Experimental investigations of internal and effective stresses during fatigue loading of high-strength steel (original) (raw)
Related papers
Étude par diffraction des rayons X des modifications microstructurales en cours de fatigue
Http Www Theses Fr, 2011
Le travail présenté ici a pour but d'évaluer les mécanismes microstructuraux liés à l'amorçage de l'endommagement par fatigue d'un acier à usage pétrolier. Les microdéformations et les contraintes résiduelles (macrocontraintes) ont été déterminées par diffraction des rayons X en temps réel pendant des essais de fatigue en flexion alternée sur des éprouvettes plates prélevées dans la paroi d'un tube neuf. Les microdeformations sont estimées à partir de mesures de la largeur de corde à mi-hauteur (LCMH) d'un pic de diffraction et les contraintes résiduelles à partir du déplacement du pic. Les essais de fatigue sont réalisés pour cinq niveaux de contraintes différents. On observe trois stades de variation pendant l'évolution des microdeformations. On montre que leur amplitude et leur durée sont proportionnelles au niveau de contrainte alternée. Des variations similaires sont observées pour les contraintes résiduelles, avec des durées identiques à celles des microdéformations. Des évolutions dans la densité et la répartition des dislocations ont été observées par microscopie électronique en transmission à l'aide de la technique du faisceau d'ions focalisés. Pour comprendre le rôle de la structure initiale, des essais de fatigue sur éprouvettes recuites ont été réalisés dans les mêmes conditions d'essai. Là encore trois stades d'évolution sont observés mais avec un premier stade inversé du fait de l'état initial du réseau de dislocations. Les résultats obtenus sont très encourageants pour la prise en compte des évolutions microstructurales dans l'établissement d'un futur indicateur de dommage de la phase d'amorçage en fatigue à grand nombre de cycles des matériaux.
Procedia Engineering, 2011
The comprehensive characterization of microstructural changes caused by cyclic loading is of major importance for the understanding of basic fatigue mechanisms and for an optimized fatigue life calculation of metallic materials. Beside conventional strain measurements in recent years temperature and electrical resistance measurements were additionally used for the determination of fatigue relevant data during constant and variable amplitude loading. This paper presents selected results of the cyclic deformation behaviour of the quenched and tempered steel SAE 4140 and the cast iron ASTM 80-55-06 (EN-GJS-600) based on mechanical stress-strain hysteresis, temperature and electrical resistance data. In general the resistivity of metals increases during cyclic loading as a function of the increasing defect density i. e. dislocation density, pores and microcracks. The electrical resistance offers additional information about the cyclic deformation behaviour also in the range of low loading amplitudes resulting in very low plastic deformation and in the case of materials with a very low ductility. Furthermore the electrical resistance is independent of a defined gauge length and also applicable to complex geometries like notched specimens. The used physical quantities are linked by cross-effects and directly related to fatigue induced microstructural changes in the bulk material.
Mechanisms of the Early Fatigue Damage in Metallic Materials
Communications - Scientific letters of the University of Zilina, 2006
The basic facts concerning the surface relief formation in fatigued metals were documented already at the beginning of the 20 century by Ewing and Humfrey [1] by observing the surface of fatigued Swedish iron. The optical microscope used in these and later studies could not reveal details of the emerging extrusions and intrusions and eventually the early initiation of fatigue cracks. Later studies using replicas in transmission electron microscope or classical scanning electron microscope (see. e.g. [2]) yield higher resolution but it is not possible to obtain quantitative information in three dimensions.
Load History in Fatigue: Effect of Strain Amplitude and Loading Path
Journal of ASTM International, 2004
The low-cycle fatigue behavior of a duplex stainless steel, 60 % -40 % , is studied under tension-compression/torsion loading at room temperature and under strain control. It is shown that the duplex stainless steel has an isotropic behavior under cyclic proportional loading. The loading path induces an extra-hardening on cyclic hardening of duplex stainless steel but lower than that on austenitic stainless steels. The effect of loading history is studied in terms of strain amplitude, mean strain, and loading path. It is shown that only histories in strain amplitude and loading path have an effect on the stabilized stress.
Analysis of the fatigue strength under two load levels of a stainless _Issue17Paper02.PDF
In questo lavoro è stato analizzato il comportamento a fatica di un acciaio inossidabile AISI 304L. Nella prima parte del lavoro sono presentati i risultati ottenuti da prove ad ampiezza di sollecitazione costante sintetizzati sia in ampiezza di tensione sia in termini di densità di energia dissipata dal materiale per ciclo, Q. Successivamente alcuni provini sono stati sollecitati ad un livello di carico superiore al limite di fatica per circa il 70% della presunta vita e poi ad un livello di tensione inferiore al limite di fatica ad ampiezza costante precedentemente determinato ed è stata confrontata l'energia dissipata nella seconda parte della prova con quella trovata in una prova ad ampiezza costante, allo stesso livello di tensione. Il confronto ha mostrato come per il materiale analizzato il parametro Q sia sensibile al danneggiamento precedentemente subito.
Concerning the Synergy of Stress and Strain-based Methods in Modern Metal Fatigue Analysis
Fatigue is without doubt one of the most complex branches of mechanical and material engineering. Complex perhaps not in the classical sense of mathematics or material behaviour modelling, but more in terms of the number of unknowns that influence any attempt to predict the fatigue life of a machine element or an entire mechanical structure. This complexity stems from a multitude of input sources; test data extraction, load representation, material structure, manufacturing methods to mention just a few. One attempt at reducing such complexity has classically been to introduce reduction- or influence factors which are then applied to material properties, the endurance limit, component size and other relevant fatigue quantities. The danger of such simplification is that it becomes error-prone due to relying heavily on subjective application of said factors. Another issue at hand is the availability of fatigue life data, which is not always adequately referenced and therefore limited in its applicability. With the help of modern numerical fatigue analysis software the complexity can be put in a systematic perspective, provided the algorithms offered are contemporary in nature and rely on a physically correct representation of the unique fatigue behaviour. AVL EXCITE Fatigue developed by Safe Technology Ltd. is such a software package, offering advanced multiaxial stress-based fatigue life prediction capabilities. This paper offers an in-depth look at the two most advanced multiaxial algorithms available in EXCITE Fatigue; the stress-based Brown-Miller algorithm and Findley’s method. These algorithms are different from traditional stress-based methods in that the life data used in fatigue life prediction is derived from a strain-based approach, providing a second-to-none level of accuracy and reliability. In addition, the most recent findings both in stress and strain-based fatigue research have been integrated on a material science level, making them more than able to address a near limitless fatigue task spectrum.