Effect of Notches on the Creep-Rupture Strength and Fatigue Strength at High Temperature

Abstract

We have carried out creep rupture tests and high temperature fatigue tests for notched bars, to examine the general effect of notches on high temperature strength over prolonged periods of time.Test materials are ten kinds of heat resisting steels, ranging from low alloy steels to super alloys, and we have given them regular heat treatment in these tests. Notched specimens have U-type and V-type circumferential groove by machine cutting, which are respectively about 1.8 and 3 in stress concentration factor.The creep rupture tests of multiple tension type were carried out aiming at 1000 hours, and fatigue tests of rotary-bending type was practised at a revolving speed of 3000rpm up to N=107 cycles.The typical values of the abave tests are tabulated in this report, with notch rupture strength ratio and strength reduction factor (β). Here, we obtained several conclusions as follows;(1) The notch rupture strength ratio is generally larger than unity, as in the short time tensile test. But, it correlates closely with ductility variation of materials, depending on progressive precipitation and aging, with the increase of applied temperature and time.(2) Within the range of the tests, all the test points of ten materials have about the same correlation for the notch rupture strength ratio to the value of plain bar contraction, at corresponding rupture time, and it falls under unity when the plain bar is contracted below approximately 30% in V-notch, and 20% in U-notch. It is convenient to assume notch effect from plain bar creep rupture tests.(3) This ratio has a tendency to decrease after it has once increased, with the increase of notch sharpness. When it has large ductility the maximum value is high, but with small ductility, it gradually falls.(4) In general, the ratio for austenitic alloys is lower than for ferritic alloys. Particularly, 25-20 stainless steels showed the highest notch sensitivity of all the test materials.(5) Concerning the high temperature fatigue, there was a tendency in the strength reduction factor (β) for ferritic alloys to decrease, and for austenitic alloys to increase at high temperature.