Key Engineering Materials Vols. 417-418

Abstract: Fretting fatigue is a complex phenomenon that occurs at component interfaces undergoing low amplitude oscillation and high contact pressure. In turbomachinery fretting occurs also at the blade tip interfaces where shrouds are machined with the aim to reduce the blade resonant vibration amplitude. The aim of this study is to investigate the damage by fretting on CMSX4 superalloy interfaces with and without plasma sprayed T800 coating. The contact pair is a hemispherical surface on a flat surface. The hysteresis cycles, namely the contact force against the relative displacement, have been measured through the experiment. The worn surfaces have been characterized by three-dimensional optical interferometry and SEM analysis. After 3×106 and 10×106 fretting cycles the surface without coating shows surface cracks and large change in the contact parameters.

Abstract: An energy-based criterion is introduced in order to investigate the nonlinear dynamic response of RC beams subject to rapid and unexpected damages. Since only static pushover analysis is necessary, the structural behavior of the damaged members can be evaluated with a new simplified procedure. It is firstly applied to the case of elastic–perfectly plastic continuous beam, when one of the supports is quickly removed. In such a situation, the proposed procedure gives satisfactory results if compared to those of a more rigorous dynamic analysis. By means of a nonlinear static analysis, which encompasses also the post-peak response, the energy-based approach is also applied to concrete beams. More specifically, it is used to predict the bearing capacities of reinforced concrete beams, having different steel rebars, when they suddenly lose a single support.

Abstract: 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.

Abstract: 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 (KIC) was also evaluated by means of microhardness measurements. The correlations between KIC 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.

Abstract: 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 extensometer. 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.

Abstract: 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.

Abstract: 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.

Abstract: Apart from its high compressive, tensile and flexural strengths reaching approximately 200MPa, 15MPa and 35MPa, respectively, Ultra High Performance Concrete (UHPC) 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.

Abstract: 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°C. TEM study of the material after the fatigue testing was performed. The dislocation activity is concentrated in the  phase. The properties and behaviour of individual dislocations seem to be similar as those observed in single -phase TiAl alloys.

Abstract: 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 Kt, notwithstanding any non-propagating crack was not recognized at notch roots for specimens endured at 108 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 Kt. Effects of Kt and the hardening behavior on fatigue strength canceled each other, and then fatigue strength became insensitive to Kt. Fatigue strength σSIT obtained by stress increment fatigue tests became higher than the ordinary fatigue strength for each specimen of Kt, that is, the coaxing effect was recognized clearly. This result indicated that the strength for fatigue fracture increased gradually during fatigue test. σSIT had a peak value at Kt of 2.0 and it decreased with increasing Kt sensitively. The increasing behavior of strength for fatigue fracture depended on Kt and fatigue fracture was controlled by circumstances of hardened material and stress amplitude considering Kt.