April 26, 2017

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Self-healing Composite Materials

Cover design submitted for the Feb. 15, 2001 issue of Nature.

Begining with his Ph.D. research at the University of Illinois, Prof. Kessler worked with an interdisciplinaryresearch group and studied a range of topics from organic chemistry, applied mechanics, and processing science to address a long standing problem in materials science: material degradation and microcracking induced failure. Current independent research at Iowa State, with funding from the Army Research Office (Young Investigator Program), is focusing on the autonomic healing of damage in polymer matrix composites.

No matter how carefully structural materials are designed and manufactured, all will eventually fail either by a catastrophic event or through natural degradation.

Recently we reported on a novel approach to the later problem in Nature. Our article describes a material with the ability to heal itself autonomically, i.e. a self-healing material. Such materials are capable of resisting and slowing down the natural degradation process, thereby prolonging their useful service life.

The self-healing concept is a simple one and is shown schematically here. A microencapsulated healing agent is embedded in a structural composite matrix containing a catalyst capable of polymerizing the healing agent. (i) Cracks form in the matrix wherever damage occurs. (ii) The crack ruptures the microcapsules, releasing the healing agent into the crack plane through capillary action. (iii) The healing agent contacts the catalyst triggering polymerization that bonds the crack faces closed

 

The healing system consists of a monomer liquid encapsulated in a polymer microsphere and a catalyst that is dispersed within the matrix material.

The monomer dicyclopentadiene (DCPD) was chosen as the healing agent. The catalyst, a ruthenium based system called Grubbs’ catalyst, initiates the ring-opening metathesis polymerization (ROMP) of DCPD.
An embedded microcapsule on the fracture plane. (Diam. ~30 microns)

The DCPD filled microcapsules are prepared by the in situ polymerization microencapsulation technique. The first step is to form an emulsion of the water-immiscible core material (DCPD) in an aqueous solution of anionic polymer under constant agitation. This is followed by the initiation of a polymerization process in the water phase to produce a capsule wall of urea-formaldehyde (UF).

The development of a self-healing fiber reinforced composite is fundamentally more difficult than the self-healing polymer reported in Nature. Although resin micro-cracks can be healed similarly, the presence of the fiber reinforcement increases the number of damage modes and the complexity of the healing process.

Fracture experiments are performed to quantify the effectiveness of self-healing interlaminar fracture damage in structural composites. We assess the healing of interlaminar fracture damage in woven composites by using double cantilever beam (DCB) and width tapered DCB (WTDCB) fracture specimen loaded in mode-I.

(A) DCB specimen during testing. (B) Microscope image of the DCB specimen during testing showing the crack tip position.
SEM micrograph of the fracture surface of a healed reference plain weave specimen.

After assessing the healing efficiency, detailed post-failure examination of the fracture surfaces are performed using optical and scanning electron microscopy to investigate the influence of complex microstructure features such as fiber bridging, fiber-matrix debonding, and catalyst agglomeration on the healing efficiency and mechanism.