Kinetic Diagrams Of Fatigue Fracture Research Articles

Evaluation of the threshold for the development of fatigue cracks in a railway steel under harmonic and operational loading

The results of experimental studies of the fatigue crack development in 20GFL steel specimens cut from a cast bolster of a freight car are presented. The ratio of the threshold stress intensity coefficient Kth determined from the kinetic diagram of fatigue fracture and from the average parameters of the operational loading process is considered using the experimental results with a simulation of operational loading. Tests were carried out upon the development of permanent blocks of crack opening in the specimen (in a rigid loading mode). The operational process is presented in the form of a block of consecutive loading cycles recorded during the test of the car frame in conditions typical for a straight section of the railway track. The threshold operational level is determined by the algorithm of gradual reduction of the loading similar to the original process. The regularities in a decrease of the rate of crack development and corresponding decrease in the load were determined. Subsequent extrapolation of the obtained experimental regularities to zero value of the crack propagation rate provided estimation of the threshold loading level, similar to the initially specified value. It is shown that the value of the threshold level of the fatigue crack development in low-alloy steel 20GFL obtained from the fatigue fracture diagram (i.e., under harmonic loading) is significantly higher than that obtained from the estimate based on the average values of the operational loading process. The considered model of operational loading gives greater damage compared to harmonic loading, on the basis of which the survivability of structural elements is usually assessed.

Modeling and calculation of the fatigue crack growth life in structural steels

The kinetics of fatigue crack growth has been studied in tensile testing of compact steel tensile specimens (S(T)-type) in the middle section of the kinetic diagram of fatigue fracture (fatigue crack growth diagram) under regular and irregular loading with different asymmetry and maximum load values. The samples were tested on a BISS Nano-25kN servo-hydraulic machine. Standard loading spectra typical for different technical objects exposed to alternating loading during operation were used. The values of the crack growth rate per cycle in the loading block were obtained. Parameters for assessing the character of irregular loading and crack closure, namely, the irregularity factor and crack closure coefficient were proposed. When calculating the effective value of the range of the stress intensity factor (SIF) at the crack mouth, we propose also to take into account the loading irregularity in addition to the closure coefficient. With this approach, the obtained fatigue crack growth diagrams can be grouped into one equivalent curve, which is characteristic of regular loading with R = 0. Moreover, grouping of the fatigue crack growth diagrams provided the use of unified parameters when calculating the crack growth kinetics, regardless of the type and parameters of loading, which rather simplified the crack growth determination. The fatigue crack growth life was predicted taking into account the crack «closure» and the nature of loading according both to the approach developed by the authors and by cyclic calculation method (cycle-by-cycle). All the data obtained are tabulated and classed according to the type of loading. The calculation results and experimental data showed good convergence, which was confirmed by the high values of the correlation coefficient.

A Study of Fatigue Crack Growth Rate in Steels in Relation to Crack-Tip Plastic Deformation and Fracture. Part 2. Parametric Relation between Fatigue Crack Growth and Plastic Strains

The first part of this study presented low-cycle fatigue and cyclic fracture toughness test methods for 10GN2MFA steel at 20 and 270°C, experimental results to construct kinetic fatigue fracture diagrams and to determine low-cycle plastic strain and fracture characteristics. Here, those results are estimated as to the application of cyclic plasticity to describe the fatigue crack growth rate. The cyclic strain-hardening coefficient and the Paris–Erdogan equation exponent for ferrite-pearlite steels are shown, to be closely related as well as the intersection point of the diagram in the range of high crack growth rates was defined. The parameters of the above relation and coordinates of the intersection point were determined for the steels of this class. In the latter case, the optimization methods were used to find the minimum sum of squares of residual functions. Based on the correlation between the cyclic strain-hardening coefficient and Paris–Erdogan equation exponent, the conclusion was drawn of the lower boundary value of this characteristic predicted for an ideal elastoplastic material (zero cyclic strain-hardening coefficient), to be equal to 1.0. As also follows, other classes of the materials (and other cycle asymmetries) can exhibit similar relations in the form of rays coming from the point (0; 1.0) at a corresponding experimental slope to the coordinate axes. From the common intersection point of the diagram (Gurney point) and their slopes to the axes for ferrite-pearlite steels, the procedure was advanced to construct the linear section of the diagram based on the results of low-cycle fatigue tests with plotting the stress amplitude vs. plastic strain amplitude curve. The procedure was approved on 10GN2MFA steel at 20 and 270°C and SAE-1020 and API5L X60 steels taken from the literature. In view of data scattering for cyclic fracture toughness test results, the estimation of the linear diagram sections is quite adequate, and the procedure of their construction is appropriate and can be extended to other classes of the materials and stress ratios.

Study of the Fatigue Crack Growth in Long-Term Operated Mild Steel under Mixed-Mode (I + II, I + III) Loading Conditions.

The paper presents an analysis of mixed-mode fatigue crack growth in bridge steel after 100-years operating time. Experiments were carried out under mode I + II configuration on Compact Tension Shear (CTS) specimens and mode I + III on rectangular specimens with lateral stress concentrator under bending and torsion loading type. Due to the lack of accurate Stress Intensity Factor (SIF) solutions, the crack path was modelled with the finite element method according to its experimental observation. As a result, the Kinetic Fatigue Fracture Diagrams (KFFD) were constructed. Due to the change in the tendency of higher fatigue crack growth rates from KI towards KIII dominance for the samples subjected to bending and torsion, it was decided to analyze this phenomenon in detail using electron-scanning microscopy. The fractographic analysis was carried out for specimens subjected to I + III crack loading mode. The mechanism of crack growth in old bridge steel at complex loads was determined and analyzed.

Application of a New, Energy-Based ΔS* Crack Driving Force for Fatigue Crack Growth Rate Description.

This paper presents the problem of the description of fatigue cracking development in metallic constructional materials. Fatigue crack growth models (mostly empirical) are usually constructed using a stress intensity factor ΔK in linear-elastic fracture mechanics. Contrary to the kinetic fatigue fracture diagrams (KFFDs) based on stress intensity factor K, new energy KFFDs show no sensitivity to mean stress effect expressed by the stress ratio R. However, in the literature there is a lack of analytical description and interpretation of this parameter in order to promote this approach in engineering practice. Therefore, based on a dimensional analysis approach, ΔH is replaced by elastic-plastic fracture mechanics parameter—the ΔJ-integral range. In this case, the invariance from stress is not clear. Hence, the main goal of this paper is the application of the new averaged (geometrically) strain energy density parameter ΔS* based on the relationship of the maximal value of J integral and its range ΔJ. The usefulness and invariance of this parameter have been confirmed for three different metallic materials, 10HNAP, 18G2A, and 19th century puddle iron from the Eiffel bridge.

Mixed mode (I+II, I+III) fatigue crack growth description in S355/P355NL1 steel

Fatigue crack growth models (mostly empirical) are usually constructed using stress intensity factor – ΔK in linear-elastic fracture mechanics. Contrary to the kinetic fatigue fracture diagrams (KFFDs) based on ΔK, Kmax new energy diagrams based on strain energy density parameter ΔH, ΔS* demonstrates no sensitivity to mean stress effect expressed by the stress ratio R. According to the dimensional analysis approach ΔH, ΔS* is replaced by elastic-plastic fracture mechanics parameter based on the energy parameter associated with ΔJ energy parameter range. Hence, the main goal of this paper is the application of the new averaged strain energy density parameter ΔS* based on hysteresis loop curves for cracked components under uniaxial as well as multiaxial stress state. The presented approach assumes that the fatigue crack growth increment in the elastic-plastic range is caused by the combination of the maximum value of the energy parameter and its positive part of the range in each cycle of loading. The usefulness and invariance of this parameter have been confirmed in many works.

Specific Features of the Fatigue Fracture of Welded Joints of 34KhN2MA Steel Formed by Electrodes with Different Phase Compositions

We study the cyclic crack resistance of the metal of welded joints of 34KhN2MA thick-plate steel obtained with the help of ferritic and austenitic electrodes. It is shown that the weld metal has a higher crack resistance than the base metal only within the second part of the kinetic diagrams of fatigue fracture. In the first and third parts, the characteristics of the weld metal are worse. The results of fractographic investigations demonstrate that the decrease in the fatigue crack-growth resistance in pure austenitic welds is explained by the presence of hot cracks in the zone between the near-root metal of the welded joint and the base metal. As for the mixed ferrite-austenitic welds, the decrease in the fatigue crack-growth resistance is explained by inadmissibly large slag inclusions creating additional stress concentration and deteriorating the characteristics of cyclic crack-growth resistance.

Fatigue crack growth of 42CrMo4 and 41Cr4 steels under different heat treatment conditions

PurposeFor nowadays construction purposes, it is necessary to define the life cycle of elements with defects. As steels 42CrMo4 and 41Cr4 are typical materials used for elements working under fatigue loading conditions, it is worth to know how they will behave after different heat treatment. Additionally, typical mechanical properties of material (hardness, tensile strength, etc.) are not defining material’s fatigue resistance. Therefore, it is worth to compare, except mechanical properties, microstructure of the samples after heat treatment as well. The paper aims to discuss these issues.Design/methodology/approachSamples of normalized 42CrMo4 (and 41Cr4) steel were heat treated under three different conditions. All heat treatments were designed in order to change microstructural properties of the material. Fatigue tests were carried out according to ASTM E647-15 standard using compact tension specimens. Later on, based on obtained results, coefficients C and m of Paris’ Law for all specimens were estimated. Similar procedure was performed for 41Cr4 steel after quenching and tempering in different temperatures.FindingsThe influence of heat treatment on the fatigue crack growth rates (42CrMo4, 41Cr4 steel) has been confirmed. The higher fatigue crack growth rates were observed for lower tempering temperatures.Originality/valueThis study is associated with influence of microstructural properties of the material on its’ fatigue fracture. The kinetic fatigue fracture diagrams have been constructed. For each type of material (and its heat treatment), the Paris law constants were determined.

Fatigue Crack-Growth Resistance of Thermally Hardened 65G Steel Under Transverse Shear

We plot the kinetic diagrams of fatigue fracture of 65G steel under the conditions of transverse shear after different modes of heat treatment within the range of fatigue crack-growth rates covering six orders of magnitude with the use of the stress intensity factor for the evaluation of the stress-strain state in the vicinity of the crack tip. The main and auxiliary characteristics of cyclic crack-growth resistance are determined with regard for the friction of the lips of a fatigue crack. We construct the diagrams of structural strength and confirm the hypothesis that the fatigue thresholds increase with the yield strength of 65G steel under the conditions of shear.

Influence of the Long-Term Operation of Gas Pipelines on the Cyclic Crack-Growth Resistance of 17G1S Steel

We establish the kinetic regularities of crack propagation under low-, medium-, and high-amplitude cyclic tensile loads in 17G1S steel taken from the “Dashava–Minsk” gas main pipelines after 40 yr of operation and from the intact (original) pipes. The kinetic diagrams of fatigue fracture are plotted for various stress ratios of the loading cycle within the range of fatigue crack growth rates covering up to six orders of magnitude. The near-threshold regions of the kinetic diagrams are investigated for the material of different zones of the walls of the intact pipes and pipes after operation. The characteristics of fatigue crack-growth resistance are determined on the basis of the constructed plots. We reveal a significant decrease in the threshold value of the stress intensity factor of the material after operation as compared with the intact material caused by the degradation of its structure and the decrease in its fatigue-fracture resistance.

Kinetics of fatigue crack growth and crack closure effect in long term operating steel manufactured at the turn of the 19th and 20th centuries

In this paper the fatigue crack growth behaviour in structural components from the old 19th century structures (e.g. bridges) has been investigated. The delivered material for investigation was extracted from a beam made of puddled iron, commonly used in the 19th century. The obtained results from several ancient railway metallic bridges (located in Lower Silesia, Poland) have shown the presence of microstructural degradation processes in puddled iron. The kinetic fatigue fracture diagrams (KFFD) have been obtained for post-operated steel. The problem of crack closure has been involved in fatigue crack growth process during the experiments and its understanding is fundamental for the analysis of stress ratio effects on KFFD. An experimental and numerical approach has been considered for the evaluation of the crack closure/opening forces based on the hysteresis loop deformation. The implemented algorithm in the numerical environment gives promising results in the description of the kinetics of fatigue crack growth of the old metallic materials with consideration of crack closure effects. The problem of fatigue crack growth rate description from the energy point of view has been presented and discussed. The Universal Graph method (based on Dimensional Analysis approach) was used for phenomenological formulation of the kinetic equation.

Investigation of the Kinetics of Fatigue Macrocracks Under Transverse Shear

We study the kinetics of shear macrocracks under the conditions of fatigue fracture by transverse shear of thermally hardened 65G steel with a troostite-sorbite structure for low-, middle-, and high-amplitude cyclic loads. We plot the kinetic diagrams of fatigue fracture with regard for the friction of the faces of a shear crack within the range of its growth rates covering up to six orders of magnitude. On the basis of the constructed dependence, we determine the principal and additional characteristics of cyclic crack resistance. We also study the contact interaction of the faces of fatigue cracks and the fractographic features of fracture under the conditions of low- and high-amplitude deformation by cyclic transverse shear.

Fatigue crack propagation behavior of old puddle iron including crack closure effects

In this paper the fatigue crack growth behavior in structural components from the old 19th century structures (e.g. bridges) has been investigated. The delivered material for investigation was extracted from a beam made of puddled iron, commonly used in 19th century. The obtained results from several ancient railway metallic bridges (located in Lower Silesia, Poland) have shown the presence of microstructural degradation processes in puddled iron. In all analyzed materials (low carbon puddled iron) microstructure degradation processes were related to: the presence of numerous precipitations of carbides and nitrides (or the carbides–nitrides) of iron inside the ferrite grains, the presence of continuous precipitations of cementite at ferrite grain boundaries. In order to restore the initial state of the microstructure, all tests were carried out in two stages of heat treatment; as-received state and after normalization (950°C, 2h, cooled in air) state. The kinetic fatigue fracture diagrams (KFFD) have been obtained. The problem of crack closure has been involved in fatigue crack growth process during the experiments and its understanding is fundamental for the analysis of stress ratio effects on KFFD. In the paper, a few experimental and numerical techniques for the evaluation of the crack closure/opening forces based on the experimental data have been compared. The implemented algorithm in the numerical environment gives promising results in description of the kinetics of fatigue crack growth of the old puddled iron with consideration of crack closure effect.

Evaluation of the Residual Life of a Three-Dimensional Solid Body Weakened by a Plane Fatigue Crack Under Cyclic Loading

On the basis of the energy approach of fracture mechanics, we construct a computational model of propagation of plane fatigue macrocracks in three-dimensional deformable elastoplastic bodies. We deduce a kinetic equation that enables us to determine the period of subcritical growth of a fatigue macrocrack corresponding to the residual life of an element of the metal structure weakened by a crack under external cyclic loading. To test the model and find the unknown physicomechanical constants of the material, we study specimens from the sections of welded joint of 1201-T aluminum alloy. The obtained kinetic diagrams of fatigue fracture of these specimens reveal good agreement between the computed and experimental results. According to the results of the theoretical and experimental investigations, we construct the nomograms of residual life of the weld metal and the base metal of the welded joint of this alloy.

Kinetics of fatigue crack growth and crack paths in the old puddled steel after 100-years operating time

The goal of the authors’ investigations was determination of the fatigue crack growth in fragments of steel structures (of the puddled steel) and its cyclic behavior. Tested steel elements coming from the turn of the 19th and 20th were gained from still operating ancient steel construction (a main hall of Railway Station, bridges etc.). This work is a part of investigations devoted to the phenomenon of microstructural degradation and its potential influence on their strength properties. The analysis of the obtained results indicated that those long operating steels subject to microstructure degradation processes consisting mainly in precipitation of carbides and nitrides inside ferrite grains, precipitation of carbides at ferrite grain boundaries and degeneration of pearlite areas [1, 2]. It is worth noticing that resistance of the puddled steel to fatigue crack propagation in the normalized state was higher. The authors proposed the new kinetic equation of fatigue crack growth rate in such a steel. Thus the relationship between the kinetics of degradation processes and the fatigue crack growth rate also have been shown. It is also confirmed by the materials research of the viaduct from 1885, which has not shown any significant changes in microstructure. The non-classical kinetic fatigue fracture diagrams (KFFD) based on deformation (??) or energy (?W) approach was also considered. In conjunction with the results of low- and high-cycle fatigue and gradual loss of ductility as a consequence (due to the microstructural degradation processes) - it seems to be a promising construction of the new kinetics fatigue fracture diagrams with the energy approach.

Influence of the Friction of Crack Faces on the Cyclic Crack-Growth Resistance of 65G Steel Under Mode II Loading

We study the fracture of 65G steel under cyclic mode-II loading of beam specimens with lateral cracks, experimentally determine the friction coefficient of crack faces for specimens made of steel with troostite structure, and plot the kinetic diagrams of fatigue fracture. The influence of the friction of crack faces on the cyclic crack-growth resistance of 65G steel under mode-II loading is established for troostite and sorbite structures depending on the growth rate of fatigue cracks.

Microstructure-Dependent Model for Calculating the Growth Rate of Physically Small and Long Fatigue Cracks

Based on the structural concept of threshold stress intensity factor ranges, the model, describing the growth kinetics of physically small and long cracks, was constructed. The model permits the calculation of lifetime on the growth of physically small and long cracks at regular cyclic uniaxial loads by the data on static mechanical characteristics and a microstructure of the initial material. The model validity was corroborated by fracture toughness results in symmetric cyclic plane bending of VT3-1 titanium alloy specimens in different microstructural states. Calculated kinetic fatigue fracture diagrams are in good agreement with experimental results.

Determination of the Characteristics of Cyclic Crack Resistance of Steels Under Transverse Shear (A Survey)

We present a survey of works devoted to the investigation of the fatigue fracture of steels under the conditions of transverse shear. Numerous methods aimed at the determination of the characteristics of cyclic crack resistance of structural steels of different strength classes are analyzed. The analysis of the results of investigations enables us to make a conclusion about the necessity of improvement of the available methods and development of new methods for the construction of the kinetic diagrams of fatigue fracture with regard for the influence of friction of the lips of fatigue cracks on the stress intensity factor K II and the elimination of stresses in the vicinity of the crack tip.

Methods for the Construction of the Kinetic Diagrams of Fatigue Fracture for Steels Under the Conditions of Trаnsverse Shear with Regard for the Friction of Crack Lips

We propose the design of a specimen for the investigation of the fatigue fracture of steels under trans- verse shear. A formula for the determination of the stress intensity factor with regard for the friction of crack lips is obtained with the use of the finite-element method. The methods aimed at the construction of the kinetic diagrams of fatigue fracture under the conditions of transverse shear are developed and tested on specimens of 65G structural steel. We plot the kinetic diagram of fatigue fracture and approx- imate its middle segment by a linear dependence in the established parameters. The life of structural elements with sharp concentrators operating the under conditions of shear stresses should be evaluated on the basis of the approaches of fatigue fracture mechanics under the conditions of trans- verse shear. This is why it is necessary to develop methodical recommendations for the reliable determination of the characteristics of cyclic crack resistance (CCCR) of the materials in transverse shear. In the case of mode I fatigue fracture of metallic materials, the methods aimed at the determination of the CCCR were standardized in (1). Some experimental investigations of the mode II fatigue fracture of steels based on the models neglecting the stage of stable growth of fatigue cracks or the friction of their lips are also known (2-5). This negatively affects the analysis of the stress-strain state (SSS) near the crack tip, the determi- nation of the CCCR of materials, and the evaluation of the service life of structural elements as a whole, which should be taken into account in the development of new methods for the construction of the kinetic diagram of fatigue fracture (KDFF) in transverse shear. The aim of the present work is to develop new methods for the construction of the KDFF and evaluation of the CCCR of steels in transverse shear with regard for the friction of crack lips.

THE ESTIMATION OF DEVELOPMENT SELF -CONSISTENT DAMAGES OF SLIMY STEEL CONSTRUCTIONS

The procedure for testing the prismatic steel specimens of heavy constructions to build the kinetic diagrams of fatigue fracture is described in the paper. The kinetic diagrams obtained have been approximated by analytical dependences with unknown parameters, which were found using least squares method.