Fracture and Fatigue Behavior of a Self-Healing Polymer Composite
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FRACTURE AND FATIGUE BEHAVIOR OF A SELF-HEALING POLYMER COMPOSITE Eric N. Brown1,2, Jeffrey S. Moore 2,3, Scott R. White 2,4, and Nancy R. Sottos1,2 1 Department of Theoretical & Applied Mechanics, Urbana IL, 61801-2983 U.S.A. 2 Beckman Institute for Advanced Science and Technology, Urbana, IL, 61801 U.S.A. 3 Department of Chemistry, Urbana IL, 61801 U.S.A. 4 Department of Aerospace Engineering, Urbana IL, 61801-2935 U.S.A. ABSTRACT Inspired by biological systems, in which damage triggers an autonomous healing response, a polymer composite material that can heal itself when cracked has been developed. The material consists of an epoxy matrix composite, which utilizes embedded microcapsules to store a healing agent and an embedded catalyst. This paper investigates issues of fracture and fatigue consequential to the development and optimization of this new class of materials. When damage occurs, the propagating crack ruptures the microcapsules, which releases healing agent into the crack plane. Polymerization of the healing agent is triggered by contact with exposed catalyst, which bonds the crack faces closed. The efficiency of crack healing is defined as the ability of a healed sample to recover fracture toughness. Healing efficiencies of over 90% have been achieved. Embedded microcapsules significantly increase the fracture toughness and reduce the fatigue crack propagation rate of epoxy. Fracture mechanisms for neat epoxy and epoxy with embedded microcapsules are presented. INTRODUCTION Thermosetting polymers are used in a wide variety of applications ranging from structural composites to microelectronics. Due to the low strain-to-failure exhibited by these polymers they are highly susceptible to damage in the form of cracks. These cracks frequently initiate deep within a structure where detection is difficult and repair often impossible, ultimately leading to catastrophic failure. White et al. [1] have introduced a novel approach to recover the fracture properties of thermosetting polymers following crack propagation through the addition of self-healing functionality. Healing is achieved through the inclusion of urea-formaldehyde microcapsules that contain dicyclopentadiene (DCPD) healing agent. A propagating crack ruptures the microcapsules and exposes Grubbs’ catalyst particles embedded in the matrix. Opening of the crack draws the healing agent into the crack plane where contact with the catalyst phase initiates polymerization. Crack healing efficiency is defined as the ability to recover fracture [3],
η=
K Ichealed KIc virgin
,
(1)
where KIc virgin is the fracture toughness of the virgin specimen and KIc healed is the fracture toughness of the healed specimen. This self-healing material has been reported to recover up to 90% of its virgin fracture toughness [2]. Moreover, the inclusion of microcapsules increases the fracture toughness of the matrix material by up to 127%. The current work investigates the issue
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of microcapsule toughening in greater depth and the corresponding effect
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