Near-threshold Crack Growth Rates Research Articles

ANALYSIS OF REDUCING THE GROWTH RATE OF FATIGUE CRACKS FROM OVERLOADS OF VARIOUS VALUE AND CHARACTER IN ALUMINUM ALLOY 2024-T3

This work highlights the results of tests with overloads and underloads on aluminum samples in the region of near-threshold crack growth rates. Crack growth curves and fatigue life values were obtained that the classical fatigue life prediction models of Elber, Barsom, Wheeler and Willenborg could not explain. Fractographic analysis verified the correctness of the fatigue fracture curves. The chemical analysis of the oxygen content on the fractured surfaces of the sample by the method of energy dispersive X-ray spectroscopy showed the possibility of a relationship between the weight fraction of active particles and residual stresses in the material and confirmed some of the provisions of the theory of brittle microfracture on the effect of oxygen and hydrogen on fatigue crack propagation.

Fatigue crack growth parameters of Laser Powder Bed Fusion produced Ti-6Al-4V

The value of fatigue predictive NASGRO models hinges on the quality of established parameters to describe the behaviour of the subject material. To establish these parameters for laser powder bed fusion (LPBF) produced Ti-6Al-4V the influence of process inherent microstructure, residual stress, and orientational dependant mesostructure needs to be considered. Examining near-threshold in combination with steady-state fatigue crack growth rates (FCGR's) offers insight into the impact of crack closure in the NASGRO formulation. This study investigates the influence of microstructure, residual stress and orientation of LPBF produced Ti-6Al-4V on establishing parameters for NASGRO models from a foundational perspective. Near-threshold crack growth rate data from Compact Tension (CT) coupons built in three orientations and as-fabricated (AF), stress-relieved (SR) and duplex annealed (DA) state are considered. Optical and scanning electron micrographs are used to consider morphological features. Results show anisotropy in material constraint, used in describing crack closure, between orientations in AF and SR state, lending to unique consideration for each orientation. DA is shown to relieve this effect. Overall, this study establishes and demonstrates the importance of uniquely considering NASGRO parameters for LPBF produced Ti-6Al-4V.

Effect of Overload on the Near-Threshold Fatigue Crack Growth Rate in a 2024-T3 Aluminum Alloy: III. Analysis of the Efficiency of the Fatigue Crack Growth Models Used to Predict the Fatigue Life under Alternating Loading

The experimental data on the fatigue lives of crack growth that were obtained in tests of aluminum specimens with overloads and underloads in the region of near-threshold crack growth rates are compared. The fatigue crack growth time is predicted using various models, including those embedded in the NASGRO and FASTRAN software packages, in order to reveal the model that is the best for calculating the fatigue life. The results obtained are compared with the prediction of a proposed combined model, which takes into account crack closure and the local stresses in the crack mouth and is based on the Neuber and Ramberg–Osgood equations. The efficiencies of the existing and proposed models used to describe the growth of fatigue cracks during random loading are estimated.

Effect of Overload on the Near-Threshold Fatigue Crack Growth Rate in a 2024-T3 Aluminum Alloy: II. Fatigue Crack Growth Simulation for Calculating the Fatigue Life under Alternating Loading

The results of overloading and underloading tests of aluminum specimens in near-threshold crack growth rate range are considered. A combined model is proposed to take into account crack closure and the influence of the local stresses at the crack mouth. The model is based on the Neuber and Ramberg–Osgood equations and can be used to estimate the fatigue life of crack growth in the near-threshold range. This model is grounded from a physical standpoint.

Effect of Overload on the Near-Threshold Fatigue Crack Growth Rate in a 2024-T3 Aluminum Alloy: I. Effect of the Character, the Magnitude, and the Sequence of Overload on the Fatigue Crack Growth Rate

The results of overload and underload tests on aluminum specimens in the near-threshold crack growth rate region are discussed. These tests are performed to choose a model for predicting the fatigue life. The influence of the character and the magnitude of overload and underload on the fatigue crack growth time is analyzed. The obtained crack growth curves and the fatigue lives cannot be explained in terms of classical models, which makes it necessary to develop a new model.

Combined action of crack closure and residual stress under periodic overloads: A fractographic analysis

The close relationship between sequence-sensitive near-tip residual stress and threshold stress intensity raises questions about load interaction models currently in use to estimate fatigue crack growth under variable amplitude loading. In an attempt to address them, experiments were performed on an Al–Cu alloy under specially designed load sequences with periodic overloads. Fractographic evidence from these tests confirms that fatigue crack closure, together with sequence sensitive variation in threshold stress intensity appear to explain all observed results. The fractographic data provide quantitative inputs for improved modeling of variable-amplitude fatigue, particularly at near-threshold crack growth rates. This study appears to suggest that conventional approaches based on the Wheeler and Willenborg residual stress models can provide reasonable estimates only by coincidence. They model the wrong parameter at lower fatigue crack growth rates and may simply not be valid at other growth rates.

The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy

The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material, alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques; these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, ΔKeff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection around SiC particles.

Micromechanism of accelerated near-threshold fatigue crack growth in Al2O3/Al-Cu composite at elevated temperature

Fatigue crack growth behavior of a peak-aged Al2O3/Al-Cu composite was examined at 150 °C and compared to the behavior at room temperature (RT). At 150 °C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach fatigue threshold, crack growth rates at 150 °C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the overaged composite.

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)* was investigated. The investigation also examined the influence of heat treatment on resultant microstructures and the near-threshold fatigue crack growth behavior. In addition, the influence of load ratios(R), material strength, and grain size on fatigue threshold was studied. Compact tension specimens prepared from Incoloy 825 with transverse-longitudinal (TL) orientation in the as-received, as well as two different heat treated conditions, were used. The heat treatment studies revealed a peak hardness condition after solution treatment at 1200 °C for 1/2 hr, followed by aging at 600 °C for 434 hr. Among all the heat treated conditions, the fatigue threshold was the highest and the near-threshold crack growth rate was lowest in this peak aged condition. Fatigue threshold values were observed to decrease with an increase in load ratio, whereas an increased grain diameter resulted in a higher fatigue threshold. An earlier mathematical model was found applicable to characterize the relationship between load ratio and fatigue threshold. Preferential etching of grain boundary suggests formation of a thin film of carbide precipitation along the grain boundary region in the aged specimens. This carbide precipitation facilitated intergranular crack growth in these samples, resulting in higher roughness-induced crack closure. The highest fatigue threshold in the peak aged condition can be attributed to this large roughness-induced crack closure process.

The effect of stress ratio on the near-threshold fatigue crack growth behavior of Ti-8A1-1Mo-1V at elevated temperature

An investigation of the effects of stress ratio and temperature on the near-threshold fatigue crack growth characteristics of Ti-8Al-1Mo-1V was performed. Computer-controlled decreasing stress intensity range tests using compact tension specimens were conducted for various stress ratios at temperatures of 76°F (24°C) and 500°F (260°C) in laboratory air. Crack opening load was determined throughout the tests based on automated compliance measurements and was used to adjust the fatigue crack growth rate data to effective stress intensity range ( ΔK eff ). The effects of stress ratio at a constant temperature were able to be explained by the concept of crack closure and ΔK eff . However, crack closure was not able to account for the effects of temperature at a fixed stress ratio of 0.1, as higher near-threshold crack growth rates were observed at 500°F (260°C) when the data were plotted as a function of ΔK eff . This difference in crack growth rate is believed to be attributable to significant crack front branching and secondary cracking as a result of the increased oxidation at 500°F (260°C). A unique value of fatigue crack growth rate threshold, ΔK th was observed for all of the test conditions when crack closure was accounted for in the data analysis.

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.

The Influence of Load Ratio and Temperature on the Near-Threshold Fatigue Crack Growth Rate Properties of Pressure Vessel Steels

Near-threshold fatigue crack propagation behavior was examined in four pressure vessel steels (SA508 Cl 2a, SA533 Gr A Cl 2, SA508 Cl 3a, and SA533 Gr B Cl 2) and two submerged arc weldments (SA508 Cl 2a and SA533 Gr A Cl 2). At a given temperature and load ratio, near-threshold crack growth rates in the four base materials and two submerged arc weldments were essentially identical. Increasing the load ratio from 0.20 to 0.50 increased the rate of near-threshold crack propagation at a given temperature. At a fixed load ratio, increasing the temperature from 24 to 288°C (75 to 500°F) increased the rate of near-threshold crack growth. The effects of load ratio and temperature on the near-threshold crack growth behavior of pressure vessel steels can be explained based on a crack closure concept.

Fatigue crack growth threshold at cryogenic temperatures: A review

The cryogenic temperature near-threshold fatigue crack growth behavior of several structural materials: aluminum alloys, copper, steels, nickel alloys and titanium alloys, was reviewed. It was observed that the resistance to near-threshold fatigue crack propagation in the alloy systems investigated generally improved with decreasing temperature. Environmental effects were not responsible for the influence of temperature on near-threshold crack growth rate behavior. Furthermore, crack closure alone could not account for this temperature effect. The dislocation dynamics model appears to offer a possible rationale to explain the improved near-threshold crack growth performance typically exhibited at cryogenic temperatures by the variety of materials examined herein. Crack closure, on the other hand, rationalized the influence of load ratio on low temperature near-threshold crack propagation behavior.

Effects of corrosive environments on near-threshold fatigue crack growth behavior of T1-6A1-4V

The near-threshold fatigue crack growth behavior of Ti-6A1-4V alloy has been investigated in low O 2 steam (< 1 ppm), high O 2 steam (40ppm), and boiling water with various concentrations of Nad and/or Na 22SO 4. At load ratio ( R) of 0.5, high O 2 steam increased the crack propagation rates in the threshold region, relative to low O 2 steam. However, at R = 0.8, the near-threshold crack growth rates in low and high O 2 steam were comparable. Values of threshold stress intensity range, ΔK th , slightly increased with an increase in the concentration of NaCl in the solution. Varying solution pH from 5.0 to 10.0 in a 0.1 g NaCl plus 0.1 g Na 2SO 4 per 100ml H 2O solution had no effect on the rates of near-threshold crack propagation. Increasing the hydrazine level from 30 to 10 7 ppb in the same salt solution also did not change the resistance to crack growth. Comparing the present results with the previous data on 403 stainless steel, the near-threshold crack propagation rate performance in Ti-6Al-4V alloy is superior to that in 403 steel in both the steam and salt solution environments.

Effects of load ratio and temperature on the near-threshold fatigue crack propagation behavior in a CrMoV steel

The influence of temperature in the range of 24 to 260 °C and load ratio on the near-threshold fatigue crack growth rate behavior of a CrMoV steel was characterized. At all temperatures investigated, the threshold stress intensity range, ΔKth, for fatigue crack growth decreased with increasing load ratio. The near-threshold crack growth rates increased significantly at 149 °C when compared with the rates at room temperature. However, the crack growth rates at 260 °C were comparable to those at 149 °C. These observations are rationalized in terms of the concepts of roughness and oxide-induced crack closure. Extensive fracture surface characterization using SEM, oxide thickness measurements by Auger spectroscopy, and roughness measurements by light-section-microscopy were conducted to substantiate the explanations.

Influence of corrosion deposits on near-threshold fatigue crack growth behavior in 2xxx and 7xxx series aluminum alloys

The role of corrosion deposits in influencing the near-threshold fatigue crack propagation behavior of 2XXX and 7XXX series aluminum alloys is examined in detail. The composition, thickness, and distribution of fracture surface oxide films are characterized with the aid of X-ray photoelectron spectroscopy, scanning Auger spectroscopy, and secondary ion mass spectroscopy analyses. It is found that the extent of crack closure due to corrosion debris in aluminum alloys is strongly dependent on the composition and aging treatment. The results suggest that environmentally-influenced near-threshold crack propagation in some aluminum alloys is controlled by twoconcurrent andmutually-compctitive mechanistic processes: a dominant role of crack closure due to corrosion deposits (which tends toarrest completely the near-threshold crack) and a strong embrittling effect (which considerablyincreases near-threshold crack growth rates) concomitantly with the crack tip oxidation phenomenon in the moist medium. The near-threshold corrosion fatigue characteristics of aluminum alloys are contrasted with those in a wide range of steels in order to gain an insight into the various

Some considerations on fatigue crack closure at near-threshold stress intensities due to fracture surface morphology

It is noted that at near-threshold levels, in addition to the role of plasticity-and oxide-induced crackclosure, fracture surface roughness or morphology may promote significant closure effects in plane strain, as similarly noted by Minakawa and McEvily.This is considered to result from the fact that, where maximum plastic zones sizes are small compared to the grain size, fatigue crack growth proceeds by a single shear decohesion mechanism (Stage I) with associated Mode II+I displacements. The resulting serrated or faceted fracture surfaces (“microstructurally-sensitive growth”) coupled with Mode II crack tip displacements thus induce high closure loads (i.e., K cl/K max ~0.5) by wedging the crack open at discrete contact points. At higher growth rates where the plastic zone encompasses many grains, striation growthvia alternating or simultaneous shear mechanisms (Stage II) produces a more planar fracture surface, with pure Mode I displacements, and a corresponding reduction in closure loads. Such concepts of roughness-induced closure are shown to be consistent with observations of the role of coarse grain sizes in reducing near-threshold crack growth rates at low load ratios and of the absence of this effect at high load ratios.

SURFACE DAMAGE AND NEAR-THRESHOLD FATIGUE CRACK GROWTH IN A Ni-BASE SUPERALLOY IN VACUUM

— Fatigue crack growth rate tests have been performed on Nimonic AP1, a powder formed Ni-base superalloy, in air and vacuum at room temperature. These show that threshold values are higher, and near-threshold (faceted) crack growth rates are lower, in vacuum than in air, although at high growth rates, in the “structure-insensitive” regime, R-ratio and a dilute environment have little effect. Changing the R-ratio from 0.1 to 0.5 in vacuum does not alter near-threshold crack growth rates very much, despite more extensive secondary cracking being noticeable at R= 0.5. In vacuum, rewelding occurs at contact points across the crack as ΔK falls. This leads to the production of extensive fracture surface damage and bulky fretting debris, and is thought to be a significant contributory factor to the observed increase in threshold values.

The influence of environment and stress ratio on fatigue crack growth at near threshold stress intensities in low-alloy steels

Fatigue crack propagation at low stress intensities has been studied in two low alloy steels in a variety of environments with particular emphasis being placed on the influence of stress ratio and strength level. It was found that fatigue crack growth rates are lower and threshold stress intensities ( ΔK 0 ) are higher in vacuum than in humid, laboratory air but, in dry gaseous environments (argon, hydrogen and air) and at low stress ratio ( R ~ 0.1), crack growth rates are faster and ΔK 0 values are lower than in laboratory air. However, the influence of stress ratio is considerably greater in laboratory air than in dry gaseous environments with the result that, at high stress ratio ( R ~ 0.8) ΔK 0 values are similar in all environments examined. Increasing material strength level resulted in higher, near-threshold crack growth rates and a reduction in ΔK 0 in both dry and humid air environments. The results are discussed in terms of the influence of crack closure and environmental effects on fatigue crack growth behaviour. The importance of corrosion debris produced in fatigue cracks at low stress intensities is also discussed.