Petr DymacekStanislav SeitlKarel MilickaFerdinand Dobes...
561-564页
查看更多>>摘要：The FEM modeling of small punch tests on miniaturized thin discs (SPT) of two heat resistant steels was performed. The FE models did represent the creep SPT, i.e. tests with constant acting force. It was shown that different values of the surface friction coefficient used in the calculations have significant impact on the calculated stress and strain state and consequently on the deformed shape of the disc. Thus, the surface friction coefficient should be considered one of the key factors for any correct correlation of SPT and uniaxial creep test results. Proper attention must be paid to the friction conditions during the long term creep deformation. An attempt to define simple approach how to relate the SPT with uniaxial tests is suggested. Some of the calculated results are compared with experiment.
查看更多>>摘要：An innovative gel-casting process was developed in order to obtain macro porous ceramics scaffolds of hydroxyapatite to be used in regenerative medicine for bone tissue reconstruction. Mechanical investigation was carried out on different formulations of dense hydroxyapatite samples in order to evaluate the effect of the gel casting process parameters on the density, the elastic modulus, the tensile and the compressive strength. The fracture critical stress intensity factor (K_(IC)) was also evaluated by means of microhardness measurements. The correlations between K_(IC) and tensile and compressive strength were examined taking into account the average and maximum size of porosity. The mechanical properties of macro and micro-porous HA are in agreement with the model of Gibson and Ashby.
查看更多>>摘要：The stress-strain curve of a material is usually obtained from the load-displacement curve measured in a tensile test, assuming no strain localisation up to maximum load. However, strain localisation and fracture phenomena are far from being completely understood. Failure and strain localisation on plane tensile specimens has been studied in this work. A deeply instrumented experimental benchmark on steel specimens has been developed. Surface strain fields have been recorded throughout the tests, using an optical extcnsometer. This allowed characterisation of the strain localisation and failure processes. Tests have been numerically modelled for a more detailed analysis. Preliminary results show a substantial influence of geometrical specimen defects on the strain localisation phenomena that may be critical on the stress-strain curves obtained and in the failure mechanisms.
查看更多>>摘要：It is well known that asphalt concrete is a self healing material: immediately after both faces of a crack are in contact, the diffusion of molecules from one face to the other starts. If there are no more loads, this process takes place until the crack has completely disappeared and the material has recovered its original resistance [1]. To increase this healing rate two methods are proposed. The first one is a passive self-healing mechanism. Embedded encapsulated chemicals are used in the binder. When microcracks start appearing in the binder due to the combination of ageing and accumulated damage, they break the capsules and the chemicals enter the binder by diffusion. These chemicals repair the material, decreasing the stiffness and increasing the healing rates of bitumen. The second approach makes use of an active self healing mechanism. Local heating inside the material is used to repair the binder and to improve the properties again. This is realized by adding conductive particles to the binder and using induction energy to increase the temperature. These methods are a fairly new concept in the asphalt industry.
查看更多>>摘要：The fatigue properties of ferritic-pearlitic-bainitic steel using specimens produced from massive forging were measured in stress controlled regime with positive mean stress. The cyclic creep curves and cyclic hardening/softening curves were evaluated. The fatigue life was plotted in dependence on the mean stress and on the plastic strain amplitude. The principal contribution to the drop of the fatigue life with the mean stress is due to the increase of the plastic strain amplitude in cycling with mean stress.
查看更多>>摘要：Apart from its high compressive, tensile and flexural strengths reaching approximately 200MPa, 15MPa and 35MPa, respectively, Ultra High Performance Concrete (LJHPC) is characterized by its high resistance against degrading factors that can delay their penetration and diffusion speeds down to 1/20 to maximum 1/10,000 compared to ordinary concrete. UHPC also exhibits self-compaction properties with a slump flow of about 220mm. Furthermore, the most remarkable feature of UHPC is the improvement of its flexural strength and toughness through the admixing of steel fiber. Accordingly, this study evaluates the effects of the placement method on the flexural performance of UHPC. As a result, the flexural strength of UHPC appears to be extremely dependent on the placement method with variation of the maximum flexural strength up to 2 to 3 times while poor influence is observed on the initial crack strength.
Tomas KrumlKarel ObrtlikMartin PetrenecJaroslav Polak...
585-588页
查看更多>>摘要：Fatigue properties of the new generation of TiAl alloys with 8at.% of Nb were studied and compared with those reported in the literature for previous generation with 2at.% of Nb. The high Nb content improves substantially the fatigue life time in the Basquin representation. Nevertheless, the fracture behaviour is still rather brittle even at 750℃. TEM study of the material after the fatigue testing was performed. The dislocation activity is concentrated in the γ phase. The properties and behaviour of individual dislocations seem to be similar as those observed in single γ-phase TiAl alloys.
查看更多>>摘要：In order to clarify notch effect on fatigue strength of an austenitic stainless steel, rotating bending fatigue tests were carried out for smooth and notched specimens at 573K. Clear endurance limits were recognized and the fatigue strength did not decrease continuously with increasing stress concentration factor K_t, notwithstanding any non-propagating crack was not recognized at notch roots for specimens endured at 10~8 stress cycles. According to hardness tests, an area of notch root hardened during fatigue test by dynamic strain aging. The hardening behavior occurred remarkably with increasing K_t. Effects of K_t and the hardening behavior on fatigue strength canceled each other, and then fatigue strength became insensitive to K_t. Fatigue strength σ_(SIT) obtained by stress increment fatigue tests became higher than the ordinary fatigue strength for each specimen of K,, that is, the coaxing effect was recognized clearly. This result indicated that the strength for fatigue fracture increased gradually during fatigue test. σ_(SIT) had a peak value at K_t of 2.0 and it decreased with increasing K_t sensitively. The increasing behavior of strength for fatigue fracture depended on K_t and fatigue fracture was controlled by circumstances of hardened material and stress amplitude considering K_t.
查看更多>>摘要：The fatigue properties of friction stir (FS) and TIG welded Al 6N01-T5 alloys were compared in the present study. The low cycle fatigue (LCF) test was performed under total strain amplitudes in the range of ± 0.6 ～ ± 1.5% and with a strain rate of 3 × 10~(-3)/s. During low cycle fatigue, the base metal showed little cyclic hardening or softening, whereas both the FS and TIG welded zones showed a large amount of cyclic hardening until fracture, although the fatigue life of the TIG welded zone was much shorter than that of the FS welded zone. The fatigue crack growth rate (FCGR) was retarded in the heat affected zone (HAZ) on the advancing side of the FS welded zone, due to the compressive residual stress in this region. The differences in the fatigue properties in these two welded zones were discussed in terms of the microstructural changes during fatigue.
查看更多>>摘要：In this paper, the mechanisms of propagation of the damage in aluminum panels repaired with bonded composite patches of different mechanical characteristics is analyzed. The aim of this study is to analyze analytically, experimentally and numerically the advantage of the use of bonded composite patches to increase the fatigue life and to reduce the state of tension at the crack tips. The experimental results show that both static strength and fatigue life of the repaired aluminum panel has significantly increased due to the bonded composite patches. The different patches and adhesive, used for cracked panels, have provided about a 100-110% improvement in the fatigue life and a 30-35% decrease in the stress intensity factor. A comparison between finite elements calculations and experimental data has been carried out. The good agreement between-the experimental data and the numerical ones has demonstrated the possibility to obtain an optimized design of bonded patches with the numerical tools.
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