Effects of gaseous hydrogen on fatigue crack growth in pipeline steel

The effects of hydrogen on crack growth rates in a moderate-strength pipeline steel subjected to cyclic loads were studied. Fatigue crack growth experiments were conducted in high-pressure hydrogen and nitrogen environments, and the influences of stress ratio, stress intensity, and cyclic loading frequency on hydrogen-accelerated fatigue crack growth were investigated. Hydrogen acceleration of intermediate-rate (Stage II) crack growth was greatest at low stress ratios and decreased to approximately zero at a stress ratio of about 0.5. However, hydrogen promoted the premature onset of accelerated (Stage III) crack growth. This appeared to be related to a hydrogen-induced reduction of fracture toughnessJ _IC.     

Effects of environment on high-temperature fatigue crack growth in a superalloy

Fatigue crack growth rate measurements were made on Inconel alloy 718 samples at 650°C in fourteen different gaseous environments. Minor amounts of either oxygen or sulfur bearing species in the environment produced large increases in crack growth rates. Aggressive environments promoted intergranular crack growth. Kinetic factors rather than thermodynamic ones appear to be the variables dominating the effects of an environment on crack growth. Environments that markedly increased the crack growth rate did not produce significant corrosion attack on unstressed samples. Thus conventional high temperature corrosion tests may not be useful for predicting service performance of stressed components.     

Influence of inclusion content on fatigue crack

Fatigue crack growth rates were measured at room temperature in dry air for three 7075-T6 aluminum alloys with different inclusion content. Volume fractions of inclusions were determined for each alloy by the point count method with two different automated systems. Plots of the fatigue crack growth rate (da/dN) vs the stress-intensity-factor range (ΔK) show a well defined change of slope at the transition between plane strain and plane stress fracture. This transition is associated with a marked increase in the amount of fracture by void growth around inclusions. The volume fraction and mean spacing of voids within the cyclic plastic zone have been determined as a function of ΔK by quantitative fractography. Fracture by voids is important when the mean spacing of such voids is approximately equal to the width of the cyclic plastic zone in the plane of the crack. It is concluded that the inclusion content increases the fatigue-crack growth rates only within the plane stress range, that is for values of the stress-intensity-factor range ΔK \s> 20 kpsi√in.     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.     

Effects of frequency and temperature on short fatigue crack growth in aqueous environments

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m^1/2) in deaerated deionized water and 0.3 M Na₂SO₄ solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na₂SO₄ solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.     

Effects of interfacial strength on fatigue crack growth in a fiberreinforced Ti-alloy composite

The fatigue crack growth behavior of a Ti-6A1-4V composite with boron fibers was previously studied in the as-received and thermally exposed conditions. Fracture strengths of the composite, fiber, and interface were characterized together with fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fiber properties as input, a recently proposed model was exercised to elucidate the effects of interfacial strength on crack growth rates in the composite. Comparison of experimental results with model calculations revealed that a weak fiber/matrix interface combined with a strong, high-modulus fiber led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Visual simulation of fatigue crack growth

An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an “ear” profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state I and II crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.     

Near-threshold fatigue crack growth behavior in copper

Near-threshold fatigue crack growth rate data were developed in annealed, quarter-hard, and full-hard copper at various load ratios, (R = σ_min/σ_max). 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.     

Near-threshold fatigue crack growth behavior in metals

Near-threshold fatigue crack growth rate (FCGR) behavior in six alloy systems (iron, aluminum, copper, magnesium, nickel and titanium) is extensively reviewed and compared. It is suggested that a unique effective threshold stress intensity range, ΔK_th,eff, exists for each alloy system in room temperature air. The value of ΔK_th,eff was found to be directly proportional to Young's modulus, E Furthermore, Young's modulus normalizes the near-threshold fatigue crack propagation behavior in the various alloy systems investigated. The present findings are consistent with the existing threshold FCGR theories that relate ΔK_th,eff to E. Additional research is required, however, to incorporate crack closure phenomena in these near-threshold FCGR theories to assess the influence of microstructure, load ratio and environment.     

Jumplike fatigue crack growth in compressor blades

It is shown that power relations between the two main fractographic characteristics of fracture surfaces forming during jumplike fatigue crack growth, namely, the crack depth and the corresponding crack front length, can be used to estimate the fracture stress during vibration tests of the compressor blades of an aviation gas turbine engine, which are made of VT3-1 titanium alloy.     

The influence of crack length on fatigue crack growth in deep sharp notches

The fatigue crack growth rateda/dN of short cracks and the transition to long crack behavior were investigated for ARMCO-iron. Deep notched specimens with very small notch radius (between 1.5 and 4 μm) were used. The experiments were performed with constant stress intensity ranges for various stress ratios; the fatigue crack growth rate was measured as a function of the crack length. The results permit a discussion of the mechanisms responsible for the different behavior of “short” and “long” cracks.     

Local yielding attending fatigue crack growth

Fatigue crack growth rate measurements were performed at 100°C on an Fe-3Si steel in three thickness conditions and at different ΔK-levels. The test pieces were subsequently sectioned and etched to reveal the plastic deformation attending crack growth both on the surface and in the interior. Unlike preceding studies, the Fe-3Si steel displayed classical cyclic crack growth: well-defined fatigue striations with a spacing close to the per-cycle growth rate, and essentially the same growth rates that have been reported for low and medium strength steels. A highly strained region, approximately one-fifth the size of the monotonie plastic zone, is identified as the cyclic plastic zone. On this basis three regions with distinct cyclic strain histories that precede the crack are identified: a microstrain region wherein the material receives ∼10³ to 10⁴ strain cycles in the range 0 < Δε _P ≲ 10^-3; a cyclic plastic zone corresponding to ∼200 cycles in the range 10^-3 < Δ∈ _P ≲ 10^-1, and a COD-affected zone that receives ∼10 strain cycles in the range 10^-1 ≲ Δ∈ _P ≲ 1. It is suggested that the damage associated with the instabilities in the fatigue substructure to overstrain contribute to the growth mechanism.     

Effects of temperature on the fatigue crack growth behavior of cast gamma-based titanium aluminides

This article reports the results of an experimental study of the effects of temperature (25 °C, 450 °C, and 700 °C) on the fatigue crack growth behavior of three near-commercial cast gamma titanium aluminide alloys (Ti-48Al-2Cr-2Nb, Ti-47Al-2Mn-2Nb+0.8 pct TiB₂, and Ti-45Al-2Mn-2Nb+0.8 pct TiB₂). The trends in the fatigue crack growth rate data are explained by considering the combined effects of crack-tip deformation mechanisms and oxide-induced crack closure. Faster fatigue crack growth rates at 450 °C are attributed to the high incidence of irreversible deformation-induced twinning, while slower crack growth rates at 700 °C are due to increased deformation by slip and the effects of oxide-induced crack closure.     

Shear-driven high-temperature fatigue crack growth in polycrystalline alumina

High-temperature crack growth behavior of a polycrystalline alumina was examined under Mode-I tension-tension cyclic loading. Locally at the crack tip, the fatigue crack was found to advance is shear by fractional sliding of grains on alternating sets of planes of the maximum shear. Evidence of a shear-driven crack growth was given in terms of topological and morphological analyses of the fatigue crack surface, grain sliding, frictional debris, and temperature-dependence of fatigue crack growth kinetics. Based on experimental observations, a new model of fatigue crack growth by alternating shear was proposed.     

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional ΔK _eff (=K _max —K _op) fails to correlate the test data and the modified ΔK eff (=K _max - _χK_op, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size.