Key Engineering Materials Vols. 488-489

Abstract: Cyclic loading of metallic engineering components at constant elevated or fluctuating temperature causes a complex evolution of damage which can hardly be described. At the same time the thermal expansion is restricted in some regions, due to the complex geometry of the components. Therefore, mechanical stresses occur and cyclic plastic deformation leads to thermo-mechanical fatigue (TMF) of the material. A careful analysis and comparison of the experimental results, based on a systematical variation of the relevant influence factors, allow to develop empirical models for computing the fatigue life of thermo-mechanical loaded components made of Ni alloys (IN718, Ni200/201). Different lifetime calculation methods were executed and analyzed to fulfil the requirements for the fatigue analysis of components made of these alloys. In addition, the model parameters can be optimized using these results. Based on the parameters, which define the usability of the investigated lifetime models, e.g. temperature, stress, plastic strain, the limiting combination of the parameters can be described. This graphical representation of the “border line” or limiting combination can also be used for any other materials. The results show that the main lifetime calculation models can only applied in a limited range. This limited range is mainly defined by temperature, strain constraining and their combination.

Abstract: In order to carry out the railway axle design according to the “Damage Tolerance” philosophy, reliable crack-growth models for these kind of components are necessary. Indeed, damage tolerance principles have received more and more attention from railway technical community, thanks to its ambitious task concerning the inspection intervals prevision of railway components subjected to non-stationary loading conditions. In this paper, a simple routine is exposed that is able to calculate the characteristic parameters of the Linear Elastic Fracture Mechanics (LEFM) for a generic cracked mechanical component. Such parameters are then used in a two parameters propagation law to estimate the necessary time for a crack to become critical.

Abstract: This Study purposed to review on the strength of geopolymer mortar and the change in liquidity depending on the mixing ratio of alkali-activator in developing the geopolymer mortar using bottom ash as binder. Alkali-activator was used through mixing 9M-Sodium hydroxide (SH) and 3 kinds of Sodium sillicate(SS) and its strength was measured by material age after curing for 48 hours at 60°C. As a result, the pressure strength was shown as 40MP when the mass ratios of 9M-Sodium hydroxide and 3 kinds of Sodium sillicate were35 vs. 65 and 50 vs. 50, but the solution of the mass ratio of 50 vs. 50 showed the rapid solid phenomenon in the process of mortar manufacturing, thereby disadvantage in construction. In addition, in case of storing the mixed solution in room temperature, as the mix ratio of 9M-Sodium hydroxide increases, the solution solidified. Accordingly, when considering on strength, liquidity and storage, it’s most preferable to fix the mass ratio of 9M-Sodium hydroxide and 3 kinds of Sodium sillicate at 35 vs. 65.

Abstract: The methods based on the properties of the two-state integrals allow one to calculate the amplitude of singular and the other terms of the Williams’ asymptotic expansion. The paper is focused on the use of the Y-integral, whose application is conditioned by the knowledge of the so-called auxiliary solution of the solved problem. On the other hand, the Y-integral can be applied to the analysis of the problems with various geometries, e.g. the analysis of the bi-material notches. The application of the Y-integral can be also extended to the matched asymptotic procedure, which allows one to predict the behavior of the cracked notches or following crack growth near the bi-material interfaces.

Abstract: Chromium coatings have been used for the protection of gas turbine blades in power plants during the last years due to their very good resistance against steam corrosion. Microhardness, scratch, adhesion and pin-on-disk sliding tests are commonly used for rapid evaluation of the mechanical properties of these coatings [1,2]. However, very limited information exists on their fatigue resistance. In this paper we present the experimental results of the impact testing examination of the above coatings. This experimental method is capable to assess the fatigue and the impact wear resistance of coatings working under cyclic impact loading conditions. From the experimental results it was concluded that the slurry Cr, Zr coatings deposited on P91 steel showed adequate fatigue strength for the above-mentioned kind of use.

Abstract: The logarithmic strain is more suitable for analyzing large strain problems because the volume invariability condition in small deformation is equivalent to volume invariability condition in large deformation when using the logarithmic strain. Large simple shear deformation has always been used in the analysis of large strain problems. In this paper, elastic large strain constitutive model was introduced based on the logarithmic strain and large simple shear deformation was analyzed by using the constitutive model given in the paper. The stress responses to large simple shear deformation were derived corresponding to four objective rates of tensors. The results show that normal stresses may maintain good monotonicity, but there exists different levels of oscillation of shear stress corresponding to various objective rates, and there was the most severe oscillation of stress when adopting Jaumann rate. The objective rate should not be the only factor bringing about oscillation of shear stress.

Abstract: Mechanical degradation of hydroxyapatite ceramics due to the slow crack growth was observed in water. Microstructural crack on the surface of hydroxyapatite initiated by grain boundary dissolution in liquid environment resulting in particle loosening and microstructural-level degradation, followed by a drastic decrease of mechanical properties. In this study, slow crack growth by the dissolution behavior of hydroxyapatite ceramics was investigated based on microstructural observation by field emission microscopy. The crack growth and morphological change of hydroxyapatite surface, especially at the initial stage of dissolution and related surface roughness were observed with immersion time. The surface dissolution occurred from the initial stage of immersion showing increase in surface roughness. Following certain period of immersion time, the surface dissolution initiated at grain boundaries and generated many separated grains.

Abstract: The appearance and growth of the microcracks in the structure is an important factor that influences the structure safety and its service life. Thus it is very important to detect the crack and monitor its growth at the beginning of the crack. Aiming at the main style of failures in metal structure - fatigue fracture, this paper research acoustic emission waveforms analysis that base on wavelet packets feature extraction, through processing acoustic emission signal to test metal fatigue fracture. First, this paper analyses the reason of metal fatigue fracture and introduces the theory of acoustic emission. Based on that, we establish the time domain module of acoustic emission signal and extract the feature of acoustic emission signal using wavelet packets. According to the experimental results bending specimen, acoustic emission techniques monitoring fatigue crack propagation is certificated not only to resemble variable rule of fatigue crack propagation but also to catch generation of fatigue crack in real time. Compared with the method of parameter extraction, this method can not only realize real-time and dynamic monitoring, but also get the result that is similar with fatigue crack expanding rate curve.

Abstract: The CBF (continuous basalt fiber) composite as new and high mechanical property material have been widely applied in actual engineering. The composite material may inevitably occur all kinds of defects during fabrication and using, so in this paper, the AE (acoustic emission) and ESSPI (electronic shearography speckle pattern interferometry) technique are adopt to study the defects or damage of the CBF composite plate. The AE technique is adopted to detect the damage phenomenon of the CBF composit plate specimen tension experiment, and the ESSPI technique is adopt to study the defects of the CBF composite plate which its defects are made by artificial. From the experiment results it can be see that these techniques can detect the position and damage of the CBF composite plate, and these study can provide useful reference for the application of the CBF composite.

Abstract: The surface displacement of a circular lining structure and multiple cracks in an elastic half space by incident SH-wave is studied in this paper based on the methods of Green's function, complex function and multi-polar coordinates. Firstly, we construct a suitable Green’s function which indicates a fundamental solution to the displacement field for an elastic half space possessing a circular lining structure and cracks while bearing out-plane harmonic line loads at arbitrary point. Then using the method of crack-division, a crack is created. Thus expressions of displacement and stress field are established at the existence of the structure and the cracks. Finally, the interaction of inclusion and two cracks is chosen as numerical examples and the influences of different parameters on the surface displacement are discussed.