Fatigue and corrosion fatigue crack growth of 4340 steel. Fatigue life is a key concern in welded steel frames for mobile equipment that experience large and varying dynamic loads. While on the subject of dislocations, it is appropriate to briefly discuss fatigue. This causes the amplitude of the applied stress to increase given the new restraints on strain.
Fatigue, how and why physics of fatigue professor darrell. A study of microcracks in overheated forgings of ultra. Proceedings of the asme 2014 international mechanical engineering congress and exposition. Fatigue crack growth data on aisi 4340 steel, obtained under sinusoidal. Crack growth behavior of 4340 steel under corrosion and corrosion fatigue conditions crack growth behavior of 4340 steel under corrosion and corrosion fatigue conditions sadananda, k vasudevan, a. For engineers who design welded steel structures subject to dynamic. Effect of frequency on fatigue crack growth response of aisi 4340.
Overviewofthemechanismsoffailure in heat treated steel components. The model for prediction of identification of critical material areas and the number of. Study of the effect of heat treatment on low cycle fatigue. Evaluation of wc10ni thermal spray coating with shot peening. Finite element simulation of shieldingintensification. Growth of small fatigue cracks in ph 8 mo stainless steel.
Fatigue crack initiation and propagation in butt joint welds of an ultrahighstrength steel formulas proposed for predicting fatigue crack intitation and propagation agree well with experimental data by b. Two different plasmanitriding treatments were performed on a quenched and tempered 42crmo4 cast steel. In aisi 4340 steel, fatigue cracks were initiated by inclusions whenever the long axis of the inclusion was normal to a direction of principal tensile stress. Constantlifetime fatigue diagram for aisisae 4340 alloy steel bars.
The objectives were to investigate crack initiation characteristics and short crack. An evaluation of shot peening, residual stress and stress relaxation on the fatigue life of aisi 4340 steel m. Crack growth behavior of 4340 steel under corrosion and. Micromechanical modelling of fatigue crack initiation in. The hardness of the 4340 var crankshaft steel ranged from 32. Corrosion fatigue crack growth cfcg experiments of aisi 4340 high strength steel were conducted using precracked compact tension specimens in dry air. The fatigue process leading to pitting is divided into crack initiation and a crack propagation period. Fatigue is one of the primary reasons for the failure of structural components. Thus, crack initiation of cyclically loaded materials occurs predominantly at internal nonmetallic inclusions nmis. Abstractthe mechanisms of fatigue crack initiation in annealed and quenched and tempered 4340 steel were characterized. In order to study the shot peening influence on fatigue life, the behavior of crsf during the fatigue process and the crack initiation points of fatigued specimens were studied. Pdf crack growth behavior of 4340 steel under corrosion. Fatigue limit, endurance limit, and fatigue strength are used to describe the amplitude or range of cyclic stress that can be applied to the material without causing fatigue failure.
The lowest observed growth rate is approximately one lattice spacing per cycle. Fatigue tests on 4340 steel showed that the endurance limit of a test specimen with a 0. Fatigue crack initiation and growth from artificial pits of different depths has been studied. The incubation period for crack initiation was measured. The influence of gaseous environment is examined on fatigue crack propagation behavior in steels. Effect of diamondlike carbon thin film on the fatigue. Elect ron microscopy of sur face rep l i cas has been employed to determine the meta l lu rg ica l mic romechan ics of fatigue c rack init iat ion at sur face and shal low subsurface inclusions in 4340 steel. The high hardness layer hinders the fatigue crack initiation. The small crack growth behavior is shown to be similar to that of a quenched and tempered aisi 4340 steel having a comparable strength. Once the cracks reached a length of approximately 0. Initiation of fatigue cracks in 4340 steel springerlink.
It is not obvious at present why only some inclusions debond, initiation and early growth of fatigue cracks in high strength steel 623 and not others of the same chemical class and approximately the same size. Aisi 4340 steel, diamondlike carbon, fatigue strength, cvd 1. The experimental results are considered in terms of current theoretical analyses of elastic stress distributions around hard inclusions imbedded in soft. Materials and properties three aerospace materials were chosen for the testing program including aluminum 7075t7351, 4340 steel, and d6ac steel. The initiation of fatigue cracks in a tib 2reinforced highmodulus, lowdensity steel was revealed by insitu scanning electron microscopy for the first time. The effects of sulfide inclusions on mechanical properties. Thus, as part of a longrange program to establish the necessary relations for predicting the corrosion fatigue behavior of structural steels, the stresscorrosion cracking susceptibility and the corrosion fatigue crack growth rates at 6 cpm of 4340 steel heattreated to yield strengths of, 180, and 220 ksi 896, 1241, and 1571 mnm 2 were. A semiempirical crack growth model incorporating crack closure effects was used to predict crack growth rates and total fatigue lives of notched 4340 steel specimens. Aug 19, 2016 initiation of fatigue cracks in 4340 steel. Initiation and growth of cracks in biaxial fatigue 233 applied crack growth planes surface strains crack stage i stage ii directions liddle and miller. During examination of metallographic sections taken from the roll, hightemperature oxides were found on the crack faces, strongly suggesting forging laps. Evaluation of fatigue strength of aisi 4340 steel under hydrostatic pressure. Langford and kusenberger, initiation of fatigue cracks in 4340 steel, metallurgical transactions, vol 4, 1977, 553 559.
Corrosion fatigue crack growth of aisi 4340 steel request pdf. Effect of shot peening process on the fatigue life of shot peened low alloy steel. Weldtoe geometries, welding residual stresses and haz heat affected zone cyclic mechanical properties are taken into account in the ni estimation procedure. Specifically, a fully martensitic 300m ultrahigh strength steel and a fully bainitic 214cr1mo lower strength steel are investigated in environments of ambient temperature moist air and low pressure dehumidified hydrogen and argon gases over a wide range of growth rates from 10sup 8 to 10.
Initiation of fatigue cracks in 4340 steel pdf document. Microscopic examinations were made of specimens with metallographically polished notches using a 400 times long working distance microscope with an xy micrometer base mounted. Electron microscopy of surface replicas has been employed to determine the metallurgical micromechanics of fatigue crack initiation at surface and shallow. Fatigue is an important mechanism for the failure of components in many engineering applications and a significant proportion of the fatigue life is spent in the crack initiation phase. The model shows very good agreement with experiment in describing the initiation and growth of a. Introduction the study of fatigue strength is an important parameter to be considered in components subject to constant variable amplitude loadings. Initiation and early growth of fatigue cracks in high strength steel. Avoid over heating and overly long dwell times while austenitizing. Dynamic crack initiation toughness of 4340 steel at. Crack initiation is a process that forms cracks on the surface of a material. 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 analysis brings in a correlation between the crack growth rate and the crack length together with the effects of the overload on the crack growth rate. Corrosion fatigue crack growth cfcg experiments of aisi 4340 high strength steel were conducted using precracked compact tension specimens in dry air, distilled water, and 3. The change of potency to nucleate cracks in high cycle fatigue hcf at a primary nonmetallic inclusion in a martensitic gear steel due to the existence of a neighboring inclusion is computationally investigated using twoand threedimensional elastoplastic finite element fe analyses. Ultrasonic fatigue tests were performed up to 10 9 cycles. In this paper, the fatigue crack initiation fci from notches and fatigue crack propagation fcp in dif. The growth of short cracks in 4340 steel and aluminumlithium 2090.
Mechanisms of fatigue crack initiation in annealed. The life of a fatigue crack has two parts, initiation and propagation. Fatigue micro cracks are mainly induced by the brittle fracture of small. The effects of a machininglike scratch on the fatigue life of 4340. Aisi 4340 steel is widely used in the aircraft industry for fabrication of structural components. Initiation and growth of fatigue microcracks were studied in vacuum degassed 4140 steel in three conditions. The improvement on the fatigue performance is mainly because the fatigue crack initiation and. The micromechanics of crack initiation and early growth in 4340 steel is studied using the sem, and it is found that growth is discontinuous and transgranular. This paper deals with the modelling of short crack initiation within a microstructure of medium carbon steel under fatigue loading, as well as with the influence of overload on the crack initiation period. Nov 01, 2015 crack growth behavior of 4340 steel under corrosion and corrosion fatigue conditions crack growth behavior of 4340 steel under corrosion and corrosion fatigue conditions sadananda, k vasudevan, a. The primary reason for the formation of cracks on any surface is fatigue. Study of the surface integrity and high cycle fatigue.
Micromechanical modeling of fatigue crack initiation in. The material will develop cell structures and harden in response to the applied load. The chemical compositions conform to the requirements for 4340 steel modified to contain maximum 0. Mechanisms of fatigue crack initiation in annealed, quenched and tempered 4340 steel. The growth of short cracks in 4340 steel and aluminum. In high strength steels, surface and subsurface defects play an important role in the reduction of the fatigue limit 2. Fatigue performance of aisi 4340 steel nicrbsife hvof. Discussion inclusions in 4340 steel are able to debond on the very first load cycle, and they can do so at stresses below the fatigue limit. Two heat treatment conditions were chosen for the steel. Pearson, initiation of fatigue cracks in commercial aluminum alloys and the subsequent propagation of very short cracks, rae tr 72236, dec 1972.
Fatigue crack initiation lives are calculated from either a basquin type or a. Modelling of overload effects on fatigue crack initiation. An evaluation of shot peening, residual stress and stress. Fatigue, how and why physics of fatigue professor darrell f.
The greatest portion of the fatigue life is spent nucleating and growing a fatigue crack to a length at which it can be detected the range of effective stress intensity factor, that is, the idea of crack closure allows the growth of fatigue cracks to be rationalized the behavior of small cracks is in many respects quite different from long. Crack initiation in the very high cycle fatigue regime of. Sem micmgraphs of fatigue crack initiation sited in 4340 steel, showing transgranular model of growth initiation and early growth of fatigue cracks in high strength steel 621 crack growth as a function of cycles were obtained, the results of these measurements are plotted in fig. Slip linesare formed during the first few thousand cycles of stress. Citeseerx citation query initiation of fatigue cracks in. Delayed failure in steel occurs by controlled initiation and growth of a crack. The objectives were to investigate crack initiation characteristics and short crack growth behavior for alli 2090 and for 4340 steel and to evaluate the ability of a closurebased crackgrowth model to predict fatigue crack growth rates and total fatigue lives for the steel. The local approach method is used to calculate the fatigue crack initiation early crack growth lives ni in high strength structural steel weldments. The approach consisted of performing fatigue crack initiation and. Fatigue and crack growth tests have been conducted on 4340 steel and 2024t3 aluminum alloy, respectively, to assess the effects of shot peening on fatigue life and the effects of shot and laser peening on crack growth. Initiation of fatigue crack and slip band crack growth stage i fatigue cracks are normally initiated at a free surface. Initiation and early growth of fatigue cracks in high.
The initiation of fatigue cracks from notches in mild. The experimental results are considered in terms of current theoretical analyses of elastic stress distributions around hard inclusions imbedded in soft matrices. Crack initiation and growth behavior in 4340 steel under static and cyclic loads in various environments are examined using the twoparameter approach developed by the authors. The fractured fatigue specimens were investigated using a scanning electron microscope in order to obtain information about the crack initiation. Eventually, the growth of one or a few crack of the larger cracks will dominate over the rest of the cracks.
The failure diagram connects the behavior of smooth. In this paper, a modified fourpoint bend kolsky bar technique is used to evaluate the dynamic crack initiation toughness of 4340 steel with a heat treatment that produces a hardness of approximately rockwell c5055. Crack initiation and propagation are controlled by interaction between hydrogen concentration. Fatigue crack initiation life prediction in high strength. The effect of prior heat treatment on the mechanical properties, namely the characteristics of plastic deformation, crack initiation and crack propagation, in the low cycle fatigue of aisi 4340 steel. The mechanism and estimation of fatigue crack initiation. Dislocations play a major role in the fatigue crack initiation phase. Mechanisms of fatigue crack initiation in annealed, quenched. Electron microscopy of surface replicas has been employed to determine. The effects of shot and laser peening on fatigue life and. Pdf effect of shot peening process on the fatigue life of. The crankshafts for aircraft engines were forged from about 4 12. Initiation of fatigue cracks in 4340 steel nasaads. The steel chosen for this study was 4340 steel heat treated to an ultimate strength.
Thus, as part of a longrange program to establish the necessary relations for predicting the corrosion fatigue behavior of structural steels, the stresscorrosioncracking susceptibility and the corrosion fatigue crackgrowth rates at 6 cpm of 4340 steel heattreated to yield strengths of, 180, and 220 ksi 896, 1241, and 1571 mnm 2 were. The existing fatigue en data have been evaluated to establish the effects of key material, loading, and environmental parameters such as steel type, strain range, strain rate. Fatigue leads to progressive and localized structural damage when any material experiences cyclic loading. Crack initiation and propagation are controlled by interaction between hydrogen concentration arnd triaxial stress state. Fatigue lives were found to depend on the size and type of initiation site, especially for the felix28 loading sequence. Initiation of a crack at an inclusion or an impurity, awa7 from the specimen and its. The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Therefore, it can be said that fatigue failure occurs in three stages crack initiation. In the second stage of fatigue, some of the tiny microcracks join together and begin to propagate through the material in a direction that is perpendicular to the maximum tensile stress.
Dislocations play a major role in the fatigue crack initiation. The results can be described by using the range of stress intensity factor for the sharp crack situation modified to take into account the notch root radius. The aim of this study is to analyse the effects on rotating bending fatigue behaviour of hard chromium electroless nickel multilayer system coated aisi 4340 high strength steel submitted to shot peening pre treatment. There is some debate, however, since some have determined that mns inclusions are ineffectual stress raisers for initiation of fatigue cracks in aisi 4340 compared with oxides 18. The experimental results were analyzed using fracturemechanical models including those for small cracks. The results show that the superior interfacial strength between the tib 2 particles and the matrix prevents the initiation of micro cracks at the interfaces.
The local approach method is used to calculate the fatigue crack initiation early crack growth lives n i in high strength structural steel weldments. Effect of a shot peening pre treatment on the fatigue. The role of crack tip shielding in retarding the initiation and growth of fatigue cracks has been examined in metallic composite microstructures consisting of. Fatigue and corrosion fatigue crack growth of 4340 steel at. Revealing the fatigue crack initiation mechanism of a tib2. The mechanism and estimation of fatigue crack initiation in. Pulsating tension fatigue tests have been carried out on edgenotched specimens of a mild steel and measurements of electrical potential drop across notches have been used to determine the number of cycles to initiate cracks. A study of microcracks in overheated forgings of ultralow. Electrochemical conditions at the tip of an advancing stress.
Back and forth fine slip movements of fatigue could build up notchesor ridges at the surface. In general, fatigue crack initiation occurs on the specimen surface. The overload is found to accelerate the short crack growth. Overheating during the austenitizing portion of the heat treatment cycle can coarsen normally fine grained steels. Initiation and growth of short fatigue cracks in austenitic sanicro 25 steel. Fatigue crack initiation and propagation in butt joint welds.
Pdf crack growth behavior of 4340 steel under corrosion and. Close up of fatigue fracture showing presence of multiple subsurface initiation sites. Fatigue crack initiation and microcrack growth in 4140 steel. Evaluation of fatigue strength of aisi 4340 steel under.