Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.     

Scatter in fatigue life due to effects of porosity in cast A356-T6 aluminum-silicon alloys

Porosity is well known to be a potent initiator of fatigue cracks in cast aluminum alloys. This article addresses the observed scatter in fatigue life of a cast A356-T6 aluminum-silicon alloy due to the presence of porosity. Specimens containing a controlled amount of porosity were prepared by employing a wedge-shaped casting mold and adjusting the degassing process during casting. High-cycle fatigue tests were conducted under fixed stress conditions on a series of specimens with controlled microstructures (especially, the secondary dendrite-arm spacing), and the degree of scatter in the results was assessed. Stochastically, such scatter was found to be adequately characterized by a three-parameter Weibull distribution function. Large pores at or close to the specimen surface were found to be responsible for crack initiation in all fatigue-test specimens, and the resultant fatigue life was related to the initiating pore size through a relationship based on the rate of small-fatigue-crack propagation. With respect to the probabilities for the pores of various sizes and locations to initiate a fatigue crack, a statistical model was developed to establish the relationship between the porosity population and the resultant scatter in fatigue life. The modeling predictions are in agreement with the experimental results. Moreover, Monte-Carlo simulation based on this model demonstrated that the average pore size, pore density, and standard deviation of the pore sizes, together with the specimen size and geometry, are all of consequence regarding scatter in fatigue life.     

Effects of fine porosity on the fatigue behavior of a powder metallurgy superalloy

Hot-isostatically-pressed powder-metallurgy Astroloy was obtained which contained 1.4 pct, fine porosity at the grain boundaries produced by argon entering the powder container during pressing. The pores averaged about 2μ,m diam and 20 μ m spacing. This material was tested at 650 °C in fatigue, creep-fatigue, tension, and stress-rupture and the results compared with previous data on sound Astroloy. The pores influenced fatigue crack initiation and produced a more intergranular mode of propagation. However, fatigue life was not drastically reduced. A large 25 μm pore in one specimen resulting from a hollow particle did reduce life by 60 pct, however. Fatigue behavior of the porous material showed typical correlation with tensile behavior. The plastic strain range-life relation was reduced proportionately with the reduction in tensile ductility, but the elastic strain range-life relation was little changed reflecting the small reduction in strength divided by modulus for the porous material.     

Microstructure-based fatigue life prediction for cast A356-T6 aluminum-silicon alloys

Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356 alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment. The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment.     

Powder Metallurgy Group Meeting,Edinburgh, 24–26 October 1983: Report of proceedings

Abstract

Porosity in sintered powder metals may contribute to fatigue strength degradation in two ways. First, pores will act as local stress concentrators and, second, they may act act as fatigue crack precursors. Accordingly, the effect of porosity on fatigue crack initiation was chosen as the thrust of the present study. Conventional powder metallurgical techniques were employed to generate various levels of porosity in a heat treatable steel of the AISI 4600 type. Porous steel specimens, in a modified compact tension configuration, were cyclically loaded and cycles to initiation noted. Initiation was defined as the generation of a fatigue crack 0·10 mm in length at the notch root. As expected, the greater the porosity content, the earlier the crack developed. There are two interdependent variables in porosity character for a given porosity content: these are the average interpore spacing and the average pore diameter. The region of concentrated stress around each pore is proportional to the cube of the diameter of the pore, whereas the total volume of material to be damaged between pores is proportional to the cube of the interpore spacing. The present study found that cycles to initiation clearly depended on the volume of highly stressed material adjacent to pores, relative to the volume of void free material between pores. The correlation suggests that porosity effects on fatigue crack initiation are primarily stress concentration effects: pores as crack precursors seem less important. PM/0323     

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing. Formerly with Carnegie Mellon University     

Identification of rolling-sliding damage mechanisms in porous alloys

Lubricated rolling-sliding damage in a relatively soft Fe-0.6 pct C alloy and a relatively hard carbonitrided iron, both produced by powder metallurgy, has been investigated. Damage mechanisms were controlled by large-scale as well as small-scale plastic deformations. A large-scale, bulk plastic deformation process produced surface densification in the Fe-0.6 pct C alloy. Formation of surface cracks by asperity-scale plastic shearing was also observed in both materials. Small-scale plastic deformation processes, restricted to the pore edges, gave rise to the formation of fatigue microcracks at the boundary between the densified and undensified region in the Fe-0.6 pct C alloy. A similar effect was found at a depth of between 550 and 1000 μm in the carbonitrided material. Moreover, in the Fe-0.6 pct C alloy, these plastic deformations also triggered the formation and propagation of macrocracks, which produced macroscopic damage by spalling. The damage mechanisms due to small-scale plastic deformations were explained on the basis of a local approach model, able to account for the influence of pores on the mechanical behavior of the materials. However, this approach could not explain the microcracks, which were found at the surface pores in the carbonitrided material. Their formation was ascribed to the interplay between the surface tensile (friction) stresses and the low matrix toughness of the material near the surface.     

High temperature fatigue properties and crack initiation and its expansion mechanism of high nitrogen bearing steel

The flexural fatigue properties of 40Cr15Mo2VN high nitrogen bearing steel at 200?? was 883MPa, which was 17% lower than that at room temperature. The results show that the types of fatigue failure are surface failure initiation and internal non- metallic inclusion. Compared with room temperature, the threshold value of fatigue stress intensity factor ??Kth at 200?? decreases by 20%, leading to the reduction of the critical non- metallic inclusion size at the start of fatigue crack initiation. The interaction of small cracks near the fatigue source in the surface initiation cracks at 200?? increases the stress intensity factor KI and accelerates the initial surface crack propagation. At the same time, according to the ratio of the nominal stress amplitude and the ultimate fatigue strength of the non- metallic inclusions in the high- nitrogen bearing steel, the influence of the size, position and temperature of the non- metallic inclusions on the fatigue life was analyzed.     

Influence of Rare Earths on Contact Fatigue of Rail Steels

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钴磷镀层表面热疲劳裂纹的萌生及扩展机理

在自约束型热疲劳试验机上对化学镀钴磷合金镀层进行了热疲劳试验,采用光学显微镜、扫描电子显微镜研究了在热应力作用下镀层表面热疲劳裂纹萌生、扩展的方式和形态。结果表明:200次冷热循环后,V型缺口处发生塑性变形,并且随冷热循环的进行,热疲劳裂纹由缺口底部萌生并沿着循环方向扩展。重点分析了钴磷合金镀层表面热疲劳裂纹的萌生及扩展机理。     

Fatigue behavior of beta processed titanium alloy IMI 685

The low cycle fatigue behavior of IMI-685 alloy withβ-processed andβ-annealed microstructures was investigated. Material with large colony structure ofα-platelets oriented in the same direction, resulting from insufficientβ-work and slow cooling rate from theβ-phase region, exhibited lower fatigue strength than material with basketweave arrangement of theα-platelets. Most of the fatigue crack initiation and propagation processes were dominated by cracking related to intense shear across a colony. The size of the shear related initial cracks could be limited by reducing the colony size, which resulted in an increased fatigue strength. In the large colony microstructure, it was possible to cause a substantial fatigue life debit by introducing a small planar defect on the surface or by applying 5 min dwell time at peak load. The combination of planar defect and dwell time caused the highest life debit. Residual porosity of negligible size caused, in the large colony microstructure, a fairly large, subsurface, cleavage-like planar defect that participated in the initiation of fatigue cracks. Due to its appearance on the fracture surface, the defect which is characterized in detail in the paper, was named cleavage rosette. J. A. HALL, formerly with the Air Force Materials Laboratory     

FATIGUE BEHAVIOUR OF SINTERED METALS AND ALLOYS

Abstract

Published information on the fatigue behaviour of sintered materials is reviewed.

Porous sintered materials exhibit similar fatigue characteristics to cast and wrought materials, including fatigue limits in ferrous materials. Their endurance ratios are slightly lower than those of similar wrought materials and they may depend on porosity content. In some cases fatigue data for sintered materials show less scatter than those for similar wrought materials. The total porosity content, which is mainly determined by compacting conditions, is the most important factor influencing fatigue behaviour. Endurance limit decreases as the porosity content increases. In the copper- and iron-base materials investigated, fatigue behaviour is influenced only slightly by powder characteristics, sintering temperature, atmosphere, and time, and by post-sintering treatments. Environmental and surface conditions seem to influence the fatigue behaviour in the same manner as pore-free materials. However, notches have a less severe effect than on pore-free materials. Fatigue fracture appears to occur in the same manner as in pore-free materials. Fatigue cracks tend to start at the free surface of the specimen in preference to the internal surfaces of pores, in agreement with theoretical predictions. Sintered low-alloy steels can be heat-treated to give a wide range of fatigue strengths, and they are less notch-sensitive than pore-free steels. The fatigue properties of sintered and pore-free materials are compared and sintered materials are shown to possess fatigue strengths in the same range as cast and wrought materials.     

Propagation of surface cracks on beam blank in the mould during continuous casting

ABSTRACT

We focus on crack propagation to investigate surface cracks in the mould during continuous casting, based on the crack initiation mechanism proposed in previous studies. The temperature and stress data of a solidified shell were extracted, and an extended finite element model based on the continuous damage theory of elastic–plastic materials was developed to simulate surface crack propagation. The results showed that, in the cracked area, stress concentration occurred at the crack tip, and the element split open and the crack propagated when the maximum principal stress in the stress concentration area reached the critical value. Prefabricated cracks in the fillet and web mainly developed into longitudinal cracks in the mould. The theoretical mechanism of this study was found to be the same as the crack propagation mechanism observed during the actual production of beam blanks. Thus, this study reveals the theoretical principle of crack initiation and propagation and can provide theoretical guidance for controlling surface cracks during beam blank continuous casting.     

Energy-Based Prediction of Low Cycle Fatigue Life of High-Strength Structural Steel

 Energy-based models for predicting the low-cycle fatigue life of high-strength structural steels are presented. The models are based on energy dissipation during average of cycles, cycles to crack propagation and total cycles to failure. Plastic strain energy per cycle was determined and found as an important characteristic for initiation and propagation of fatigue cracks for high-strength structural steels. Fatigue strain-life curves were generated using plastic energy dissipation per cycle (loop area) and compared with the Coffin-Manson relation. Low cycle fatigue life was found similar from both methods. The material showed Masing-type behavior. The cyclic hysterisis energy per cycle was calculated from cyclic stress-strain parameters. The fracture surfaces of the fatigue samples were characterized by scanning electron microscope and the fracture mechanisms were discussed.     

The crystallography of fatigue crack initiation in coarse grained astroloy at 20°C

The effects of crystallographic orientation on fatigue crack initiation has been examined for coarse-grained Astroloy at 20°C. Specimens were cycled by three-point bending at stress ranges between 5 and 95% of the proportional limit until fatigue cracks were detected. The crystallographic orientation of individual grains within which fatigue cracks initiated was determined by use of selected area electron channeling. Grains forming cracks were found to have surface normals near the 〈100〉, 〈011〉, and 〈113〉 directions. Conversely, grains which did not initiate cracks were not similarly grouped in orientation. Calculations of the Taylor factor using the Bishop-Hill approach revealed that fatigue crack initiation in Astroloy occurred at grains with low values of the Taylor factor.     

Effect of Hot Deformation on Formation and Growth of Thermal Fatigue Crack in Chromium Wear Resistant Cast Iron

Chromium wear resistant cast iron is widelyusedin engineering, mining and power industry forits high strength,hardness and wear resistance .Inproduction process , some wear-resistant parts ser-ving in alternative stress due to rapid heating andcooling rate ofteninduces thermal fatigue andresultsinfailure .The badthermal fatigue property of chro-mium wear resistant cast iron is due to eutectic car-bides which distribute as continuous net in matrix.Recent researches showthat hot deformation can …     

Microstructural examination of

Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508 class 3 low-carbon steel. A 3-mm-long crack was introduced in compact tension (CT) fatigue test specimens under four different loads in order to obtain crack tip plastic zones at different stress intensity factor ranges, ΔK = 18, 36, 54, and 72 MPa√m. The microstructure of the plastic zones around the crack tip were examined by trans- mission electron microscopy (TEM) and selected area electron diffraction (SAD). Micro- orientation of the dislocation cells in the plastic zones of all of the CT samples increased to 4 deg from the level of an as-received sample. Four-point bending fatigue tests were performed for plate shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4 deg in the damaged area where cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning is a prerequisite for the fatigue crack initiation and propagation in SA508. These observations may lead to better under- standing of how fatigue initiation processes transit to cracks.     

Prediction of Fatigue Lifetime under Multilevel Cyclic Loading Based on a Short Crack Growth Model in a Low Carbon Steel SAE 1017

This contribution deals with the initiation and growth of short fatigue cracks in cyclic loaded notched specimens made of the 0.15 wt% carbon steel SAE1017. On the basis of experimentally determined data, a damage model based on cyclic crack growth has been developed, which accounts for the anomalous behaviour of short fatigue cracks. In this model the initiation and propagation of a critical crack is regarded as damage. This approach allows to calculate fatigue life for constant amplitude tests as well as for multilevel tests and irregular loading. Deviations from Miner's rule, which have been often observed for two‐level tests, are attributed to the varying fraction of crack initiation and propagation phase for different loads. The inaccuracy of Miner's rule deduced from two‐level tests is of secondary importance for service life calculation when compared with the negligence of amplitudes below the fatigue limit. The proposed model yields shorter service life than the elemental version of Miner's rule.     

The Significance of Crack Initiation Stage in Very High Cycle Fatigue of Steels

Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10^?8 and 10^?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix.