Air fatigue of notched 1018 steel in the endurance limit range

Reverse bend, constant deflection fatigue tests were performed in ambient temperature air on notched 1018 steel specimens. Endurance limit for these exhibited scatter in excess of 25 pct, and this was correlated with concentration of manganese sulfide inclusions at the notch root vicinity. For specimens designated as “high inclusion” the endurance limit was 95 MN/m² (14 ksi); and for “low inclusion” ones it was 130 MN/m² (19 ksi). By monitoring crack length as a function of number of cycles it was observed that unbroken specimens tested above approximately 85 MN/m² (12.5 ksi) contained nonpropagating cracks. Also, high inclusion specimens fatigued at stresses above their endurance limit, but less than about 140 MN/m² (20 ksi) exhibited fatigue crack hesitation; that is, cracks initiated and grew to microscopic size but then arrested for a finite number of subsequent cycles. The endurance limit for high inclusion specimens was then a stress above which arrested cracks repropagate and below which they do not. Repropagation of hesitant fatigue cracks is projected to result from inclusion related cyclic stress damage near the crack tip, which was possibly stimulated by atmospheric interactions.     

压痕对车轴钢疲劳极限的影响

利用硬度计在光滑沙漏状车轴钢疲劳试样上制造压痕,同时利用电火花在试样上加工缺陷,通过疲劳试验研究两种缺陷尺寸与试样疲劳极限之间的关系.将两类试样的测试结果和基于材料硬度、缺陷投影面积的Murakami模型计算结果进行对比.利用扫描电镜观察试样疲劳断口.结果表明,与计算结果相比较,压痕局部塑性变形导致的加工硬化和残余应力对试样的疲劳强度没有影响,裂纹依然从应力集中最大的压痕底部起裂.电火花缺陷表面粗糙度较大引起二次缺口效应,表面硬脆的重铸白层上还有微孔和微裂纹存在,导致此类试样疲劳强度低于模型计算结果,裂纹从电火花缺口底部多处萌生.     

Large strain mechanical behavior of 1018 cold-rolled steel over a wide range of strain rates

The large-strain constitutive behavior of cold-rolled 1018 steel has been characterized at strain rates ranging from to 5 × 10⁴ s⁻¹ using a newly developed shear compression specimen (SCS). The SCS technique allows for a seamless characterization of the constitutive behavior of materials over a large range of strain rates. The comparison of results with those obtained by cylindrical specimens shows an excellent correlation up to strain rates of 10⁴ s⁻¹. The study also shows a marked strain rate sensitivity of the steel at rates exceeding 100 s⁻¹. With increasing strain rate, the apparent average strain hardening of the material decreases and becomes negative at rates exceeding 5000 s⁻¹. This observation corroborates recent results obtained in torsion tests, while the strain softening was not clearly observed during dynamic compression of cylindrical specimens. A possible evolution scheme for shear localization is discussed, based on the detailed characterization of deformed microstructures. The Johnson-Cook constitutive model has been modified to represent the experimental data over a wide range of strain rates as well as to include heat-transfer effects, and model parameters have been determined for 1018 cold-rolled steel.     

Thermal activation model of endurance limit

A thermal activation model of the endurance limit is proposed in the present study. It can quantitatively explain the effects of temperature and frequency on the endurance limit of metals at or below room temperature. Theoretical analysis indicates that the endurance limit, σ_ac, which is considered as a parameter characterizing the resistance of metals to cyclic microplastic deformation, has the same thermally activated nature as the plastic flow stress has and it can be resolved into two independent components: the long-range internal stress (the athermal component), μ(ε_apc), and the short-range effective stress (the thermal component), σ_a ^*(T, ε_p). The former is considered as a material constant insensitive to temperature and strain rate (or frequency). The latter, the temperature- and strain rate-dependent part of the endurance limit, is approximately identical with the effective stress component of plastic flow stress (or cyclic yielding stress). In light of the thermal activation model, the temperature and strain-rate dependence of monotonic and cyclic flow stresses in a low alloy steel (16Mn) and a precipitation-hardening aluminum alloy (LY12CZ) were experimentally investigated. The results indicate that the effective stress components of monotonic and cyclic flow stresses are identical, if the temperature and strain rate are held unchanged, and that both of them are approximately independent of the magnitude of plastic strain. On the basis of the thermal activation model, an expression predicting the endurance limit below room temperature is offered. The predicted values of the endurance limit agree with the test data of steels and aluminum alloys available in literature.     

Microstructural characterisation of nickel rich areas and their influence on endurance limit of sintered steel

Abstract

Nickel is an often used alloying element in powder metal steel to achieve high hardenability. However, when nickel is added, the slow diffusion rate between iron and nickel leads to the formation of nickel rich areas (NRAs). Two steel alloys were studied: a Fe–6·4Ni–0·7Mo–0·7C with standard sized nickel powder additions and a Fe–2·4Ni–0·7Mo–0·7C with a finer sized nickel powder. Microstructural characterisation of the parts revealed that sufficient hardenability was achieved for both materials, but that NRAs were observed when standard sized nickel is used. X-ray energy dispersive spectrometry and electron diffraction show that the NRAs are composed of martensite and austenite under rapid cooling conditions. Three-point bending fatigue tests were carried out on both alloys to evaluate the effect of these soft austenitic areas on the fatigue properties of powder metal steel parts. The analysis of the endurance limit results shows that NRAs are not a governing factor.     

Demonstration of an endurance limit in cast 319 aluminum

An investigation of the fatigue properties of cast W319-T7, an Al-Si-Cu alloy used in automotive engine components, was conducted using ultrasonic testing equipment with operating frequencies of 20 kHz. The stress-life (S-N) behavior at room temperature was determined for three solidification conditions of this alloy, where stresses for fatigue lives ranging from 10⁵ to 10⁹ cycles were determined. The results are compared to fatigue data acquired using servohydraulic equipment operating at 40 Hz. No influence of loading frequency has been observed. A discrete endurance limit is indicated for each of the three solidification conditions of W319-T7. The scientific and practical implications of this result are discussed. A material model presented previously is modified by introducing a crack growth threshold condition in order to predict the observed endurance limits. The model is shown to effectively predict the influence of solidification time on the fatigue properties of W319-T7.     

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 10¹⁰ cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 10⁹ cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10^-4) of the fatigue limit. The slip bands continued to develop up to about 9 × 10⁹ cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 10¹⁰ cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 10¹⁰ cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.     

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 10¹⁰ cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 10⁹ cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10^-4) of the fatigue limit. The slip bands continued to develop up to about 9 x 10⁹ cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 10¹⁰ cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 10¹⁰ cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.     

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.     

Effect of grain size,cold work,loading frequency and temperature on fatigue limit of mild steel

Fatigue tests are performed on mild steel (0.07 pct ?C) having different grain sizes or 5 and 10 pct cold work at frequencies of 83.3 Hz (LF) and 23 kHz (HF) at room and elevated temperatures. The fatigue limit was found to be a function ofd ^?1/2, whered is grain size, for HF loading at different temperature conditions. The ratio between fatigue limits determined at various frequencies and different temperatures during testing was found not to be strongly dependent on grain size. Prior cold work increased the HF fatigue limit atRT. HF tests at room temperature displayed higher fatigue limits than for LF loading. Increases in temperature during HF tests significantly decreased the fatigue limit for different grain sizes and various amounts of cold work. These results are explained by physical-metallurgical models.     

Effect of grain size, cold work, loading frequency and temperature on fatigue limit of mild steel

Fatigue tests are performed on mild steel (0.07 pct ‡C) having different grain sizes or 5 and 10 pct cold work at frequencies of 83.3 Hz (LF) and 23 kHz (HF) at room and elevated temperatures. The fatigue limit was found to be a function ofd ^−1/2, whered is grain size, for HF loading at different temperature conditions. The ratio between fatigue limits determined at various frequencies and different temperatures during testing was found not to be strongly dependent on grain size. Prior cold work increased the HF fatigue limit atRT. HF tests at room temperature displayed higher fatigue limits than for LF loading. Increases in temperature during HF tests significantly decreased the fatigue limit for different grain sizes and various amounts of cold work. These results are explained by physical-metallurgical models.     

Initiation of fatigue cracks in 4340 steel

Electron microscopy of surface replicas has been employed to determine the metallurgical micromechanics of fatigue crack initiation at surface and shallow subsurface inclusions in 4340 steel. The experimental results are considered in terms of current theoretical analyses of elastic stress distributions around hard inclusions imbedded in soft matrices. A model for crack nucleation at surface inclusions is suggested, based upon debonding of the inclusion/matrix interface at the tensile pole of the inclusion, followed by growth of the debond seam toward the inclusion “equator”. Subsequent generation of a matrix crack is associated with the initiation of “point” surface defects some distance from the inclusion/matrix boundary, but in the plane of the inclusion “equator”. Surface slip plays no discernible role in nucleation of surface inclusions, but slip bands are formed at crack sites above sub-surface inclusions. However, the microcracks continue to form through the link-up of point surface defects.     

Electrochemical detection of fatigue cracks in steel

In an earlier study, the current flow during reanodization of fatigued specimens of aluminum was shown to provide a simple method of detecting fatigue cracks. An immersion scanning technique identified the location of the cracks, and the flow of charge was a measure of the crack length. This paper describes the application of a similar technique to SAE 1065 steel. Anodic surface oxide films are formed in a passivating electrolyte of boric acid and sodium borate. These films are ruptured during fatigue of the steel substrate, and the rupture sites are identified by the flow of current during subsequent reanodization. Fatigue cracks as small as 0.9 mm long are detected and the flow of charge during reanodization of a fatigue crack is again a quantitative measure of the total length of the crack.     

Ratcheting behavior of notched stainless steel samples subjected to asymmetric loading cycles

The ratcheting response of 316 stainless steel samples at the vicinity of notch roots under single- and multi-step load-ing conditions is evaluated. Multi-step ...     

Microcracking and fatigue in a carburized steel

A carburized coarse-grained AISI 8620 steel was subjected to three postcarburization heat treatments: a) direct oil quench from the carburizing temperature (1700°F), b) direct oil quench, reheat to 1550°F and oil quench, and c) slow cool, reheat to 1550°F and oil quench, reheat to 1450°F and oil quench. The latter two treatments refined the austenitic grain size over that resulting from the direct quench and caused a reduction in the size of the marten-site plates and of the number and/or size of the microcracks within the plates. The refine-ment of the microstructure and the reduction of the number of microcracks resulted in greatly improved fatigue resistance,i.e., from fatigue limits of 140 to 250 ksi maximum cyclic stress for the direct quench and double reheat conditions, respectively. Subcritical crack growth was transgranular, but the mode of unstable crack propagation was mixed transgranular and intergranular in the direct quench and single reheat specimens. Obser -vations of microcrack coalescence and fracture surface features suggest that microcracks are instrumental in the transgranular mode of failure. C. A. Apple, formerly Postdoctoral Fellow, Lehigh University, Bethlehem, Pa. 18015     

Skeletal muscle fatigue and physical endurance of young and old mice

In order to evaluate the role played by muscular and extramuscular factors in the development of fatigue in old age, the time course of fatigue in isolated skeletal muscles and spontaneous motor activity and endurance of whole animals were monitored using young (3-6 months) and old (34-36 months) CF57BL/6J mice. The isolated extensor digitorum longus (EDL) and soleus muscles from old mice had smaller (P < 0.05) mass and developed lower (P < 0.02) maximal tetanic tension at 100-Hz stimulation than the muscles of young mice. During stimulation at 30 Hz every 2.5 s, a 50% decline in original tetanic tension occurred by 109 s in young EDL and 129 s in old EDL, but by 482 s in young soleus and 1134 s (projected) in old soleus, indicating more (P < 0.05) resistance to fatigue in old than young soleus. However, the old mice showed significantly fewer (P < 0.002) spontaneous ambulatory movements than the young mice. On a treadmill with a belt speed of 10 m/min at an inclination of 0 degrees, the old mice could only run for 22 min compared to 39 min ran by young mice (P < 0.02). They took more rest periods (P< 0.02) than the young mice. In a quantitative swimming monitor, the old mice swam for a shorter (P < 0.05) time than young mice (20.4 min compared to 28.6 min). Integrated swimming activity at 20 min was smaller (P < 0.05) in old mice than in young mice (413 g/s compared to 628 g/s). Hence increased fatigue in old age is not caused by impairment of processes within the muscles, but by impairment of central or extramuscular processes.     

Environmentally assisted fatigue crack propagation in steel

The influence of various gasenous environments on fatigue crack propagation has been determined for three quenched and tempered steels with yield strength levels of 800 to 1400 MN/m². The crack growth rate was increased by an order of magnitude in low pressure (13 KPa) hydrogen, and by a factor of two in most mildly aggressive environments relative to the growth rate in vacuum. The gases oxygen, acetylene, carbon monoxide, and nitrous oxide were dominant in a combined environment with hydrogen while methane and carbon dioxide had only a small effect on crack propagation when added to hydrogen. The crack propagation in acetylene was intermediate between that in hydrogen and the mildly aggressive environments. The increase in fatigue crack propagation rate in the hydrogen environment was dependent on the temperature and the cyclic stress intensity. The fracture mode was transgranular for all conditions except the hydrogen influenced HP-9-4-20 fractures. These results are discussed relative to various stages of the hydrogen embrittlement mechanisms. In pacticular, the results are discussed with respect to the adsorption-dissociation of the environment, transport of the gaseous specie within the plasticly deformed zone by mobile dislocations and interaction with segregated impurities within the metal. H. L. MARCUS, formerly with Science center, Rockwell International, Thousand Oaks, CA     

Skeletal muscle fatigue and endurance in young and old men and women

The effects of increasing age on skeletal muscle fatigue and endurance were assessed in 22 healthy young (14 men and 8 women; mean age, 28 +/- 6 years) and 16 healthy old (8 men and 8 women; mean age, 73 +/- 3 years) individuals. All subjects performed 100 repeated maximum dynamic knee extensions at 90 degrees.s-1 (1.57 rad.s-1) using an isokinetic dynamometer (Cybex II). Peak torque was recorded during every contraction, and for each individual the maximal voluntary contraction (MVC), the fatigue rate, the endurance level, and the relative reduction in muscle force were determined. MVC and endurance level were significantly lower in old men and women, but there was no discernible difference in relative muscle force reduction and fatigue rate between young and old individuals. We conclude that thigh muscles of older individuals are weaker than those of younger individuals, but relative to their strength, older individuals have similar properties as younger individuals with respect to muscle fatigue and endurance.     

Load sharing of the phases in 1080 steel during low-cycle fatigue

By means of X-ray diffraction, the stress response of the individual phases in a 1080 steel were measured. Specimens with pearlitic, spheroidal, and tempered martensitic microstructures were subjected to low-cycle fatigue and the stress-strain hysteresis loops were separated into components for the carbide and matrix phases. Calculations of the microstresses formed by differential plastic deformation of the matrix and inclusions accurately model the spheroidite. Measured microstresses in the pearlite are smaller than the predicted values, probably due to yielding of the cementite and limitations on modeling the morphology. Work-hardening rates associated with the microstresses also qualitatively agree with the measurements. The tempered martensite cyclically softens with fatigue loading. The increased plastic strain range in the tempered martensite with cyclic softening is accompanied by an increase in the microstresses. These microstresses are significantly larger than predicted. Formerly Research Assistant, Department of Materials Science and Engineering, McCormick School of Engineering, Northwestern University