Key Engineering Materials Vols. 385-387

Abstract: Nearly 90% of failures of machines and mechanical parts are caused at the area of stress concentrated in the structural components. Hence, it is important to investigate the method of improving fatigue strength for notched parts. In this paper, the fatigue tests have been performed to investigate the effect of the plastic-working on fatigue strength of specimens with notch. The main results obtained in this study are as follows: (1) The fatigue limit of notched specimen chamfered by the plastic-working increases by 45% than that of specimen without chamfer. (2) The reason of enhancing the fatigue strength may be to attributed to the work-hardening and compressive residual stress which suppress the fatigue crack initiation and propagation in early stage.

Abstract: The formulations of fatigue crack growth prediction are still mostly based on phenomenological models. A commonly used formula in the field of high cycle fatigue is the Paris- Erdogan law. For given experimental conditions (such as temperature, stress ratio or environmental conditions) the parameters C and m have to be experimentally determined and considered as material constants. Thus, for a given material, the fatigue crack growth rate (FCGR) depends only on the applied range of the stress intensity factor. In a threshold region a significant shift in the data of the fatigue crack propagation rate can be observed. The shift is induced by different test specimen geometry. To analyses it the authors will present their own laboratory fatigue crack growth rate test data measured on two different specimens with different levels of constraint and for different steels. It is demonstrated that fatigue characteristics (i.e. C, m and Kth) obtained from different specimen geometries are not only properties of the materials but depends on the specimen geometry.

Abstract: This paper presents the use of statistically rigorous algorithms combined with electromechanical (E/M) impedance approach for health monitoring of engineering structures. In particular, a statistical pattern recognition procedure is developed, based on frequency domain data of electromechanical impedance, to establish a decision boundary for damage identification. In order to diagnose damage with statistical confidence, health monitoring is cast in the context of outlier detection framework. Inappropriate modeling of tail distribution of outliers imposes potentially misleading behavior associated with damage. The present paper attempts to address the problem of establishing decision boundaries based on extreme value statistics so that the extreme values of outliers associated with tail distribution can be properly modeled. The validity of the proposed method is demonstrated using finite element method (FEM) simulated data while a comparison is performed for the extreme value analysis results contrasted with the standard approach where it is assumed that the damage-sensitive features are normally distributed.

Abstract: The degradation process in fretting fatigue is due to mechanical and chemical attack between two contacting surfaces, being directly related to wear, corrosion and fatigue. There are many parameters that influence the fretting fatigue phenomenon out of which relative displacement, δ , normal load, n F , and tangential load t F , are the most important and consequently the most studied ones. This paper describes the fretting fatigue phenomenon occurring on a high strength aluminium alloy, Al7175. The aim of this study is to achieve a better understanding of the fretting fatigue behaviour by observing the evolution of the fatigue life of the specimen with the normal load, the tangential load and the relative displacement amplitude.

Abstract: The J-integral for cracks in structures storing elastic, dielectric and thermal energies is derived from thermodynamical fundamentals. The FEM is applied to solve thermoelectrome- chanical boundary value problems, accounting for piezoelectric coupling, thermoelasticity and the pyroelectric effect. Examples demonstrate the influence of the thermal conductivity of cracks on the loading of the crack tip.

Abstract: In the present paper, singular stress field at the vertex on the interface in three-dimensional bonded joints is analyzed using BEM and eigen analysis. The order of stress singularity is determined solving an eigen equation based on FEM formulation and the stress distribution is expressed using the result of the eigen-value analysis. A relationship between the thickness of interlayer and residual thermal stresses is presented. Then, a three-dimensional intensity of singularity is determined.

Abstract: The generalized quasi-variational principles with two kinds of variables of time initial value problem were established in nonlinear non-conservative elasto-dynamics. Then, the analytic solution of time initial value problem of a typical non-conservative elasto-dynamics was studied by applying the obtained quasi-complementary variational principle.

Abstract: Cylindrical specimens of cast polycrystalline nickel base superalloy Inconel 738LC were cyclically strained under total strain control at 23 and 800 °C to fracture. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained at both temperatures. Surface relief was studied in specimens fatigued to failure using scanning electron microscopy. Cyclic hardening/softening behaviour depends both on temperature and strain amplitude. Low amplitude straining was characterized by saturation of the stress amplitude. In high amplitude straining a pronounced hardening was found which was followed by saturation at room temperature and by cyclic softening at 800 °C. The cyclic stress-strain curves can be fitted by power law. They are shifted to lower stresses with increasing temperature. Fatigue life curves can be approximated by the Manson- Coffin and Basquin laws. The Manson-Coffin and Basquin curves are shifted to lower lives with increasing temperature. Slip markings were detected on specimen surface at all test temperatures. When temperature grows the density of slip markings is reduced.

Abstract: Orthotropic bi-materials interfacial crack was studied, by constructing new stress function and using composite complex function method of material fracture. When secular equations’ discriminant 1 0
< and 2 0
< , the theoretical solutions of stress fields without oscillation singularity and displacement fields without embedding of upper and lower sides are derived.

Abstract: The formulae between electrical resistance change and damage in metals were studied on the basis of different damage mechanisms. This DC electrical resistance is a more sensitive parameter to the initiation of microcracks during the irreversible damage accumulation process. Measurement of low electrical resistance due to accumulative damage is discussed. Agreement between evaluations of ductile damage via electrical resistance change and elasticity modulus measurement is good.