A fracture mechanics analysis of the fatigue reliability of tubular welded joints.
Fatigue failure is generally the result of crack initiation at a
surface followed by stable crack propagation leading finally to
unstable fracture. The total fatigue life is often a random variable
due to the randomness of fatigue loading. the variability of
material properties and the final failure conditions. Fracture
mechanics provides a rational description of fatigue. based on which
an analytical method has been developed for assessing the
reliability of components under random loading. This method can be
used for predicting the distribution of crack lengths after a given
number of fatigue cycles have been applied to a initial crack; the
distribution of the number of cycles needed to grow a crack to a
specific length; or the distribution of fatigue lives terminated at
a random final crack length.
In order to assess the fatigue reliability of tubular welded joints.
the stress intensity factors of surface cracks have been determined
by shell analysis with the cracked section modelled by line springs.
The surface cracks are generally subjected to mixed mode loadings
and the line spring method allows the stress intensity factor for
individual modes to be assessed separately. The applicability of the
line spring method to stress concentration areas has been verified in plane strain T joints, which has provided the confidence to apply
this method to tubular welded joints. The calculations are compared
with available experimental data.
The fatigue life distributions of a tubular welded T joint for three
loading modes have been predicted based on an assumed initial crack
length. The S-N curve produced from the analytic prediction compares
favourably with experimental data in terms of an equivalent hot spot
strain range. In the cases studied, the hot spot strain range is
seen to be a reasonable but not very accurate parameter for
determining fatigue life. In conclusion, fracture mechanics has been
demonstrated to be a valid and important method for predicting the
residual fatigue life of damaged tubular welded joints