Showing papers on "Paris' law published in 1982"

Some Considerations on Fatigue Crack Closure at Near-Threshold Stress Intensities Due to Fracture Surface Morphology

Abstract: 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.

Prediction of Fatigue Crack Growth under Variable-Amplitude and Spectrum Loading Using a Closure Model

Abstract: The present paper is concerned with the application of an analytical crack-closure model in the study of crack growth under various load histories. The model was based on a concept like the Dugdale model, but modified to leave plastically deformed material in the wake of the advancing crack tip. The model was used to correlate crack growth rates under constant-amplitude loading, and to predict crack growth under variable-amplitude and aircraft-spectrum loading on 2219-T851 aluminum alloy sheet material. Predicted crack-growth lives agreed well with experimental data. For 80 crack growth tests subjected to various load histories, the ratio of predicted-to-experimental lives, N P /N T , ranged from 0.5 to 1.8. The mean value of N P /N T was 0.97 and the standard deviation was 0.27.

Stress Intensity Factors, Crack Profiles, and Fatigue Crack Growth Rates in Residual Stress Fields

Abstract: A linear elastic fracture mechanics approach to crack growth rate prediction implies the need to calculate accurate, effective stress intensity (K) factors, and hence effective R-values, (K m i n /K m a x ), for components containing residual stress. To this end the weight function and associated superposition techniques are described, with emphasis on stress intensity and crack shape prediction for residual stress problems. Stress intensity factors are presented for various geometries with residual stress fields. The nonlinear, crack surface 'overlapping' effect is noted, and the case of cracks emanating from notches in residual stress fields is shown to be an associated problem. The application of such results in crack growth rate prediction is addressed. The characteristic crack growth rate features of several different loading systems are predicted, and shown to agree with available experimental data. Finally, the qualitative changes in the form of standard S-N curves for welded details are predicted, and shown to conform with limited available S-N curve experimental data.

Mixed‐mode fatigue thresholds

Abstract: — Near threshold fatigue crack growth under mixed-mode loading and elastic plane-strain conditions has been studied in 316 stainless steel in laboratory air at room temperature. Particular emphasis was placed on the influence of the mode II component. Crack growth from the starter crack, although initially coplanar, branches to be perpendicular to the maximum normal stress. However the threshold for the branched crack growth is controlled not only by mode I displacement, but also by the mode II component. Upper and lower bound curves are obtained for the threshold condition and discussed in terms of crack tip reversed plastic deformation, crack surface rubbing and oxide-induced closure. A theoretical method for predicting the lower bound curve is proposed and compared with the maximum normal stress and strain energy density criteria. The new theory shows the best agreement with experimental results, giving a safe prediction for design purposes.

Effects of Residual Stress on Fatigue Crack Propagation

Abstract: Experimental results on the effects of compressive and tensile residual stresses on Mode I fatigue crack growth are briefly reviewed. Prediction methods that attempt to account for the observed effects are compared. Current limitations of the methods and their relative advantages and drawbacks for use in design analysis are discussed. The possible role of residual stress re-equilibration on growth behavior, caused by crack extension itself, is also discussed.

The influence of a duplex microstructure in steels on fatigue crack growth in the near-threshold region

Abstract: The influence of a duplex microstructure on fatigue crack growth in the near threshold region has been studied for AISI types 1018, 1045, and 10B35, as well as 225 Cr-1 Mo steels For a duplex microstructure which consists of a continuous martensitic phase encapsulating ferrite, an increase in threshold level and yield strength in both the AISI 1018 and 225 Cr-1 Mo steels was observed However, with increase in carbon content and consequent decrease in the volume of ferrite, the threshold levels were not as significantly affected, although the yield strengths were higher

Mode III Fatigue Crack Propagation in Low Alloy Steel

Abstract: To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 104 cycles at nominal yield to 106 cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity(△K III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTDIII per cycle.

Near-threshold fatigue crack growth behavior in copper

Abstract: Near-threshold fatigue crack growth rate data were developed in annealed, quarter-hard, and full-hard copper at various load ratios, (R = σmin/σmax). Increasing theR value decreases the resistance to threshold crack growth. At a fixed value ofR, annealed copper has the slowest near-threshold crack propagation rate while full-hard copper has the fastest crack growth rate. Waveform (sine and triangle) and specimen geometry (WOL, CT, and CCT) do not appear to affect the rates of near-threshold crack propagation. The influences of load ratio and material strength on threshold crack growth behavior can be rationalized by crack closure.

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

Abstract: 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

Fatigue Reliability: Variable Amplitude Loading

Abstract: Models which designers use to predict fatigue under variable amplitude and random loading are summarized. Basic terminology associated with random process theory is presented. Models for computing fatigue damage associated with various statistical distributrions of stress ranges are reviewed. The Palmgren-Miner rule is described and statistical summaries which provide a description of the performance of Miner's rule is presented. A closed form expression for fatigue damage under wide band stress spectra is given. Finally, methods of predicting fatigue crack growth under variable amplitude stresses are reviewed.

Effects of grain size and microstructure on threshold values and near threshold crack growth in powder-formed Ni-base superalloy

Abstract: Threshold stress intensity values, ranging from ∼6 to 16 MN m −3/2 can be obtained in powder-formed Nimonic AP1 by changing the microstructure. The threshold and low crack growth rate behaviour at room temperature of a number of widely differing API microstructures, with both ‘necklace’ and fully recrystallized grain structures of various sizes and uniform and bimodal γ′-distributions, have been investigated. The results indicate that grain size is an important microstructural parameter which can control threshold behaviour, with the value of threshold stress intensity increasing with increasing grain size, but that the γ′-distribution is also important. In this Ni-base alloy, as in many others, near threshold fatigue crack growth occurs in a crystallographic manner along {111} planes. This is due to the development of a dislocation structure involving persistent slip bands on {111} planes in the plastic zone, caused by the presence of ordered shearable precipitates in the microstructure. However, as the stress intensity range is increased, a striated growth mode takes over. The results presented show that this transition from faceted to striated growth is associated with a sudden increase in crack propagation rate and occurs when the size of the reverse plastic zone at the crack tip becomes equal to the grain size, independent of any other microstructural variables.

Surface damage and near-threshold fatigue crack growth in a Ni-base superalloy in vacuum

Abstract: — 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.

High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939

Abstract: The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz.

A comparison of the strain intensity and cyclic j approaches to crack growth

Abstract: Two parameters describing the growth of fatigue cracks are compared. They are the cyclic J integral ΔJ and the strain intensity expressed as an equivalent stress intensity ΔKeq-. By referring to cyclic stress-strain data obtained from hysteresis loops in high strength ferritic steels at room temperature and austenitic and ferritic steels at elevated temperature it is shown that: (i) for short cracks the parameters are simply related and (ii) both parameters adequately link fatigue crack growth rates observed in the separate high strain fatigue (HSF) and linear elastic fracture mechanics (LEFM) regimes. Correction factors for thumbnail cracks and the conditions under which the relations need further modification are discussed.

Effect of Environment on Fatigue and Creep Crack Growth in Inconel X-750 at Elevated Temperature

Abstract: The fatigue crack growth rates (FCGR) of Inconel X-750 were measured in air and in vacuum at 25 °C and 650 °C as a function of test frequency. The wave shape was triangular and the frequency varied from 10 Hz to 0.01 Hz. The creep crack growth rates (CCGR) were also measured on single edge notch specimens at 650 °C in air and in purified argon. For a givenAK, the FCGR increases when temperature increases and frequency decreases. At low frequency the FCGR approach the creep crack growth rates. The mode of fracture changes from transgranular at 10 Hz to intergranular at 0.01 Hz. The effect of air environment is to accelerate the transition from transgranular to intergranular fracture modes with decreasing frequency. The role of oxidation in accelerating crack growth rate in fatigue and in creep is discussed in detail.

Fatigue fracture of polycarbonate

Abstract: The fatigue fracture process in polycarbonate can be different for cracks grown from notches and from surface crazes During the discontinuous crack growth process (which occurs during the very slow crack growth regime), full-width cracks grown from notches have a very different crack tip plastic zone when compared to that observed in localized cracks grown from surface crazes These localized surface cracks have a pair of sharply defined shear bands at the crack tip (forming the “epsilon” plastic zone) that are absent from notched-grown cracks, The presence of the shear band pair in this plastic zone produces two major modifications The discontinuous crack growth is extended to higher stress intensity ranges due to the (crack tip) craze stabilization by the shear band pair Moreover, a shear fatigue fracture mode, absent in the notched specimens, occurs upon termination of the discontinuous growth process The implications of this comparison are that estimates of fatigue lifetimes of smooth samples based on fatigue crack growth data (on notched specimens) may be erroneous

Constant and variable amplitude fatigue behavior and fracture of A572 steel at 25°C(77°F) and −45°C(−50°F)

Abstract: Constant amplitude fatigue crack propagation behavior and variable amplitude fatigue crack initiation and propagation behavior and R-curves were obtained at room temperature and −45°C(−50°F) for hot-rolled ASTM A572 steel. Both fatigue crack initiation and propagation resistance were better at the cold temperature. SEM fractographic analysis revealed little difference in the fatigue crack growth modes with ductile striations and secondary cracks occurring at both temperatures. Final R-curve and fatigue fracture surfaces, however, consisted of ductile dimples at room temperatures and transcrystalline cleavage at −45°C(−50°F). The hot-rolled A572 steel was found suitable for −45°C(−50°F) operating temperature despite the fact that this cold temperature is in the lower shelf CVN energy region. Fatigue design based upon room temperature conditions appears to be reasonable for this steel down to −45°C(−50°F).