The work is devoted to the determination of local stresses arising in the vicinity of a micropore containing methane under high pressure. Equations are proposed for describing the physical processes that have a decisive influence on the failure of a hollow steel cylinder loaded with internal hydrogen pressure. It is important to note that the high-temperature hydrogen attack (HTHA) itself (cracking of the hollow steel cylinder under the influence of local stresses) is not considered. The local failure criterion is the sum of the hoop stress arising under the influence of hydrogen pressure within the steel cylinder cavity and the hoop local stress arising under the influence of methane pressure within the micropore. To describe the accumulation of methane concentration in the hollow steel cylinder, a diffusion equation with one initial and two boundary conditions is used, where the mass transfer condition is assumed at the outer boundary radius. Porosity, defined as the relative increment in the area of an elementary ring, is considered as a measure of material discontinuity. A new displacement relationship was established from the expression for porosity, where a hyperbolic dependence of displacement on radius was previously accepted. A relationship between pressure, concentration, porosity, and temperature was proposed as the equation of state for methane within a micropore, satisfying the corresponding initial conditions. It should be emphasized that this is the first time that the equation of thermal state for a gas has been considered in practice. It is hypothesized that for localized failure to occur, the sum of the hoop stress and the hoop local stress must reach the break creep strength. Calculations showed that at the micropore boundary, the hoop local stress significantly exceeds the hoop stress. This circumstance led to the conclusion that hoop local stress must play a key role in the cracking of a hollow steel cylinder.
The paper suggests a method integrating an applied version of the theory of inelasticity into a finite element complex for computations of the stress-strain state, strength and service life of structural elements operating under long-term loading. The paper presents the experimental results of smooth specimens made of bronze alloy under creep conditions at different loading levels to obtain material characteristics used to identify the material parameters of BrH08-Sh bronze alloy at temperatures of 400, 500 and 600 °C. The possibility of applying the theory variation is considered by conducting computational and experimental studies on cylindrical specimens with a stress concentrator in the form of a V-shaped annular groove with a radius of 1 mm. Tests of specimens under creep conditions were carried out at temperatures of 400, 500 and 600 °C until failure. The experiment was simulated and the distribution of stress intensity and accumulated inelastic deformation in the finite element model of a specimen with a groove was obtained. The integration of the model into the finite element complex was carried out using the algorithm, which allows calculating stresses and strain at the end of the computation step using the components of the strain increment tensor. The algorithm makes it possible to separate the areas of monotonic and cyclic deformation in the space of the inelastic strain tensor using the memory surface. Comparison of the computed and experimental data shows that this version of the applied theory of inelasticity allows for a sufficiently accurate modeling of creep and assessment of the long-term strength of structural elements with stress concentrators.
This study evaluates fracture toughness of polymer fiber-reinforced composites in terms of the critical stress intensity factor (SIF) at the onset of the splitting crack initiation at the tip of a transverse notch under four-point bending of unidirectional carbon fiber-reinforced plastic specimens. In addition to the nonlinearity analysis of loading diagrams, as adopted in linear fracture mechanics, advanced techniques including acoustic emission (AE), digital image correlation (DIC), and numerical processing of experimental loading diagrams were developed to more accurately determine the moment of splitting the crack initiation. The loading diagrams were processed to identify the local load reduction, corresponding to the formation of a longitudinal splitting crack. A sliding window method based on the standard deviation of the load from an approximating function was used for processing. This moment was further determined using AE techniques with algorithms for analyzing disruptions in time series data. The most informative AE parameters for this task were identified as the average duration and specific energy of the recorded pulses. By dividing the critical plane into three distinct segments, each associated with different sources of AE, the moment of time series disruption was identified, correlating with the onset of the splitting crack formation in the composite specimen and the emergence of a new segment. For verification, the DIC method was employed to analyze strain fields at the notch tip. Through the integrated processing of AE impulse streams, deformation diagrams, and strain fields, the average critical load was determined for a series of specimens corresponding to the formation of a splitting crack in the composite. This critical load was used to calculate the splitting fracture toughness of the carbon fiber-reinforced composite in terms of the critical SIF at the notch tip.
Controlled synthesis of composites attracts attention due to the possibility obtaining a homogeneous product in terms of composition and structure. This also applies to intermetallic composites. In this case, the heat release in reactions is summarized with the heat due to external heating, which can cause self-sustaining regimes such as thermal explosion or combustion. Non-equilibrium thermokinetic processes are accompanied by mechanical stresses and strains, which can affect the process dynamics and synthesis result. In this paper, a coupled model is analysed for the bulk synthesis of a composite from Ni-Al blend, which takes into account the interrelation of thermal, chemical and mechanical phenomena. The model includes a heat conduction equation with an additional heat source of chemical and non-chemical nature. The kinetic equation is added to the model to deter the mass fraction of the synthesis product. To estimate thermal stresses and strains, the known solution is used for the mechanical equilibrium problem of a multiplex cylinder corresponding to the model reactor. A cylinder is uniformly heated from the side surface. The analytical solution of the equilibrium problem after substitution into the thermokinetic model makes it possible to identify equivalent reaction parameters and equivalent thermophysical properties depending on the mechanical moduli. For the numerical implementation of the obtained nonlinear model, a numerical algorithm was developed; a program has been compiled and debugged that allows for detailed parametric research. In the calculations, the temperature fields, the final fraction of the reaction product, as well as the values of stresses and strains were determined in the whole investigated area at different instants of time. As a result, it is shown that taking into account the coupling of thermal, chemical and mechanical processes changes all process characteristics including stress values of different physical natures. The stress values depend on the reaction initiation conditions and geometrical parameters of the reactor.
The paper considers the methodology and principle of refining the load-carrying capacity or ultimate strength of composites using an acoustic emission recording system for ambiguous diagrams. The aim of this work was to increase the objectivity of the results of experimental studies of the mechanical characteristics of composite fiber-reinforced materials with an ambiguous loading diagram based on additional measurements during standard testing of specimen. Obtaining a set of new, more objective data on the behavior of promising composite materials during deformation and fracture. As a result of studying the mechanical behavior of knitted 3D carbon fiber specimen using standard methods, it was possible to record and formulate the concept of an ambiguous deformation (loading) diagram. An algorithm has been developed for refining the ultimate strength (load-bearing capacity) of composite materials in the case of ambiguity in deformation or loading diagrams. The ultimate strength or load-carrying capacity are understood solely in the interpretation of the standard in question. The algorithm is based on the concept and algorithm for determining the critical value of the cumulative energy of acoustic emission during the deformation of a composite material specimen - an acoustic analogue of the ultimate strength (load-carrying capacity). The algorithm is an analysis of the extrema of the approximation of the time-domain sweep of the energy parameter of acoustic emission, a polynomial function with the allocation of a primary localized array of this parameter. For the analysis, the hypothesis is considered that the basic parameter of the ultimate strength (load-carrying capacity) - Pcr is located in the region of half the distance between the maximum and minimum of the primary localized array of the time-based scan of the energy parameter of acoustic emission. A comparative analysis of the strength values (load-carrying capacity) obtained without and with the use of a hypothesis for promising 3D carbon fiber reinforced plastics was carried out. The significant efficiency of the described method is demonstrated, often with multiple changes in the values of the ultimate strength or load-carrying capacity of composite materials.
