The Effects of Programme Loading on Fatigue Crack Propagation

Abstract

The object of the present research was to find and evaluate the
effects of programme loading on fatigue crack propagation in EN24 steel.
A potential drop technique was developed to measure very small changes
in the crack length. The constant load amplitude test results,
analysed on the basis of fracture mechanics, served as a basis for
comparison with the results of the programme loading tests. The block
programme loading schedules were designed to find the effects of load interaction and stress levels on the crack propagation rate. Microscopic
and fractographic examinations were conducted to understand the
influence of micro-structures and strength levels on the fatigue
fracture process.

The results showed that the micro-structural features of the three
heat-treatments considered exerted a secondary influence on the crack
growth rate. The capacity of the low strength ductile steels to accommodate
large amount of strain at the crack-tip by plastic deformation
and their ability to dissipate energy by numerous crack-branching and
island formation were associated with the higher crack growth resistance
of these steels as compared with the high strength martensitic steel.
At any ΔK level, the crack growth rate was initially constant followed
by a gradual slowing down which depended on the loading variables. With
a growing fatigue crack at a constant ΔK, a decrease in the maximum
stress caused a transient slow growth period whereas an increase in
σmax did not show any appreciable acceleration of the crack growth rate.
Based on a proposed mechanism of fatigue crack growth, an equation of
the type da/dn= A(ΔK)M/R' , where R' = σys/σmax , was found to unify the results of different loading conditions seat This equation does not
account for the slow growth periods and would, therefore, yield a
conservative estimate of fatigue life.

Date of Award	1973
Original language	English