Oxide-induced Crack Closure Research Articles

Short fatigue crack growth mechanism in Ni-Co based superalloy at elevated temperatures and in oxidative atmospheres

In this study, to elucidate the oxidation effect on the short fatigue crack growth (SFCG) characteristics of Ni-Co based TMW-4M3 superalloy, fatigue tests were conducted at room/elevated temperature in air/vacuum and three-dimensional microscopic observation of the SFCs using a plasma focused ion beam – scanning electron microscope (PFIB – SEM) system. Fatigue lives tested under vacuum at elevated temperature were comparable to those at room temperature while those tested at elevated temperatures in air showed shorter fatigue life in higher stress regions and longer fatigue life in lower stress regions than the others. In situ observation of SFCs at elevated temperatures in air revealed SFCG deceleration in the small ΔK regions and acceleration in large ΔK regions. SFCs opening/closing behaviours at elevated temperatures measured by digital image correlation (DIC) showed the crack opening stress to be higher at elevated temperature in air, possibly due to oxide-induced crack closure. However, the crack closure effect did not fully explain the difference in FCG rate between room and elevated temperatures in air. Three-dimensional investigations revealed SFCs to form at elevated temperatures in air, showing straight transgranular FCG to be insensitive to microstructure in slow growth regions, and intergranular FCG to precede that in the surrounding material in fast growth regions, in contrast to microstructural SFCs features at room temperature. It appears that slow and straight unique SFCG at elevated temperatures might occur due to intermittent brittle fracture of oxides formed at the crack tip. This suggests a possible role of the oxide layer at the crack tip in broadening the plastic deformation distribution, thus contributing to FCGR deceleration.

Fatigue crack tip corrosion processes and oxide induced closure

This paper reviews the corrosion and hydrolysis reactions that occur inside a fatigue crack at ambient temperatures and pressures in humid air and aqueous solutions. This study examines the individual reaction steps, intermediate phases formed, and the time required for the metal ions to transition into hard oxide phases. This study also finds that when a fatigue crack closes at ambient temperatures, the corrosion products inside the crack should consist of thin (≈2 nm) hard oxide films on the fracture surfaces and viscous layers of water with metal ions in various states of the transition from aqueous ions to stable end-product phases, but with insufficient strengths to resist load transfer at the crack tip. This analysis assumes that metal ion complexes form at the crack tip and transition toward the end-product phases in the wake of the crack in a manner analogous to that reported for plane surfaces exposed to humid air and aqueous solutions. It is concluded that the contribution of the viscous layers to an oxide induced crack closure (OICC) mechanism should be negligible. The impact of this finding on the interpretation of R-ratio effects on fatigue crack threshold and oxide thickness measurements is discussed. The OICC mechanism should be possible at high temperatures and in the near threshold fatigue crack growth region where hard oxides can grow to sizes relevant to the cyclic crack opening displacement (COD).

Near-threshold fatigue crack growth behavior of the weld metal and heat-affected zone in an ex-service CrMoV steel joint

Threshold value (ΔKth) and fatigue crack growth behavior at different temperatures and stress ratios were investigated in weld metal (WM) and heat-affected zone (HAZ) of an ex-service CrMoV steel joint. Compared with the ΔKth at 25 ℃, the ΔKth at 500 ℃ and 540 ℃ were still high, because significant oxide-induced crack closure (OICC) occurred in WM and HAZ during high-temperature tests. The high-temperature ΔKth of HAZ were higher than those of WM, due to their more severe oxidation and more significant OICC. This was owing to the lower Si and Cr content and smaller grain size in HAZ.

Determination of fatigue crack propagation thresholds for steel in presence of environmental effects

The experimental determination of the resistance to fatigue crack propagation in steel, particularly in the so-called near-threshold regime, is a crucial issue for safety-relevant components which are designed to operate for a large number of loading cycles before undergoing periodic inspections. This work summarizes the most relevant results of extensive experimental campaigns conducted at BAM and IPM over the last years, which have been devoted to the determination of fatigue crack propagation characteristics on medium and high strength steels. The influence of environment, oxide-induced crack closure and test methodologies in the near-threshold regime are discussed.

Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.

Near-threshold fatigue crack propagation without oxide-induced crack closure

An accurate value for the threshold stress intensity factor range, ΔKth, is a key parameter for studying crack-like defects. However, it is difficult to obtain accurate ΔKth values due to oxide-induced crack closure. In this study, we report conditions for minimizing the effects of oxide-induced crack closure near the threshold region, where a concave curve of the effect on the loading frequency on oxide-induced crack closure is achieved. The resulting conditions allow for an accurate determination of ΔKth, which is a key material parameter relating to the pertinent loading ratio.

Near-threshold fatigue crack growth behavior of 10% Cr martensitic steel welded joint with 9% Cr weld metal in high temperature air

The near-threshold fatigue crack growth (FCG) behavior under different stress ratios (R) of a 9%–12% Cr steel welded joint was studied in 538 °C air. The most significant oxide-induced crack closure (OICC) occurred in weld metal (WM) due to the lowest Cr content, resulting in the highest threshold value (ΔKth) of WM. The base metal (BM) with coarser grains and denser lath structures had more obvious roughness-induced crack closure (RICC) and more severe crack deflection, which led to a higher ΔKth of BM than heat affected zone (HAZ). The ΔKth of WM, HAZ and BM decreased as R increased.

Fatigue crack growth model including load sequence effects – Model development and calibration for railway axle steels

Beside well-known influence factors like residual stresses, component and crack geometry also load sequence effects significantly impact the crack propagation rate. In this paper two major load sequence effects, namely overloads and small loads, and their particular influence on the fatigue crack growth are investigated in detail. Both effects lead to a retardation in the crack propagation rate by increasing the plasticity-induced (overload) or the oxide-induced (small loads) crack closure. To develop a holistic fatigue crack growth model including the short crack behaviour as well as load sequence effects, the NASGRO equation is modified accordingly. This improved fatigue crack growth approach is calibrated for two common material grades for railway axles, namely EA4T and EA1N, and implemented into crack growth assessment software tools.

Microscopic analysis of non-propagating fatigue crack tips

Herein, the phenomena around a non-propagating fatigue crack tip in polycrystalline interstitial-free steel were investigated on a microscopic scale. A non-propagating fatigue crack was introduced in a standard compact tension specimen using a load-shedding procedure. The near-tip area was subsequently investigated via scanning electron microscopy and transmission electron microscopy. The wake in the tip area was covered with an oxide layer. Discrete dislocations, whose densities were relatively low, were only found in front of the crack. These results may suggest that an increase of the opening point was attributable to the oxide-induced crack closure rather than the plasticity-induced one.

Quantitative dependence of oxide-induced crack closure on air humidity for railway axle steel

The paper analyses fatigue crack closure in the near-threshold regime of the EA4T railway axle steel. The dependence of fatigue crack growth threshold on air humidity was experimentally obtained. Varying air humidity led to changes of fatigue threshold values Kmax,th in the range from 4 to 7 MPa m0.5, which was explained by the effect of oxide-induced crack closure (OICC). Such a wide range of threshold values results in significant changes of residual fatigue lifetime of railway axles (by one or two orders of magnitude) operated under various humidity conditions. The Newman's model of plasticity-induced crack closure is not sufficient to explain high portions of crack closure near threshold and their variation, since roughness-induced crack closure (RICC) and OICC has also be taken into account. Detailed explanations and interpretations of the parameters quantifying crack closure are presented. A quantitative model for separation of the crack closure components was applied to threshold data from literature for dry and humid atmosphere as well as for new experimental data of the EA4T steel tested at various levels of humidity. The levels of OICC determined from threshold values were compared to those calculated from thicknesses of oxide debris measured on fracture surfaces. The oxide layers were measured after the experiments in scanning electron microscope using focused-ion-beam cuts. The results were plotted as dependences on absolute air humidity and were in a good agreement. Very low threshold values measured in dry air can occur in operation when the temperature is around 0 °C which is not exceptional in many countries.

Crack Closure in the Near-threshold Region in Metallic Materials

Resistance to fatigue crack propagation is divided into the intrinsic (effective) component and the extrinsic component, represented by crack closure, which is very significant in the near-threshold regime. Crack growth rate measurements were done on the compact tension specimens made of the ferritic-pearlitic steel for automotive at the stress ratio R = 0.1 and R = 0.8. Crack closure was quantified from the experimentally measured data using the effective crack growth rate curve da/dN-ΔKeff and a new expression of the crack closure ratio Rcl = Kcl/Kmax depending on plasticity-induced crack closure, roughness-induced crack closure, oxide-induced crack closure and the exponent of the Paris power law. The presented methodology helps to analyse the crack closure mechanisms and to calibrate parameters used in the crack closure models.

Oxide induced crack closure in the near threshold regime: The effect of oxide debris release

Within this work the influence of oxide particle transport during fatigue loading on the fatigue propagation rate was investigated in detail by means of load-shedding tests in the near threshold region (crack propagation rate between 0.1 and 10 nm per cycle). To this purpose, standard fatigue specimens (SENB - single edge notch bending) were machined. At selected specimens, a stretchable nitrile rubber patch was glued onto the sample surfaces in the ligament region. This patch was expected to prevent the emergence of iron oxide particles near the sample surface, and indeed subsequent fracture surface analysis showed obvious differences in the shape of the arising oxide debris layer. It is shown that the build-up of oxide debris on the fracture surface is higher in the specimen with the nitrile rubber patch applied. This leads to increased oxide induced crack closure and, in turn, to a higher fatigue crack growth threshold. In this context, also the influence of the specimen thickness on the fatigue crack growth threshold is examined. Finally, also the influence of the load ratio on the oxide debris layer is briefly discussed.

Crack Size and Shape Considerations Related to Near-Threshold Fatigue Crack Growth Rate Behavior

Abstract It is well recognized that crack size or load history or both have an important influence on near-threshold fatigue crack rate behavior at a low stress ratio. This investigation will compare the behavior of a radial crack, such as a surface flaw or a corner crack, to that of a linear crack to see if crack shape plays a role in physically small crack to long crack behavior. Recent research has unveiled a methodology to experimentally partition plasticity from roughness- and oxide-induced crack closure, showing that nonplasticity-induced closure mechanisms have a greater influence at low Kmax by orders of magnitude. Knowledge of this is key to understanding differences between long crack and physically small crack behavior in the near-threshold regime. In this study, the well behaved 2024-T351 aluminum alloy was used to generate threshold data using numerous specimen geometries, flaw geometries, crack sizes, and test procedures to highlight not only the significance of crack size but also the role of crack shape. It will be shown both experimentally and analytically that a radial crack (surface flaw, corner crack) has significantly less remote closure and, therefore, a lower threshold than a through crack (compact tension, middle crack tension), even if the crack size is the same.

High-R low growth rate fatigue crack propagation at elevated temperatures

While fatigue cracking is widely considered as the primary cracking mode under cyclic loading conditions, this is not the case for high-R (Kmin/Kmax) low growth rate fatigue crack propagation at elevated temperatures. Creep cracks can sometimes be observed in such situations when the stress intensity factor range ΔK is around the fatigue crack growth threshold stress intensity factor ΔKth. A Time Dependent Failure Assessment Diagram (TDFAD) has been adopted in this study for predicting creep cracking in high-R cyclic loading conditions at elevated temperatures. In addition, attention is also paid to oxide-induced crack closure which is greatly influential at high temperatures. Three power plant steels with different chromium contents, namely 1%Cr, 9%Cr and 18%Cr steels, have been investigated at temperatures between 550°C and 650 °C. The experiments were performed by reducing ΔK continuously to ΔKth. Good accordance has been found between fracture surface observations and the TDFAD approach predictions of creep crack initiation. Apart from creep cracking, high-R ΔKth values have also been found to be influenced by oxide-induced crack closure at elevated temperatures. The enhancement is studied through the relationship between the crack tip opening displacement and stress intensity factor.

Crack closure in near-threshold fatigue crack propagation in railway axle steel EA4T

The paper is focused on fatigue crack propagation in the threshold area of the EA4T railway axle steel. Railway axles represent one of the most critical components for safety of trains. The main aim of this paper is to describe fatigue crack closure mechanisms in EA4T, which significantly influence the threshold value – a crucial parameter for determination of residual fatigue lifetime of railway axles. Investigated fatigue crack closure mechanisms are: plasticity induced crack closure, roughness-induced crack closure and oxide-induced crack closure. One part of this paper is focused on comparison of crack closure determined by NASGRO relationship (including plasticity induced crack closure mechanism only) with the experimentally obtained one. Difference between these two approaches is explained by presence of roughness and oxide induced crack closure. Based on theoretical models different sources of fatigue crack closure mechanisms was separated and evaluated. The significance of oxide-induced crack closure was confirmed based on experimental measurement of fatigue crack propagation in low humidity air. Finally, the effect of a reduced threshold value in low-humidity air on residual fatigue lifetime of railway axles is demonstrated.

Effects of Crack Closure and Cyclic Deformation on Thermomechanical Fatigue Crack Growth of a Near α Titanium Alloy

In this study, closure corrected in-phase (IP) and out-of-phase (OP) thermomechanical fatigue crack growth rates at two temperature intervals viz. 573 K to 723 K (300 °C to 450 °C) and 723 K to 873 K (450 °C to 600 °C) of Timetal 834 near α titanium alloy are presented. It is found that closure mechanisms significantly influence the stage I crack growth behavior. Surface roughness-induced crack closure (RICC) predominantly modifies the crack growth rate of near-threshold region at 573 K to 723 K (300 °C to 450 °C) test conditions. However, oxide-induced crack closure further strengthens RICC at 723 K to 873 K (450 °C to 600 °C) TMF loading. In stage II crack growth behavior, the alloy shows higher crack growth rates at 723 K to 873 K (450 °C to 600 °C) OP-TMF loading which is attributed to the combined effect of cyclic hardening occurring at the crack tip and weakening of interlamellar regions due to oxidation.

酸化物誘起き裂閉口が無視できる場合の疲労き裂進展下限界特性の板厚依存性

In order to investigate the effect of specimen thickness on near threshold crack propagation without oxide induced crack closure, ΔK decreasing tests under the stress ratio 0.1 and the loading frequency 5 Hz, specimens thickness 12.7 mm and 6 mm conditions were carried out. In case of Cr-Mo Steel, no clear oxide films were observed on macroscopic fracture surface on both specimens. EDX results showed that no oxides were detected on the near threshold fracture surface of both specimens. The crack propagation showed almost the same behavior in both high ΔK and near threshold areas. The threshold values were 4.6 on both specimens. These results showed that there were no effects by specimen thickness on both high ΔK and near threshold range.

酸化物誘起き裂閉口が無視できる低応力比下でのき裂進展特性

酸化物誘起き裂閉口が無視できる低応力比下でのき裂進展特性

The influence of corrosion pits on the fatigue life of 17-4PH steam turbine blade steel

Fatigue life tests on smooth and pre-pitted specimens and near threshold fatigue crack growth rate measurements are performed on 17-4PH steam turbine blade steel. The influence of environment and stress ratio is investigated. A decrease in the fatigue life and a slowdown of the crack growth rate in the near threshold regime is found in salt solution compared to air. The latter can be explained by oxide-induced crack closure. The critical stresses for fatigue failure in the presence of corrosion pits can be calculated using a methodology that was already successfully applied to 12% Cr steam turbine blade steel.

Characterisation of short fatigue cracks in titanium alloy IMI 834 using X-ray microtomography

A first attempt at the three-dimensional evaluation of naturally initiated surface connected and internal fatigue cracks is presented. Fatigue crack initiation and growth in air and vacuum environments have been investigated through X-ray microtomography in air and vacuum environments at elevated temperatures (350°C), accompanied by post-mortem electron microscopy of the fracture surfaces. In vacuum (<10−5mbar), multiple internal and surface-connected crack initiation was observed, but only the surface-connected cracks grew. In contrast, fewer cracks formed in air, these were mostly surface-connected and all were observed to grow. In all instances the initiation features were associated with globular primary α. An improved fatigue life was found in vacuum, which was mostly a consequence of delayed initiation, but was also due to slower fatigue crack propagation. The non-propagation of internal cracks was taken to imply that even the good laboratory vacuum obtained here was insufficient to simulate the conditions obtained for an internal crack in a component. The crack shape evolved towards a semi-circular shape a/c=1 in air during fatigue crack growth, whilst the vacuum cracks remained semi-elliptical (a/c≃1.4). This was taken to imply that oxide-induced crack closure played a role in fatigue crack growth in air.