BIAXIAL CYCLIC DEFORMATION BEHAVIOUR OF STEELS

Abstract— The cyclic stress-strain curves for 1% Cr-Mo-V steel and AISI 316 stainless steel were determined under biaxial loading conditions at various temperatures and strain rates. It is shown that these curves may be correlated in terms of the maximum shear stress and strain amplitudes. It is argued that, even though metals obey the von Mises yield criterion for monotonie loading, the micromechanisms of slip which produce the stabilized cyclic stress-strain behaviour are governed by the Tresca criterion.     

工业纯钛低温拉伸和循环变形中的孪生行为

在-196℃下对钛进行了拉伸和低周循环变形,观察分析了变形后试样的微观组织.结果表明,工业纯钛在-196℃拉伸变形后,强度比在室温下拉伸变形有了明显的提高,塑性也有明显的增加;在-196℃下循环变形时,循环应力-应变曲线位于-196℃静拉伸应力-应变曲线的上方,显示出明显的循环硬化特征.微观组织观察表明,-196℃拉伸及循环变形试样中存在着大量的孪晶,且孪晶数量随着循环应变幅及循环周次的增加而增加.在工业纯钛-196℃下的拉伸及循环变形中孪生起重要作用.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

Base material fatigue data for low alloy forged steels used in the subsea industry. Part 2: Effect of cathodic protection

In air S–N fatigue data for forged low alloy steels as used in the subsea industry are presented in Part 1 of this paper. The test scope in Part 1 included testing to quantify the effect of the surface roughness, mean stress and material strength on the high cycle fatigue strength of low alloy steels with a tensile strength in the range of 600–800 MPa. A method for estimating the in air S–N curve from the tensile strength (material grade), surface roughness (machining) and mean stress (such as residual stresses, pressure testing, pre-load and external loads) is presented in Part 1. In this Part 2, fatigue test results for low alloy steels and one carbon steel tested in seawater with cathodic protection with a potential of −1050 mV versus an Ag/AgCl reference electrode are presented. The fatigue testing has been performed using smooth specimens. The tested smooth specimens have (actual) tensile strengths in the range from 627 to 790 MPa. Penalty factors for the tested smooth specimens in seawater with cathodic protection with respect to in air performance (Part 1) are presented and compared with penalty factors used in fatigue design codes such as DNVGL-RP-0005 (former DNV-RP-C203) and BS 7608. The obtained environmental reduction factors are found to be in accordance with the penalty factors used in BS 7608 provided that the maximum stress in the cycle is less than 94% of the yield stress for the material. The penalty factors used for forged steels in DNVGL-RP-0005 are non-conservative compared to the test outcome for the steel tested in an artificial 3.5% NaCl seawater solution. For higher stress levels, larger penalty factors than used in BS 7608 are required. It is found that the obtained S–N based environmental reduction factors are of similar magnitude as BS 7910 fatigue crack growth based reduction factors for CP.     

Probabilistic anisomorphic constant fatigue life diagram approach for prediction of P–S–N curves for woven carbon/epoxy laminates at any stress ratio

A general engineering methodology to construct a family of anisomorphic constant fatigue life (CFL) diagrams with probability of failure as the parameter that allows efficiently predicting P–S–N curves at any stress ratios is developed and validated for a plain weave fabric carbon/epoxy laminate. Constant amplitude fatigue tests are first performed to obtain statistical samples of fatigue life at different stress levels and stress ratios, respectively. Static tensile and compressive strength data are also collected. The Kolmogorov–Smirnov and Anderson–Darling goodness-of-fit tests suggest that both two-parameter lognormal and Weibull distributions are acceptable as the distributions for the static strength and fatigue life data, respectively, at the significance level of 5%. Then, we attempt to develop a methodology for efficient construction of the anisomorphic CFL diagrams for different constant values of probability of failure. It requires the P–S–N curves for any percentile points of the distribution for the critical stress ratio. To come up with this requirement, a probabilistic scaling law is formulated. It takes account of the probability-of-failure dependence of the critical stress ratio and the stress-ratio dependence of the P–S–N curve for the critical stress ratio. Finally, the anisomorphic CFL diagrams for different constant values of probability of failure are predicted using the proposed methodology, and they are shown to be in good agreement with the experimental results. It is also demonstrated that the P–S–N curves can efficiently and accurately be predicted for the woven CFRP laminate at any stress ratios using the proposed probabilistic anisomorphic CFL diagram approach.     

Influence of reverted austenite on fatigue life of 18 % nickel maraging steel

Maraging steels containing 18 % nickel are based on a low-carbon iron-nickel-cobalt-molybdenum system. These steels demonstrate a remarkable combination of high strength and high toughness. To investigate the effect of overaging and resulting austenite reversion on the fatigue behavior of C250 maraging steel, specimens in the solution annealed state were overaged at 510 °C for various intervals, resulting in volume fractions of reverted austenite ranging from 2.6 % to 11.4 %. The staircase method was used to calculate the fatigue limit. S−N curves were generated by testing a minimum of four stress levels and at least three samples at each stress level. Basquin's equation was used to fit the experimental stress-life data, and the Basquin exponent and fatigue strength coefficient were calculated. The presence of 2.6 % reverted austenite in the microstructure improved the fatigue limit and fatigue ratio, without significantly reducing tensile strength. The results can be interpreted in terms of reverted austenite having a crack blunting effect on the propagating cracks. A small amount of reverted austenite in the range of 2 %–3 % vol has a beneficial effect on the fatigue life.     

MECHANISMS AND FATIGUE PERFORMANCE OF TWO STEELS IN CYCLIC TORSION WITH AXIAL STATIC TENSION/COMPRESSION

Abstract— Fatigue tests conducted under fully reversed cyclic torsion, with and without superimposed axial static tension/compression loads, were carried out using hour-glass smooth specimens in laboratory air. A high strength spring steel and a 316L stainless steel, were employed to evaluate the effects of mean stress on fatigue performance. Experimental test results show that a biaxial tensile/compressive mean stress had no influence on the cyclic stress-strain response in both materials. However a biaxial tensile mean stress was found to be detrimental to fatigue life of the high strength spring steel but had no effect on the total fatigue life of 316L stainless steel. A compressive mean stress was found to be beneficial to the life of both steels. The fatigue behaviour of the two materials was investigated by experimental observations and the application of theoretical analyses of short crack growth behaviour. Based upon the analysis of surface acetate replicas it has been found that fatigue crack growth is material/stress-state dependent. A biaxial tensile static stress promoted a change in the direction of the Stage I (mode II) crack from the longitudinal direction to a plane normal to the specimen axis in the high strength steel but not in the stainless steel. Consequently a different growth behaviour of Stage I (mode II) cracks was observed for the two materials. The effect of a biaxial mean stress on fatigue crack growth behaviour of the two materials is analysed and described in some detail.     

Evaluation of local fatigue strength in spot weld by small specimen

It has been reported that high strength steel sheet cannot improve fatigue strength of components with a spot weld. The purpose of this study is to discuss the dominant factors on the fatigue strength of spot weld in order to clarify the reasons. A new fatigue testing technique is developed for a small specimen with a total length of less than 3 mm, and the local fatigue strength of heat‐affected zone (HAZ), which is the crack initiation site in the joint, in a mild steel sheet (270MPa‐grade) and a high strength steel sheet (590MPa‐grade) are evaluated by this technique. The fatigue strength of HAZ is almost equal in both steels although the tensile strength of the 590MPa steel is higher than that of the mild steel. The stress in the tensile‐shear spot‐welded joint under cyclic loading and the residual stress by the spot‐welding are evaluated by finite element analyses. The residual stress is tensile in both steels. However, the plastic deformation takes place in the joint of the mild steel and this releases the residual stress. On the other hand, the stress in the 590MPa steel is elastic and the residual stress decreases the allowable alternating stress. The stress under the condition of the experimental fatigue limit of the joint considering the residual stress coincides well with the fatigue limit diagram of HAZ, which means that the fatigue limit of the joint is determined by the fatigue limit of HAZ and the residual stress.     

The effect of PVD coatings on the tensile strength and low‐cycle fatigue resistance of stainless steel and titanium alloys

PVD coatings applied to components form hard, stronger layers and generate high residual compressive stresses that limit the plastic deformation in surface layers of the base metal thus increasing its tensile strength and resistance to fatigue loading. The purpose of this paper is to experimentally determine the influence of the deposition of 2 to 16.5‐μm‐thick PVD coatings of TiN, Cr, (Cr+TiN), (TiC)N, (TiAl)N onto specimens of stainless steel 321 and titanium alloys of types MILT‐81556A and (10‐2‐3; 4966) on their tensile strength and low‐cycle fatigue resistance when the development of large elastic–plastic strains takes place. The tensile and low‐cycle fatigue tests were conducted under conditions of axial zero‐to‐tension cycle of the stress‐controlled loading on flat 1‐ to 1.5‐mm‐thick specimens in the initial state (uncoated specimens) and after application of a PVD coating, including those after pretensioning or after cyclic prestraining in the low‐cycle fatigue range. The deposition of PVD coatings is found to enhance the characteristics of tensile strength and low‐cycle fatigue resistance in the quasi‐static fracture range. The deposition of PVD coatings on specimens cyclically prestrained to the values of 53–86% of the number of cycles to fracture, changes the cyclic properties of the material and predetermines the fatigue fracture mode only.     

Influence of microstructure and load ratio on cyclic fatigue and final fracture behavior of two high strength steels

The results of a recent study aimed at understanding the conjoint influence of load ratio and microstructure on the high cycle fatigue properties and resultant fracture behavior of two high strength alloy steels is presented and discussed. Both the chosen alloy steels, i.e., AerMet® 100 and PremoMet™ 290 have much better strength and ductility properties to offer in comparison with the other competing high strength steels having near similar chemical composition. Test specimens were precision machined from the as-provided stock of each steel. The machined specimens were deformed in both uniaxial tension and cyclic fatigue under conditions of stress control. The test specimens of each alloy steel were cyclically deformed over a range of maximum stress at two different load ratios and the number of cycles to failure recorded. The specific influence of load ratio on cyclic fatigue life is presented and discussed keeping in mind the maximum stress used during cyclic deformation. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic mode and to concurrently characterize the intrinsic features on the fracture surface. The conjoint influence of nature of loading, maximum stress and microstructure on cyclic fatigue life is discussed.     

FATIGUE BEHAVIOUR OF AUSTENITIC Cr-Mn-N STEELS

Abstract— Solid solution hardening and strain hardening are the dominating strengthening mechanisms to achieve a high yield stress and ultimate tensile strength in Cr-Mn-N steels for drill collars. The fatigue limit can be improved more effectively by solid solution hardening than by strain hardening, but the attainable hardness due to soluble elements is restricted by other metallurgical demands. Strain hardening significantly increases the strength and the fatigue limit is improved too, but to a lesser extent. The reason for this behaviour is the introduction of internal stresses of the I, II and III kind by forging the drill collars. This results in macroscopic stresses which are varying over the cross section and in stresses of a microscopical scale which lead to an early loss of linearity in the elastic line of the stress strain curve and to the well known Bauschinger effect. Cyclic softening and hardening is accompanied by a rearrangement of the dislocation structure as revealed by transmission electron microscopy. The results indicate that a multiaxial and homogeneous cold working to produce a stable dislocation structure and to avoid large directional internal stresses is extremely important to achieve high fatigue strengths. Crack growth and crack closure measurements were performed for determination of the effective cyclic threshold stress intensity range for evaluation of the influence of the grain size on the fatigue limit.     

The choice of cyclic plasticity models in fatigue life assessment of 304 and 1045 steel alloys based on the critical plane-energy fatigue damage approach

The present study intends to examine various cyclic plasticity models in fatigue assessment of 304 and 1045 steels based on the critical plane-energy damage approach developed earlier. Cyclic plasticity models of linear hardening, nonlinear, multi-surface, and two-surface were chosen to study fatigue damage and life of materials under proportional and non-proportional loading conditions. The effect of additional hardening induced due to non-proportional loading in 1045 steel and particularly in 304 steel was further evaluated as different constitutive models were employed. In the present study, the plasticity models were calibrated by the equivalent cyclic stress–strain curves. The merits of the models were then investigated to assess materials deformation under proportional and non-proportional loading conditions. Under non-proportional loading, the cyclic plasticity models were found to be highly dependent upon the employed hardening rule as well as the materials properties/coefficients.The stress and strain components calculated through constitutive laws were then used as input parameters to evaluate fatigue damage and assess the fatigue life of materials based on the critical plane-energy approach.The calculated values of stress components based on constitutive laws resulted in a good agreement with those of experimentally obtained under various loading paths of proportional and non-proportional conditions in 1045 steels. In 304 steel, the calculated stress components were however found in good agreement when plasticity models were employed for proportional loading conditions. Under non-proportional loading, the application of the multi-surface plasticity model in conjunction with the fatigue damage approach resulted in more reasonable results as compared with other plasticity models. This can be attributed to the motion of the yield surface in deviatoric stress space in the multi-surface model encountering additional hardening effect through estimated higher stress values under non-proportional loading conditions.Predicted fatigue lives based on the critical plane-energy damage approach showed such range of agreements as ±1.05–±3.0 factors in 1045 and 304 steels as compared with experimental life data when various constitutive plasticity models were employed.     

Cyclic deformation behaviour and morphology of polypropylene

The viscoelastic-plastic behaviour and stress relaxation in polypropylene after uniaxial simple tension or cyclic preloading are studied by an electrohydraulic, servo-controlled testing machine. The stress-strain curve data of simple tension at different strain rates show that the magnitude of stress depends strongly on strain rate. The stress relaxation behaviour after simple tension and cyclic preloading indicates complex features and reveals differences in the deformation of molecular chains in polypropylene subjected to different cyclic preloadings. The stress-strain curves and the stress relaxation curves under three mean strains are fairly different from each other. The microstructural changes in samples subjected to different cyclic preloadings were determined by scanning electron microscopy (SEM). The results of the stress-strain curves and the stress relaxation behaviour are discussed in comparison with the observation of SEM fractographs.     

Study of the fatigue lifetimes and crack propagation behaviour of a high speed steel as a function of the R value

High speed steels, such as the alloy H‐13, when used as forging dies are subjected to both wear and cyclic loading, and both of these factors can affect the useful life of such dies. It follows that it is of some importance to determine the fatigue characteristics of such steels. However, fatigue studies of such alloys are limited, especially with respect to fatigue crack propagation (FCP) behaviour as a function of mean stress, and therefore more detailed studies are necessary. In the present study, the fatigue lifetimes and the crack propagation behaviour of a high speed steel were experimentally investigated in laboratory air under different stress ratios, R. A modified linear‐elastic fracture mechanics (LEFM) approach was applied to analyze the experimentally‐obtained FCP behaviour. The predicted S–N curves and crack growth behaviour for a wide range of R ratios agree well with the experimental data, and the modified LEFM approach is therefore considered to be useful for evaluation of the fatigue behaviour of this class of high strength steels.     

An improved method for estimation of Ramberg–Osgood curves of steels from monotonic tensile properties

In the present study, first a method for estimating cyclic yield strength is improved and compared using the experimental data of 121 steels from literature. Correlations between cyclic deformation properties (K′ and n′) and monotonic tensile data are then investigated, and a simple method requiring only the monotonic tensile properties is developed for estimation of the Ramberg–Osgood curve. Prediction capability of the proposed method is not only evaluated using the aforementioned 121 steels, but also compared with several commonly used methods that are available in the literature. The proposed correlations are shown to predict the cyclic deformation properties of most of considered steels reasonable well.     

Fatigue limit and life evaluation formulae for compressive mean stress states

Considering the difference of stress concentration behaviour near micro-defects in materials, a unified fatigue limit evaluation formula has been developed, which can be applied to compressive mean stress states without the necessity of introducing additional material’s properties except for the modification parameter of mean stress. A generalised life evaluation formula has also been proposed and the dependence of fatigue life on mean stress can be simply expressed by the effect of fatigue limit corresponding to the mean stress. By considering the effect of fatigue limit corresponding to the compressive mean stress, the fatigue life can be evaluated by the same generalised formula as that developed for tensile mean stress state, without the necessity of carrying out additional S–N curve fatigue tests.     

Comparison of low cycle fatigue behavior of 304 stainless steels induced by tensile and torsional prestrain

The effect of tensile and torsional prestrains on fatigue lives of 304 stainless steels is compared. The fatigue life as a function of prestrain amplitudes exhibits an ‘S’ curve, and the inflexion point of the S curve is affected by prestrain modes and cyclic strain amplitudes. An interesting phenomenon is that when the onset stress of secondary hardening reaches 300 MPa, intergranular martensite begins to be formed. Combining martensite distribution, fatigue life and cyclic stress responses, the onset stress of secondary hardening is proposed to reflect the location of martensite nucleation sites. Compared with tensile prestrain, torsional prestrain results in higher stress. In addition, higher prestrain and cyclic amplitudes also lead to higher cyclic stress and hence earlier nucleation of intergranular martensite. Furthermore, the influence mechanism of prestrain modes and cyclic strain amplitudes on the inflexion point of S curve is revealed.     

FATIGUE DAMAGE IN AUSTENITIC-FERRITIC DUPLEX STAINLESS STEELS

Fatigue damage in two austenitic-ferritic duplex stainless steels, with the structure of a natural composite and different levels of nitrogen content, was studied in low-cycle fatigue. Both steels show initial cyclic hardening followed by softening and a long stabilisation period. The cyclic stress-strain curve increases with the nitrogen content while Manson-Coffin curves of both steels intersect at medium fatigue lives. The study of the surface relief reveals intensive slip markings both in ferrite and in austenite. Their density is influenced by the nitrogen content. Both the intensity and density of the persistent slip band (PSB) markings are higher in the ferrite. Crack initiation was found to appear predominantly in PSBs in the ferritic grains at the low strain amplitudes, and in the ferritic and austenitic grains at the highest strain amplitudes. The level of the cyclic stress-strain response and the fatigue lives are discussed in terms of the cyclic strain localisation and of the effect of texture and nitrogen content on the strength and fatigue damage. The increased strength of the austenitic phase, due to high nitrogen alloying, results in cyclic slip localisation in the ferrite, and the decrease of fatigue life, compared with the steel with the lower nitrogen content.     

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation.
The fatigue strength of the steel wires between 10⁶ and 10⁹ cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 10⁷ cycles. For Cr–V spring wire, the stress–life ( S – N ) curve becomes horizontal at a maximum stress of 800 MPa after 10⁶ cycles, but the S – N curve of the Cr–Si steel continues to drop at a high number of cycles (>10⁶ cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.