Key Engineering Materials Vols. 324-325

Abstract: The behavior of crack growth with a view to fatigue damage accumulation on the tip of cracks is discussed. Fatigue life of welded components with initial crack in bridges under traffic loading is investigated. The study is presented in two parts. Firstly, a new model of fatigue crack growth for welded bridge member under traffic loading is presented. And the calculate method of the stress intensity factor necessary for evaluation of the fatigue life of welded bridge members with cracks is discussed. Based on the concept of continuum damage accumulated on the tip of fatigue cracks, the fatigue damage law suitable for steel bridge member under traffic loading is modified to consider the crack growth. The proposed fatigue crack growth can describe the relationship between the cracking count rate and the effective stress intensity factor. The proposed fatigue crack growth model is then applied to calculate the crack growth and the fatigue life of two types of welded components with fatigue experimental results. The stress intensity factors are modified by the factor of geometric shape for the welded components in order to reflect the influence of the welding type and geometry on the stress intensity factor. The calculated and measured fatigue lives are generally in good agreement, at some of the initial conditions of cracking, for a welded component widely used in steel bridges.

Abstract: Fatigue tests are conducted on 304 stainless steel and 45C steel under fully reversed strain control conditions with two different loading modes. 45C steel exhibits cyclic softening under each phase loading. While for 304 stainless steel, much additional hardening is observed in out-of-phase loading. The damage values for failure of 45C steel is similar to the previous research, however, 304 stainless steel is not. Fatigue life is predicted based on the linear damage rule, the double linear damage rule, and the plastic work model of Morrow. The damage value is different in the same loading mode for the two materials according to linear damage rule.

Abstract: Triangular cantilevers are used as small force sensors. Prediction of location and size of multiple cracks from experimental results will be of value to users and designers of cantilever deflection force sensors. We extend a method for prediction of location and size of multiple cracks in rectangular cantilevers to deal with triangular cantilevers in this paper. The cracks are assumed to introduce local flexibility change and are modeled as rotational springs. The beam is divided into a number of segments, and each segment is associated with a damage index, which can be calculated through the relationship between the damage index and strain energy of each segment and the changes in the frequencies caused by the cracks. The location of cracks can be obtained with high accuracy with sufficient segment numbers. The size of a crack can be calculated through the relationship between the crack size and its stiffness, which can be obtained from the damage index related to the crack. The maximum error in prediction of the crack position in the case of double cracks is less than 15%, and it is less than 25% in prediction of the crack size.

Abstract: In this paper, the size effect of thickness on fracture toughness in the fracture behavior of a kind of copper foils was studied experimentally and analytically. Copper foil specimens with thicknesses ranging from 0.02mm to 1mm were adopted. The fracture toughness defined as the value of the critical J integral at cracking initiation was shown to depend on foils’ thickness within a certain range of thickness. For elucidating the relationship between fracture toughness and thickness, microscopic experimental analysis was carried out. Metalloscope results also showed that microscopic structure of different specimens with different thicknesses is also a factor affecting the fracture toughness. In order to explore the effect of pure thickness on fracture, specimens with the same microscopic structure were also investigated.

Abstract: Some numerical parameters-sensitivity analysis has been conducted to evaluate the stability and propagation of the interface cracks at heel of concrete gravity dam. In this paper, utilizing the software ANSYS to simulate the stress and displacement fields of the tip of the interface cracks between concrete gravity dam and foundation, the stress intensity factor (SIF) of the interface crack is analyzed using facture mechanics. Three impacting factors have been discussed, such as the crack length, the angle of crack, and the water height. Critical length and loads of interface crack propagation are obtained using composite fracture criteria. The results indicate that the coarse interface retards the propagation of interface crack, and redounds to stability of gravity dam. It is found that the interface crack often propagates alone the interface between dam and foundation, simultaneously the branch crack kinks to foundation at the specific condition.

Abstract: A newly developed Z-fracture criterion is introduced in this paper by introducing the new concepts of in-plane dilatational strain energy density factor, in-plane average strain and reciprocal function. The Z-fracture criterion is applied to predict the mixed mode crack propagating direction in carbon fiber reinforced plastic (CRP) plate. The test results show that the Z-fracture criterion can be successfully used to predict the mixed mode crack propagation direction in CRP plate.

Abstract: In this paper, the characterisation of damage in an epoxy adhesive has been investigated. Bulk adhesive samples were used in this study for two reasons; firstly the stress distribution in the bulk adhesive sample is simpler than that in a joint, secondly, the specimen’s dimensions meet fatigue test specimen standards. Low cycle fatigue (LCF) tests with a load ratio of 0.1 and a frequency of 5 Hz were performed on bulk adhesive dumbbell specimens. Damage curves, relating damage in the specimen to number of cycles to failure, were plotted using an isotropic damage equation in which damage is a function of stress, which decreases as damage progresses. The damage curves were then fitted using a LCF damage evolution law. This equation was derived from a dissipation potential function using Continuum Damage Mechanics (CDM) theory. Curve fitting was performed using a Robust Least Square technique rather than ordinary linear least square because the damage curve has extreme points (usually at the breaking point). Two damage parameters α and β were found from the curve fitting process. This process resulted in different values of α and β for different stress levels. The logarithmic α and β points were then plotted respect against stress level and linear regression was used to determine α and β as a function of stress. With this function, damage parameters for other stress level can be predicted.

Abstract: The effect of transverse shear on the deformation of thick laminated sandwich plates under cylindrical bending is studied, based on the first order shear deformation theory (FSDT) with the application of shear correction factor (SCF). It is shown that depending on the mechanical and geometrical properties of the layers, the contribution of the transverse shear stress to the maximum deflection of the plate is variable and in some cases accounts for up to around 88% of the total deflection. The analytical results are compared and verified with finite element analysis.

Abstract: This paper presents a modified Weibull stress model, which accounts for the effects of plastic strain and stress triaxiality at the crack tip region. The proposed model is applied to predict cleavage fracture in a modified A508 pressure vessel steel. It is demonstrated that the Weibull modulus (m) remains constant in the temperature range considered, while the threshold Weibull stress (σw-min) decreases with an increase in temperature due to reduction of the yield stress and the scale parameter of the Weibull model (σu) increases with temperature reflecting the influences of temperature on both material flow properties and toughness. The proposed model accurately predicts the scatter of the measured fracture toughness data and the strong effects of constraint and temperature on cleavage fracture toughness.

Abstract: A moving crack in an infinite strip of orthotropic anisotropy functionally graded material (FGM) with free boundary subjected to anti-plane shear loading is considered. The shear moduli in two directions of FGM are assumed to be of exponential form. The dynamic stress intensity factor is obtained by utilizing integral transforms and dual-integral equations. The numerical results show the relationships among the dynamic stress intensity factor and crack velocity, the height of the strip, gradient parameters and nonhomogeneous coefficients.