摘要: 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.
摘要: 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.
摘要: 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
摘要: 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.
摘要: 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.
摘要: 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.
摘要: 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.
摘要: 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.
摘要: 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.