Crack initiation and propagation in torsional fatigue of circumferentially notched steel bars

Circumferentially notched bars of austenitic stainless steel, SUS316L, and carbon steel, SGV410, with three different notch-tip radii were fatigued under cyclic torsion without and with static tension. The torsional fatigue life of SUS316L was found to increase with increasing stress concentration under the same nominal shear stress amplitude. Electrical potential monitoring revealed that the crack initiation life decreased with increasing stress concentration, while the crack propagation life increased. This anomalous notch-strengthening effect was ascribed to the larger retardation of fatigue crack propagation by sliding contacts of fracture surfaces. The superposition of static tension on cyclic torsion causes notch weakening. The notch-strengthening effect in torsional fatigue was not found in carbon steels, SGV410. The difference in the crack path of small cracks near notch root between stainless steel and carbon steel gives rise to the difference in the notch effect in torsional fatigue. The factory-roof shape observed on fracture surfaces of SUS316L became finer with higher stress amplitude and for sharper notches. The superposition of static tension makes the factory-roof shape less evident. Under higher stresses, the fracture surface was smeared to be flat. The fracture surfaces of SGV410 became smoother with increasing stress amplitude and notch acuity. The three-dimensional feature of fracture surfaces clearly showed the difference of the topography of fracture surfaces. The topographic feature was closely related to the amount of retardation of crack propagation due to the sliding contact of fracture surfaces.     

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation.
The fatigue strength of the steel wires between 10⁶ and 10⁹ cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 10⁷ cycles. For Cr–V spring wire, the stress–life ( S – N ) curve becomes horizontal at a maximum stress of 800 MPa after 10⁶ cycles, but the S – N curve of the Cr–Si steel continues to drop at a high number of cycles (>10⁶ cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.     

Crack propagation during fatigue experiments

In the present paper a very simple phenomenological theory of crack propagation during fatigue experiments is presented. It is shown that an important role is played by work-hardening to explain both the existence of a fatigue limit and the well known deviations from Miner's Rule in the case of tests conducted at varying maximum applied stresses. Although approximate, the theory can account for the main phenomena involved in fatigue experiments.

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Zusammenfassung Es wird eine sehr einfache Grundlagentheorie zur Beschreibung der Fortpflanzungsbedingungen eines Risses unter Dauerbeanspruchung dargelegt.Die Grundlage dieser Theorie besteht darin die Rissfortpflanzung mit der mechanischen Hysteresis in Zusammenhang zu bringen.Es wird auf die wichtige Rolle der Verformung hingewiesen, sowohl für der Begriff der Dauerfestigkeit als auch für die wohlbekannten Abweichungen von dem Miner Gesetz, im falle von Versuchen bei maximaler Beanspruchung mit veränderlichen Amplituden.Trotz einer Anzahl von Vereinfachungen, gibt these Theorie eine gute Beschreibung der wichtigsten Vorgänge beim Dauerversuch.

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Résumé On présente une théorie phénoménologique extrêmement simple pour décrire les conditions de propagation des fissures en fatigue. Cette théorie repose sur le principe d'associer la propagation des fissures an cycle d'hystérésis mécanique. On montre que l'écrouissage joue un rôle important, à la fois dans la notion de la limite d'endurance, et dans les inexactitudes, bien connues, de la règle de Miner, lorsque des essais sent conduits à des contraintes maximales d'amplitude variable.Quoique assez grossière, cette théorie rend compte des principaux phénomènes qui interviennent dans des expériences de fatique.

     

Crack propagation and fatigue in zirconia-based composites

This paper reviews existing published studies on crack propagation behavior of zirconia-based composites. The first part of the paper is concerned with slow crack growth (SCG) under static loading. SCG in zirconia ceramics is shown to be a consequence of stress corrosion by water molecules at the crack tip. The influence of transformation toughening on SCG is discussed in terms of a stress intensity factor acting to reduce the net driving force for propagation. This proposition is in agreement with results obtained on 3Y-TZP and Mg-PSZ ceramics. A master curve is proposed which could be applied roughly to all zirconia ceramics. The influence of zirconia addition to alumina ceramics (ZTA ceramics) is also discussed. The second part of the paper deals with SCG under cyclic loading. A mechanical degradation of all zirconia-based composites is observed by a decrease of crack shielding. This degradation of zirconia-based composites under cyclic loading leads to increased velocities as compared to the static fatigue case. A master curve is also obtained, as in the case of static fatigue. Cyclic fatigue results are interpreted in terms of stress corrosion at the crack tip assisted by a decrease of the reinforcement.     

Crack propagation in a commercial steel

In this work, the fatigue crack propagation behaviour of commercial steel sheet specimens containing a circular hole, under uniaxial loading conditions at room temperature was investigated.The experimental data have been analysed in terms of variability of material constants. The results were presented in the form of power relationship between the crack growth rate and the stress intensity factor range.

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Résumé Dans ce travail, on a étudié le comportement à la propagation des fissures de fatique d'éprouvettes de tôle mince en acier de construction, présentant un trou circulaire et sujettes à une mise en charge uniaxiale à température ambiante.On a analysé les données expérimentales en termes de variabilité des constantes du matériau. Les résultats sont présentés sous forme d'une relation parabolique entre la vitesse de croissance de la fissure et la variation du facteur d'intensité de contrainte.

     

Mechanisms for corrosion fatigue crack propagation

ABSTRACT The corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high‐strength low‐alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open‐circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non‐single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (K_max) exceeded a certain well‐defined critical value characteristic for a given material‐solution combination. When K_max was lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen‐induced cracking (HIC). Therefore, critical values of K_max, as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated as K_HIC and ΔK_HIC. HIC was the main mechanism of corrosion FCG at K_max > K_HIC (ΔK > ΔK_HIC). For most of the material‐solution combinations investigated, stress‐assisted dissolution played a dominant role in the corrosion fatigue crack propagation at K_max < K_HIC (ΔK < ΔK_HIC).     

Microcrack nucleation, growth, coalescence and propagation in the fatigue failure of a powder metallurgy steel

Detailed quantitative micrographic data are presented for Stages I and II of a Powder Metallurgy Fe-1.5Cr-0.2Mo-0.7C steel specimen fatigued in bending with R  =−1 at 24 Hz and a stress amplitude of 312 MPa. The fatigue limit was ∼240 MPa, at which stress level no microcracks were detected in static loading. Testing was interrupted at 100 cycles and at further 29 intervals until failure after 49 900 cycles. For each arrest, surface replicas were made in the two regions where maximum stress was applied. Microcracks could nucleate below 100 cycles, when their sizes ranged from <5 to ∼20 μm. Fractographic examination identified the failure-originating site, which was then associated with the crack system observed on the 'last' pre-failure micrograph. Detailed examination of the eventual failure region showed nucleation, at various cycle intervals, of 18 microcracks, their subcritical growths, arrests and coalescences with continuing cycling to form a critical crack 2.25 mm deep. Stepwise microcrack growth was probably rapid – to the next arrest or coalescence. For each (micro)crack size stress intensity factors, K _a s, were estimated and, at the end of Stage II, for the coalesced crack, K _a reached K _1C, independently estimated to be ∼36 MPa m^1/2.     

The influence of hammer peening on fatigue in high-strength steel

An investigation of the influence of hammer peening on fatigue crack initiation and propagation in high-strength steel has been performed using edge notch specimens. The crack initiation time was found to decrease after peening; however, the fatigue crack growth stage remained unchanged or decreased depending upon the peening parameters. The results have been used to develop an analytical model to predict the fatigue crack growth rate in peened materials, based upon the compressive residual stress effect.     

Nanoscopic fatigue and stress corrosion crack growth behaviour in a high-strength stainless steel visualized in situ by atomic force microscopy

In situ atomic force microscope (AFM) imaging of the fatigue and stress corrosion (SC) crack in a high‐strength stainless steel was performed, under both static and dynamic loading. The AFM systems used were (1) a newly developed AFM‐based system for analysing the nanoscopic topographies of environmentally induced damage under dynamic loads in a controlled environment and (2) an AFM system having a large sample stage together with a static in‐plane loading device. By using these systems, in situ serial clear AFM images of an environmentally induced crack under loading could be obtained in a controlled environment, such as in dry air for the fatigue and in an aqueous solution for the stress corrosion cracking (SCC). The intergranular static SC crack at the free corrosion had a sharp crack tip when it grew straight along a grain boundary. The in situ AFM observations showed that the fatigue crack grew in a steady manner on the order of sub‐micrometre. The same result was obtained for the static SC crack under the free corrosion, growing straight along a grain boundary. In these cases, the crack tip opening displacement (CTOD) remained constant. However, as the static SC crack was approaching a triple grain junction, the growth rate became smaller, the CTOD value increased and the hollow ahead of the crack tip became larger. After the crack passed through the triple grain junction, it grew faster with a lower CTOD value; the changes in the CTOD value agreed with those of the crack growth rate. At the cathodic potential, the static SC crack grew in a zigzag path and in an unsteady manner, showing crack growth acceleration and retardation. This unsteady crack growth was considered to be due to the changes in the local hydrogen content near the crack tip. The changes in the CTOD value also agreed with those of the crack growth rate. The CTOD value in the corrosive environment was influenced by the microstructure of the material and the local hydrogen content, showing a larger scatter band, whereas the CTOD value of the fatigue crack in dry air was determined by the applied stress intensity factor, with a smaller scatter band. In addition, the CTOD value in the corrosive environment under both static and dynamic loading was smaller than that of the fatigue crack; the environmentally induced crack had a sharper crack tip than the fatigue crack in dry air.     

Crack resistance and micromechanisms of fracture of thermomechanically treated high-strength steel

We study the influence of thermomechanical treatment with deformation by the method of hydrostatic extrusion on the parameters of crack resistance of 45KhN2MFSh high-strength steel and plot the dependences of the critical stress intensity factorK _Ic and critical crack opening displacements δ_c on temperature. It is shown that these curves have the threshold character. The results of microfractographic analysis demonstrate that changes in crack resistance observed as temperature decreases are accompanied by changes in the micromechanisms of fracture in the regions of the onset of crack propagation, which may take place under the condition of changes in the second-order stress-strain state. We show that the temperature curves of the parameters of crack resistance can be efficiently used in determining the temperature of brittle-ductile transition. In the considered case, this temperature does not depend on the size of the specimen and the loading mode and characterizes the structural state of the cracked material. As compared to conventional modes of thermal treatment, thermomechanical treatment guarantees much higher values of crack resistance, especially at low temperatures, and decreases the threshold of cold brittleness for 45KhN2MFSh steel by 20°C. The indicated increase in crack resistance is explained by the hereditary influence of the deformational substructure on the structural and morphological parameters of martensite. Khar'kov State Technical University of Automobiles and Roads, Khar'kov. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 82–86, May–June, 1997.     

Crack layer analysis of fatigue crack propagation in rubber compounds

A methodology to characterize the resistance of rubber compounds to crack propagation (fracture toughness) is presented. A constitutive model based on the crack layer theory is utilized to extract the specific energy of damage ^*, a material parameter characteristic of the material's resistance to crack propagation and the dissipative characteristic, . The model expresses the rate of crack propagation as% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaaiaacaqabeaadaqaaqaaaOqaamaalaaabaGaam% izaGqaciaa-fgaaeaacaWGKbGaa8Ntaaaaaaa!3AFA!\[\frac{{da}}{{dN}}\]= % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaamaalaaabaGaeq% OSdiMaamOsamaaDaaaleaacaaIXaaabaGaaGOmaaaaaOqaaiaadMha% caGGQaGaamOuamaaBaaaleaacaaIXaaabeaakiabgkHiTiaadQeada% WgaaWcbaGaaGymaaqabaaaaaaa!41A5!\[\frac{{\beta J_1^2 }}{{y*R_1 - J_1 }}\]where da/dN is the cyclic rate of fatigue crack propagation (FCP), J ₁ is the energy release rate (tearing energy) and R ₁ is the resistance moment which accounts for the amount of damage associated with the crack advance. Microscopic examination revealed that crack tip microcracking is the dominant damage mechanism. Hence, R ₁ was evaluated as the area (m²) of microcracking surfaces per unit crack advance.Fatigue crack propagation data for a particular rubber compound have been analyzed using the present model. The proposed equation describes the entire FCP history in the compound. According to this model, ^* and for the compound investigated, are found to be 9.3 kJ m^-2 and 9.7×10^-9 m⁴/J-cycle, respectively.

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Résumé On présente une méthodologie pour caractériser la résistance de composés de caoutchouc à la propagation des fissures du point de vue de la ténacité à la rupture. Un modèle constitutif basé sur la théorie de la couche de fissuration est utilisé pour obtenir l'énergie spécifique d'endommagement ^*, un paramètre du matériau représentatif de sa résistance à la propagation d'une fissure, et une caractéristique de dissipation . Le modèle exprime la vitesse de propagation d'une fissure de fatigue par cycle da/dN en fonction de ces deux paramètres, de la vitesse de relaxation de l'énergie de cisaillement J ₁, et du moment résistif R ₁ qui tient compte de état de l'endommagement associé à la progression de la fissure. Un examen microscopique révèle que la microfissuration à l'extrémité de la fissure est le mécanisme déterminant de l'endommagement. Dès lors, on évalue R ₁ en fonction de l'aire de microfissuration (en m²) par unité de progression de la fissure.Des données de propagation de fissure de fatigue sont analysées à l'aide du présent modèle pour un composé de caoutchouc particulier. L'équation proposée décrit l'entièreté de la propagation de la fissure dans le composé. Des valeurs numériques pour ^* et pour de respectivement 9,3 kJ m^-2 et 9,7×10^-9 m^-4/J-cycle sont trouvées.

     

Crack layer analysis of fatigue crack propagation in ABS polymer

Differences in damage formation during fatigue crack propagation in acrylonitrile-butadiene-styrene polymer, between tests both fulfilling and not fulfilling linear elastic fracture mechanics requirements, were related to differences in crack propagation behaviour through the crack layer (CL theory. At both test conditions, damage consisted of crazing and shear yielding of the matrix, as well as elongation of rubbery domains. For a given crack length, the lower load level showed a higher intensity of craze damage. CL analysis showed that the process-dependent dissipation coefficient, , is inversely proportional to the lifetime. Further, despite drastic differences in the amounts of each damage species, both tests were estimated to have the same specific enthalpy of damage (^*=105 cal g^–1), a material constant that is a measure of the intrinsic resistance to damage formation at the crack tip.