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S-N Curve (s-n + curve)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2009
L. T. LU
ABSTRACT Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 106 cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size. [source]

Bauteiloberfläche und Schwingfestigkeit , Untersuchungen zum Einfluss der Randschicht auf die Dauerschwingfestigkeit von Bauteilen aus Stahl

MATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 5 2006
U. Kleemann Dipl.-Ing.
surface; surface layer; fatigue strength; surface stress-concentration factor Abstract Die Berechnung der Schwingfestigkeit hat in den letzten Jahren für die Bauteilentwicklung an Bedeutung gewonnen. Aus Zeit- und Kostengründen wird angestrebt, den experimentellen Festigkeitsnachweis auf die Freigabe von Sicherheitsteilen zu beschränken. Die Schwingfestigkeit von glatten, polierten Werkstoffproben (Spannungs- und Dehnungswöhlerlinie) kann heute mit guter Treffsicherheit abgeschätzt bzw. entsprechenden Katalogen entnommen werden. Die Übertragbarkeit der Schwingfestigkeit von Werkstoffproben auf reale Bauteile ist jedoch mit erheblichen Schwierigkeiten verbunden, da eine Reihe von Einflussgrößen zu berücksichtigen sind wie Geometrie und Größe, Mittelspannung, Beanspruchungsart, Mehrachsigkeit, Randschicht (Oberflächentopographie, Eigenspannungen, Gefüge, Härte), Temperatur, korrosive Medien u.,a.. Der Einfluss dieser Größen ist komplex und lässt sich nur sehr grob durch eine Multiplikation von Einflussfaktoren beschreiben. Der heutige Stand im Technischen Regelwerk zum Oberflächeneinfluss, z.,B. FKM-Richtlinie ,Rechnerischer Festigkeitsnachweis für Maschinenbauteile", basiert auf einem Kenntnisstand, der 50 Jahre zurückliegt. Der Ausgang für das Forschungsvorhaben war die Forderung der Industrie nach einer verbesserten rechnerischen Erfassung des Einflusses der Oberflächenbearbeitung bei Zerspanung. Hierzu wurde auf einen Vorschlag von Liu zurückgegriffen, der die Oberflächentopographie neben der Rauheit durch eine Oberflächenformzahl kennzeichnet. Zur Erfassung des Werkstoffes wird eine charakteristische Strukturlänge eingeführt, die sich aus der Werkstoffwechselfestigkeit und dem Schwellenwert für makroskopischen Rissfortschritt berechnet. Weiterhin wurde überprüft, welche Festigkeitshypothesen in der Lage sind, den biaxialen Eigenspannungszustand an der zerspanten Oberfläche realistisch zu erfassen. Damit kann ein Konzept vorgeschlagen werden, mit dem die Dauerfestigkeit zutreffend berechnet werden kann, wenn die statische Festigkeit, die Oberflächentopographie und die Eigenspannungen bekannt sind. Zur Validierung werden Schwingversuche an drei Stählen und zwei Sphärogusslegierungen bei unterschiedlichen Randschichteigenschaften durchgeführt. Structural component surface and fatigue strength , Investigations on the effect of the surface layer on the fatigue strength of structural steel components For the development of structural components, the importance of calculating the fatigue strength has steadily increased during recent years. In order to save time and cost, efforts are in progress for limiting experimental strength testing to the release of safety components. The fatigue strength of smooth, polished material specimens (stress and strain S-N curve) can now be estimated with high accuracy, or can be obtained from the corresponding catalogs. However, the results of fatigue strength determinations on material specimens cannot be applied to real components without considerable difficulty, since a number of decisive parameters must be taken into account. These factors include the geometry and size, mean stress, type of load, multiaxiality, surface layer (surface topography, residual stresses, structure, hardness), temperature, corrosive media, etc. The effect of these parameters is complex, and a multiplication of the various decisive factors yields only a very rough description. The current state of the art in the catalog of technical rules on surface effects, such as the FKM guideline, "Computational Demonstration of Strength for Machine Components", is based on results which were obtained 50 years ago. The original incentive for the research project was the industrial demand for an improved computational method for determining the effect of surface machining by cutting processes. For this purpose, recourse was made to a proposal by Liu, who characterises the surface topography, besides the roughness, with the use of a surface stress-concentration factor. A characteristic structural length is introduced for describing the material; this length is calculated from the fatigue strength of the material and the threshold value for macroscopic crack propagation. Moreover, a check was made to determine which strength hypotheses are capable of realistically describing the biaxial residual stress state on the machined surface. Thus, a concept can be proposed for accurately calculating the fatigue strength, provided that the static strength, the surface topography, and the residual stresses are known. For validation, alternating-load tests are to be performed on three types of steel and two nodular cast alloys with different surface layer properties. [source]

Accelerated fatigue properties of unidirectional carbon/epoxy composite materials

POLYMER COMPOSITES, Issue 2 2006
Hsing-Sung Chen
It has been confirmed that polymer matrix composites possess viscoelastic behavior. This means that one could accelerate the fatigue testing by changing the stress amplitude, frequency, or temperature. This study is to investigate the accelerated fatigue properties, which are resulted from the viscoelastic behavior, of carbon/epoxy composites and to predict their fatigue life. For this purpose, a series of fatigue tests of unidirectional specimens are conducted at room temperature under different stress ratios and stress frequencies. A group of sigmoid S-N curves, which are suitable for the whole fatigue life, and the corresponding parameters are developed for different cyclic loading conditions. A transformation method, which can transform a reference S-N curve to the corresponding S-N curve of the assigned fatigue conditions, is established by the parameters. And this S-N curve can be utilized to predict the fatigue life of the composite at the assigned stress ratio or stress frequency. The comparison between the linear and sigmoid S-N curves is also carried out to show the advantages of the latter model in the whole fatigue life. POLYM. COMPOS., 27:138,146, 2006. © 2006 Society of Plastics Engineers [source]

Corrosion fatigue of spot-welded austenitic stainless steels in 3.5% NaCl solution

MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 12 2004
M. E. Somervuori
Abstract Corrosion fatigue and fatigue properties of spot-welded austenitic stainless steels EN 1.4301 and EN 1.4318 in 2B or 2F and 2H conditions were investigated in 3.5% sodium chloride (NaCl) solution and in air. The shear-loaded specimens were single spot overlap joints. The effect of steel grade, load, frequency, temperature and type of chloride on fatigue strength of the 1.0 mm thick steel specimens was evaluated by using the Taguchi Method®. Increase of the load, rise of temperature and lowering of the frequency accelerate corrosion fatigue of the spot-welded steel samples. Type of chloride had only a minor effect on fatigue strength. The 2B grade spot-welded steel samples exhibited better fatigue strength than the 2H grade samples of the same steels. On the basis of the results obtained by the Taguchi Method® the S-N curves were defined for the spot-welded 1.9 mm thick steels in 3.5% sodium chloride solution at 50°C. For reference the fatigue experiments were performed in air at the ambient temperature. Comparison of the results shows that corrosive environment decreases remarkably the fatigue strength of the spot-welded steels. The EN 1.4301 2H and EN 1.4318 2H steels have no distinctive difference in their corrosion fatigue strength even though they show a different fatigue behaviour in air. The microscopic investigations indicate that the fatigue cracks in the spot welds initiate from either side of the recrystallised area in the HAZ outside the spot-weld nugget both in air and in the corrosive environments. Pre-exposure in the corrosive environment seems to have no major influence on the crack initiation, because the cracks do not initiate at the heat-tinted area of the crevice where the crevice corrosion occurs. [source]