Key Engineering Materials Vols. 348-349

Abstract: In this paper the fatigue behaviour of Ti-6Al-4V alloy coated with a TiN arc-deposited PVD film was studied. Rotating bending tests (R = -1) were carried out on standard “hourglass” specimens to evaluate the fatigue limit at 200000 load cycles. Conventional and notched (120° Vnotch transversal to the rotating axis at the minimum cross section area) specimens were tested, both coated and uncoated, to investigate the effect of the coating on the fatigue limit of the titanium alloy, with and without the surface notch. Fracture surfaces were observed by SEM. The coating did not improve the titanium alloy fatigue life.

Abstract: The paper is devoted to fatigue crack propagation in layered materials. The influence of plasticity induced crack closure on threshold values for fatigue crack propagation through interfaces between different materials layers is studied. The main aim was to estimate the influence of the loading ratio R on threshold values for crack propagation through a bi-material interface as a function of the elastic mismatch of both materials. The finite element method (FEM) is used for numerical calculations. Results obtained for different loading ratios, materials, boundary conditions and magnitude of applied loading can be generalized and used for the design of composite bodies with different material layers.

Abstract: Based on the stress analyses on the surrounding rockmass of underground caverns, the stress states on the surface and inside the surrounding rockmass are simulated respectively using biaxial compression tests and truly triaxial compression tests, and the fracture characteristic of rock and the relation between the angle of rock rupture,θ ,and stress state are studied. According to the testing results, the stress state condition of different type of rock bursts generating in different regions of the surrounding rockmass. The research results show that the rock burst of flake-splitting type takes place around the cavern’s surface under the biaxial compression stress state whereas that of shearing-dislocation type takes place inside the surrounding rockmass under the stress state of truly triaxial compression.

Abstract: Probabilistic models are developed to determine changes in the survival probability particularly when prior loading contributes to changes in residual stress and fracture toughness. Experimental evidence for variations in toughness and residual stress is examined and then incorporated into probabilistic models. The structure’s reliability is consistently better after prior loading.

Abstract: Application of coating fragmentation test for determination of coating properties is considered. A probabilistic model of coating fragmentation under uniaxial tensile loading is applied for coating and interface property identification of thin brittle coating/polymer substrate system. A finite element model is developed to simulate the process of buckle formation in coating strips during fragmentation test. The measured buckle geometry parameters and buckle density evolution as a function of the applied strain is used to estimate the interface toughness.

Abstract: Under constant amplitude loading, a single variable (K) or Kmax are required in crack growth relationships. The transferability of fatigue laws, obtained under constant amplitude loading to variable amplitude fatigue, requires at least an additional variable, whose evolution with crack length accounts for the interactions effects between cycles of different types. This paper presents an analysis of fatigue crack growth tests on M(T) specimens made of a medium carbon steel. The specimens are subjected to repeated blocks of cycles made up of one or several overloads separated by a variable number of baseline cycles and two baseline stress ratios. The main objective of this study is to better understand the mechanisms at the origin of interactions effects due to the presence of overloads (or underloads) at different locations of each block loading. Results have shown that the interaction effects are closely related to the cyclic plastic behaviour of the material and also the so-called Bauschinger effect.

Abstract: In this paper, supposing the rockmass is an idealistic elastic object, the methods to calculate I type and II type strength factors of the crack tip are obtained by analyzing the stress state of the inner surface of a close crack in rockmass under compressive and shear stress and applying complex variable function stress method and superposition principle. Furthermore, supposing the friction coefficient, f , of the crack surface is a constant and the normal pressure, P(x) , on the crack surface obeys to normal distribution, the propagation rules of a close crack under compressive and shear stress are also deduced.

Abstract: Fatigue life assessment of critical bridge members using online monitoring data has been investigated in recent years. To make a quick and efficient evaluation, a representative block of strain cycles is usually defined. However, such block sometimes fails to cover the change in fatigue damage by temperature fluctuation and traffic growth, which is important to the life prediction of the welds in a long-span suspension bridge. To find the influence of temperature and traffic volume on the fatigue damage, an equivalent vehicle load method is dedicated through finite element analysis, and the statistical disposition of traffic flow has been conducted, using data from the Runyang Bridge. The influence of traffic growth can be estimated and eliminated from the total damage so that the temperature effect on the fatigue damage is finally obtained. It is found that temperature has a linear effect on the fatigue damage. According to the fitted relationship between fatigue damage, temperature and the traffic flow, a more objective fatigue assessment is undertaken, based on the rain-flow counting method and the Palgren-Miner rule.

Abstract: The possibility of sensing the damage in smart carbon fiber concrete (SCFC) during the fatigue loading process by electrical resistance measurement is investigated in this paper. The damages occurred and accumulated in the carbon fiber concrete specimens under the fatigue loading destroy the local conductive network in the areas near the micro-cracks and cause the whole volume electrical resisitivity increasing. It is verified that the resistance measurement has a great potential to monitor and evaluate the damages in carbon fiber concrete during the whole fatigue process on line.

Abstract: Chaos and chaos synchronization of the centrifugal flywheel governor system are studied in this paper. By mechanics analyzing, the dynamical equation of the centrifugal flywheel governor system are established. Because of the non-linear terms of the system, the system exhibit both regular and chaotic motions. The evolution from Hopf bifurcation to chaos is shown by the bifurcation diagrams and a series of Poincaré sections under different sets of system parameters, and the bifurcation diagrams are verified by the related Lyapunov exponent spectra. This paper addresses control for the chaos synchronization of feedback control laws in two coupled non-autonomous chaotic systems with different coupling terms, which is demonstrated and verified by Lyapunov exponent spectra and phase portraits. Finally, numerical simulations are presented to show the effectiveness of the proposed chaos synchronization scheme.