Fatigue crack propagation in microcapsule-toughened epoxy
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The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔKT, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.
KeywordsFatigue Crack Crack Growth Rate Fatigue Crack Growth Fatigue Crack Propagation Fatigue Crack Growth Rate
The authors gratefully acknowledge support from the AFOSR Aerospace and Materials Science Directorate Mechanics and Materials Program (Award No. F49620-00-1-0094), the National Science Foundation (NSF CMS0218863), and Motorola Labs, Motorola Advanced Technology Center, Schaumburg Ill. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the AFOSR or Motorola Labs. The authors would also like to thank Profs. J.S. Moore and P.H. Geubelle of the Autonomic Materials Laboratory of the Beckman Institute of Advanced Science and Technology and Dr. A. Skipor of Motorola Labs for technical support and helpful discussions. Electron microscopy was performed in the Imaging Technology Group, Beckman Institute, of the University of Illinois at Urbana-Champaign, with the assistance of S. Robinson. LAUR-04-2668.
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