Key Engineering Materials Vols. 525-526

Abstract: The residual fatigue life of a submarine pressure structure is investigated, based on the combination between the methods of conventional Monte Carlo and classical probabilistic fracture mechanics. Firstly, Monte Carlo method is employed to obtain the reliability of given initial fatigue life. Secondly, the two induced factors M_A1 and M_A2 in the paper are estimated according to the initial fatigue life and the reliability. Thirdly, based on the two factors, the residual fatigue life based on other reliability is obtained by using classical probabilistic fracture mechanics method. Numerical examples show that the proposed method is more efficient without accuracy loss for residual fatigue life compared with Monte Carlo method. This method can also be employed to predict the residual fatigue life on other analogue structures.

Abstract: Coupling techniques for components of different materials is spreading in mechanical industry; the test case studied in this work deals with the connection of an aluminium alloy component with a carbon fibre composite one. In particular, the first component is made of an aluminium-zinc alloy and exhibits an isotropic behaviour, while the second is made of a carbon fibre reinforced polymer (CFRP) and shows a strongly anisotropic behaviour; both materials are widely used in engineering applications. A titanium bolt connects the parts. This work is focused on the influence of the geometrical parameters which characterize the coupling between the components. In particular, a study has been carried out on the influence of the shank-hole clearance, the bolt head size, the bolt preload and the shape of the bolt head. A numerical model has been built and statically tested; the results have been compared with the experimental ones from literature. Once validated, the same numerical model has been used to evaluate the performance of the joint in presence of a change of the above mentioned characteristic parameters. The required numerical analyses have been performed using Abaqus/Standard® numerical code.

Abstract: The bending fatigue tests were performed using the small sized specimens of AZ31 magnesium alloy fabricated by the focused ion beam (FIB) processing to investigate the scale effect on the fatigue behavior. For the fatigue test of the small sized specimens, the fatigue testing apparatus was constructed by the piezo actuator and the high-resolution microscope for controlling the very small displacement. The specimen was micropillar-shaped with the rectangular cross section of 3μm x 8μm and the height of 40μm. The fatigue strengths of the small sized specimens were higher than those of the bulk sized specimens. The fracture surfaces were also investigated carefully compared with those of the bulk specimen.

Abstract: The fatigue behavior of cast aluminum alloy, A356-T6, microstructurally modified by the friction stir processing (FSP) was investigated. The FSP conditions were set to be the tool rotational speed of 500 rpm and traveling speed of 200 mm/min, in which the strain rate was relatively low. Plane bending fatigue tests have been performed using the as-cast and friction stir processed (FSPed) specimens. Fatigue strengths in the finite life region and the fatigue limit of the FSPed specimens were highly improved compared with the as-cast ones resulting from the elimination of casting defects by the FSP. However, the crack growth rates of the FSPed specimens were faster than those of the as-cast ones due to the softening of the material by heat input during the FSP. The effects of FSP with low stain rate were discussed based on the microstructural consideration.

Abstract: Particulate composite with soft polymer matrix and rigid mineral fillers are one of most frequently used construction and engineering materials. The main focus of a present paper is an estimation of the load influence on behavior of micro-crack placed in close proximity to the particle with interphase in soft matrix. The particulate composite with polymer matrix filled by magnesium-based mineral fillers is investigated by means of the finite element method. A non-linear material behavior of the matrix was considered. Numerical model on the base of representative plane element (RPE) was developed. The conclusions of this paper can contribute to a better understanding of the behavior of micro-crack in particulate composites with soft polymer matrix.

Abstract: The most relevant loading conditions for real polymer pipe systems are not only internal pressure, but also loading caused by sand embedding including bending or different kinds of point loads. It has been shown that service lifetime of buried pipes can be reduced especially due to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated here and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper the effect of the hard indenter, Poissons ratio, hoop stress level and pipe wall thickness on the crack shape was estimated using numerical simulations of the creep crack propagation based on finite element method. Relation between crack length and crack width was found and expressed by simple relationship. A deeper understanding of the point load effect in order to prevent unexpected failure of the pipelines is of paramount importance for pipeline design.

Abstract: This work presents numerical methods used for predicting crack paths in technicalstructures based on the theory of linear elastic fracture mechanics. The FE-method is usedin combination with an efficient remeshing algorithm to simulate crack growth. A post pro-cessor providing loading parameters such as the J-integral and stress intensity factors (SIF) ispresented. Path-independent contour integrals are used to avoid special requirements concern-ing crack tip meshing and to enable efficient calculations for domains including interfaces andinternal boundaries. In particular, the interaction of cracks and internal boundaries and inter-faces is investigated. The simulation combines crack propagation within elastic bodies and atbi-material interfaces. The latter is based on a cohesive zone model. The presented numericalresults of crack paths are verified by experiments.

Abstract: Recently it was shown that the temperature plays very important role in coupling with the impact of shock wave caused by explosion in a free space. The deterioration of fiber reinforced concrete structures, FRC, at the face to which the main impact of the coupled load is directed is not easy to describe. In couple of previous papers of the author the problem was solved under condition that the structure behaves elastically. It appeared that a numerical tool known as free hexagon method, which is based on soft contact problem in combination with boundary elements, seems a powerful approach solving any disconnecting media. The soft contact is identified by nonlinear spring rules. Similar idea is used in the suggested paper for solving the coupled problem envisaged, but the mesh of free hexagons is split into the air and the structure. If the impact of explosion and the subsequent shock wave cause is viewed separately from the changes in material due to temperature a very fast changes in stresses and movements in material are observed in the first case while the changes due to temperature develop much slower. On the other hand in combination of both influences the temperature increase the activity inside of structures considerably. In the paper the theory is briefly described and selected examples illustrate the mobility of the application of the method to a problem of FRC plate.

Abstract: We present a method for crack detection and stress intensity factor measurement in plate structures by using strain gauges and applying the dislocation method. The presented approach is based on the strain measured at different locations on the surface of the structure. This allows both the identification of crack position parameters, such as length, location and angles with respect to a reference coordinate system and the calculation of stress intensity factors (SIF). The method solving the direct problem is based on the idea of representing the crack by a line of point dislocations. The latter are formed by applying a constant displacement between adjacent points located at either side of the crack. Thus, the approach is based on the weighted superposition of elastic Greens functions representing the strain field due to the presence of a crack, where the weights are being identified by inverse problem solution. Since the strain fields are controlled by both external loads and the crack growth the unknown parameters are crack length, position and inclination as well as loading quantities. The particle swarm algorithm (PSO) came out to be most suitable for parameter identification in a high dimensional space.

Abstract: To take advantage of the toughness mechanism of DN gels and explore the possibility for engineering application as the structural member, the information on the mechanical behaviour of DN gels under various loading conditions is indispensable. Therefore, in this paper, we at first constitute a model of DN gel by paralleling a slider element with a nonlinear rubber elasticity spring element based on the nonaffine molecular chain network model, where each element represents the first and the second network of DN gel respectively. The theoretical stress-strain relation of this model shows a strain softening and subsequent strain hardening response, which has been considered as an agent of the propagation of the necking during the simple tension of glassy polymer. Continuously, based on this model, we propose a constitutive equation for DN gel and a three-dimensional simple tension simulation is performed. The computational results show that the propagation of the necking together with the macroscopic mechanical response of DN gel can be reproduced by the proposed model very well.