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

erroneous extrapolations in the transition from stable to unstable crack growth zone and the
extreme non-linearity of this transition. With smaller control parameters smaller jumps occur and
the cycle jump scheme is able to extrapolate accurately the stable-unstable crack transition zone.
H100 specimens due to slightly different crack growth rate relations and stable crack growth, as
also observed in the fatigue experiments, show much less dependency on the control parameters.
Additionally, with the chosen initial crack length, the H100 specimens have already passed the
highly non-linear region of transition from very slow crack growth rates to much larger growth
rates.
200
200 q=qG=0.05
q=qG=0.15
q=qG=0.25
Crack length (mm)
150
100
50
0
q=qG=0.01
q=qG=0.05
q=qG=0.15
q=qG=0.25
0 20000 40000 60000 80000 100000
Cycles
Cycles
(a)
(b)
Figure 5.31: The effect of the control parameters on the simulation of (a) H45 and (b) H100
STT specimens.
In order to model the initial highly non-linear crack growth zone for the H100 STT specimens,
simulations were conducted on the specimens with H100 core and 5 mm smaller initial crack
length (20 mm crack length) for a range of different control parameters, see Figure 5.32.
Deviations similar to those of the STT specimens with H45 core are seen this time for the
qG=q=0.05, 0.15 and 0.25 control parameters, illustrating one of the main limitations of the
developed cycle jump scheme. In the case of highly non-linear behaviour of the structure, the
control parameters should be chosen carefully to be able to simulate the non-linear zone
accurately. This limitation makes the sensitivity and convergence analysis an essential part of
incorporating the cycle jump method in the simulation of general fatigue crack growth in
structural analysis. Simulation results using qG=q=0.05 control parameters are presented
together with experimental results in Figure 5.33. The simulations of the specimens with H100
core show fair accuracy compared to the experimental results. However, large deviations are
seen between the simulations and experimental results for the specimens with H45 core. The
deviations start at the beginning of the unstable crack growth and remains constant throughout
the stable crack growth zone. The reason for this deviation can be found in the interface fatigue
characterisation. Since the interface fatigue characterisation was only made for the stable linear
part of the crack growth rate diagram (the Paris regime), the resulting da/dN vs. G relation is
112
Crack length (mm)
150
100
50
0
1 mm
0 20000 40000 60000 80000 100000