Very high cycle fatigue properties of high‐strength spring steel 60SiCrV7

The very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔK_GBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔK_GBF for uncharged specimens are close to crack growth threshold ΔK_th, which indicates that ΔK_GBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔK_GBF.     

FATIGUE BEHAVIOR OF A RAIL STEEL

The fatigue behavior of a hot-rolled, control-cooled, plain carbon eutectoid rail steel has been characterized. The data include monotonic and cyclic stress-strain curves, low cycle fatigue data and near-threshold fatigue crack growth rate behavior in air and in vacuo. The effects of environment and mean stress on the near-threshold fatigue crack growth rates of rail steel are significant. At a low stress ratio (R), ΔK_o is lower in vacuum (7 MPa √m) than in moist air (10 MPa √m). At high R, ΔK_o is higher in vacuum (6 MPa √m) than in air (4 MPa √m). The beneficial effect of moist air on FCGR at low ΔK and low R is attributed to an increase in closure due to fracture surface roughness and oxide film.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

Vacuum effects on fatigue crack growth in submicrometre‐thick freestanding copper films

To clarify vacuum effects on fatigue crack growth in freestanding metallic thin films, experiments were conducted on approximately 500‐nm‐thick copper films inside a field emission scanning electron microscope. Fatigue crack growth accompanied by intrusion/extrusion formation occurred in vacuum, and da/dN was smaller than in air in the middle‐ΔK region (ΔK ≈ 1.7‐3.1 MPam^1/2). Conversely, in the low‐ΔK region (ΔK ? 1.7 MPam^1/2), da/dN was larger in vacuum than in air. Further, fatigue crack growth in vacuum occurred below the fatigue threshold in air (ΔK_th,air). A nonpropagating crack after reaching ΔK_th,air continued to propagate in vacuum when the environment changed from air to vacuum. This indicates that fatigue crack growth resistance is smaller in vacuum than in air under the same effective driving force. The fatigue damage area near the crack paths in vacuum in the low‐ΔK region became wider, suggesting that the nucleation of fatigue damage was enhanced in vacuum.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

COMPARISON OF PLASTIC WORK OF FATIGUE CRACK PROPAGATION IN LOW CARBON STEEL MEASURED BY STRAIN-GAGES AND ELECTRON CHANNELING

Abstract— The plastic work to propagate a fatigue crack by a unit area, U, measured by the foil strain gage technique requires an extrapolation to estimate the contribution closer than 100 μm to the crack tip. This is due to the size of the strain-gages used, 200 × 210 μm. Conversely, the electron channeling technique for determining U is useful mainly close to the crack tip where subgrains form. In the present work U was measured by both techniques in the same low carbon steel at ΔK= 8 MN/m^3/2. The contribution to U from closer than 100 μm of the crack tip was determined to be 1·7 × 10⁶ J/m² using electron channeling and 2·0 × 10⁶ J/m² by extrapolation. The measured contribution to U from further than 100 μm from the crack tip was 3·6 × 10⁶ J/m² giving 5·3 × 10⁶ J/m² for U. Thus, a large amount of energy is absorbed outside the region where sub-grains form. The non-hysteretic plastic work was found to be about four orders of magnitude smaller than the hysteretic plastic work, and may be neglected. A map of the plastic zone results from the strain-gage measurement. Rice's theory predicts the measured plastic zone sizeif the proper material's strength is employed in the formula.     

Effects of oxide on fatigue crack growth behaviour of type 347 stainless steel in PWR water conditions

The fatigue crack growth behaviour of type 347 stainless steel in pressurized water reactor (PWR) conditions was investigated at two different temperatures, 25 and 316 °C. The fatigue crack growth rate was slightly increased at the elevated temperature in air. In the simulated PWR water environment, the fatigue crack growth rate was changed in a rather complicated manner with a change of the dissolved oxygen (DO) content. The DO content did not significantly change the fatigue crack growth rate at 25 °C. With the lower oxygen content of 5 ppb, which is the practical limit of deoxygenated water, the fatigue crack growth rate was similar at both 25 and 316 °C. The fatigue crack growth rate was significantly decreased at 316 °C with the higher oxygen level of 100 ppb. Under 316 °C water conditions, oxides were observed on the fatigue crack surface where the size of oxide particles was about 0.2 µm at 5 ppb and about 1 µm at 100 ppb. The thickness of the oxide layers also increased with the increase of DO. Moreover, the ΔK threshold (ΔK_th) also increased as the DO increased from 5 to 100 ppb. The dissolved hydrogen levels did not affect the measured crack growth rate at the given test conditions. The decrease of the fatigue crack growth rate with higher DO content is attributed to a crack closure resulting from the formation of larger oxides near the crack tip at a rather fast loading frequency of 10 Hz that was used in this study.     

High cycle fatigue characteristics of 2124-T851 aluminum alloy

The fatigue crack growth rate, fracture toughness and fatigue S-N curve of 2124-T851 aluminum alloy at high cycle fatigue condition were measured and fatigue fracture process and fractography were studied using optical microscopy (OM), X-ray diffraction (XRD) technique, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that at room temperature and R = 0.1 conditions, the characteristics of fatigue fracture could be observed. Under those conditions, the fatigue strength and the fracture toughness of a 2124-T851 thick plate is 243 MPa and 29.64 MPa · m^1/2, respectively. At high cycle fatigue condition, the higher the stress amplitude, the wider the space between fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area, and the greater the ratio between the intermittent fracture area and the whole fracture area.     

Fatigue crack propagation behavior of multiphase microstructure in a quenched and partitioned medium-carbon bainitic steel

A medium-carbon steel was treated by the bainitic isothermal transformation plus quenching and partitioning (B-QP) process to obtain bainite/martensite/retained austenite multiphase microstructure, and its fatigue crack propagation (FCP) behavior was evaluated in contrast with BAT (bainite austempering) sample with fully bainite microstructure. Results show that B-QP sample exhibits a lower FCP rate and higher fatigue threshold ΔK_th (12.6 MPa·m^1/2). Moreover, the FCP path of B-QP sample displays a strongly tortuosity and more crack branching due to more filmy retained austenite (7.2%) and higher percentage of high angle misoriented boundaries (68%). The larger crack tortuosity and the secondary cracks as result of crack branching are primarily responsible for the lower FCP rate of B-QP sample. In addition, the FCP rate curve of B-QP sample shows a pronounced small plateauing at the near-threshold zone, which can be ascribed to the mechanical twinning that occurred in the filmy retained austenite.     

Gigacycle fatigue properties of 1800 MPa class spring steels

Fatigue tests up to 10⁸ cycles were carried out for two spring steels (Heats A and D1) and one valve spring steel (Heat F) with tensile strength, σ _B, of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in Heats Dl and F were controlled. The surface‐type fracture occurred at shorter lives below 10⁶ cycles, while the fish‐eye‐type fracture occurred at longer lives. The fatigue limit, σ _W, at 10⁸ cycles was 640 MPa for Heats A and D1 and 700 MPa for Heat F. Al₂O₃ inclusions for Heat A and both TiN inclusions and matrix cracks, i.e. internal facets, for Heat F were observed at the fish‐eye‐type fracture sites, while only matrix cracks were observed for Heat Dl. ODA, i.e. optically dark area, which is considered to be related to hydrogen effects, were formed around Al₂O₃ and TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 573 K in high vacuum of 2 × 10^–6 Pa. The heat treatment eliminated matrix cracks for Heat D1 and the fatigue limit at 10⁸ cycles recovered to the estimated value of 920 MPa from the equation σ _w= 0.53 σ _B for the surface fracture. These results suggest that inclusions control and hydrogen influence the gigacycle fatigue properties for these high strength steels. In addition, it is expected that the creation of a martensite structure with a high resistance to hydrogen effects in the inclusion‐controlled steel could achieve the higher fatigue limit estimated for the surface‐type fracture.     

Rapid determination for fatigue parameters of AZ31B magnesium alloy based on evolution of temperature under high cyclic fatigue

Abstract

Based on the infrared thermography method, experiments are carried out to investigate the evolution of temperature field of the extruded AZ31B magnesium alloy specimens under high cyclic fatigue load. The experimental results show that the superficial temperature of specimen under cyclic fatigue load changes with the number of cycles. According to the characteristics of surface temperature change, we propose a formula to calculate the residual fatigue life using energy approach. The proposed formula to assess the fatigue parameters (fatigue limit, residual fatigue life, fatigue life and S–N curve) achieves good results for AZ31B magnesium alloy. Furthermore, the fatigue limits (Δσ_eSN?=?90·3 MPa) derived from the traditional method through 10⁷ cycles were compared with the values predicted by the infrared thermographic method (Δσ_eTM?=?87·3 MPa) and the energy approach (Δσ_eΦ?=?86·2 MPa), and the comparison results of percentage differences are 3·3 and 4·5% respectively.     

Hydrogen-enhanced cracking of 2205 duplex stainless steel

Tensile and fatigue crack growth tests of 2205 duplex stainless steel (DSS) were performed in laboratory air, gaseous hydrogen at 0.2 MPa and saturated H₂S solution. The longitudinal specimen showed a lesser degradation of tensile properties than the transverse ones in saturated H₂S solution. The orientation of specimens with respect to rolling direction had little influence on the fatigue crack growth rate (FCGR) of the alloy in air. Furthermore, 2205 duplex stainless steel was susceptible to hydrogen‐enhanced fatigue crack growth. Transmission electron micrographs, in addition to X‐ray diffraction, revealed that the strain‐induced austenite to martensite transformation occurred near the crack surface within a rather narrow depth. Fatigue fractography of the specimens tested in air showed mainly transgranular fatigue fracture with a small amount of flat facet fracture. Furthermore, extensive quasi‐cleavage fracture of 2205 duplex stainless steel was associated with the hydrogen‐enhanced crack growth.     

Shear mode threshold for a small fatigue crack in a bearing steel

The fatigue behaviour of small, semi‐elliptical surface cracks in a bearing steel was investigated under cyclic shear‐mode loading in ambient air. Fully reversed torsion was combined with a static axial compressive stress to obtain a stable shear‐mode crack growth in the longitudinal direction of cylindrical specimens. Non‐propagating cracks less than 1 mm in size were obtained (i) by decreasing the stress amplitude in tests using notched specimens and (ii) by using smooth specimens in constant stress amplitude tests. The threshold stress intensity factor ranges, ΔK_IIth and ΔK_IIIth, were estimated from the shape and dimensions of non‐propagating cracks. Wear on the crack faces was inferred by debris and also by changes in microstructure in the wake of crack tip. These effects resulted in a significant increase in the threshold value. The threshold value decreased with a decrease in crack size. No significant difference was observed between the values of ΔK_IIth and ΔK_IIIth.     

An effect of strength of railway axle steels on fatigue resistance under press fit

High-cycle fatigue tests with an evaluation of fatigue limit were carried out on large model components of bars with press fitted hubs of diameter 63/59 mm. Bars were made of three railway axle steels EA1N, EA4T and 34CrNiMo6 with considerable different strength from 586 MPa to 1041 MPa, respectively. Detection and measurement of crack growth under hubs by ultrasonic method was performed during the tests. In spite of the differences in strength and alloying of tested bars, differences in mean value of fatigue limit were not significant. This result was connected with specific damage mechanism and microcracks initiation under hubs with fretting effects. Short fatigue crack growth under hubs occurred at stress intensity factor range ΔK considerably bellow threshold value ΔK_th of long cracks. Simultaneous growth of main cracks from more than one point of surface circumferential area under hub was quite frequently observed.     

EFFECTS OF AIR AND INERT ENVIRONMENTS ON THE NEAR THRESHOLD FATIGUE CRACK GROWTH BEHAVIOR OF ALLOY 718

The near threshold fatigue crack growth behavior of alloy 718 was studied in air and helium environments at room temperature and at 538°C. Tests were performed at 100 Hz and at load ratios of 0.1 and 0.5. At room temperature and at 538°C, the ΔK_th values in helium were lower than in air. The ΔK_th values in air decreased with increasing load ratio. These results can be explained with a model that involves the accumulation of oxide in the crack which enhances crack closure. In the air tests, the oxide build-up on the fatigue fracture surfaces at ΔK_th was of the order of magnitude as the crack tip opening displacement. In the helium tests, no significant build-up of oxide on the fracture surface at threshold was found.     

High cycle fatigue mechanisms in a cast AM60B magnesium alloy

ABSTRACT We examine micromechanisms of fatigue crack initiation and growth in a cast AM60B magnesium alloy by relating dendrite cell size and porosity under different strain amplitudes in high cycle fatigue conditions. Fatigue cracks formed at casting pores within the specimen and near the surface, depending on the relative pore sizes. When the pore that initiated the fatigue crack decreased from approximately 110 µm to 80 µm, the fatigue life increased two times. After initiation, the fatigue cracks grew through two distinct stages before final overload specimen failure. At low maximum crack tip driving forces (K_max < 2.3 MPa√m), the fatigue crack propagated preferentially through the α‐Mg dendrite cells. At high maximum crack tip driving forces (K_max > 2.3 MPa√m), the fatigue crack propagated primarily through the β‐Al₁₂Mg₁₇ particle laden interdendritic regions. Based on these observations, any proposed mechanism‐based fatigue model for cast Mg alloys must incorporate the change in growth mechanisms for different applied maximum stress intensity factors, in addition to the effect of pore size on the propensity to form a fatigue crack.     

Abnormal da/dN – ΔK curves: New failure mode with Ti – alloys

Fatigue crack propagation (FCP) -rates and -threshold values have been determined on the titanium alloys IMI 834, IMI 685 (turbine disk materials) and Ti-6Al-6V-2Sn (plate material). K_max-constant tests were executed in laboratory air at room temperature and run with 50 Hz on C(T) specimens. It was found that FCP-rates in K_max-constant tests followed the well known FCP behavior up to a certain limiting value K_max, denoted as °K_max. Below °K_max, the FCP-rates da/dN decrease with decreasing ΔK down to the threshold value ΔK_T (ΔK for 10^?7 mm/cycle). For K_max-constant tests with K_max > °K_max, the FCP-rates initially decreased with decreasing ΔK, but reached 10^?7 mm/cycle at smaller ΔK. For K_max ≧ ∧K_max > °K_max, FCP-rates of 10^?7 mm/cycle were never reached as ΔK decreased to and below ΔK_T. Instead, as ΔK approaches or gets smaller than ΔK_T, the FCP-rates stay either constant or increase again. The limit value °K_max for this abnormal FCP-behavior had been determined for IMI 834 to be 22 to 28 MPa√m, for IMI 685 to be 46 MPa√m and for Ti-6A1–6V-2Sn to be 26 MPa√m. The important result from a practical stand-point is the large difference in °K_max for comparable Ti alloys, i.e., IMI 834 and IMI 685.     

Low and high‐cycle fatigue properties of an ultrahigh‐strength TRIP bainitic steel

The scope of this study is to characterize the mechanical properties of a novel Transformation‐Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low‐cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 10² and 10⁵ fatigue cycles. The data are used to determine the parameters of the Coffin–Manson as well as the cyclic stress–strain curve. No significant stress‐induced austenite transformation is detected. The high‐cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 10⁶ fatigue cycles at two loading ratios, namely R = ?1 and R = 0. At fatigue lives longer than 2 × 10⁵ cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions.     

Pit-to-crack transition under cyclic loading in 12% Cr steam turbine blade steel

Fatigue measurements were performed up to the very high cycle fatigue regime in order to investigate pit-to-crack transition in 12% Cr steam turbine blade steel. Pre-pitted and smooth specimens were tested in air and aerated 6 ppm Cl⁻ solution. S–N curves for different stress ratios were determined and a stress ratio dependent critical pit size for pit-to-crack transition with subsequent failure was found. Early crack initiation and small crack growth were observed in the process of development and by fractography using field emission scanning electron microscopy. Fatigue crack growth rates (FCGR) for cracks emanating from pits were determined. Good similarity of FCGR curves for short and long cracks was obtained by normalising the stress intensity factor ranges with the threshold values.     

Fatigue crack grovvth in fibre reinforced titanium MMC laminate at room and elevated temperatures

Abstract

The fatigue crack growth resistance of a [0/90°]_2S Ti-6Al-4V (wt-%) SCS-6 cross ply laminate has been assessed as a function of varying the initial nominal stress intensity factor range (?K), the test temperature, and the environment. In all cases, through thickness cracks have been grown from unbridged defects. Fatigue crack growth rates are higher at elevated temperatures of 300 and 450° C in air. However, tests carried out at a temperature of up to 450° C in vacuum have shown that crack arrest conditions are similar to those observed from specimens studied at room temperature and at a temperature of 300° C in air. In these cases, initial ?K_ini transition values between fatigue crack arrest and eventual specimen catastrophic failure are close to 10 MPa m^1/2. In contrast, at a temperature of 450°C in air, even for tests performed at a frequency of 10 Hz, the limiting value of initial ?K_ini to give crack arrest is less than 6 MPa m^1/2. This has been attributed to the action of an aggressive environment, and particularly to the attack of the carbon coating layers. In addition, correlations have been found between fibre pull out lengths and changes in both temperature and environment; these are negligible after tests at 450°C in air. Finally, for such composites, sudden increases in fatigue crack growth rates have been attributed unequivocally to the failure of bridging fi bres, which were detected using acoustic emission.