Non-propagating Fatigue Crack Research Articles

AN INFLUENCE OF ROLLER STRENGTHENING ON MULTCYCLIC FATIGUE OF SPECIMENS WITH CUTS OF VARIOUS DEPTH AND A PRESSURIZED HUB

The influence of roller strengthening on a residual stresses distribution and multicyclic fatigue of solid cylindrical specimens of 50 mm diameter made of steel 20 with cuts of various depths and a pressurized hub has been examined. As a result, an endurance limit of roller strengthened specimens diminishes. In order to preserve the effect of hardening with a cuts depth increase, its necessary to increase the thickness of a layer with compressive residual stresses. Surface hardening influence on an endurance limit of specimens has been estimated by two criteria: axial residual stresses on a concentrators surface and average integral residual stresses, calculated through the dangerous sections surface layer thickness that equal a critical depth of a non-propagating fatigue crack. Its been stated that its more reasonable to use the average integral residual stresses criterion for an evaluation of roller strengthening influence on an endurance limit of specimens with cuts of various depth and a pressurized hub.

THE INFLUENCE OF CUT’S DEPTH ON RESIDUAL STRESSES DISTRIBUTION AND AN ENDURANCE LIMIT OF SPECIMENS UNDER SURFACE HARDENING

The influence of cuts depth on residual stresses distribution and an endurance limit under bending of hollow cylindrical specimens made of steel 20 with external diameter 50 mm and interior diameter 40 mm after outstripping superficial plastic deforming by roller strengthening of two regimes has been examined. Its been stated that the value of compressive residual stresses in a dangerous section of specimens decreases on cuts depth increase. As a result, an endurance limit increment of hardened specimens with cuts diminishes. In order to preserve the effect of hardening by increasing the endurance limit under outstripping superficial plastic deforming with an increase in a cuts depth, it is necessary to increase the thickness of a smooth part with compressive residual stresses layer. Its been shown that in order to estimate the increase in the endurance limit of hardened specimens, one should use not residual stresses at the bottom of a cut, but average integral residual stresses through the parts dangerous section surface layer thickness that equal a critical depth of a non-propagating fatigue crack.

THE INFLUENCE OF HYDRO BLASTING ON THE EN-DURANCE LIMIT OF SPECIMENS WITH A CUT UNDER BENDING AND STRETCHING-COMPRESSION

The hydro blasting influence on an endurance limit of cylindrical specimens with circular cuts of a semicircular profile of 0,3 mm radius under bending and stretching-compression in a case of symmetric cycle has been examined. Specimens of 10 mm diameter for fatigue tests were made of steels 30ХГСА, 12Х18Н10Т, ЭИ961,45 and aluminum alloys В93, Д16Т.The influence of surface hardening on an endurance limit of specimens with cuts was valuated by two criteria: residual stresses on the concentrator surface and the average integral residual stresses calculated through the parts dangerous section surface layer thickness equal the critical depth of a non-propagating fatigue crack. 
 On results of residual stresses and specimens with cuts endurance limits determination its been stated that coefficients of surface hardening influence on an endurance limit increase under bending and stretching-compression by two criteria are approximately equal. Besides, the coefficient of the influence by the average integral residual stresses criterion changes within essentially lesser limits than the coefficient of the influence by the residual stresses on the concentrator surface criterion. So the average integral residual stresses criterion should be used for the evaluation of a surface hardening influence on an endurance limit of parts with concentrators.
 On an example of specimens within and without peening in a dangerous section its been shown that peening doesnt influence the endurance limit of a surface hardened part. The compressive residual stresses only influence the endurance limit of these parts.

THE ESTIMATION OF ROLLER STRENGTHENING INFLUENCE ON THE ENDURANCE LIMIT OF SHAFTS WITH PRESSURIZED HUB

The influence of roller strengthening on endurance limit under bending of the 25 mm and 50 mm diameter shafts made of steel 20 and 25 mm diameter shafts made of steel 40X with a pressurized hub has been examined. Its been stated that the endurance limit of roller strengthened shafts with a pressurized hub depends not only on a value of compressive residual stresses in their dangerous section but on the character of its distribution. The valuation of a surface hardening influence on the endurance limit of shafts by the surface residual stresses criterion and the average integral residual stresses criterion has been shown that the average integral residual stresses criterion calculated through the parts dangerous section surface layer thickness equal the critical depth of the non-propagating fatigue crack can be recommended for the hardened shafts with a pressurized hub endurance limit increase prediction. Its been shown that on a diameter of a shaft with a pressurized hub raising it is necessary to increase a thickness of a hardened surface layer with compressive residual stresses.

Generalized Diagram of the Ultimate Nominal Stresses (Endurance Limit) and the Corresponding Dimensions of the Non-Propagating Fatigue Cracks for Sharp and Blunt Notches

The paper generalizes the original data and the experimental data from the references on the investigation of the mechanisms of initiation and behavior of the non-propagating fatigue cracks in the notches of various shapes and dimensions. The dependence of the ultimate nominal stresses (endurance limit) and their corresponding dimensions of the fatigue cracks on the theoretical stress factor for the notches of various shapes and dimensions is proposed to be schematically presentedin the form of the generalized diagram. The ultimate stresses should be understood as the endurance limits only within the zone of sharp notches provided that their value does not depend on the theoretical stress concentration factor. The limiting dimensions of the non-propagating cracks in the zone of sharp notches should be understood to mean the maximum dimensions of the cracks that are consistent with the ultimate stresses. The generalized diagram indicates the fact that the nominal endurance limit decreases in the zone of blunt notches, while the dimensions of the non-propagating cracks increase with an increase in depth and decrease in the notch radius (with the increase of the theoretical stress concentration factor), whereas they are controlled by the notch depth within the zone of sharp notches. With the increase of the notch depth, the ultimate stresses decrease, while the dimensions of the non-propagating cracks increase. At the constant depth of the notch, the ultimate stresses and dimensions of the non-propagating cracks are independent on the notch radius and value of the theoretical stress concentration factors.

Brittle coating effects on fatigue cracks behavior in Ti alloys

In order to study the coating effects on fatigue crack initiation mechanism in Ti-alloys, two types of brittle coatings, CrAlN and TiN, were deposited on the surface of TC4 titanium alloy by physical vapor deposition. The tension-tension fatigue tests and the in-situ observations of fatigue crack morphology were performed to study the coating effects on the fatigue crack initiation and propagation in the Ti-6Al-4V alloy. It was found that the 510–530 MPa TC4 fatigue limit is reduced to 315–330 MPa due to the CrAlN coating. The brittle coatings impeded the deformation of the TC4 samples at the beginning stage of fatigue tests, while coating cracking promoted the elongation of the tested samples. Fatigue crack was found to be initiated in the brittle coatings and propagated to the coating-substrate interface, inducing micro-damage of the substrate surface. The fracture surface of coated and uncoated samples was quite different, and the formation of non-propagating fatigue cracks was also observed. The coating cracking-induced low cyclic stress substrate damage model was proposed. This study should be of significance for the coating improvement and provides a theoretical basis for improving fatigue properties of coating materials.

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.

ECCI Characterization of Dislocation Structures at a Non-propagating Fatigue Crack Tip: Toward Understanding the Effects of Mn-C and Cr-N Couples on Crack Growth Resistance

We attempted to clarify the underlying mechanisms of the enhanced small-fatigue-crack resistance of Fe-Mn-C twinning-induced plasticity (TWIP) steel and high-nitrogen austenitic steel. To this end, we performed electron channeling contrast imaging near the tips of non-propagating fatigue cracks in Fe-18Cr-14Ni steel without a significant amount of interstitials, Fe-23Mn-0.5C TWIP steel, and Fe-25Cr-1N austenitic steel. The fatigue crack non-propagation limits of the TWIP steel and high-nitrogen steel were higher than that of the steel without a significant amount of interstitials; the higher limits of the TWIP steel and high-nitrogen steel are attributed to the Mn-C and Cr-N interactions, respectively. The enhanced small-fatigue-crack resistance of the Fe-23Mn-0.5C steel is attributed to local hardening at the crack tip caused by an increase in the dislocation density via dynamic strain aging. The enhanced dislocation planarity of the Fe-25Cr-1N steel, which is a result of the Cr-N interaction, is a significant factor that influences (i.e., increases) the crack resistance. The enhanced dislocation planarity results in dislocation pile-up stress at the crack tip, thereby preventing dislocation emission from the crack tip.

Dependence of the limit endurance of specimens with notches from the depth of hardened surface layer

It is shown that, growth of the layer with compressive residual stresses at advanced surface plastic deformation of smooth samples leads to an increase in the endurance limit of the notched specimens due to an increase in the compressive residual stresses in their dangerous section. The study was conducted on the specimens with outer diameterDequal to 10 and 25 mm and on a 25 mm specimen with inner diameterd= 15 mm which were subjected to air shot blasting and roll-in. Residual stresses in smooth specimens were determined by the mechanical method. Circular incisions (R= 0.3 and 0.5 mm) were applied on hardened smooth samples with a shaped cutter. Residual stresses in notched specimens were determined by a numerical method calculating the redistribution of the residual stresses upon the incision. As the diameter of the samples increases with the same strengthening surface treatment,the depth of the layer with the compressive residual stresses increases due to an increase in the rigidity of the samples. The effect of surface hardening on the endurance limit of notched specimens was evaluated according to the criterion of the average integral residual stresses, which takes into account both the magnitude and completeness of the residual stresses. This criterion was calculated from the depth of the non-propagating fatigue crack that occurs in the surface layer in the dangerous section of the specimen during the fatigue test at the limit endurance. Fatigue tests are carried out in symmetrical cycle, the test base being 3 × 106cycles of loading. Estimation of the increment of the limit endurance of surface-hardened cylindrical specimens with semicircular notches gave the results acceptable for practice. The use of the proposed method can significantly reduce long-term and expensive fatigue tests.

Prediction of endurance limit of hardened parts with account of operational factors

Fracture endurance limit is one of the main parameters used in calculating the strength of surface hardened parts. The influence of various operational factors along with hardening should be taken into account in predicting this parameter. Prediction of the endurance limit under stress concentration was carried out according to the criterion that takes into account the influence of residual stresses on the surface of the dangerous section of a hardened part, as well as the criterion of the average integral residual stresses over the thickness of the hardened surface layer equal to the critical depth of a non-propagating fatigue crack. The effect of surface hardening on the endurance limit was analyzed taking into account such operational factors as the type of deformation, working temperature, the asymmetry of the loading cycle of specimens made of steels and aluminum alloys. Fatigue tests of cylindrical specimens with circular notches of a semicircular profile were carried out under torsion, bending and tension-and-compression. It was established that the use of the criterion of the average integral residual stresses is quite a good indicator of the effect of surface hardening on the fracture endurance limit taking into account the operational factors studied.

Non-propagating fatigue cracks in austenitic steels with a micro-notch: Effects of dynamic strain aging, martensitic transformation, and microstructural hardness heterogeneity

The non-propagation limit of a microstructurally small fatigue crack was investigated with respect to dynamic strain aging (DSA), martensitic transformation, and microstructural hardness heterogeneity. In this study, we selected four model alloys: Fe-19Cr-8Ni-0.05C, Fe-19Cr-8Ni-0.14C, Fe-23Mn-0.5C, and as-hot-rolled Fe-30Mn-3Si-3Al steels. Transformation-induced cyclic hardening results in the most significant improvement of the non-propagation limit, i.e., in the case of the Fe-19Cr-8Ni-0.05C steel. Within different contexts, DSA, transformation-induced crack closure, and hardness-heterogeneity-enhanced plasticity-induced crack closure could also realize superior non-propagation limits. The effects of DSA and hardness heterogeneity can be combined with the effects of transformation, which is expected to create a new venue of material design and selection in terms of the crack non-propagation limit.

Analysis of the mechanism of fatigue crack arrest in a cylindrical notched specimen

The phenomenon of fatigue crack arrest in surface-hardened cylindrical specimens with stress concentrators is examined from the perspective of linear fracture mechanics. The stress intensity coefficient (SIC) is considered as the criterion, its scope determines the speed of fatigue crack growth in the case of cyclic loading. The calculations of the study were carried out by the Finite Elements Modeling method using the calculation complex ANSYS. The calculation complex ANSYS that uses the Finite Elements Modeling method in the form of displacements was applied to determine the stress intensity coefficient under bending of cylindrical surface-hardened and non- hardened specimens with the diameter of 10 mm with a semicircular notch with the radius of 0,5 mm. It is established that the stress intensity coefficient has two extremes in the initial area of the fatigue crack development: the minimum and maximum values. The results of the calculation indicate that the minimum value of SIC is lower than its threshold value below which the crack does not develop in the case of a cylindrical surface-hardened notched specimen. This fact is in good agreement with the results of tests of these specimens as non-propagating fatigue cracks were revealed in these specimens only. It is also established that the depth of a crack is approximately 0,02 of the minimum size of the section of a cylindrical specimen for the maximum value of the stress intensity coefficient. The study carried out validates the use of the criterion of average integral residual stresses to calculate the increase of the endurance limit of surface-hardened specimens and parts with stress concentrators due to compressive residual stresses.

Microstructural hardness heterogeneity triggers fatigue crack non-propagation in as-hot-rolled Fe-30Mn-3Si-3Al twinning-induced plasticity steel

We investigated the fatigue properties and associated crack growth behaviors of as-hot-rolled and solution-treated Fe-30Mn-3Si-3Al twinning-induced plasticity (TWIP) steels. The as-hot-rolled TWIP steel showed a higher fatigue strength than that of the solution-treated steel. Moreover, the as-hot-rolled steel showed a non-propagating fatigue crack at the fatigue limit, while the solution-treated steel did not. The high resistance to fatigue crack growth cannot only be attributed to an increase in hardness, but is also associated with heterogeneous hardness distribution. The effect of hardness heterogeneity was numerically supported by Dugdale model-based simulations.

Fatigue crack non-propagation assisted by nitrogen-enhanced dislocation planarity in austenitic stainless steels

Rotating bending fatigue tests were conducted to assess the fatigue crack propagation behavior of the Fe-25Cr-1N and Fe-18Cr-14Ni austenitic steels in terms of the microstructure, crack propagation paths, and non-propagating fatigue crack characteristics. The Fe-25Cr-1N steel exhibited a non-propagating fatigue crack at the fatigue limit (310MPa), but this did not occur in the Fe-18Cr-14Ni steel at the fatigue limit (110MPa). The non-propagating fatigue crack observed in the Fe-25Cr-1N steel was produced by roughness-induced crack closure. This phenomenon was caused by the enhanced planar dislocation and high dislocation pile-up stress resulting from the suppression of cross-slip, which inhibited the dislocation emission from the crack tip. The Fe-25Cr-1N steel exhibited a lower fatigue crack growth rate than the Fe-18Cr-14Ni steel because of the enhanced dislocation planarity produced by the Cr-N interaction. The Cr-N interaction affected the fatigue crack growth behavior as follows. The short crack region exhibited a planar glide dislocation pattern, but multiple slip systems were activated as the crack lengthened. As the dislocation pattern remained planar on each slip plane, the crack propagation occurred along the {111}γ slip planes, even in the long crack. Moreover, the dislocation pile-up at the grain boundaries caused grain boundary subcracks, which can induce crack toughening through mechanisms such as stress redistribution. These positive effects contributed to the lower fatigue crack growth rate in the Fe-25Cr-1N steel than the Fe-18Cr-14Ni steel.

Material property controlling non-propagating fatigue crack length of mechanically and physically short-crack based on Dugdale-model analysis

Non-propagating fatigue crack lengths were analytically calculated under stress control conditions using plasticity-induced crack closure analysis with the Dugdale model. In addition, a non-dimensionalization method was applied in terms of the Burgers vector and a monotonic plastic zone size under small-scale yielding conditions, which was validated for various initial crack lengths and material properties. When the yield strength was increased, the non-propagating fatigue crack lengths were found to increase for a short crack and decrease for a long crack. The non-dimensionalization enabled the analytical derivation of a generalized non-propagating fatigue crack length, which can be utilized for fatigue designs. The material property controlling the threshold stress intensity factor range of mechanically and physically short-crack was discussed.

The influence of static crack-opening stress on the threshold level for shear-mode fatigue crack growth in bearing steels

An investigation was conducted into the influence of statically-applied, mode I, crack-opening load on the threshold condition for propagation of a shear-mode fatigue crack in a bearing steel. Torsional fatigue test was carried out at an R of −1 using a hollow cylindrical specimen into which a semi-elliptical, small slit was axially introduced. A static axial compressive stress was simultaneously applied to suppress crack branching. A coplanar, shear-mode, non-propagating fatigue crack emanating from the slit was attained by appropriate control of shear stress amplitude. Internal pressure was then applied to generate a hoop stress as a static crack-opening stress, σθ static. Consequently, the threshold shear-mode stress intensity factor range, ΔKτth, was significantly decreased with increase of the static mode I stress intensity factor. To further understand the contribution of σθ static to the reduction in ΔKτth, microstructural observations for the cross-sections of a non-propagating crack were conducted using a scanning electron microscope in conjunction with the electron backscatter diffraction analysis. The results revealed that the excess loading of σθ static accounts for the change in the crack-path, resulting in a further reduction in ΔKτth.

Investigation of preliminary surface rolling influence on the fatigue resistance of threaded specimens

The influence of preliminary surface plastic deformation on the endurance limit of parts made of steels 40X and 30ХГСА with the thread М16х2 has been examined. The investigation has been carried out by the computational-experimental method using the results of determining residual stresses mechanically on the specimens cut out of smooth workpieces and by modeling threaded specimens using finite-elements models. The method of thermal elasticity has been used for the calculation of the strain-stress state in the PATRAN/NASTRAN environment, whereby the distribution of initial deformations in the hardened layer is replaced by the appropriate temperature field. The graphs of distribution of axial residual stresses across the thread roots are plotted on the basis of the calculation. The increment of the endurance limit for threaded parts has been predicted by the average integral residual stress criterion. The calculation results are compared with the experimental data of testing the specimens. The specimens made of 30ХГСА steel without a nut were tested under the symmetrical cycle according to the pure bending pattern, the specimens made of 40X steel with a nut were tested under the zero-to-tension positive cycle in the case of stretching. Non-propagating fatigue cracks have been found in the specimens that withstood the tests and were destroyed.

GS0208-217 二種のTWIP鋼における微小き裂進展挙動の比較研究 : Mh-C相互作用による疲労限創出の可能性

The high cycle fatigue properties of Fe-30Mn-3Si-3Al and Fe-22Mn-0.5C twinning-induced plasticity (TWIP) steels were evaluated by rotary bending fatigue testing and a replica technique. More specifically, small fatigue crack propagation and non-propagating fatigue crack phenomenon were comparatively studied in terms of occurrence of dynamic strain aging. In fact, dynamic strain aging occurs in the Fe-22Mn-0.5C TWIP steel because of the presence of Interstitial-substitutional (I-S) atom interactions. Probably due to the occurrence of dynamic strain aging, only the Fe-22Mn-0.5C TWIP steel showed non-propagating fatigue crack phenomenon at the fatigue limit. In contrast, a small fatigue crack of the Fe-30Mn-3Si-3Al TWIP steel continuously propagated even at the fatigue limit which was defined that the specimen does not fracture until 10^7 cycles.

Assessing the influence of surface hardening on the endurance limit of specimens made of steel 40X using residual stresses of reference specimen

The possibility of the employment of reference specimens hardened simultaneously with the parts has been examined for the hydroblasting influence estimation on the parts with concentrators endurance limit on the example of hollow cylindrical parts made of steel 40 with circular cuts of semicircular profile with the radius 0,3 mm. The investigation has shown that the difference between residual stresses values found by calculating and experimental methods doesnt exceed 8% in smooth specimens, in specimens with cuts 6%. Under the hydroblasting influence on the endurance limit estimation in conditions of bending in the case of symmetric cycle for hollow cylindrical specimens made of steel 40Х with cut of radius 0,3 mm its been established that the employment of residual stresses distribution calculated by the initial deformations of reference specimen let predict the specimens endurance limit increase under stress concentration conditions to an approximation of 8%. It has been shown that its reasonable to use the average integral residual stresses criterion calculated through the dangerous section surface layer thickness equal the non-propagating fatigue crack critical depth for the prediction of the endurance limit increase of hardened by hydroblasting specimens made of steel 40 with stress concentrators.

Dependence of the endurance limit on the character of residual stresses distribution in the surface layer of hardened parts

The influence of the character of compressive residual stress distribution on the endurance limit of surfacehardened specimens made of various steels and alloys under bending has been examined. Specimens madeof ВНС40 and 38Х2МЮА steels with V-shaped notches were hardened by nitriding, foundation specimensmade of ВКС-5 alloy by cementation and ionic cementation, flat specimens made of ЭИ698ВД alloy by airshot-blasting. The influence of surface hardening on the endurance limit was assessed using two criterions,namely: residual stresses on the dangerous section surface and the average integral residual stresses. It was establishedthat the endurance limit of surface-hardened specimens (parts) is determined not by the value of compressiveresidual stresses on the surface but by the completeness of the diagram of the residual stress over the partsurface layer thickness on condition that the surface layer thickness is equal to the critical depth of the nonpropagatingfatigue crack. The average integral residual stress criterion calculated over the thickness of a surfacelayer of the dangerous section equal to the critical depth of the non-propagating fatigue crack gives a rather accurateestimation of the influence the character of compressive residual stress distribution has on the endurancelimit of hardened specimens (parts). In practice the estimation of the increment of surface- hardened specimens(parts) endurance limit by the criterion of residual stresses on the dangerous section surface may lead to a significanterror.