Papers by Keyword: Fatigue Crack Growth

Abstract: Actuality. The accumulation of damage due to fatigue, plastic deformation, and wear significantly reduces the service life of railway rolled metal products. The development of a fatigue crack to its critical length (main cracks) leads to failure at stress levels much lower than the material's strength limit. In industrial-grade steels, there may be chemical micro-inhomogeneity of the main element—carbon. Objective of the study: To determine the effect of chemical micro-inhomogeneity (carbon content variation of 0.02%) on fatigue failure characteristics (crack growth rate, threshold stress intensity factor, fatigue life, and critical defect size) of railway wheel steels of grades ER7 and ER8 according to EN 13262. Results. Segments of the fatigue crack growth rate (FCGR) diagram were constructed to characterize the development of fatigue cracks. The crack growth rate on the second linear section of the diagram and the critical value of the stress intensity factor at which failure occurs were determined. It was found that on the linear portion, which describes the crack growth process, the indicator values vary slightly (up to 10%), indicating that the crack growth rate differs minimally between these steels. Fatigue life—the number of loading cycles until failure—was also determined, and the critical size of the fatigue crack was calculated. A carbon content fluctuation within 0.02% by mass leads to a reduction in fatigue life by approximately 10% for ER7 steel and about 20% for ER8 steel, and a reduction in the critical crack size by around 8% for ER7 and 18% for ER8. Conclusion. Chemical micro-inhomogeneity with carbon content variation in the range of 0.02% in ER7 and ER8 railway wheel steels leads to a decrease in fatigue life (as determined from specimens with cracks) and in the critical size of the fatigue crack (up to 20%). However, it has only a minor effect (about 10%) on the stable fatigue crack growth rate.

Abstract: The application of high strength structural steels in welded structures is growing steadily and intensively. Quenched and tempered (Q+T) as well as thermomechanically treated (TM) steel base materials are developing faster than the filler metals for fusion welding processes, and therefore the selection of filler metal deserves special attention. Welded structures made of high strength steel are often subjected to cyclic loading, which can cause initiation and propagation of fatigue cracks and can lead to fatigue fracture failure of the structural element or the structure. This characteristic must also be taken into account when selecting the filler metal. In order to study this issue, welded joints were made from base materials in the 700-1300 MPa strength category using gas metal arc welding (GMAW) process. The applied filler metals were of the undermatching, matching or overmatching type, depending on the strength of the base material. Fatigue crack propagation (FCP) tests were performed on specimens machined from the welded joints, in which notch locations and crack propagation directions were statistical in nature. Therefore, the fatigue crack propagation directions were parallel and perpendicular to the longitudinal axis of the welded joints and located in different zones of the heat affected zone (HAZ). From these investigations, the two parameters (C and n) of the Paris-Erdogan equation were determined for each specimen and statistical samples were formed from the base material-filler metal matching pairs. During the evaluation of the results, it was found that the matching phenomenon has significant effect on the fatigue crack propagation behavior of the welded joints and that this effect depends on the strength category of the base material. Based on these results, recommendations for the applicable base material-filler metal pairings were proposed.

Abstract: The compact tension (CT) and tensile specimens of the AISI 4140 steel in cold rolling condition (untreated steel) were austempered by immersing it into the salt bath at 362°C for 60 minutes. The tensile strength properties and the fatigue crack growth (FCG) resistance were performed to investigate the effect of the austempering process in AISI 4140 steel. A significant increase in the yield strength for austempered steel is about 8.7 % and the elastic strain energy increases by 55.7 %. Austempered steel's fatigue crack cycle is longer than that of untreated steel. Data of stress intensity factor range (ΔK, MPa.m^1/2) and FCG rate (da/dN, m/cycle) was constructed in double log plot x-y axes for determining the materials constants m and C according to Paris’s law equation using a linear regression method. From the curve of ΔK versus da/dN, the constant m value for austempered steel (m = 3.45) shows better resistance than untreated steel (m = 3.77). On the other hand, the constant C value of 1.409×10^-12 for austempered steel is one order magnitude higher than that of untreated steel (C = 4.151×10^-13). The resistance of austempered steel against fatigue crack growth can be attributed to the formation of a bainite structure.

Abstract: In the present research, the fatigue crack growth (FCG) of AISI 1020 steel with and without pre-deformation were characterized by using MTS Landmark 100 kN under fatigue loading at ratio (R) = 0.3, P_max = 0.7 and f = 10 Hz at room temperature. Tensile test results show that 6.25% pre-deformation given on the steel increases of yield strength. In contrast, the ultimate tensile strength, elastic modulus and elongation decrease. The FCG rate (da/dN) of AISI 1020 steel without pre-deformation determined at stage II is 6.12´10^-11ΔK^2.94 m/cycle and steel with 6.25% pre-deformation is 8.03´10^-10ΔK^2.02 m/cycle. According to microstructural observation for the pre-deformation steel, plastic deformation formed on the steel in the axial direction affects the FCG rate of the steel, leads to crack retardation for certain period of time. SEM fractographic observation on the fracture surface of the steel shows that a transgrannular crack length of 12 mm for 42,000 cycles occurs at ferrite grains. The steel failed when the crack length reached ~18.1 mm within 43,500 cycles and continuing up to 43,549 cyles, the steel experienced static failure.

Abstract: Cyclic pressure fatigue tests of silicon nitride ball were performed in two phases. In the first phase, compressive and tensile stresses were applied to two cracks on the ball surface, respectively. In the second phase, tensile stress was applied to the crack that was applied to compressive stress in the first phase to grow. The effect of cyclic compressive stress on crack growth was investigated through this series of tests. The results are as follows. The ball did not fracture in either Phase 1 or Phase 2 tests. The crack did not propagate when the maximum compressive load of approximately 5 kN was repeatedly applied across the crack surface. In addition, the crack applied with compressive stress before tensile stress, and the crack which was not, both grew to about 520 μm during N ranging from 0 to 1.2×10⁷ fatigue cycles. The crack applied with compressive stress before tensile stress at fatigue cycles N = 10³ grew about 170 μm longer than the crack to which stress was not applied.

Abstract: To research effects of cyclic loadings and the loading magnitude on the crack opening-closing behavior, crack opening displacement evaluations for fatigue cracks on a silicon nitride ball were carried out. The compressive loading magnitude, which was less than 2.7 kN, did not affect crack opening-closing behavior. The crack opened at the early cycles of fatigue during 0 to 10³ cycles.

Abstract: Mode I and Mode II fatigue crack growth on the equator of silicon nitride balls were tested under cyclic compressive loads. The mode I crack propagated in a straight direction along compressive axis. The angle of the mode II crack changed toward the direction perpendicular to tensile stress direction. The effect of mode I on cracks in differential mode II was strong after cycles.

Abstract: Fatigue crack growth in NR/BR compound and the effect of two different types of recycled rubber powder (RRP) i.e. micronized cryo-ground 74 μm and ambient-ground 400 μm were studied using fracture mechanics approach. Absolute and relative hysteresis losses using single-edge notch tensile (SENT) specimens were determined with a displacement-controlled strain compensating for permanent set of the samples throughout the Fatigue Crack Growth (FCG) experiments. Results indicated a correlation between absolute/relative hysteresis loss and fatigue crack growth rate under specific dynamic strain amplitudes. Differences in relative hysteresis loss showed that additional energy dissipation, due to multiple new crack surfaces at the crack tip, contributes to the FCG of the RRP compounds. At higher tearing energy, beside other factors affecting the FCG performance of the RRP compounds, both higher absolute and relative hysteresis loss are slightly detrimental to the crack growth rates.

Abstract: The fatigue crack growth of Al-Zn-Mg-Cu alloy can be adjusted by different aging treatments. In the present work, a high Zn-containing Al-Zn-Mg-Cu alloy was treated by single, double and triple stage aging treatments and typical T6, T79 and T77 states were selected by tensile properties. Fatigue crack growth under these aging states was tested and related fracture morphology and precipitation characteristics were observed. The results showed that fatigue crack growth resistance for the alloy was T6<T79<T77. The corresponding fracture morphology also showed the difference of fatigue striations and the measurement of them provided an additional evidence. The precipitation proved that the alloy with T6 state possessed GPI zone, GPII zone and η' phase while that for T76 state was GPII zone, η' phase and η phase. As for the T77 state, the precipitate types were GPII zone and η' phase. The matrix precipitate for T6 state was smaller and denser than that for T79 and T77 states while that for T77 state possessed a dense distribution than that for T79 state. The measurement of precipitate size distribution also proved it. The grain boundary precipitates for T79 and T77 states were similar, which had a more intermittent distribution than that for T6 state.

Abstract: The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (<1μm) structure in a microalloyed (MA) steel as well as an interstitial-free (IF) steel, which could impart a good combination of yield strength and tensile ductility. At the same time, an effort has been made to evaluate the fracture toughness characteristics by conducting 3-point bend tests and computing the K_Q, K_ee and J-integral values of ultrafine grained (UFG) samples and correlating them with the microstructure, besides evaluating the other mechanical properties. Fatigue strength in the high cycle fatigue (HCF) regime were also investigated and fracture mechanisms analyzed and comparison established between differently processed samples. The microstructural analysis was performed using transmission electron microscopy (TEM) and Electron backscatter diffraction (EBSD) and results corroborated with the mechanical properties. Superior combinations of yield strength (YS), ductility (% El.), fracture toughness (K_ee) and high cycle fatigue strength (σ_f) were obtained under certain conditions, i.e., i) MA steel: intercritical (α+γ) phase regime (~A_r1) controlled and 15-cycle multiaxially forged (MAFed) (YS=1027MPa, %El.=8.3%, σ_f=355MPa and K_ee=90MPa√m), and ii) IF steel: ferritic region (<A_r1) controlled 18-cycle MAFed (YS=881MPa, %El.=11.2%, σ_f=255MPa and K_ee=97MPa√m). In the case of MA steel, an enhancement of the fatigue and fracture toughness properties can be ascertained following the formation of uniformly distributed nanosized fragmented cementite (Fe₃C) particles (~35nm size) present in the submicron sized (average ~280nm size) ferritic microstructure. In contrast, in the case of IF steel, this is ascribed to the development of submicron sized ferrite grains (average ~320nm) along with a high density of dislocation substructures. These fine dislocation cells/substructures along with the nanosized Fe₃C particles could effectively block the initiation and propagation of cracks and thereby enhance the fatigue endurance and fracture toughness of the steel. Superior fracture toughness along with high mechanical properties in submicron-grained condition render the two steels highly useful for high-strength structural applications.