Page 440 - Biaxial Multiaxial Fatigue and Fracture

424 i? ITOH AND Z MIYAZAKI

estimates unconservative lives for nonproportional LCF. Type 304 stainless steel is known as a
material which shows large additional hardening under nonproportional loadings [5,9-1 I].
Fatigue lives are drastically reduced by the additional hardening which depends on the strain
history. The maximum reduction is a factor of 20 when comparing with the proportional fatigue
life. On the other hand, 6061 aluminum alloy shows small additional hardening, which results
in only a small reduction in fatigue lives due to nonproportional loading. Therefore,
nonproportional loading reduces the LCF lives accompanied by the additional hardening
depending on the material and the strain history, so the nonproportional parameter must take
account of the additional hardening. A couple of nonproportional parameters which include the
stress range or stress amplitude have been proposed [3,12,13]. Stress terms in the parameters
can be calculated using inelastic constitutive equations [9,14-161, but in general it is not a simple
procedure and it requires many material constants. There is no well-established method of
estimating nonproportional LCF life only based on strain history.
One of the authors [4,8] carried out nonproportional LCF tests using hollow cylinder specimen
of Type 304 stainless steel and 6061 aluminum alloy, and proposed a nonproportional LCF
parameter depending only the strain history, AEN~. This strain parameter is able to evaluate the
dependence of fatigue lives on strain history and material, and correlates the fatigue lives within
a narrow scatter band for 15 kinds of proportional and nonproportional strain paths. However,
the parameter was only applicable to the life prediction under a limited nonproportional strain
history, so that some modifications of it were required in order to apply the life prediction to
more complex nonproportional loadings.
In this paper, a simple damage model for life prediction is developed by combining A&NP with
Miner’s law as well as a new cyclic strain counting method for the nonproportional LCF with
more complex strain history. The applicability of the present model is examined for life
evaluation of nonproportional LCF for Type 304 stainless steel, 6061 aluminum alloy and other
materials obtained from different research institutes.

EXPERIMENTAL PROCEDURE

The materials tested were Type 304 stainless steel (304SS) and 6061 aluminum alloy (6061Al).

Fig. I. Shape and dimensions of the hollow cylinder specimen tested (mm).