation of metals. The proceeding fatigue damage was evaluated using the change of the load-free electrical resistance, which is strongly influenced by the defect density of the individual material state. A new test procedure was applied for the fatigue assessment under random loading on the basis of cyclic deformation curves, similar to single step loading. A physically based fatigue life calculation “PHYBAL” was developed, which requires only three fatigue tests for the rapid and nevertheless precise calculation of S-N (Woehler) and fatigue life curves.

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