Key Engineering Materials Vols. 592-593

Abstract: The domain of the generalized stress intensity factors dominance ahead of the notch tip can be rather small with respect to the length of the perturbing cracks initiated from the tip of the notch. Thus the non-singular terms of the stress asymptotic expansion at the notch tip would play an important role in the notch tip stability. Following the procedures dealing with complex potential theory and path-independent two-state integrals developed for the singular stress analysis of the stress concentrators one can evaluate their magnitude and include them to the energy release rate of the preexisting crack initiated from the notch tip applying the matched asymptotic procedure. The presented analysis should lead to better understanding of the notch stability process and precising of the notch stability criteria.

Abstract: The paper presents the results of fatigue crack growth rate tests performed in magnesium AZ91 alloy cast of different wall thicknesses. Due to the limited size of investigated cast the tests were carried on mini-samples of 4x4x1 mm gauge section dimensions. The samples were cut from the 5, 10 and 30 mm thick walls. The optical displacement measurement technique, namely Digital Image Correlation, and inverse method were applied for determination of the stress intensity factors and the crack tip coordinates during tests. There were also performed uniaxial tensile tests in mini-samples for the determination of the mechanical properties changes related to different cast thicknesses. The relation between the microstructure and the results of mechanical tests was discussed.

Abstract: The effect of bainite content on the short crack growth behavior of three ferrite bainite dual phase steels has been examined using a rotating bending machine together with examinations of the microstructural constituents associated with the fatigue crack path. The magnitude of ΔK_thsc for ferrite bainite dual phase steels, in general, is found to increase with increasing amount of bainite. Critical examinations of the crack paths indicate that it is inter-granular for steels with < 70% bainite while it is intra-granular for steels containing > 70% bainite.

Abstract: Polycrystalline materials may suffer internal damage due to diffusion of chemically aggressive species during service. Diffusion rates are greatly enhanced on grain boundaries (GB). This can be modelled with discrete networks, where the GB structure is represented by links with local diffusivities. We present a site-bond model for concentration-driven diffusion that can be used to study the accumulation of chemical species at GB, leading to deterioration and eventual cracking. We employ realistic distributions of GB energies and corresponding diffusivities from published works. We show how the model can be used to predict macroscopic diffusivities with little experimentation. We demonstrate how the grain boundary structure controls the extent of internal damage resulting from the diffusion of chemical species.

Abstract: The sensitivity to liquid sodium embrittlement (LME) of T91 martensitic steel, one of the selected structural materials for future sodium fast reactors has been investigated. The study took into account the role of microstructure. Small punch tests and three points bending tests were carried out in a purified and controlled atmosphere. Precipitation state and dislocations structure resulting from a tempering at 550° C provoked LME of the T91 steel between 200 and 550 °C. Secondary Ions Mass Spectroscopy investigation suggested that sodium penetration at prior austenitic grains boundaries promoted by plastic deformation occurred and caused brittle crack initiation. Brittle cracks propagated in sodium preferentially along martensitic laths-boundaries. J integral calculations confirmed a drop in toughness of T91 tempered at 550°C by liquid sodium up to 80 %.

Abstract: We present a theoretical study of the fracture of two-dimensional disc-shaped samples due to projectile penetration focusing on the geometrical structure of the crack pattern. The penetration of a cone is simulated into a plate of circular shape using a discrete element model of heterogeneous brittle materials varying the speed of penetration in a broad range. As the cone penetrates a destroyed zone is created from which cracks run to the external boundary of the plate. Computer simulations revealed that in the low speed limit of loading two cracks are generated with nearly straight shape. Increasing the penetration speed the crack pattern remains regular, however, both the number of cracks and their fractal dimension increases. High speed penetration gives rise to a crack network such that the sample gets fragmented into a large number of pieces. We give a quantitative analysis of the evolution of the system from simple cracking through fractal cracks to fragmentation with a connected crack network.

Abstract: Factors determining the durability of a material include among others its thermal shock resistance, as a non-homogenous temperature field may trigger the formation and propagation of cracks. To evaluate thermal shock resistance by means of one of the resistance criteria: the R_st or R₄ parameter, it is necessary to know the work of fracture (γ_WOF), which is determined in a cracking test. Technical testing conditions influence the recording of crack propagation in a material sample. Ordinary and refractory castables are a group of materials in which the effect of temperature on the course of their cracking is among others related to the presence of deformations, internal stresses and cracks. Tests were performed for mullite, andalusite and boxite doped chromia castables, having known phase compositions, for which the temperature of liquid phase appearance in the matrix was calculated. A considerable increase in the value of castables γ_WOF at 900°C was noted compared to the values at 300°C and 600°C. In the group of the examined castables the highest γ_WOF value was obtained for andalusite castable. The applied methods of testing enabled simultaneous presentation of crack propagation in terms of energy (load vs. deformation) and kinetics (load vs. time).

Abstract: We present a theoretical study of the creep rupture of heterogeneous materials based on a fiber bundle model which provides a direct connection between the microscopic fracture mechanisms and the macroscopic time evolution. In the model, material elements fail either due to immediate breaking or undergo a damage accumulating ageing process. We found that on the micro-level the competition of the two failure modes gives rise to bursts of breakings with power law distributed size and waiting time between events. We demonstrate that approaching macroscopic failure the system accelerates which can be fully described as a non-homogeneous Poissonian process for long range load sharing, however, when localization occurs breaking events get clustered. Bursts are composed of sub-avalanches which lead to a non-trivial temporal shape comparable to measurements. The pulse shape proved to be sensitive to the range of load sharing.

Abstract: PSB formation and its relevance for an eventual fatigue limit of polycrystalline electrolytic copper was studied in the very-high cycle fatigue regime with the ultrasound fatigue loading method. PSBs are formed at much lower stress/strain amplitudes than reported in earlier literature, if a high enough number of cycles is applied. Fatigue fracture takes place at approximately 50% higher amplitudes than needed for PSB formation, which is likewise in contrast to former literature results. Non-propagation of small cracks, originating from intrusions or PSB-induced non-propagating grain-boundary cracks are made responsible for this different material response.

Abstract: Persistent slip markings (PSMs) were experimentally studied in 316L steel fatigued to early stages of the fatigue life. High resolution SEM, combined with focused ion beam (FIB) technique and atomic force microscopy (AFM) were used to assess the true shape of PSMs in their early stage of development. General features of PSMs in fatigued metals are extrusions and intrusions. Their characteristic features were determined. They were discussed in relation with the theories of surface relief formation and fatigue crack initiation based on the formation, migration and annihilation of point defects in the bands of intensive cyclic slip - persistent slip bands (PSBs)