Change In Fatigue Strength Research Articles

ТЕХНОЛОГИЧЕСКИЕ ОСОБЕННОСТИ ВОССТАНОВЛЕНИЯ ШЕЕК КОЛЕНЧАТЫХ ВАЛОВ В СВЯЗИ С ИХ СТАРЕНИЕМ В ПРОЦЕССЕ ЭКСПЛУАТАЦИИ

Numerous publications in the open press on the restoration of crankshaft necks by various methods are known. They provide recommendations for applying coatings to worn necks. However, most work lacks studies on the change in fatigue strength and wear resistance of crankshafts. (Research purpose) The research purpose is studying the technological features of recovering the crankshafts due to their aging during operation. (Materials and methods) As objects of research, crankshafts of diesel engines of various repair sizes were selected. The surface hardness of the necks was studied using portable hardometers in the zone of the greatest and least wear. The data were processed using mathematical methods and compared with allowable hardness values for new and worn necks. Fatigue tests were carried out on machines when the bending moment was loaded along a symmetrical cycle before the appearance of a fatigue crack and final destruction for various coating methods. (Results and discussion) The surface hardness of the necks of extremely worn shafts is reduced by 10 HRC compared to the hardness for new shafts. The reason for the decrease is due to the operation of the neck in dry friction conditions, which causes heating of the surface and, as a result, tempering. The limit of endurance of limit worn shafts in comparison with the limit of endurance of new crankshafts is reduced by 20-25 percent. This is due to the accumulation of fatigue failures. Laser hardening is proposed to increase life of worn-out shafts with reduced hardness. Technological features have been developed for restoring the necks of extremely worn crankshafts. (Conclusions) The aging of crankshafts occurs with a decrease in surface hardness by 10 HRC units and a decrease in the endurance limit by 20-25 persent. Invention proposes laser thermal resistance of necks with reduced hardness, which increases wear resistance by 1.9- 2.6 times.

Influence of Impurity Morphology on the Fatigue Strength of High-Purity Structural Steel Melted in an Electric Furnace

A modern user requires low operating costs, but also reliability from machines and technical devices. Reliability during the service life depends on the quality of construction solutions, but also largely on the quality, properties and adaptation to the working conditions used in the construction of construction materials. During the operation of technical objects, their a highly predictable wear occurs. The problem is the phenomena of premature wear and damage of elements. The causes of failure of technical facilities are usually complex and depend on many factors. They can include the human factor and the one related to the quality, selection, production and technological processes of the materials used in the construc-tion of the facility. In real technical facilities, many premature failures are caused by material fatigue, which is related to the quality and distribution of impurities in the material. The paper presents the change in fatigue strength for rotational bending of low-carbon structural steel hardened and tempered at different temperatures as the effect of the size and distance between impurities on the fatigue strength of high-quality carbon structural steel melted in the industrial conditions in an electric arc furnace.

Influence of microstructural defects and the surface topography on the fatigue behavior of “additively‐subtractively” manufactured specimens made of AISI 316L

AbstractAs additive manufacturing offers only low surface quality, a subsequent machining of functional and highly loaded areas is required. Thus, a sound knowledge of the interrelation between the additive and subtractive manufacturing process as well as the resulting mechanical properties is indispensable. In this work, specimens were manufactured by using laser‐based powder bed fusion (L‐PBF) with substantially different sets of process parameters as well as subsequent grinding (G) or milling (M). Despite the substantially different surface topographies, the fatigue tests revealed only a slight influence of the subtractive manufacturing on the fatigue behavior, whereas the different laser‐based powder bed fusion process parameters led to pronounced changes in fatigue strength. In contrast, a significant influence of subtractive finishing on the fatigue properties of the defect‐free continuously cast (CC) reference specimens was observed. This can be explained by a dominating influence of process‐induced defects in laser‐based powder bed fusion material, which overruled the influence of surface machining. However, although both laser‐based powder bed fusion parameter sets resulted in substantial defects, one set yielded similar fatigue strength compared to continuously cast specimens.

Effects of FOD on fatigue strength of 7075-T6 hourglass specimens

The term “Foreign Object Damage (FOD)” refers to the damage associated with the impact of particles on aircraft engine components that causes changes in fatigue strength as a result of induced stress concentrations, residual stresses and microstructural changes. This paper presents the Finite Element (FE) models created to study the stress field induced by the impacts of a steel sphere on a 7075-T6 hourglass specimen. The impacts occur at the minimum cross section, in the normal and oblique directions. The results reveal that in the event of an oblique impact the axial tensile stresses are greater and closer to the crater created by the impact. The superposition of the residual axial stresses to the bending stresses by fatigue test allows to identify the areas in which the maximum stresses are reached and therefore crack initiation is expected.

Tensile and high-cycle fatigue performance of HRB500 high-strength steel rebars joined by flash butt welding

In this research, extensive testing results of the specimens of HRB500 high-strength steel rebars joined by flash butt welding were demonstrated, including metallographic and micro-hardness tests, monotonic tensile tests, high-cycle fatigue tests, and fracture surface analysis. The chief aim of the present research is to investigate the applicability of flash butt welding for HRB500 high-strength steel rebars. Thus, these tests were executed on the welded specimens, which were previously manufactured to evaluate the tensile and high-cycle fatigue properties. Meanwhile, a series of experimental technical details were proposed in the test process. Some conclusions can be obtained as follows: welding heat can change the metallographic structures of HRB500 high-strength steel rebars, and the welding heat-affected zone on the steel rebars with diameter of 16 mm is totally about 10 cm in length. Besides, the deformability and tensile strength of the welded specimens decline with their diameter degrees increasing, and brittle fractures occur in those welded specimens with diameter of 32 mm. Furthermore, three-parameter S-N curves of the welded specimens and the steel rebars base materials are obtained based on the fatigue test results. Fatigue strengths of welded specimens decrease with their diameters increasing, and reduction of high-cycle fatigue strength is more apparent for large-diameter welded specimens. Welded specimens are more prone to pre-mature fatigue failure than base materials of HRB500 high-strength steel rebars, and the rate of change in fatigue strength of welded specimen decreases with the increase of loading cycles. At last, mechanism of crack propagation in fatigue process is analyzed and further evaluated by fracture surface analysis with the aim of wider application.

スポット溶接継手の疲労寿命に及ぼすナゲット径と打点位置及び板厚のばらつきの影響

In this paper, the amount of variation in the fatigue life due to variations in the spot welding location and the nugget diameter of three spot-welded joints subjected to bending loading was determined quantitatively by sensitivity analysis. As a result, the effect on fatigue life due to variations in spot welding location was found to approximately the same as that of the nugget diameter. Further, the amount of change in fatigue strength due to the thickness for the tensile shear joints with nugget diameter5√t include components due to changes in nugget diameter. Both components have been separated by a sensitivity analysis. The ratio of the sheet thickness component and the nugget diameter component was 7:3. These analytical results were confirmed by the fatigue test.

The effect of weld bead dimensions on the fatigue strength of cruciform joints

The choice of weld bead size in the case of welded cruciform joints can be problematic, especially when the sheets forming the joint are of differing thickness. Technological standards generally recommend a weld bead thickness less than the minimum thickness of the sheets to be joined, whereas structural standards do not envisage any dependency between joint static and fatigue strength and weld bead dimensions, unless these are so reduced as to lead to failure starting and propagating in the weld bead itself rather than the base metal plates. The scope of this study is the theoretical and experimental analysis of the change in fatigue strength with varying weld bead thickness and minimum welded plate thickness.

Effects of Surface Roughness and Abnormal Surface Layer on Fatigue Strength

The purpose of this research is to evaluate the effects of surface roughness in normalized steel and the effects of abnormal surface layer in carburized steel on the fatigue strength that is considered as the limit of no crack initiation. Normalized and Carburized test pieces with crowned round notches were used in the fatigue tests, the normalized test pieces have different surface roughnesses and the carburized test pieces have different surface microstructures on the notch surface. The changes in fatigue strength due to the differences in the notch surface roughness and surface microstructure were investigated. As the conclusions, fatigue strength will be certainly increased by decreasing the surface maximum height, and removing the abnormal surface layer formed during carburized treatment leads to a marked increase in fatigue strength.

Change in Fatigue Strength of Biomedical β-Type Titanium Alloy through Heat Treatment Processes

Relationships between fatigue properties and microstructures of hot-forged Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) in under aged, peak aged, and over aged conditions at aging temperatures of 673K and 723K are investigated. The changes in the fatigue properties of TNTZ subjected to thermomechanical treatments, which includes an aging treatment at 673K or 723K for 259.2ks after a severe deformation process by cold rolling, are also investigated.At an aging temperature of 673K, ω phase precipitates at the early stage of aging, but α phase precipitates at relatively longer aging time. The precipitation site of α phase changes from intra-grain to grain boundary at around peak aging time when TNTZ is aged at 673K and 723K. The elastic modulus of TNTZ increases simply with increasing aging time at both 673K and 723K. The fatigue strength of TNTZ increases considerably when α phase precipitates compared with when ω + α phases co-exist. The fatigue strength of TNTZ decreases slightly due to the coarsening of α phase precipitated in β grain and its grain boundary. TNTZ aged at 723K for 259.2ks after cold rolling exhibits the highest fatigue strength in both the low- and high-cycle fatigue life regions. Furthermore, the fatigue limitof about 770MPa (fatigue raito : 0.71) is nearly equal to that of hot-forged Ti-6Al-4V ELI alloy subjected to aging after solution treatment with equiaxed α structure.

Evaluation of fatigue strength of friction stir butt-welded aluminum alloy joints inclined to applied cyclic stress

Recently, fatigue tests have been performed to investigate the fatigue behavior of friction stir butt-welded aluminum alloy joints. However, the joints of more than 10 mm thick plates have rarely been tested, nor has the effect of the inclined angle of the friction stir welding line to the applied stress been studied. In this research, fatigue tests were carried out on friction stir butt-welded joints with or without welding flash. The joints had a friction welding line that was inclined angles of 0°, 30°, 60° and 90° to the perpendicular direction of the applied stress. Observations of the fatigue failure modes and fracture surfaces indicated that fatigue cracks tended to initiate from the base material or the heat-affected zone when the inclined angle was 0°, regardless of the presence of welding flash. When the inclined angle was 30°, 60° or 90°, most cracks in the joints with welding flash initiated at the stirred zone, whereas cracks in joints without welding flash tended to initiate from the base material or the stirred zone. A comparison of the S– N data indicated that the change in fatigue strength was small, regardless of the inclined angles of the friction stir welding line or the presence of welding flash.

Effect of Roller Working on Fatigue Properties of Pure Aluminum(Fatigue 1)

According to the statistical analysis by one of the authors, more than 90% of failure is caused by fatigue, and more than 90% of failure causes are initiated from the notched portion. It is very important to improve the fatigue strength of the notched portion in the practical components. Roller working is one of the effective plastic working method to improve the fatigue strength of materials such as carbon steels. In this study, roller work process was used to notched pure aluminum specimens for estimating its effect on fatigue properties. The result of fatigue test indicates that the fatigue strength by 10^7 cycles of specimens after roller working increases by 30% compared to non-roller worked specimens, but with the increase of roller working value, the fatigue strength by 10^7 cycles decreases gradually. The crack propagation behavior was also investigated by taking replicas and observations under an optical microscope. The change in fatigue strength was discussed according to the concept of compressive residual stress at the notch root and the surface condition after roller working.

Preloading Technique in Dynamic Fatigue Testing of Glass and Ceramics with an Indentation Flaw System

The solution of fatigue strength as a function of preloading in dynamic fatigue testing was obtained numerically for an indentation flaw system. The dynamic fatigue strength is dependent on preloading and fatigue parameter (K). The effect of preloading on dynamic fatigue strength decreases with increasing fatigue parameter: for n 20 the effect is negligible up to a preloading of 90%. The solution was verified by dynamic fatigue experiments conducted with soda‐lime glass and alumina specimens with as‐indented flaws in room‐temperature distilled water. The result indicates that one can apply a preloading corresponding up to 90% of fatigue strength for most glass and ceramic materials with no change in fatigue strength, resulting in a dramatic saving of testing time.

The Influence of Annealing on Fatigue Strength in Polyethylene

To investigate the influence of morphology on fatigue strength of polyethylene, the change in fatigue strength and relaxation behavior by annealing was evaluated using high-density polyethylene, which has few short branches, and medium-density polyethylene, which has many short branches. For medium-density polyethylene, which showed higher fatigue strength, he annealing had a small effect on β relaxation behavior and the fatigue strength was increased by annealing at 110°C. Alternately, for high-density polyethylene which showed lower fatigue strength, annealing decreased the fatigue strength and enlarged the intensity of β relaxation. It was thought that the fatigue strength of high-density polyethylene was dependent on entanglement by fine crystals and that the disappearance of that by annealing yielded lower fatigue strength and the enlargement of β relaxation intensity.

極低温構造材料の高サイクル疲労特性

High cycle fatigue tests at cryogenic temperatures were performed for titanium alloys and austenitic steels. The S-N curves shifted to higher stress or longer life side with a decrease in test temperature with some exceptions in forged Ti-6Al-4V alloys. There was little change in fatigue strength between 77K and 4K for the forged Ti-6Al-4V alloys. Fatigue strength at 106 cycles increased in proportion to a gain in strength by temperature decrease. However, a ratio of the farigue strength to tensile or yield strength depended on material and its processing. From the viewpoint of combination of yield strength and fatigue strength, austenitic steels had a higher fatigue strength than titanium alloys at a given yield strength. Among the titanium alloys, the rolled ones exceeded the forged ones.

Changes in fatigue strength due to cyclic prestress: II. Fatigue tests of aluminium alloy specimens and evaluation by means of the sequential Wohler curve concept: Pusche, A.C. IfL-Mitt. Mar.–Apr. 1989 28, (2), 43–49 (in German)

The propagation of small cracks in fatigue has received considerable attention over the past decade. Microstructurally and even mechanically small cracks have been shown to consistently exhibit higher crack growth rates than predicted using standard threshold and Paris growth law concepts, based on linear elastic fracture mechanics (LEFM) applied to mechanically long cracks. This has been commonly attributed to several factors, including the influence of microstructure, the breakdown of LEFM parameters for representation of the crack tip field, and the transient development of plasticity-induced closure towards some steady-state value associated with long crack behavior. Other mechanisms related to microstructural effects such as roughness-induced closure and crack face bridging/interference are also potential contributors. Quantitative attempts to explain the fatigue propagation of small cracks in terms of plasticity-induced closure, along with adoption of an additional component of the driving force (e.g. crack tip opening displacement) to reflect the contribution of cyclic plastic strain have met with some success in correlation of the so-called “anomalous” propagation behavior, including crack deceleration and acceleration transients. However, these models rely on the adoption of highly idealized assumptions regarding the self-similarity of crack growth, neglect of local anisotropy and heterogeneity associated with microstructure, etc.; in spite of these compromises, they still involve a considerable degree of complexity. Here, we adopt the viewpoint that multiple, microstructure interactions and closure effects may simultaneously influence the propagation of small cracks; moreover, driving force parameters based on self-similar crack growth arguments of elastic-plastic fracture mechanics (EPFM) for mechanically long cracks, such as the cyclic J-integral or crack tip opening displacement, may apply in principle, but not rigorously, as the driving force for small cracks. As an engineering approach, we consider a recent extension of the multiaxial microcrack propagation model first proposed by McDowell and Berard[1,2] for the growth of microstructurally small and mechanically small fatigue cracks[3] in multiaxial fatigue. Integrated between initial and final crack lengths, the model is fully consistent with standard strain-life laws of fatigue crack initiation mechanics under various states of stress, [1,2] and therefore bridges the mechanics of classical initiation and LEFM/EPFM to some extent. The existence of a fatigue limit (nonpropagating crack limit) is neglected in this particular work. It is shown for uniaxial loading of both 1045 steel and Inconel 718 that the model is able to describe, to first order, the anomalous high propagation rates of small cracks and convergence with long crack d a/ dN - ΔK data as the crack transitions from small to mechanically long scales. The limits of validity of engineering schemes based on decomposition of total fatigue life into “initiation” and propagation phases that rely on strainlife and long crack propagation laws are discussed[4]. Moreover, it is shown that the model essentially reflects a closure transient in the context of a cyclic J-integral approach, similar to the EPFM plasticity-induced closure modeling concepts set forth by Newman[5] and McClung et al. [6] for small cracks in fatigue. However, the present model implicitly reflects multiple forms of crack tip shielding effects, not just plasticity-induced closure. Finally, the model is shown to provide realistic treatment of cumulative damage in two level loading sequences, as reflected by comparison with the damage curve approach of Manson and Halford[7].

The corrosion fatigue properties of surgical implants in a living body.

Fatigue fracture of artificial implants in the human body, caused by the repeated application of stress, is well documented. It is known that the fatigue strength of implant materials decreases when they are exposed under in vivo corrosion conditions. There are, however, no investigations concerning the effect of body fluids on the fatigue characteristics of commonly used biomaterials. Accordingly, fatigue tests on machined stainless-steel AISI 316, and COP alloy rods have been conducted in the right lower leg of rabbit. These specimens were pierced through the hole drilled at the middle of the tibial bone. A cyclic tensile stress of frequency 5 or 10 Hz was applied to the rods. From the results, it was found that the fatigue strength at 5 x 10(6) cycles for AISI 316 under the in vivo environment was 680 MPa compared to 830 MPa in air and similarly for COP alloy, was 680 MPa in the living body compared to 800 MPa in air. These remarkable changes in fatigue strength associated with the in vivo environments are considered to be due to the corrosive action of body fluids on the biomaterials.

Effect of long service on the fine structure of the surface of gas turbine engine rotor blades

1. The most significant structural changes are concentrated in a thin surface layer, which is related to the character of distribution of thermal loads under conditions of the corrosive action of the medium. 2. Structural damages accumulating during service have a two-stage character: a) the removal of microdistortions and an increase in the dimensions of the blocks as a result of the removal of mechanical work hardening in the surface layer of new blades (the density of defects decreases); b) disintegration of the elements of the substructure back to dimensions smaller than in the original conditions; the microdistortions remain insignificant (lines become broader). The effect observed is caused by an increase in the boundary density of dislocations. 3. The similarity of the structural changes in different zones of the trailing edge of the blade in the last stages of service and the two-stage character of the change in fatigue strength is an indication of the difficulty of simulating long-term failure of heterophase alloys by short-term tests.

Effect of heat treatment on the static and fatigue strength of polycaproamide

The changes in strength and the polymorphous transformations which occur during the aging in air of polycaproamide heat-treated under various conditions were investigated. It was shown that the strength after prolonged aging in air with normal relative humidity depends on the structural changes occurring in the heat-treatment process. The structural changes lead to changes in fatigue strength and also in the mechanism of the breakdown of polycaproamide on aging.

2相ステンレス鋼の疲労強度について

The present work was carried out so as to investigate the change in fatigue strength by pre-working, welding of stainless steel which has two phases, austenite and ferrite. The fatigue test was done by pulsating tension and the structural changes associated with the fatigue were also examined by means of X-ray, electron-microscopy and EPMA.The results obtained are summarized as follows:(1) The fatigue strength of austenitic-ferritic stainless steel was considerably stronger than that of austenitic stainless steel and that a difinite fatigue limit was obtained similary with carbon steel by means of age-hardening of ferrite during the fatigue test. The influence of martensitic transformation by pre-working on the fatigue strength was negligibly small.(2) In the γ-phase Cr was lower than Ni, while in the α-phase Ni was rather lower. The variation of composition by welding was considerably violent in the heat affected zone.(3) This alloy was not sensitive to a mechanical or welding notch. The decrease of fatigue strength was due largely to the change of composition and structure by welding heat.(4) The change of fatigue strength of this alloy was little by welding. Moreover the solution-treatment temperature of about 1100°C after welding was deemed adequate.(5) As stainless steel with two phases was relatively unstable in structure, a complicated striation pattern was obseved in the fatigue fractured surface.

13%Cr鋼の機械的強度に及げす非金属介在物およびMoの影響

The experiments herein introduced were made to clarify the tensile, impact, fatigue and creep strength of 13% Cr steel at high temperatures, and the effect of non-metallic inclusions and the Mo on the mechanical strength of the alloy.In the experiments on the effect of non-metallic inclusions, four types of specimens were used, which contained 0•13 to 0•20% C, 12•69 to 13•13% Cr and various quantities of nonmetallic inclusions. For testing the effect of Mo, five types of specimens containing 0•09 to 0•17% and trace to 1•06% Mo were employed.The results are summarised as follows.(1) Tensile strength is not affected by the non-metalljc inclusions.(2) With the increase of temperature, the tensile strength and the fatigue strength decrease, but endurance ratio, (i. e. fatigue strength divided by tensile strength, ) increases.(3) The impact value is lowered by inclusions only slightly at room temperature but considerably at high temperatures.(4) Rotating -bending fatigue strength decreases with the increase of inclusions on the stressed surface.(5) Addition of Mo increases the tensile strength and particularly creep strength at high temperatures, but affords no change in fatigue strength. 13% Cr steel containing 1.06% Mo has about twice the creep strength of that containing no Mo.