Probabilistic Approach for Nondestructive Detection of Fatigue Crack Initiation and Sizing

Early detection of a growing crack is one of the concerns in structural integrity and can be used to predict the remaining useful life of a structure. Acoustic Emission (AE) is a nondestructivetesting method with potential applications for locating and monitoring fatigue cracks. A novel AE signal analysis approach is proposed in this paper to detect crack initiation and assess small crack growth behavior. A probabilistic AE-based model for small fatigue cracks was developed and the uncertainties of the model were estimated. The outcome of this research can be used to evaluate the integrity of structures and assess structural health by estimating the probability density function of the length of detected cracks. This paper discusses the methodology used, experimental approach, results obtained and predictive models developed.

Analysis of Acoustic Emission Parameters Changes with the Growth of Fatigue Racks in Steel Samples

This work presents the analysis of damage assessment for dangerous industrial objects of acoustic emission method. Samples of low carbon and low alloy steels were researched. The aim of this work is to experimentally determine damage assessment for the state of developed and unacceptable sources of acoustic emission. Cracks of various lengths have been obtained by low-cycle loading. According to research, it is necessary to use several criteria simultaneously to increase the reliability of inspection of each growing crack.

Influence of reinforced concrete forming features on mechanical characteristics

The article found that defects in cement and aggregate grains can not only cause stress concentration and contribute to cracking, but also can stop their propagation. It was determined that, in addition to zones with reduced strength, a zone with an increased viscosity can become a brake on the path of a growing crack, having a modulus of elasticity much less than the modulus of elasticity of the base material. The influence of moulding of reinforced concrete structures on its mechanical characteristics was revealed. It can be stated that mineral additives of various nature, regardless of hydraulic activity degree, optimize the deformative properties of cement stone and concrete with practically constant compressive strength.

Kinetic Theory of the Fracture of the Coal (Rock) Edge by the Gas-Filled Cracks. Instantaneous Loading-Out

The kinetic theory of the fracture of brittle materials is applied to the study of undercritical and critical growths of cracks in gas-filled rocks. In this type of the materials, the gas filtration from the environment to the cavity of a growing crack plays an important role. The proper account for this factor combined with the dynamics of the stressed state of the bed allows the estimation of the rate of growth of the main crack on the assumption of the Griffith criterion validity. It is found that, immediately after the instantaneous loading-out of the bed in the course of excavation, the cracks of certain size and orientation are exploded with the succeeding growth dependent on the gas entry into the cracks. The time of the filtration growth of the cracks has been estimated. The intervals of the control parameters (formational gas pressure, crack size, overburden pressure, surface energy of coal/rock, modulus of elasticity), where the spontaneous fracture of the bed becomes possible, have been found. The results open a way to the forecast of instantaneous outbursts of coal, rock, and gas.

Stress-Driven Migration of Low-Angle Grain Boundaries Near Crack Tips in Nanocomposites Containing Incoherent Nanoinclusions

Theoretical model describing stress-driven migration of low-angle grain boundaries (GBs) in the vicinity of growing crack in metal matrix nanocomposites with reinforcing (metallic or ceramic) incoherent nanoinclusions is proposed. Using two-dimensional discrete dislocation dynamics approach profiles of migrating GBs are analytically calculated and critical stress for transition into unstable migration mode is found. It is shown that the presence of crack always promotes stress-driven migration and thus grain growth.

REFINED STUDY OF STRESS-STRAIN STATE NEAR THE CRACK TIP UNDER CYCLIC LOADING IN A DAMAGED MEDIUM

The article is devoted to the fatigue growing crack problems in damagedmedia and mutual effects of damage on the evolution of the stress-strain statenear the crack tip and vice versa. The new asymptotic study of fatigue crackgrowth in an isotropic linear elastic material based on the continuum damagemechanics in the coupled (elasticity damage) formulation under plane strainand plane stress conditions is proposed. 1) The new numerical solution of thetwo-point boundary value problem for non-linear ordinary differential equationsto which the fatigue crack growing problem reduces is obtained; 2) The newanalytical presentation of stress, strain and continuity fields both for plane strainand plane stress conditions is given. The results obtained differ from Zhao andZhang’s solution where the original formulation of the problem for plane stressconditions has been proposed.

ASYMPTOTICS OF STRESS AND CONTINUITY FIELDS NEAR A FATIGUE GROWING CRACK IN A DAMAGED MEDIUM IN CONDITIONS OF STATE OF PLANE STRESS

In the paper asymptotic solution to the problem of growth of fatigue crack in conditions of repeated loading in a damaged medium in the coupled elasticity-damage statement of the problem is given. Asymptotic expansions of stress fields and continuity fields in which two summands are retained in asymptotic representation are derived. The problems of determination of amplitude coeffl-cients of obtained asymptotic expansions are discussed.