Corrosion fatigue short crack growth behaviour in a high strength steel

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.     

Propagation of physically short cracks in a bainitic high strength bearing steel due to fatigue load

Physically short cracks in a bainitic high strength bearing steel were fatigue loaded. The rapid propagation rate of early open short cracks agreed with that of long closure free cracks. After some rapid growth, the short cracks entered a transition period to the rate of growth limited long cracks. Potential drop showed that the short cracks were open to the tip throughout the growth sequence, which excluded crack face closure in the wake as the growth limiting mechanism in this material. Instead the short crack effect was related to residual stresses and other mechanisms at the crack tip. Crack manufacturing procedures were determined for straight long and short start cracks in the present material. LEFM with effective material parameters and limit compensation predicted the short crack lives.     

Influence of nonmetallic inclusions on fatigue crack growth in a structural steel

A study of the fatigue behaviour of a hardened and tempered steel, at two inclusion levels, has been carried out according to the linear elastic fracture mechanics criteria. The influence of inclusions on the fatigue crack growth rate has turned out to be a function of the local stress intensity factor range,K _I, at which fracture propagates. At lowK _I values, to which are related crack growth rates less than 10^–5 mm cycle^–1, the crack growth rate in the steel with higher inclusion content is lower than in the steel with lower inclusion content. AsK _I increases, an inversion in the difference between the two rates occurs. In the dirtier steel, the higherK _I, the higher the growth rate than in the other steel. The difference between the two rates becomes nil just below the fast propagationK _Ic level. By fractographic analysis, it has been possible to find out how inclusions affect fatigue behaviour.     

Notch fatigue crack initiation studies in a high strength nickel base superalloy

It is with great pleasure that I offer this paper for the Special Issue honoring Professor Takeo Yokobori on the occasion of his 70th birthday. I regard Professor Yokobori as a personal friend, having known him for over twenty five years professionally and socially. His impact on the discipline of fracture mechanics has been most significant, not only by his own technical contributions but, perhaps, more so because he put into motion the means whereby those interested in the field of fracture mechanics could meet together and to share their work with others. I refer, of course, to the formation of the International Conference on Fracture at Sendai, Japan in 1965 and to the present international journal, Engineering Fracture Mechanics. The following paper is therefore dedicated to Professor Yokobori as my contribution in honoring his distinguished career.     

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 10⁷ cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.     

Effect of tensile overloads on fatigue crack growth of high strength steel wires

Fatigue of the tensile armor wires is the main failure mode of flexible risers. Techniques to increase the life of these components are required to improve the processes safety on oil exploration. This work evaluates the crack growth retardation of high strength steel wires used in flexible pipelines. Fracture toughness tests were performed to establish the level of stress intensity factor wherein the wires present significant plastic deformation at the crack tip. The effect of tensile overload on fatigue behavior was assessed by fatigue crack growth testing under constant ΔK control and different overload ratios with two different load ratios. The outcomes show that the application of controlled overloads provides crack retardation and increases the fatigue life of the wires more than 31%. This behavior is also evident at stress ratio of 0.5, in spite of the crack closure effect being minimized by increasing the applied mean stress.     

Prediction of KIc in a high strength bainitic steel

In this work the stress intensity factor (K_Ic) has been determined for a high strength bainitic steel, making reference to ASTM E 399 standard, which involves the use of complex geometry specimens. An experimental value of 1627 N mm^−3/2 has been obtained for K_Ic, which is equivalent to 51.4 MPa m^1/2. It has been verified that Lescovsek’s expression, which predicts K_Ic as a function of hardness (HRc) and the CVN Charpy impact test value, can be applied to steels with a bainitic microstructure, as the CVN value is more influential than HRc. The acceptable prediction of Lescovsek’s expression shows the way to improve the K_Ic value in bainitic steels.     

S–N curve characteristics and subsurface crack initiation behaviour in ultra-long life fatigue of a high carbon-chromium bearing steel

The S – N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S – N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S – N curve composed of two different S – N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S – N curve are discussed.     

Short-crack growth in corrosion fatigue for a high strength steel

Fatigue crack growth tests of HY130 steel were conducted under either constant-load-range or constant-stress-intensity-range conditions, by using four-point-bend and compact-tension specimens with a crack of length 0.4–41 mm. The tests were conducted in 3.5% NaCl solution under either the freely corroding condition or with the specimen coupled to a sacrificial zinc anode.Crack growth rates for the zinc-coupled case were higher than those for the freely corroding case, for either the long crack or short crack specimen. The growth rates of short cracks were faster than those of long cracks for the same environment. The maximum amount of crack growth acceleration due to crack size effect was about a factor of two for these alloy-environment combinations. With increasing crack length, the rate for short cracks converges to that for long cracks. Since there was no crack-size effect in air, the additional enhancement in the rate of crack growth for short cracks must be chemical in nature.The upper limit of chemically short cracks (as delineated by the point of convergence of rates) was shorter for the freely corroding case than for the zinc-coupled case, and was longer for the higher stress intensity range, for both the constant-load-range and the constant-stress-intensity-range tests. The observed effect of crack size on crack growth rates is discussed in terms of the hydrogen embrittlement mechanisms.     

Measuring overload effects during fatigue crack growth in bainitic steel by synchrotron X-ray diffraction

In this work we present the results of in situ synchrotron X-ray diffraction measurements of fatigue crack-tip strain fields following a 100% overload (OL) under plane strain conditions. The study is made on a bainitic steel with a high toughness and fine microstructure. This allowed a very high (60 μm) spatial resolution to be achieved so that fine-scale changes occurring around the crack-tip were captured along the crack plane at the mid-thickness of the specimen. We have followed the crack as it grew through the plastic/residually stressed zone associated with the OL crack location. We observed two effects; one when the enhanced plastic zone is ahead of the crack and one after it has been passed. Regarding the former it was found that the compressive stress at the crack-tip initially falls sharply, presumably due to the increased plastic stretch caused by the OL. This is associated with a concomitant fall in peak tensile stress at K_max, the elastic excursion between K_min and K_max remaining essentially unchanged from before OL. Subsequently discontinuous closure as seen previously for plane stress caused by crack face contact at the OL location limits the elastic strain range experienced by the crack tip and thereby retards crack growth.     

Fatigue crack initiation associated with inclusions in a low-carbon steel

On secondment from El-Zagazig University, Egypt.     

Modeling fatigue crack nucleation at primary inclusions in carburized and shot-peened martensitic steel

The mechanics of high cycle fatigue crack nucleation (formation of a stable crack that can grow away from the influence of the notch root of the inclusion) at subsurface primary inclusions in carburized and shot-peened martensitic steel subjected to cyclic bending is investigated using three-dimensional (3D) finite element (FE) analysis. FE models are constructed using a voxellation technique to address the shape, size, and distribution of primary inclusions within clusters. The critical depth for fatigue crack nucleation is predicted considering the gradient in material properties arising from carburization, prestrain and compressive residual stress distribution due to shot peening, and the gradient of applied bending stress. The influence of inclusion shape and interface condition (intact or debonded) with the matrix on the driving force for fatigue crack nucleation is examined. It is observed that the inclusion shape has minimal influence on the predicted results while the effect of the interface condition is quite significant. For partially debonded interfaces, the predicted critical depth from surface for fatigue crack nucleation agrees qualitatively with experimental observations.     

Initiation and early growth of fatigue cracks in high strength steel

The micromechanics of crack initiation and early growth in 4340 steel is studied using the SEM, and it is found that growth is discontinuous and transgranular, with the distance between growth steps corresponding to the prior austenite grain size spread. The lowest observed growth rate is approximately one lattice spacing per cycle. It is shown that cyclic growth of even the smallest cracks can be described using the fracture mechanics formalism, if the influence of debonded inclusions is taken into account.     

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.     

Effects of tempered structure on the near threshold fatigue crack growth behavior of a coarse grained high strength steel

Near threshold fatigue crack growth behavior of a high strength steel under different temper levels was investigated. It is found that the observed variations in ΔK_th could predominantly be attributed to roughness induced crack closure. The closure-free component of the threshold stress intensity range, ΔK_eff,th showed a systematic variation with monotonic yield strength.     

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.     

The role of inclusions in fatigue crack initiation in an aluminum alloy

A literature survey gave preliminary evidence that large second phase particles were cracked under static loading and that these might subsequently be sources of fatigue crack initiation. In order to determine more fully the effect of this type of inclusions on crack initiation bulk samples of 2024-T3 material were strained to 6, 8, 10 and 12% and subsequently reheat-treated. Reverse bending fatigue specimens were then prepared for crack initiation studies.Observations by optical microscopy showed that the prestrain had caused several voids at the inclusions partly by cracking of the inclusions while the more probably event was failure of the matrix-inclusion interface. It was shown that the voids were sources of fatigue crack initiation. Tests on as received material indicated initiation to occur also at inclusion clusters. Repeated polishing revealed the complex three-dimensional nature of the inclusions and its significance for the local direction of crack growth.

---

Zusammenfassung Es hat sich im Literatur gezeigt daß groben intermetallischen Einschlüsse unter einem statischen Last angerissen wurden und daß nachdem an diese Stellen vielleicht Ermüdungsrissen entstehen können.Um den Einfluß dieser Teilchen besser zu deuten, wurden Proben von 2024-T3 Material einer plastischen Dehnungsamplitude von 6, 8, 10 oder 12% unterworfen, nachdem die Proben aufs neue eine Wärmebehandlung gegeben ist. Biege-ermüdungsversuche wurden denn gemacht zur Untersuchung des Entstehens der Ermüdungsrisse.Optische Mikroskopie hat dann gezeigt daß die Vordehnung sämtlichen Mikrohohlräume an der Teilchen hervorgerufen hat, zum Teil durch Anreissen des Teilchens; des mehr annehmbare Vorfall war jedoch Dekohäsion zwischen den Einschlüsse und der Matriz. Es hat sich gezeigt daß an der Stelle der Hohlräume Rissen entstehen. Prüfung des selben Materials in der Anfertigungszustand hat gezeigt daß auch dann die Rissen entstehen an der Stelle von zusammengetroffenen Teilchen. Wiederholtes Polieren zeigte den komplizierten drei-dimensionalen Aufbau der Teilchen und dessen Bedeutung für die lokalen Richtung der Rißfortschritt.

---

Résumé Un examen de la documentation technique a mis en évidence que sous une charge statique, des criques se forment dans des grosses inclusions et qu'à partir de celles-ci peuvent s'initier des criques de fatigue. Pour établir d'une façon plus complète l'effet de ce type d'inclusions sur l'initiation des criques, des échantillons de 2024-T3 on été sollicités en traction jusqu'à 6, 8, 10 et 12% et ensuite de nouveau soumis à un traitement thermique. Des éprouvettes étaient prélevées et des essais de fatigue faits en flexion pour étudier l'initiation des criques.Des examens au microscope ont montré que la précontrainte avait produit plusieurs trous aux inclusions, en partie par la formation des criques, le phénomène le plus probable étant cependant la défaillance de l'interface matriceinclusion. Il a été montré que les trous sont à l'orgine de l'initiation des criques de fatigue. Des essais sur du matériau tel qu'il était reçu ont indiqué que l'initiation se produit aussi sur des amas d'inclusions.Un polissage répété a révélé la complexe nature tri-dimensionelle des inclusions et son importance pour la direction locale de l'accroissement des criques.