Predicting the effects of material properties gradient and residual stresses on the bending fatigue strength of induction hardened aeronautical gears

This paper proposes an experimental methodology to characterize complex parts presenting various gradients using aeronautical induction surface hardened spur gears. A 3D fatigue model taking into account residual stresses, microstructure variations, and surface roughness is then proposed for the prediction of the bending endurance limit. The model is based on the well-known Crossland criterion; calibrated with representative axial and torsion laboratory specimens. The results are compared with testing performed on a custom-made single tooth bending fatigue (STBF) rig. Fracture surface analysis using electronic microscopy is used to investigate the crack initiation sites. It is shown that residual stresses can have a significant impact on bending fatigue and that two induction treatments can present very different fatigue resistance even if the shape and depth of the hardened layer is identical in the root. The proposed methodology could be adapted to other geometries and surface treatments.     

Application of the weakest-link concept to the endurance limit of notched and multiaxially loaded specimens of carburized steel 16MnCrS5

Based on the weakest‐link concept a method is developed, from which the endurance probability of every surface and volume element of a case‐hardened part, which is loaded near the endurance limit, can be calculated. A prerequisite for the calculation is knowledge of the hardness and residual stress distribution, the surface roughness and the surface oxidation depth. By multiplication of the endurance probabilities of neighboured elements the endurance probability of a limited region or of the whole part can be calculated, which includes an endurance limit determination. It is shown, that this model can be applied successfully to specimens of a carburized steel. The necessary model parameters can be gained from a set of reference specimens. Because of the possibility to formulate an endurance probability for every volume and surface element, there are no geometrical restrictions on the parts to be assessed.     

Effect of cyclic hardening on fatigue performance of slide burnished components made of low‐alloy medium carbon steel

The slide burnishing process causes cyclic loading of the surface being treated, which provokes cyclic hardening. Using a forced‐controlled indentation test, the sixth “loading‐unloading” cycle was stabilised. The effect of the number of passes and the cyclic loading coefficient (CLC) on the fatigue performance of slide burnished specimens was investigated. Rotating bending fatigue tests were conducted using nine groups of hourglass shaped specimens, which were slide burnished through a different number of passes and CLC values. A stabilised cycle of the surface layer achieved with six passes, lead to largest fatigue limit, whereas the CLC exerted negligible influence on the fatigue performance. The observed phenomenon was explained through different residual stress relaxation rates, due to the rotating bending load, as well as with the obtained surface layer microstructure. The residual stress relaxation was investigated through rotating bending fatigue tests, using cylindrical fatigue specimens, followed by X‐ray stress analysis.     

Improving fatigue performance of Ti-6Al-4V alloy via ultrasonic surface rolling process

The effect of a gradient nanostructured (GNS) surface layer obtained by ultrasonic surface rolling process (USRP) on the fatigue behavior of Ti-6Al-4 V alloy has been studied in this paper. Microstructure, surface topography, surface roughness and residual stress measurements were performed to characterize the surface under different conditions. Rotating bending fatigue tests were carried out to evaluate the fatigue behavior of different treatments. The results present a remarkable fatigue performance enhancement for the Ti-6Al-4 V alloy with a GNS surface layer obtained by application of USRP with respect to the untreated condition, notwithstanding its considerable surface roughness due to severe ultrasonic impacts and extrusions. Mechanical surface polishing treatment further enhances the beneficial effects of USRP on the fatigue performance. The significantly improved fatigue performance can mainly be ascribed to the compressive residual stress. Simultaneously, the GNS surface layer and surface work hardening have a synergistic effect that accompanies the effect of compressive residual stress.     

Werkstoffkundliche Gesichtspunkte zur Beurteilung des Schwingfestigkeitsverhaltens induktiv randschichtgehärteter Bauteile

Metallurgical viewpoints concerning the fatigue behaviour of surface induction hardened components The fatigue behavior of induction hardened surface‐hardened components and component‐like specimens is conclusively explained by a comparison of load‐stress and fatigue‐resistance profiles (Woodvine analysis). The defining criterion is the critical strengthened depth. Fatigue‐resistance studies have to be carried out from the specific viewpoint of short‐time austenitization. The level of fatigue resistance which can be achieved is determined by the procedure used. The criteria which lead to the highest level are known. With regards to rotating bending fatigue resistance, this highest level is, for the untempered state, independent of steel type, surface condition and stress concentration factors, and reaches a level of about 900 MPa. With increasing tempering heat, this level drops under the influence of notches and roughness.     

Residual strength at −40 °C of a precracked cold‐formed rectangular hollow section made of ultra‐high‐strength steel – an engineering approach

This study investigated the residual strength of a precracked cold‐formed rectangular hollow section made of novel ultra‐high‐strength steel. The primary goal was to experimentally discover the residual strength of the structure when used in low temperature service conditions. The secondary goal was to predict the residual strength by using a J‐integral approach with nonlinear finite element calculations and to compare these predictions with measured results. The experimental tests were carried out with a beam in four‐point bending loading. The test specimens were taken from a cold‐formed rectangular hollow section fabricated from direct quenched (untempered) ultra‐high‐strength steel S960 QC omitting the annealing in the fabrication process. The tests for final failure were carried out at ?40 °C, with the exception of the first pilot test. There were two kinds of tests: (1) the beam was cyclically loaded until the final fracture or the fatigue precrack was first introduced and (2) the specimen was then subjected to a quasistatic bending loading condition until it failed. The new experimental results matched well with our predictions, and both confirmed the high toughness of ultra‐high‐strength steel in beam construction studied, even at a low ambient temperature.     

Fatigue strength and fracture mechanism of steel modified by super-rapid induction heating and quenching

Four kinds of surface hardened-specimens (ordinary structural steel with carbon content of 0.45% C) having hardened thicknesses of 0.7–1.8 mm were prepared using a ‘super-rapid induction heating (SRIH) system’. Rotation bending fatigue tests were performed with special focus on the effect of a hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 10⁷ cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000 MPa. This is because the fatigue crack originating from inside the hardened layer leads to the final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved on the specimen with thick hardened layers, such as those about 1.8 mm thick. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to final fracture (hardened-layer fracture mode).     

Monotonic and Cyclic Deformations of Case‐Hardened Steels Including Residual Stress Effects

Abstract: Monotonic and cyclic deformations of case‐hardened steel specimens under axial loading were investigated experimentally and analytically. A finite element (FE) model for the case‐hardened specimens was constructed to study multiaxial stresses due to different plastic flow behaviour between the case and the core, as well as to evaluate residual stress relaxation and redistribution subsequent to cyclic loading. The multiaxial stress is shown to increase the effective stress on the surface, and, therefore, unfavourable to yielding or fatigue crack nucleation. The residual stresses are shown to relax or redistribute, even in the elastic‐behaving region, when any part of a case‐hardened specimen or component undergoes plastic deformation. Multi‐layer models were used to analyse and predict monotonic and cyclic deformation behaviours of the case‐hardened specimen based on the core and case material properties, and the results are compared with the experimental as well as FE model results. The predicted monotonic stress–strain curves were close to the experimental curves, but the predicted cyclic stress–strain curves were higher than the experimental curves.     

Fatigue of alumina under contact loading

Bars cyclically loaded by opposite concentrated forces via rollers are appropriate test specimens for the determination of fatigue under contact loading. As practical applications of the proposed test, the contact strengths and numbers of cycles to failure under contact loading were determined for three Al₂O₃. In additional tests the residual strength after a pre-loading under contact loading conditions was determined in four-point bending tests.     

Prediction of fatigue limit of induction surface hardened 1.05Cr–0.23Mo steel alloy using extreme value statistics

Fatigue characteristics of the surface hardened steel are different from that of normal steel, so the prediction of the fatigue limit of surface hardened steel is very complicated. In this paper, specimens are tested using rotary bending, and the surface of 1.05Cr–0.23Mo steel alloy is hardened by induction surface hardening. Variation of the distribution of microvickers hardness and residual stress is discussed, and the difference of S-N diagram between surface hardened steel and unhardened steel is examined. The maximum defect size of surface hardened specimen is calculated by the extreme value statistics to predict conservative fatigue limit. Actual shape of defect in the specimen is three dimensional, so a conversion method from 2D to 3D defect size based on examination volume and inclusion size is used to predict statistical maximum defect size. The predicted results can be defined as a lower fatigue limit which may be useful to predict conservative fatigue limit of surface hardened specimen.     

Das Konzept der lokalen Dauerfestigkeit und seine Anwendung auf martensitische Randschichten,insbesondere Laserhärtungsschichten

The local fatigue strength concept and its application to martensitic surface layers, especially laser hardened layers Starting from the known local fatigue strength concept problems of its application to martensitic surface layers, especially laser hardened layers have been discussed. A method for calculation of the fatigue limit of surface hardened specimens or components from quench-hardenable steels has been proposed, including the influence of surface roughness on fatigue strength. There is given a new formula for estimation of the mean and residual stress sensitivity. The limits of the concept have been shown and its practical use has been demonstrated exemplarily.     

Evaluating the fatigue limit of metals having surface compressive residual stress and exhibiting shakedown

The objective of the study described in this article is to evaluate the effect of shakedown of surface compressive residual stresses introduced by shot peening on fatigue limit of stainless steel. First, the tension‐compression fatigue tests were conducted on ASTM CA6NM specimens under controlled load and displacement conditions to acquire a fatigue limit diagram under various compressive mean stress. The results showed that shakedown of negative mean stress occurs under controlled displacement. We then carried out in‐plane–bending fatigue tests under controlled load conditions on welded ASTM 309 stainless steel specimens with surface compressive residual stress introduced by ultrasonic shot peening. The results provide a fatigue limit of 415 MPa, which agrees with the value of 404 MPa calculated based on a modified Goodman line considering shakedown. Therefore, it is suggested that the surface layer is restricted by the internal bulk that creates controlled displacement conditions and the shakedown of surface compressive residual stress occurs.     

Modelling the effects of surface finish on fatigue limit in austenitic stainless steels

Existing short fatigue crack models have been reviewed to determine the most suitable fatigue model to analyse the effect of the surface finish on the fatigue limit of Type 304 austenitic stainless steels. A mechanistic model firstly proposed by Navarro and Rios (N‐R model) was selected as the most suitable generic model, because the model can include the effects of surface finishing parameters such as surface roughness and residual stress depth profile on the fatigue limit. The N‐R model has been implemented for fatigue specimens with various surface finishing conditions, and the effect of the surface finish on the fatigue limit was simulated. The material/surface properties required for the implementation were fully characterized by experiments. The applicability of the model to this study was also discussed. It is concluded that a development of the model would be required for proper prediction of the surface effects on fatigue in austenitic stainless steels.     

Relationship between decarburisation and fatigue strength of through hardened and carburising steels

Abstract

The effect of surface decarburisation upon the fatigue strength in rotating bending of a through hardened low alloy steel and a carburised carburising steel was investigated. The relationships between surface carbon content, depth of decarburisation, microhardness, residual stress, surface finish, and fatigue limit were examined. Whereas the fatigue limit was found to be independent of the depth of decarburisation over the range investigated (up to 1 mm), linear relationships were found between fatigue limit and surface carbon concentration and micro hardness. Furthermore, the presence of residual tensile stresses at the surface were found to lead to a reduction of fatigue limit. The fatigue limit of the rough machined specimens was found not to be significantly lower than that of the polished specimens.

MST/1179     

Impact of slide diamond burnishing additional parameters on fatigue behaviour of 2024‐T3 Al alloy

One of the methods for increasing fatigue life of symmetric rotary metal components is slide diamond burnishing (SDB). This method is implemented on conventional and computer numerical control machine tools by means of simple equipment, which is its main advantage. The SDB basic parameters are diamond insert radius, burnishing force, feed rate, and burnishing velocity. The additional ones are number of passes, working scheme, and lubrication conditions. The effect of SDB additional parameters on the fatigue behaviour of 2024‐T3 Al alloy was experimentally studied. Groups of smooth and notched hourglass‐shaped specimens were slide burnished using different combinations of additional SDB parameters and then were subjected to bending fatigue tests. The residual stresses, introduced by SDB, were measured by X‐ray diffraction technique. The near‐surface microstructure of the slide‐burnished specimens was investigated. Based on the results obtained, it was established that SDB produces two main effects, which depend on SDB additional parameters. The essence of the macroeffect is creation of residual compressive stresses in the superficial and subsurface layers. This stresses retard the formation and growth of fatigue macrocracks and thus increase the lifetime of slide‐burnished components. The microeffect is expressed in modifying the microstructure of the surface and subsurface layers, correspondingly, refining the grain and homogenizing and reducing the pores in the material. Such microstructure is characterized by increased plasticity and fatigue crack resistance. The fatigue life depends on the combination of these two effects. Thus, the desired fatigue behaviour of the slide‐burnished component can be ensured through an appropriate selection of the governing additional SDB parameters.     

表面超声滚压处理对高速列车车轴钢疲劳性能的影响

对EA4T型高速列车车轴钢棒状旋转弯曲疲劳试样实验段磨削加工后进行了表面超声滚压处理。观察了处理前后试样的表面形貌及表层微观组织,测量了处理前后试样的表面粗糙度、表层硬度及表层残余应力。利用旋转弯曲疲劳实验得到处理前后试样的疲劳极限。结果表明:表面超声滚压处理后,试样的疲劳极限由352MPa提高到401MPa。疲劳极限的提高主要由于表面超声滚压处理后试样表面粗糙度降低、表层强度及残余压应力增加。     

Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

Fatigue testing under fully reversed axial loading (R=?1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.     

Simulation‐based optimization of the local material state in the field of cyclically highly stressed case hardened construction details with notch effect*

Steel components very often show construction details, such as cross holes and rounded shaft shoulders which lead to local stress concentrations of the multiaxial stress state in case of mechanical loads (notch effect). Under cyclic loading these stress concentrations (hot spots) can cause crack initiation, crack propagation and finally failure of structural components. The fatigue strength of cyclically loaded components can be considerably increased by the heat treatment case hardening. The shape of the construction detail has a significant influence for the sub‐processes of the case hardening. This can be related to the carbon diffusion process during carburizing and the local heat transfer during quenching. As a result, the local material state following a case hardening process is often not optimal with respect to phase composition and residual stress field. In order to optimize the hardening process a heat treatment simulation based on the Finite Element Method was coupled with procedures for sensitivity analysis and optimization. Taking into account the operational loading conditions for the component, it was possible to adapt technological parameters of the case hardening process for the specific shape of the construction detail, leading to a substantially increased fatigue strength and therewith improvement of the efficiency of the case hardening process itself.     

Fatigue assessment of high strength leaf springs based on the FKM guideline

Analytical fatigue strength calculations based on the FKM guideline have been performed for hot tapered and stress‐shot‐peened high‐strength leaf spring specimens subjected to three‐point fatigue bending. The ultimate tensile strength of the decarburized specimens' surface has been approached by means of Rockwell‐C hardness measurements, and used as input for the approximation of its fatigue limit and mean stress sensitivity. Surface roughness and residual stress measurements were performed to take account for the technological life influencing factors. Fatigue tests at a constant mean stress and various stress amplitude levels were performed to determine the specimens' S–N curve and validate the calculation's accuracy. Comparison of calculated with experimentally determined fatigue lives, though satisfactorily, pinpoints the necessity for more accurate implementation of the stress‐shot‐peening process within the FKM guideline.     

A new method for modelling the support effect under rotating bending fatigue: application to Ti‐6Al‐4V alloy,with and without shot peening

The subject of this paper is to investigate the capability of the relative stress gradient to properly represent the beneficial effect of residual stress states on the fatigue life of Ti‐6Al‐4V specimens, with notches of different severity. The research was developed considering notched and un‐notched specimens with different geometries and different shot‐peening treatments. The results were determined by running fatigue experimentation under rotating bending and by developing a novel predictive model based on the relationship between the local fatigue limit and a generalized form of the relative stress gradient, accounting for the peening‐induced residual stresses. The proposed tool for fatigue limit estimation was completed by a stochastic analysis, which considered the variability of the involved parameters, in particular the residual stress entity. This made it possible to finally determine the component failure probability in a general, efficient and accurate way.