Seminar - Xiao-Bo Yu- Fatigue Crack Propagation under Mixed Mode Loads

Tuesday, July 20, 1999, 3 - 4 pm
Civil Engineering Lecture Room 1

Abstract

This seminar will summarise the major results of the Ph.D thesis by
postgraduate student, Xiaobo Yu.

The research work presented in the thesis is focused on the intrinsic and
extrinsic mechanisms of fatigue crack propagation under mixed mode
loading conditions. The thesis work involves: a) experimental study of
fatigue crack growth behaviour, b) modelling analysis of crack surface
interference (CSI), and c) experimental study of CSI. The experimental
study was performed on notched thin-walled tubular specimens of mild
steel.

The crack growth behaviour was investigated under three load conditions:
i) cyclic mode I plus steady or low frequency mode II, ii) cyclic mode II
plus steady mode I, and iii) cyclic mode I plus cyclic mode II with various
loading paths. The crack growth direction, rate, and surface topography are
evaluated for the above conditions. It is concluded that a fatigue crack in
steel may propagate in a shear mode, in addition to the conventionally
recognised opening mode. The shear mode propagation is promoted by
non-proportional mixed loads. It is stable, associated with small scale
yielding, and can be up to 6 times faster than an equivalent mode I
propagation. In addition, sliding movement along the asperity facets is
proposed, as a supplementary to the conventional roughness-induced crack
closure mechanism.

The modelling analysis of CSI provided a quantitative description of the
proposed mechanism of sliding movement along slanted asperity facets. The
effects of plasticity/oxidation crack surface build-ups and unsynchronised
CSI development were incorporated in the analysis. Two load conditions
were considered: cyclic mode I plus steady mode II, and cyclic mode II plus
steady mode I. The modelling predictions were related to the observations
of fatigue crack growth behaviour.

The experimental study of CSI was designed to provide verification or
otherwise of the validity of the proposed sliding movement mechanism. CSI
was measured by near-tip strain gauges and a new method was developed
for the uncoupling of mode I and mode II CSI from the raw strain readings.
Two load conditions were tested and the experimental results agreed well
with the modelling prediction.

The work described above leads to two significant conclusions in relation
to mixed mode fatigue. The first lies in the intrinsic mechanism: it is
strongly suggested that shear mode propagation in steel can take place in a
stable manner and it is promoted by non-proportional mixed mode loads.
The second is related to the extrinsic mechanism: it is concluded that sliding
movement along asperity facets is possible and therefore the CSI-induced
cyclic wedge opening and mode II stress intensities should also be
considered in addition to the conventionally recognised shear attenuation
and mode I crack closure.