Pure Geometrical Size Effect in fatigue tests with constant stress amplitude and in programme tests

Abstract

The investigation considers the geomctrical size effect for hcat-treated 37Cr 4 stecl specimens in the case of one-level and multi-level fatigue tests. It was shown that the ratio Notch FactoriSCF of geometrically similar specimens increases asymptotically with the notch radius, tending to a value less than 1. Thc formula of Srieler for the ratio SCFi Notch Factor do not describe correctly the decrease of the fatigue limit with specimen diameter. A more exact description o f the size effect is obtained with the aid of a two-parameter formula, which takes into account the support effect of the cyclic strain hardening and of the stress gradient.An effcct of spccimen size on fatiguc life was also investigatcd in the case of multi-level tests. It seems that this effect is associated with a smaller diameter interval than the effect of size on the Fatigue limit. This is conncctcd with the higher plastification of the notclicd arca in programme tests with maximum stress much higher than the fatigue limit. The programme fatigue life and the damage sum were considerably larger for the smaller (d = 8.9 mm) than fur the larger (d = 22.26 mni) specimens. but further increase of thc specimen diameter see~ns to have a slight effect. Rein geometrischer GroReneinfluR im

A probabilistic model for LCF

“…the number of cycles to crack initiation should decrease with increasing specimen size. This effect is based on the assumption, that the number of possible crack initiation sites increases with specimen size, confer [12,13]. As in our case cracks are generally initiated at slip bands in surface grains, the quantity that determines the size effect is the specimen surface.…”

Analyse der Wirkung von Kerben, Mittel‐ und Eigenspannungen auf die Schwingfestigkeit des hochumgeformten Werkstoffbereichs von Spaltprofilen

“…Der in Kerben vorliegende spannungsmechanische Einfluss von Plastifizierungen ist bei Anwendung des örtlichen Dehnungskonzepts bereits durch die Verwendung der örtlichen elastisch-plastischen Beanspruchungen berücksichtigt. Unterscheidet sich die Größe des höchstbeanspruchten Bereichs in den Proben und dem Bauteil wesentlich, ist der statistische Größeneinfluss zu berücksichtigen [3,13,18,19,20,21]. Dies ist auch beim Vergleich der ungekerbten Proben und der gekerbten Proben notwendig.…”

“…Hierbei kann die gekerbte Probe stellvertretend für ein Bauteil betrachtet werden. Der statistische Größeneinfluss wurde bei Spaltprofilen mit Hilfe des Weibullschen Fehlstellenmodells [18] berücksichtigt [3,13,22] x, y, z), N) bezeichnet. Sind die von der aufgebrachten Last L(t) abhängigen örtlichen Beanspruchungen P SWT (x, y, z, L) eines Schwingspiels sowie die spezifische Streuung der lokalen Werkstoffzustände j bekannt, lässt sich für eine bestimmte Schwingspielzahl N die Ü berlebenswahrscheinlichkeit des Bauteils P Ü ,Bauteil bestimmen.…”

Notch‐Size Effect in fatigue of steel specimens – verification of some calculation methods

“…The verification of the different calculation methods which can give an answer to the above mentioned questions must be carefully and thoroughly performed, because of the large scatter of test results (especially for high stress concentration factors KT and small notch radii), and the often inexact estimates of KT and of the fatigue limits of notched specimens obtained from a small number of tests, which result in inaccurate values of the ratios In a recent investigation a verification was performed for some well known, typical KF-formulas. Rotating-bending and tension-compression test results of the Institute of Material Science -Zfw -Darmstadt [15][16][17] obtained on a large number of specimens, with statistical evaluation of the fatigue limit, were used as basis for the verification. The tested specimens had different diameters and stress concentration factors, but identical high quality surface finish.…”