Effect of submillimeter size holes on the fatigue limit of a high strength tool steel

The very high cycle fatigue and small fatigue crack growth behaviour of a generic tool steel material for diesel fuel injector application are described. The small crack growth tests for the tool steel material with and without the hardening heat treatment revealed the mechanisms of crack propagation and threshold behaviour. Based on the small fatigue crack propagation threshold value, an elastic plastic fracture mechanics methodology for the prediction of the endurance limit of specimens with submillimeter holes is proposed. The advantages of the new methodology are discussed in relation to existing methodologies for endurance limit prediction of specimens with small holes.     

Microstructural influence on the scatter in the fatigue life of steel reinforcement bars

Fatigue damage accumulation and failure of steel reinforcement bars (rebars) is a stochastic process. Scatter can be influenced by the sensitivity of the short crack growth to the microstructural features, especially near the fatigue limit. This work investigates the scatter inherent to the microscopic conditions near the fatigue limit of ferrite–pearlite and martensite microstructures found in the outer layer of rebars. An adapted Navarro–De Los Rios model within a Monte-Carlo framework is used to simulate the short crack growth in material grains. Grain size variation, grain orientation factor and multiple phases i.e., ferrite–pearlite and martensite were considered in the model. The results are compared with the scatter found in fatigue tests on hot-rolled-cold worked (HR-CW) as well as quenched and tempered (QST) rebars. It is shown that microstructural effects explains part of the observed scatter in the fatigue tests.     

On the relationship between fatigue limit,threshold and microstructure of a low-carbon Cr-Ni steel

The relationship between the fatigue limit stress range, Δσ_w, the threshold stress intensity factor, ΔK_th, and microstructure of low-carbon 12CrNi3A steel has been investigated. Non-propagating microcracks were observed on the surface of smooth specimens which has been subjected to at least 5 × 10⁶ cycles at the fatigue limit stress. The size of the cracks depended on the characteristic sizes of the microstructure of the material. Scanning electron microscopy showed that the fractographic characteristics in the near-threshold region of fatigue macrocrack growth were similar to those in the fatigue microcrack initiation region. This implies that the fatigue limit and fatigue threshold of the material have a similar physical meaning, both signifying the resistance of the material to the propagation of fatigue cracks. The relationship ΔK_th = 1.12Δσ_W √πα was shown to be valid, where a is a material parameter relating to microstructure, rather than to the length of a macrocrack. The results also showed that the value of a depends on the material and microstructure, and that both Δσ_W and ΔK_th will change if the microstructural characteristics of the material change.     

Effects of annealing and quenching on fatigue behaviour in type 444 ferritic stainless steel

In order to understand the effects of annealing and quenching on fatigue behaviour in type 444 stainless steel, fully reversed axial fatigue tests have been performed using smooth specimens of heat‐treated materials in laboratory air and 3%NaCl aqueous solution. Three materials subjected to different heat treatments, annealing at 960 and 1000 °C, and water‐cooling at 960 °C, were prepared. In laboratory air, the fatigue limit of the annealed specimens was lower than that of the as‐received specimen and decreased with increasing annealing temperature. The subsequent grain coarsening from the heat treatments was primarily responsible for the lower fatigue strength in the annealed specimens. The fatigue strength of the water‐cooled specimen was lower than that of the corresponding annealed specimen. In the annealed specimens, cracks were generated within ferritic grains, while in the water‐cooled specimen, at or near grain boundaries. In 3%NaCl solution, the fatigue strengths of all specimens decreased compared with those in laboratory air. Only in the water‐cooled specimens, crack initiation at grain boundary and intergranular crack growth were observed, indicating the most sensitive to corrosion environment.     

珠光体对ZG120Mn13钢拉伸断裂过程的影响

为探究珠光体降低高碳高锰钢机械性能的原因,本文采用金相组织分析、机械性能测试和断口微观形貌分析等实验方法,研究了奥氏体基体上含体积分数23%珠光体的ZG120Mn13高碳高锰钢的拉伸性能及其裂纹形核和扩展过程.结果表明:通过时效处理,在奥氏体基体上析出的条状、颗粒状以及沿晶界连续分布的珠光体将使ZG120Mn13钢的强度和塑性大幅度下降.机械性能的降低与其力学行为有关,当基体为单一奥氏体时,裂纹将在大量孪生变形后,在孪晶界、孪晶与晶界交界处形核,并沿孪晶界长大而相互连接、扩展.而奥氏体基体上存在珠光体时,裂纹主要在珠光体团内形核,并通过相邻珠光体间奥氏体的塑性耗竭、切断而得以扩展.     

Effect of interlamellar spacing on the mechanical properties of 0.65% C steel

The mechanical properties of a steel containing a nearly fully pearlitic structure have been examined as a function of the interlamellar spacing. The steel had been heat-treated at different austenitization temperatures in order to obtain varying interlamellar spacings. It was observed that hardness and yield strength follow a Hall–Petch type of relationship with respect to the interlamellar spacing but the ultimate tensile strength (UTS), percent elongation and impact toughness did not do so. It was noted that, below a critical size of interlamellar spacing, the UTS, impact toughness and ductility remained invariant to the interlamellar spacing. The results have been explained on the basis of a microstructure–thermal residual stress relationship.     

The nature of initiation and propagation S-N curves at and below the fatigue limit

The traditional stress-life method of life prediction relies on an S – N curve of stress versus total life. However, the total life of a sample can be divided into two phases—an initiation phase and a propagation phase that leads to ultimate failure. Although this break-up of total life into two phases has been recognized in theory, there has been no experimental method to generate initiation and propagation S – N curves. In this paper a methodology to generate initiation and propagation S – N curves is presented. Acoustic emission technology is used to detect the transition from the initiation phase to the propagation phase. The phenomenon of fatigue limits is also explored and it is shown that the fatigue limit of the traditional S – N curve corresponds to the fatigue limit of the initiation phase and that initiated cracks continue to propagate at stress levels below the initiation endurance limit. It is also shown that no damage is accrued at stress levels below the fatigue limit. A method to extend the propagation life curve below the initiation endurance limit is also presented. The proposed two-phase S – N curve will greatly extend the life-predicting capability of the stress-life method and can explain some of the contradictions observed in experiments.     

Interaction effect of adjacent small defects on the fatigue limit of a medium carbon steel

Structural steels contain various material irregularities and natural defects which cause local stress concentrations from which fatigue cracks tend to initiate. Two defects in close proximity to each other may affect local stress distributions, and thus, begin to interact. In this paper, the effect of interacting small cracks on the fatigue limit is systematically investigated in a medium carbon steel. The growth of interacting cracks, as well as the characteristics of non-propagating cracks and microstructural aspects, was closely examined via the plastic replica method. It was found that although the fatigue limit is essentially controlled by the mechanics of interacting cracks, based on their configuration, the local microstructure comprised ferrite and pearlite has a statistical scatter effect on the behaviour of interacting cracks and non-propagating thresholds. With respect to the fatigue limit, when two defects were in close proximity, they behaved as a larger single defect. However, with greater spacing between defects, rather than mechanical factors, it is the local microstructure which determines the location and characteristics of non-propagating cracks.     

Effect of prior austenite grain size and pearlite interlamellar spacing on strength and fracture toughness of a eutectoid rail steel

Abstract

The influence of prior austenite grain size d_γ, and true interlamellar spacing of pearlite S_t on the strength and fracture toughness of a eutectoid rail steel has been investigated. Specimens were machined from rail sections and heat treated to produce a wide variation in d_γ and s_t. Mechanical properties studied included 0·2% proof stress σ_0·2, ultimate tensile strength σ_u, tensile ductility δ, cleavage fracture stress σ_f, and plane strain fracture toughness K_1c. All tests were performed at a temperature of ?80°C. The values of σ_0·2 and σ_u increase as s_t decreases. The proof stress is related to the mean free distance λin the pearlitic ferrite by a Hall–Petch equation. A microstructural dependence similar to that of σ_0·2 is shown by σ_f and for all but the finest pearlites σ_f is interpreted as a shear stress controlled cleavage nucleation stress. The value of K_1c first decreases with decreasing s_t and then increases for the finest spacings. This behaviour is attributed to a change in the micromechanism of cleavage nucleation as the pearlite spacing changes from coarse to fine. The value of d_y has very little effect on K_1c, but δ decreases progressively as d_γ increases. The effect of d_γ on K_1c is negligible because the fracture process zone is much smaller than the grain size and therefore the grain boundaries cannot influence the fracture processes occurring at the crack tip. The tensile ductility is interpreted as the strain necessary to develop an internal microcrack which then propagates as a quasibrittle fracture. The size of the microcrack is shown to be related to the pearlite nodule size which in turn is related to d_γ.

MST/396     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

Prediction of fatigue resistance of a hot-dip galvanized steel

The paper clarifies the effect of a galvanizing coating on the fatigue strength of a ferritic steel. Depending on experimental conditions and on the microstructure of the coating, a reduction in fatigue strength is observed especially when the coating is thick. Cracks in the galvanizing coating rapidly form under cyclic loading and then propagate into the steel substrate. This completely modifies the distribution of crack lengths. Very short cracks are not observed in the steel when galvanized. It is shown that the propagation of a crack in the substrate from the coating is only possible when the crack completely crosses the coating. By assimilating the coating thickness to a crack in the steel substrate, the fatigue resistance of hot-dip galvanized steel can be predicted using the Kitagawa–Takahashi diagram.     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Effect of microstructure and low cycle fatigue deformation on tensile properties of P91 steel

Isothermal furnace heat treatments were carried out to simulate the microstructures of inter-critical, fine grain and coarse grain heat-affected zones of P91 steel weld joint at different soaking temperatures ranging from just above AC₁ (837 °C) to well above AC₃ (903 °C). Interrupted low cycle fatigue tests were performed on the specimens of P91 steel up to 5 %, 10 %, 30 %, and 50 % of the total fatigue life at the strain amplitude of ±0.6 %, strain rate of 0.003 s⁻¹ and temperatures of 550 °C and 600 °C. Subsequently, tensile tests were conducted on the interrupt tested specimens at the same strain rate and temperatures. Soaking at the inter-critical temperature region reduces / deteriorates the tensile and yield strengths of base metal compared to fine grain and coarse grain regions. The inter-critical heat-affected zone accounted higher damage contribution towards the overall tensile behavior of the actual P91 steel weld joint. Substructural coarsening during strain cycling at elevated temperatures attributes to the rapid reduction in the initial yield strength up to 10 % of fatigue life of P91 steel. A higher amount of plastic strain accumulation during low cycle fatigue deformation resulted in a decrease in fatigue life of the inter-critical heat-affected zone of P91 steel.     

Short crack growth and fatigue life in austenitic-ferritic duplex stainless steel

The kinetics of short crack growth has been studied in austenitic‐ferritic 2205 duplex stainless steel. Smooth cylindrical specimens and specimens with shallow notch were subjected to constant plastic strain amplitude loading. The crack growth was studied in notched specimens. The notch area has been mechanically and electrolytically polished to facilitate the observation of crack initiation and growth. The initiated cracks were observed in an SEM (scanning electron microscope). The crack growth was studied using long distance QUESTAR optical microscope equipped with high‐resolution camera. In constant plastic strain amplitude loading the microcracks were initiated and their growth kinetics has been studied. The characteristic features of the crack growth at different plastic strain amplitudes were recorded. Two approaches to analyse the crack growth rates were adopted. The comparison of the prediction of the fatigue life using the plastic‐strain‐dependent crack growth rate was compared with Manson–Coffin law and the relation between parameters of this law and parameters of the short crack growth law were established.     

The effect of overload on the fatigue crack growth behaviour of 304 stainless steel in hydrogen

Fatigue crack growth (FCG) behaviour and its characteristics following tensile overloads were investigated for AISI 304 stainless steel in three different atmospheres; namely dry argon, moist air and hydrogen. The FCG tests were performed by MTS 810 servohydraulic machine. CT specimens were used for the tests and crack closure measurements were made using an extensometer. FCG rates of 304 stainless steel at both dry argon and moist air atmospheres have shown almost the same behaviour. In other words, the effect of moisture on FCG of this material is very small. However, in a hydrogen atmosphere, the material showed considerably higher crack growth rate in all regimes. In general, for all environments, the initial effect of overloads was to accelerate the FCG rate for a short distance (less than a mm) after which retardation occurred for a considerable amount of time. The main causes for retardation were found as crack blunting and a long reinitiation period for the fatigue crack. Regarding the environmental effect, the overload retardation was lowest in a hydrogen atmosphere. This low degree of retardation was explained by a hydrogen embrittlement mechanism. In a general sense, hydrogen may cause a different crack closure mechanism and hydrogen induced crack closure has come in to the picture. Scanning electron microscope and light microscope examinations agreed well with the above results.     

微观组织对贝氏体钢疲劳裂纹扩展行为的影响

为了研究组织对疲劳裂纹扩展行为的影响,对3种不同贝氏体组织钢进行了疲劳裂纹扩展实验,并采用SEM和EBSD等方法对裂纹进行了分析.结果表明,板条贝氏体组织在近门槛区和稳定扩展区阻碍裂纹扩展的能力最强,具有最小的裂纹扩展速率.板条贝氏体组织中的大角度晶界使裂纹更容易发生偏折,导致断口表面粗糙度增加,裂纹扩展受到较强的粗糙度诱导裂纹闭合效应的作用.随着ΔK的增大,塑性诱导裂纹闭合效应取代粗糙度诱导裂纹闭合效应开始占据主导作用,是板条贝氏体组织中裂纹扩展速率对ΔK的变化较敏感的原因.     

The fatigue crack growth of a ship steel in seawater under spectrum loading

Fatigue crack growth of ABS EH36 steel under spectrum loading intended to simulate sea loading of offshore structures in the North Sea was studied using fracture mechanics. A digital simulation technique was used to generate samples of load/time histories from a power spectrum characteristic of the North Sea environment. In constant load-amplitude tests, the effects of specimen orientation and stress ratio on fatigue crack growth rates were found to be negligible in the range 2 × 10^?5 to 10^?3 mm/cycle. Fatigue crack growth rates in a 3.5% NaCl solution were two to five times faster than those observed in air in the stress intensity range 25 to 60 MPa √m. The average fatigue crack growth rates under spectrum loading and constant-amplitude loading were in excellent agreement when the fatigue crack growth rate was plotted as a function of the appropriately defined equivalent stress intensity range. This procedure is equivalent to applying Miner's summation rule in fatigue life calculations.     

Multiaxial fatigue of quenched and tempered U2 steel: Testing and fatigue life prediction

Tension‐compression, tension‐tension, torsional, and 90° out‐of‐phase axial‐torsional fatigue tests were performed on a quenched and tempered U2 steel. All tests were conducted under force/torque control because macroscopic plastic strains were insignificant in the life range of interest (from 10⁴ to 2 × 10⁶ loading cycles). Stress‐based versions of the Fatemi‐Socie critical plane parameter and of the Smith‐Watson‐Topper parameter with a critical plane interpretation were evaluated using the experimental data. The Smith‐Watson‐Topper parameter was not able to correlate the test data. The Fatemi‐Socie method correlated most of the test data within factor‐of‐three boundaries. A modified Crossland invariant‐based parameter made of two interaction rules between the shear stress amplitude and the maximum hydrostatic stress, and of a definition of shear stress amplitude based on the maximum prismatic hull method, yielded fatigue life estimates in reasonable agreement with the experimental observations.     

Estimation of the endurance fatigue limit for structural steel in load increasing tests at low temperature

The endurance fatigue strength of structural steel S355 was investigated in fatigue tests according to the method of increasing stress amplitude. The so‐called ‘load increasing test’ is based on the direct correlation between the fatigue limit and the temperature changes caused by local plastic deformation ahead of the tip of a micro crack, which was initiated as a result of cyclic loading. In the present work the fatigue limit for testing temperatures 40°C and ?20°C was estimated not only from the temperature measurements but also from the electrical potential measurements. Further, the obtained results were validated in standard fatigue tests with constant stress amplitude and a very good agreement was found.