Key Engineering Materials Vols. 348-349

Abstract: Recently the decisive role of plastic strain amplitude for the initiation and the growth rate of short cracks has been demonstrated. The plastic strain amplitude can be related to the rate of short crack growth and also to the fatigue life. Since the cyclic stress-strain response of a material determines the plastic strain amplitude it influences basically its fatigue life. The experiments in stress and plastic strain controlled loading and short crack growth are presented and used to demonstrate the importance of the cyclic plastic response for the evaluation of the fatigue life.

Abstract: The present work studies the effect of the microstructure and of the stress gradient on the early fatigue crack growth in a two-phase austenitic-ferritic stainless steel. Fatigue tests were performed on two geometries of double edge notched specimens. Direct optical observations of the surface are used to measure the surface crack growth rates. The results are presented and the effects of stress and strain gradients on the crack propagation kinetics are discussed.

Abstract: An overview on modeling of high-cycle fatigue is given and experimental findings of the damage accumulation are discussed. Finally we sketch an isotropic constitutive model for the description of the damage accumulation due to high-cycle fatigue.

Abstract: The fatigue vulnerability estimation method including the effects induced by the corrosion rate and the traffic variation with time has been proposed to evaluate the reduction of fatigue strength in steel bridge members. A fatigue limit state function considering corrosion and traffic variation is established to determine whether the steel bridge members have been damaged by both corrosion and fatigue or not. Monte-Carlo simulation is used for reliability analysis which provides the data used to obtain fatigue vulnerability curves. The results show that the damage accumulation induced by fatigue in steel bridges could be assessed by fatigue vulnerability curve considering influences of corrosion and traffic variation.

Abstract: In this paper detailed results of computational 3D fatigue crack growth simulations will be presented. The simulations for the crack path assessment are based on the DBEM code BEASY, and the FEM code ADAPCRACK 3D. The specimen under investigation is a SEN-specimen subject to pure anti-plane or out-of-plane four-point shear loading. The computational 3D fracture analyses deliver variable mixed mode II and III conditions along the crack front. Special interest is taken in this mode coupling effect to be found in stress intensity factor (SIF) results along the crack front. Further interest is taken in a 3D effect which is effective in particular at and adjacent to the two crack front corner points, that is where the crack front intersects the two free side surfaces of the specimen. Exactly at these crack front corner points fatigue crack growth initiates in the experimental laboratory test specimens, and develops into two separate anti-symmetric cracks with complex shapes, somehow similar to bird wings. The computational DBEM results are found to be in good agreement with these experimental findings and with FEM results previously obtained. Consequently, also for this new case, with complex 3D crack growth behaviour of two cracks, the functionality of the proposed DBEM and FEM approaches can be stated.

Abstract: The progressive failure analysis of composite pressure vessel under hydrostatic pressure has been carried out. The composite pressure vessel has been modeled by using layer structural shell element. After the failure of the weakest ply, the stiffness is reduced by either fiber failure or matrix failure. The stiffness of failed element has been totally discarded and other element are considered to remain unchanged after the weakest ply failure. The stress of the laminate at the same point is evaluated again to see if the laminate can carry additional load. This ply-by-ply analysis progresses until the ultimate strength of the pressure vessels is reached. A parametric study has been done on the progressive failure analysis.

Abstract: Notched tensile tests {orientation: tensile axis [001], direction of notching [010]or[110] } were performed to investigate the crack growth process in Ni-single crystal with hydrogen cathodic charging. Y-shaped hillocks and striation-like patterns were observed on the fracture surfaces of hydrogen embrittled specimens using the scanning electron microscope (SEM). The Y-shaped hillocks are exactly matched on both fracture surfaces of a specimen. In addition, cleavage fracture is occurred between a Y-shaped hillock and next one. It means that the cleavage fracture is occurred at first, and the Y-shaped hillock is secondary formed. It is considered that the striation-like pattern is formed by the localized slips and cleavage fracture between them. Peach-Koehler’s law leads that the dislocations on the equivalent slip systems with (111　)　[101] are subjected to the highest force of all dislocations at the crack tip. Furthermore, Ni-hydride was not observed on the surface of specimen just after charging hydrogen. Thus, it was concluded that Y-shaped hillock and striation-like pattern were occurred in Ni-single crystal with charging hydrogen under condition that Ni-hydride is not formed.

Abstract: The research into the stress state of gear rim has been based on the developed model of pinion-wheel system and the finite element application. For a spur gear structure of middle web position, the assumption of load distribution in the contact area that reflects, first of all, the tooth support deformation, has been not required for the imposed boundary conditions. To study the influence of web thickness stiffening effect on the inner rim surface stress, the ranges of rim and web thickness values have been considered. The area of stress along the rim circle and tooth facewidth has been shaped and discussed in relation to maximum rim stress appearance.

Abstract: It is currently interesting to use thermal or electrical conductive asphalt concrete for snow-melting and maintenance of asphalt pavements in winter. The addition of conductive fillers may have negative effects on the mechanical properties of asphalt concrete. The performance of conductive asphalt concrete is greatly affected by the initial crack and its propagation. Laboratory tests for neat and conductive asphalt concrete include Dynamic Modulus Test (DMT) and Indirect Tensile Fatigue Test (ITFT). DMT tests indicates that the value of dynamic modulus of asphalt concrete decreases with the increasing graphite content. It means that the conductive asphalt concrete achieves lower stiffness. It can be concluded from ITFT tests that the fatigue life (load cycle times) of conductive asphalt concrete is more than neat ones when stress level is under 1.0 MPa. Meanwhile higher fatigue resistance of conductive asphalt concrete is observed when carbon fibers (2 weight % of total aggregate) were added together with 22% graphite, especially at low stress levels.

Abstract: In the present paper, the meshless local Petrov-Galerkin (MLPG) method is extended to two-dimensional (2-D) continuously nonhomogeneous piezoelectric solids with cracks under dynamic loading conditions. To eliminate the time-dependence, the Laplace-transform technique is applied to the governing partial differential equations which are satisfied in the Laplace-transformed domain in a weak-form on small fictitious subdomains. A meshless approximation is used for spatial variations of the displacements and the electric potential.