Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

4.5 Conclusion
A cycle jump method for accelerated simulation of fatigue crack growth in a bimaterial interface
was presented in this chapter. The proposed method is based on finite element analysis for a set
of cycles to establish a trend line, extrapolating the trend line which spans many cycles, and use
the extrapolated state as an initial state for additional finite element simulations. Two finite
element routines for accelerated fatigue crack growth simulation were developed. The first
routine is suitable for 2D crack growth and the second is applicable to any 3D fatigue crack
growth simulation with an arbitrary crack front shape. To assess the computational efficiency
and accuracy of the developed finite element routines, they were used to simulate face/core
interface fatigue crack growth in a sandwich beam (2D) and a sandwich panel (3D). The results
were compared with a reference analysis simulating all individual cycles.
By application of the cycle jump method, fatigue crack growth in the interface of a sandwich
beam was simulated for 500 cycles as a numerical example. The computational efficiency and
accuracy of the cycle jump method was discussed and verified based on the three parameters:
crack length, difference between maximum and minimum energy release rate in a cycle (G) and
mode-mixity phase angle against the reference analysis. The effect of the control parameters
governing the implementation of the cycle jump method on the computational efficiency and
accuracy was studied. The results suggest that the computational efficiency of the simulations
increases considerably with increasing the control parameters. However, the accuracy of the
simulations decreases for crack length, G and mode-mixity phase angle determination. For the
control parameters qG=q=0.05 the cycle jump method requires 175 cycles to simulate 500
cycles, resulting in a 65% reduction in computational time with reasonably good accuracy
(around 1% error).
The second routine (3D) was used to simulate fatigue debond propagation in sandwich panels
with an elliptical face/core debond at the centre of the panels. To make the simulation suitable
for practical applications and due to lack of experimental methods for characterization of the
effect of the mode III energy release rate, GIII, on the crack growth rate, only mode I and II
components of the strain energy release rate were used in the crack growth routine. However, to
analyse the effect of mode III loading at the crack tip, the mode III strain energy release rate was
determined along the debond front. It was shown that the mode III crack tip loading is
considerable close to the longer radius of the ellipse for an elliptical debond with large a/b radius
ratios, which implies the importance of the development of new experimental methods for
characterisation of the effect of mode III loading at the crack tip on the crack growth rate in such
debond geometries.
To examine the accuracy and computational efficiency of the developed 3D cycle jump method,
a reference simulation, simulating all individual cycles and simulations based on the cycle jump
method with different control parameters were conducted. It was shown that with good accuracy
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