Kinetics Of Crack Propagation Research Articles

Comprehensive analysis of the performance of welding joints of gas pipelines with different viscosities

Full-scale tests of gas pipelines with different viscosities were carried out in landfill conditions, which are as close as possible to the natural conditions of operation of gas pipelines. The kinetics of crack propagation along welded joints - their trajectory, the transition from the weld seam to the base metal of the pipe and vice versa, the speed of movement of cracks in viscous and brittle metal and defined zones of plastic deformation - were experimentally investigated. It is known that tests on Menaget, Charpy and other samples, which have thicknesses that do not correspond to the thickness of the pipe walls, do not reflect the real picture of visco-plastic and brittle fractures, which does not allow to develop a methodology or model for predicting the final resource (non-accident) of long-term gas pipelines period of operation. At the same time, theoretical and laboratory researches do not always adequately answer the questions directly related to ensuring the durability and trouble-free operation of pipelines. It is likely that in the laboratory conditions of enterprises or scientific institutions it is difficult to reproduce and take into account all the factors that characterize the growth and spread of destruction in a real operating gas pipeline. Therefore, the data of laboratory studies must be checked and necessarily clarified according to the results of pneumatic tests of long pipe sections, i.e., at present, the need to combine laboratory and field tests of pipes of the gas pipeline network is urgently needed. Such tests are not massive, but as a result of their performance, important information is obtained regarding the behavior and properties of the metal in the conditions of loading and operation, which are closest to operational conditions. In field tests, pipes with a diameter of 1000x18 mm were used, which underwent controlled rolling (steel class X70 - grade 06G2BA) at a working pressure of RR = 9.7 MPa (in the northern - low-temperature version). Tensile tests, studies of impact toughness, strength and plasticity of the weld metal were carried out according to standard methods described in works [2-5]. During the experiments, a large number of sensors of temperature, pressure, strain, stress, crack propagation speed, etc. were used. The obtained results regarding the kinetics of the destruction of natural pipes in landfill conditions lay the groundwork for the development of a mathematical model of the engineering forecast of the residual (non-accident) resource of pipelines operating in oil and gas fields.

The kinetics of crack propagation in CVD graphene film

The preparation of graphene by chemical vapor deposition (CVD) is often accompanied by the growth of some structural defects, one of which is crack. The understanding of crack formation is key to the preparation of high-quality graphene films. In this work, the crack defects of graphene grown on liquid copper substrates by chemical vapor deposition (CVD) were examined. Then, the effects of growth temperature, growth time and total pressure on kinetics of cracks were analyzed, respectively. It is found that (I) The crack appeared during the period of graphene growth, not the cooling process of graphene; (II) the crack preferred to propagate at the vertex angle of etched hexagonal holes; (III) the crack tended to orient along the same direction—the zigzag direction. Finally, a dynamic mechanism of crack growth was proposed. Our study provides significant insights into the tearing mechanism of defective graphene.

Fatigue fracture of steel with ferrite-martensite composite structure

The article presents results of the fatigue tests of natural steel composite material for cyclic bending in a zero loading cycle. Natural ferritemartensite composite (NFMC) has a structure of alternating layers of viscous ferrite and strong martensite, which determines special mechanism of crack deceleration under loading. Zero loading cycle presumes presence of tensile forces directed only in one side, which makes it possible to avoid work hardening of crack edges during its growth. A diagram of fatigue fracture was constructed using data obtained on kinetics of fatigue crack propagation and its growth rate, depending on the number of vibration cycles. Comparison of test results for the samples made of steel of the same chemical composition was carried out. In one case, the secondary sorbite structure ran through the traditional heat treatment. In the other, quenching of the initial row ferrite-pearlite structure in intercritical temperature range, led to obtaining ferrite-martensite composite layered structure. These materials had the same hardness, but the difference in structure organization caused the NFMC structure steel advantage in terms of resistance to fracture under cyclic loading. When crack approaches the martensite-ferrite interface, delamination occurs in ferrite due to tensile stresses parallel to the crack plane. Growth of a crack stops before additional energy is supplied for a new crack generation under conditions close to the uniaxial stress state. Method for determining characteristics of kinetics of crack growth under fatigue loading is presented and recommended for testing steels and alloys under conditions of cyclic load changes.

Fatigue-Crack Propagation of High-Density Polyethylene Homopolymers: Influence of Molecular Weight Distribution and Temperature

The present study focuses on the influence of the molecular weight distribution (MWD) on the crack-growth kinetics of fatigue-crack propagation in high-density polyethylene (HDPE) homopolymers. Compact-tension specimens of HDPE homopolymer grades, with polydispersities ranging from 2 to 45 and weight-averaged molar mass ranging from 49 to 450 kg/mol, are tested in cyclic loading at temperatures ranging between 23 and 92 °C. Through a variation of sample thickness, linear elastic-fracture mechanics is shown to apply for the chosen geometry (compact tension). It was found that the crack-propagation kinetics obey the Paris–Erdogan law, for which the Paris–Erdogan exponent m is (highly) similar for all grades tested (m = 3.9), implying that the Paris–Erdogan prefactor A is the governing parameter for the crack-growth kinetics. Relatively poor correlations are observed when the prefactor A is plotted as a function of both the tie-molecule fraction derived from the theoretical model by Huang and Brown, J. Mater. Sci. 1988, 23, 3648, and the average number of effective physical cross-links per chain as derived by Tervoort et al., Macromolecules 2002, 35, 8467. A far better correlation is observed between prefactor A and the weight-average molecular weight (Mw), which improved further when Mw is corrected for the width of the MWD, taking into account the z-average molecular weight Mz, through the ratio Mz/Mw. A power-law correlation of prefactor A with Mw and the width-corrected Mw reveals slopes of −3.4 and −3.3, respectively. Because a molecular slip within the fibrils would require chain transport through the crystalline blocks, the temperature dependence of the fatigue-crack-growth kinetics is investigated to identify the underlying molecular processes. This investigation reveals the existence of a high-temperature and a low-temperature deformation process, both of which can be related to chain-slip mechanisms through their respective activation energies (125 and 50 kJ/mole), as their activation energies are considerably lower than that required for chain scission (430 kJ/mol). This, combined with the power-law exponent of −3.4, would suggest a possible connection between the underlying failure mechanisms of craze fibrils and reptation-like dynamics. Furthermore, experiments at elevated temperatures on a selection of homopolymer grades suggest that the MWD has no influence on the temperature dependence of fatigue-crack propagation for HDPE homopolymers.

Size-Induced Constraint Effects on Crack Initiation and Propagation Parameters in Ductile Polymers.

Fracture mechanics are of high interest for the engineering design and structural integrity assessment of polymeric materials; however, regarding highly ductile polymers, many open questions still remain in terms of fully understanding deformation and fracture behaviors. For example, the influence of the constraint and specimen size on the fracture behavior of polymeric materials is still not clear. In this study, a polymeric material with an elastic plastic deformation behavior (ABS, acrylonitrile butadiene styrene) is investigated with regard to the influence of constraint and specimen size. Different single-edge notched bending (SENB) specimen sizes with constant geometrical ratios were tested. The material key curve was used to investigate differences in the constraint, where changes for small and large specimen sizes were found. Based on a size-independent crack resistance curve (J–R curve), two apparent initiation parameters (J0.2 and Jbl) were determined, namely, the initiation parameter Jini (based on the crack propagation kinetics curve) and the initiation parameter JI,lim (based on an ESIS TC 4 draft protocol). It was found that J0.2 and Jbl could be used as crack initiation parameters whereby Jini and JI,lim are indicative of the onset of stable crack growth.

Prediction of crack propagation kinetics through multipoint stochastic simulations of microscopic fields

Prediction of crack propagation kinetics in the components of nuclear plant primary circuits undergoing Stress Corrosion Cracking (SCC) can be improved by a refinement of the SCC models. One of the steps in the estimation of the time to rupture is the crack propagation criterion. Current models make use of macroscopic measures (e.g. stress, strain) obtained for instance using the Finite Element Method. To go down to the microscopic scale and use local measures, a two-step approach is proposed. First, synthetic microstructures representing the material under specific loadings are simulated, and their quality is validated using statistical measures. Second, the shortest path to rupture in terms of propagation time is computed, and the distribution of those synthetic times to rupture is compared with the time to rupture estimated only from macroscopic values. The first step is realized with the cross-correlation-based simulation (CCSIM), a multipoint simulation algorithm that produces synthetic stochastic fields from a training field. The Earth Mover’s Distance is the metric which allows to assess the quality of the realizations. The computation of shortest paths is realized using Dijkstra’s algorithm. This approach allows to obtain a refinement in the prediction of the kinetics of crack propagation compared to the macroscopic approach. An influence of the loading conditions on the distribution of the computed synthetic times to rupture was observed, which could be reduced through a more robust use of the CCSIM.

Varieties and kinetics of fatigue cracking of fixed offshore platforms for oil and gas production

The article investigates the different types of cracks and kinetics of propagation of cracks in main structural components and welds of the support blocks of fixed offshore platforms. Describes the process of metal fatigue for offshore installations. The article provides methods for calculating of the stress intensity factor and describes the methods of calculation for some cases. Based on the analysis of scientific studies, the author provides methods allowing to calculate two parameters describing the kinetics of crack propagation in relation to the elements and welded joints of offshore fixed platforms, namely the methods of calculation of threshold and critical stress intensity factor. On the basis of the conducted research the author constructed the kinetic chart of the development of fatigue cracks in relation to the elements and welded joints of the supporting blocks of fixed offshore platforms. In addition, the author gives the equation in which it is possible to determine the growth rate of fatigue cracks in various areas of the kinetic diagram.

A Simple Energetic Approach to Describe Propagation of Fast‐running Cracks in Thermoplastic Polymers

Applying dynamic fracture mechanics, a simple analytical model for the evolution of the crack propagation velocity during unstable crack growth has been developed. Based on the energy balance concept, rarely used in dynamic fracture mechanics, this model allows the estimation of the crack growth energy and the process zone size to be 1.2 J at breaking of the specimen and 0.65 mm in diameter, respectively, for polycarbonate at room temperature. Furthermore, the model is able to describe the influence of the specimen geometry of the crack propagation kinetics adequately in principal agreement with experimental results found in literature.

Oxidation assisted intergranular cracking in 718 Nickel Superalloy: on the mechanism of dynamic embrittlement

Nickel Superalloy 718 was specially designed for high temperatures. However, due to an embrittlement phenomenon, its application is limited up to 650°C. This work presents a contribution to the understanding of the embrittlement mechanism focusing on the crack propagation kinetics responsible for the brittle fracture. This manifestation is known as OAIC (Oxidation Assisted Intergranular Cracking), although the phenomenology resulting in such degradation is not well understood. The approach to the problem was accomplished in the solubilized condition by mechanical testing, fractography and activation energy calculation.Generally, high temperature dissociation of NbC in the presence of oxygen is pointed as the source of niobium that ultimately lead to fracture by Nb2O5 formation at grain boundaries. However, metallographic analysis of fractured samples between 600 and 925°C at different strain rates indicated otherwise. Samples tested at 850°C showed minimum ductility and fast intergranular crack propagation, at a short time incompatible with carbide dissociation and Nb2O5 formation. Furthermore, there is no evidence of sufficient NbC along grain boundaries to supply elemental niobium to the formation of the Nb2O5 continuous film. Besides, onset to OAIC as a function of temperature and strain rate led to a 223kJ/mol activation energy, which can be associated to niobium self diffusion within the matrix. Considering that the dynamic aging phenomenon and the γ″ precipitation occur concomitantly with the deformation, the results suggest that niobium segregation together with preferential γ″ precipitation at grain boundary and the consequent stress state build up should play also a decisive role in the embrittlement.

Assessment of subcritical crack growth in hydrogen-containing environment by the parameters of acoustic emission signals

Estimation models (differential equations, initial and final conditions) for determining the crack propagation kinetics in hydrogen-containing environments using the acoustic emission (AE) signal parameters are proposed. The formulation of these models is based on the main ideals of the AE method, dependence between the crack increment area and a sum of AE-signals amplitude, main criteria of fracture mechanics and laws of thermodynamics.

A rate-dependent breakage model based on the kinetics of crack growth at the grain scale

Experimental evidence indicates that granular soils under high confining stress accumulate deformation over time, with processes such as creep, relaxation and strain-rate sensitivity being controlled by the growth of internal flaws and the delayed fracture of individual particles. In this paper, a connection between the theory of subcritical crack propagation and the continuum modelling of crushable soils is established by exploiting an analogy between the processes that promote energy release upon the fracture of single particles and those that are responsible for delayed comminution in soil samples. Models describing the kinetics of crack propagation have been reinterpreted from a thermodynamic standpoint, showing that the energy loss during crack growth includes higher-order terms which augment the dissipation by surface area creation. This result has inspired the introduction of similar higher-order terms in the dissipation function of a continuum breakage model, thus hypothesising that the rate dependence of collective crushing can be seen as the macroscopic outcome of the growth of cracks in individual grains. This choice leads to a rate-dependent continuum model characterised by a breakage growth rate controlled by the same coefficient that governs the kinetics of crack growth in the grain-forming minerals (the so-called stress corrosion index). It is shown that the model exhibits good agreement with the creep and relaxation response measured for a range of soils tested under oedometric and triaxial conditions. These results corroborate the hypotheses about the link between subcritical crack growth and the energetics of delayed comminution and set a vision to model delayed fracture processes occurring in fluid-infiltrated reactive environments.

Кинетика возникновения трещин в осаживаемых цилиндрических образцах

Introduction. The article is devoted to the crack occurrence analysis in a powder body, and identification of the basic laws of this process in the freely upset material. The work objective is to study kinetics of crack initiation and propagation in freely upset powder materials, and microstructure of alloy and iron powders in a variety of areas in order to determine the conditions for defect-free deformation of the material under free upsetting. The analysis of the crack development in a powder body is of great practical importance, and therefore, the investigation of this problem is essential for the fabrication of accepted parts. Materials and Methods. New powder materials from the Swedish company Höganäs which are characterized by the improved deformation characteristics are considered. Besides, these materials differ in occurrence of alloy elements, such as Ni and Mo which affect cracking. Microstructures of the upset samples are analyzed using the metallographic microscope. Research Results . Cracks in the upset materials are classified as cleavage cracks. The crack origin is determined by the absence of factors for the development of the sample metal component plasticity due to the occurrence of defects and pores in its body. It is established that rapid cracking is confirmed by the nature of the crack when it scarcely has extension into the sample material, and, moreover, ends at an angle to the area for development. Discussion and Conclusions. The analysis of the microcrack development allows identifying morphological structural differences of the central and fringe regions of the powder material. The peripheral area is characterized by different values of grains and increased content of micropores, and the directivity of clusters of micro-discontinuities and non-metallics, in the line of the former physical interface of the metal powder particles.

Short-Term Stable Crack Propagation through Polyolefin Single- and Bilayered Structures - Influence of Welding, Composition and Direction of Crack Propagation

The overall stable crack initiation and propagation behaviour of specimens cut from plastic pipes that were composed of different polyolefin materials were investigated using concepts of elastic–plastic fracture mechanics including the crack propagation kinetics. The effect of specimen shape, orientation, welding, lading rate, composition/microstructure and direction of crack propagation on the crack resistance (R) behaviour of these materials has been thereby assessed.

Effect of corrosive media on the cracking resistance of pipe steel under load

The kinetics of fatigue crack propagation has been studied in full-thickness specimens cut from a main pipeline under the simultaneous action of a cyclic load and a hydrogen sulfide-containing corrosive medium. The degradation mechanisms of the material are discussed using the results of optical microscopy.

Evaluation of the Residual Life of Hydrogenated Pipes According to the Parameters of Acoustic Signals

We propose a technique for the evaluation of the residual life of a hydrogenated element of a pipe containing a cracklike defect on its inner surface. The kinetics of crack propagation is studied with regard for the influence of the hydrogen concentration on the pipeline wall. We establish the location of the cracklike defect in the pipe wall and its initial sizes by the acoustic-emission method. For the determination of the hydrogen concentration in the metal of pipeline, we propose to use the magnetoacoustic emission method. It is shown that the fluctuations of internal pressure in the oil (or gas) pipeline and the hydrogenation of its walls depending on the initial sizes of the cracklike defect may induce its fatigue propagation

Kinetics of Fatigue Crack Propagation at the Tooth Root of a Cylindrical Gear Wheel

The present paper proposes an analytical method for determining the course (path) of crack propagation. A fatigue crack arises at the tooth root of a cylindrical geared wheel due to loading connected with gear work conditions. The cracking model was built using the adequate NASGRO2/3 formula; moreover, for comparison purposes, Forman and Paris-Erdogan’s laws were utilized. The length of an increase (propagation) in the crack was directly calculated by means of the boundary element method.

Conditions for the Transition from Nonlocalized to Localized Damage in Metals and Alloys. Part 3. Determination of the Transition Conditions by the Analysis of Crack Propagation Kinetics

Special features of initiation and growth of short cracks under stresses above the fatigue limit are studied for a wide range of metals and alloys. The authors analyze the growth kinetics of short fatigue cracks, which is assessed by the dependence of crack length or crack growth rate on the number of loading cycles. The transition from the stage of short crack propagation (nonlocalized fatigue damage) to the stage of a main crack propagation (localized fatigue damage) is shown to be characterized by a more intensive increase of the crack growth rate. A procedure for determination of the main crack sizes corresponding to the transition from nonlocalized to localized fatigue damage has been substantiated. It has been found out that the fatigue crack sizes corresponding to this transition at stresses above the fatigue limit decrease with increasing stresses and remain smaller than those at stresses equal to the fatigue limit.

Assessment of the Kinetics of Crack Propagation in the Header-Steam Generator Connector Welded Joint NO. 111 by Plasticity Resource

The results of simulation of crack kinetics of the PGV-1000 header-steam generator connector weldment of NPP WWER-1000 reactor units calculated using plasticity resource under normal operational conditions and hydrotesting are presented. It is shown that the proposed approach to calculation of the crack propagation kinetics makes it possible to obtain a conservative estimate of the structural elements’ lifetime, including with account for corrosion effect.

Influence of the Interface and the Additional Layer on the Stable Crack Propagation through Polyolefin Bilayered Structures

In this work, model structures coextruded from polyolefin materials with different resistance against elastic and plastic deforma- tion (tensile modulus and work of deformation up to the yield point) as well as different resistance against stable crack initiation and propagation were experimentally analyzed using the multiple-specimen crack resistance (R) curve approach under quasistatic loading conditions. Additionally, numerical calculations were done using the finite element method. R curve ratios were calculated where the R curve of material 1 in the bilayer was related to the R curve of material 1 in the single-layer and so on. Depending on the mechanical performance of both layers, the additional layer highly affects the crack propagation kinetics inside the first layer when the crack propagates across the bilayered structure. Because the resistance against stable crack propagation is related to the size and shape of the plastic zone and the energy dissipated inside, the mismatch of the resistance against stable crack propagation (R curve ratios) is highly correlated to the mismatch of the resistance against plastic deformation.

Prediction of the kinetics of crack propagation and the residual durability of vessels in gaseous hydrogen

We propose an experimental–theoretic approach to the prediction of the kinetics of crack propagation and determination of the residual durability of vessels in gaseous hydrogen. Kinetic diagrams of hydrogen cracking (KDHC) of VNL-1М (05Kh13N8M3) cast martensitic steel under various pressures of the hydrogen atmosphere and various concentrations of hydrogen preabsorbed at high temperatures are constructed. The threshold values of stress intensity factors are established. The residual durability of a pipeline and a high-pressure gas cylinder are calculated. It is established that, as the hydrogen pressure and concentration increase, the threshold value of the stress intensity factor decreases in the first section of a KDHC and the rate of subcritical propagation of a crack increases in the second section.