Fig 8. The FCG rates in Rice Husk
composite. a) in terms of amplitude, √ΔG. b) the √ΔG versus R plots for
selected crack growth rates. c) √ΔG vs √Gmax plots at
selected crack growth rates showing the L-shaped curve with limiting √ΔG
and √Gmax values. d) shows the cack growth trajectory
deviating from the pure fatigue line.
the two mechanisms involved. It also compares the Epoxy trajectory data
of Osorio discussed earlier.
Fig. 8 shows the FCG behavior of a polymer composite with rice husk
fibers as a strengthening phase [24]. The crack growth rates were
determined at three R ratios, 0.1, 0.3, and 0.5. Interestingly the
authors have used ΔG instead of ΔK, where G corresponds to the crack tip
driving force. From our analysis point, it would not make any difference
in terms of the two-parametric requirement. In this case, the
corresponding two parameters will be √ΔG and √Gmax as
the stress intensity factor K is proportional to √G.
Fig. 8a shows the crack growth rates in terms of √ΔG. The √ΔG – R plots
for crack growth rates near the threshold are shown in 8b. The linear
portion is extended up to R = 0.7, and a horizontal line is drawn with a
possible constant √ΔG value at high R-ratios. The corresponding √ΔG -
√Gmax plot, Fig. 8c resembles the typical L-shaped plot
of ΔK-Kmax defining the limiting values of the
parameters for each crack growth rate. The crack growth trajectory of
this rice-husk composite is shown in Fig. 8d. The trajectory as expected
deviates from the pure fatigue line indicating the
√Gmax-controlled mechanism governing the crack growth.
The behavior of this composite follows that of other polymer materials.
This data further proves that the two parametric nature involving
amplitude and peak stress is fundamental to FCG.
4. Summary and Conclusions:
It was well established that fatigue requires two load parameters for
proper analysis. For fatigue crack growth these parameters correspond to
the amplitude ΔK and the peak stress intensity factor,
Kmax. Crack closure is not required to account for the
load ratio effects. This two-parametric approach is shown to be
applicable to account for the crack growth behavior in several polymeric
materials. In contrast to metals and alloys, polymers deform either by
crazing, viscoelastic deformation, localized brittle fracture, or their
combination. The analysis establishes that the two-parametric nature of
fatigue crack growth remains the same and is relevant for all materials.
5. Acknowledgements: The authors acknowledge the funding support from
TDA’s ONR contract # N68335-16-C-0135. Special acknowledgments are due
to Mr. Bill Nickerson, Program Officer of ONR, who encouraged this
research and provided valuable insights and guidance.
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