Key Engineering Materials Vols. 592-593

Abstract: The reference texture is a subset of the image texture in SEM fractographs of fatigue fractures. It is common to all fractures caused by loadings in which significant events occur sufficiently regularly and frequently. The reference crack growth rate is unambiguously related to the reference texture. A particular loading is characterized by the ratio of the reference and conventional crack growth rates called reference factor. Its value may be related to the sequence of successive sizes of cyclic plastic zone, while the mechanism of the effect of overloads follows the models of Wheeler and Willenborg. Application to a set of three test specimens from stainless steel AISI 304L loaded by various loading regimes is shown.

Abstract: The general scheme of crack growth modeling and periodicity of aviation structure elements checks was developed. The major factors determined the scale of inaccuracy calculation ( εN*) based on calculations of crack growth duration (N*) and inspections intervals (τ0) were marked and estimated with the block diagram.

Abstract: Engineering models to estimate a life of structure components which are simultaneously subjected to aggressive hydrogen environment influence and cycling, substantially, use two different approaches to problem-solving, i.e. the influence of aggressive hydrogen environment on material and the fatigue. A developed engineering model to estimate the life of structure components assumes that either the influence of aggressive hydrogen environment, or the fatigue initiates a local fracture of structure component. The engineering model enables a calculating of a structure component life.

Abstract: Today frictional shear resistance along pre-existing ruptures (faults) is considered as the lower limit on rock shear strength for confined conditions. The paper proposes a mathematical model of recently identified shear rupture mechanism which can provide propagation of faults through the highly confined intact rock mass at shear stress levels significantly less than frictional strength of pre-existing faults. The model demonstrates that due to the self-unbalancing structure of the rupture head, representing the core of this mechanism, the failure process caused by the mechanism is always spontaneous and violent. It allows a novel point of view for understanding the nature of spontaneous failure processes including earthquakes.

Abstract: We present jet approach to microstructures, in particular to Cosserat continuum. We compare classical methods with the representation of Cosserat bodies by frame bundles. We demonstrate that nonholonomic, semiholonomic and holonomic bundles occur in such description.

Abstract: The phenomenological model of microcracks evolution in laminated rocks was developed under an isothermal approximation. The general form of the constitutive relations of porous media with microcracks was obtained. This form is necessary and sufficient for carrying out the principle of objectivity and the principle of thermodynamic consistency. The approximation of infinitesimal deformations of medium was investigated. The elastic potential of transversal isotropic brittle rocks was constructed. It involves latent energy of damage and terms which characterize elastic energy release due to damage evolution.

Abstract: Formation of thin macrofractures and high porosity bands parallel to the compression axis in the unconsolidated sedimentary rocks is treated on the basis of unified approach by considering migration of microdefects (pores) with respect to particles of the medium. The migration of pores is driven by a common cause, namely, a trend of a system to lower its total energy. The mechanism of how discontinuities develop along the maximum compressive stress T_max is discussed and quantitatively investigated. A single pore splits into two separate holes which move away along the T_max axis. The trace left by moving hole is interpreted as a macro-discontinuity. Multiple pores migrate so that they form a system of chains extending along the T_max axis. We associate these chains with observed high porosity bands.

Abstract: Advanced ceramics are a class of material used in extreme conditions, such as high speed turning of aerospace alloys and rock drilling. Their high hardness makes them suitable for these uses, however their lower toughness means that failure due to fracture and chipping is a problem. They are composed of micron-sized particles of a primary hard phase together with either a ceramic or metallic matrix material. A combined experimental-numerical method was used to investigate the role of microstructure on the fracture of advanced ceramics. Two dimensional, statistically representative microstructures of the advanced ceramics are created using Voronoi tessellation. The synthetic microstructures are compared to real microstructures in terms of particle size distribution and particle aspect ratio. Simulation results indicate that the computed elastic parameters are within the Hashin-Shtrikman bounds and agree closely with analytical predictions made with the Eshelby-Mori-Tanaka method. It is found that the local stress and strain distribution within the model is significantly affected by the underlying microstructure, which in turn affects fracture properties. Hence, tailoring the microstructure can optimise the bulk strength parameters of the material.

Abstract: We study the impact fragmentation of two-and three-dimensional disordered solids in a discrete element model of heterogeneous brittle materials focusing on the spatial distribution and mass-velocity correlation of fragments. Our calculations revealed that depending on the energy of impact the breakup process can have two different outcomes: at low impact energy the sample gets damaged, however, to achieve fragmentation the imparted energy has to surpass a critical value. Based on large scale computer simulations we show that the position of fragments inside the original body with respect to the impact site determines their mass and velocity in the final state. A novel relation of the mass and velocity of fragments is revealed: In the damage phase the mass and velocity of fragments are strongly correlated, however, in the fragmented phase correlation emerge solely for large fragments. The correlation function decays as a power law with a universal exponent in an excellent agreement with recent experimental findings.

Abstract: Mathematical modeling of thermal fracture of functionally graded/homogeneous bimaterial structures with a system of arbitrarily located cracks is performed and based on the previously suggested theoretical approach [1-which used the integral equation method. It is supposed that the structure is subjected to thermal loading (a thermal flux) and mechanical loading (a tension). The properties of the functionally graded material (FGM) are described by a continuous exponential function. The main fracture characteristics (stress intensity factors and fracture angles) are presented as functions of the geometry of the problem and special inhomogeneity parameters of FGMs. Some typical crack patterns for FGM/homogeneous bimaterial structures resulting from experiments available in literature are studied in detail. Thermal fracture of actual material combinations of FGMs such as: ceramic/ceramic, e.g., TiC/SiC, MoSi₂/Al₂O₃ and MoSi₂/SiC, and also ceramic/metal FGMs, e.g., zirconia/nickel and zirconia/steel, is investigated.