Chapter 1
Autonomous Healing and Indication of Transverse Crack Damage
in Carbon Fiber Composite Laminates
Kelly M. Chang and Nancy R. Sottos
Abstract The performance of fiber-reinforced polymer (FRP) composites is limited by susceptibility to transverse
microcracking and interfacial debonding. In this work, we introduce a microcapsule-based self-reporting and self-healing
strategy for simultaneous detection and repair of cracks in FRPs. This dual functionality is achieved via the microencapsulation of a solvent-based healing agent doped with aggregation induced emission (AIE) luminogens and the subsequent
dispersion of such microcapsules in carbon prepreg containing a thermoplastic-toughened epoxy matrix. Composite specimens are fabricated with a [0/90/0] stacking sequence from self-healing prepreg tapes and loaded in transverse tension until
crack saturation is achieved. The transverse cracks rupture the microcapsules and release of the encapsulated solvent into the
crack plane. Crack healing is achieved by the dissolution and redistribution of thermoplastic-rich regions into the damage
volume. Evaporation of the solvent leaves solid thermoplastic in place of the crack and allowing AIE luminogens to aggregate
and fluoresce. Using a small load frame that mounts under an optical microscope, we measure full-field surface strains during
loading of the composite specimens via digital image correlation (DIC). The self-reporting functionality is evaluated by
correlating the presence of microcracks with regions of damage-induced fluorescence. The healing efficiency of the composite
specimens is assessed by comparing the applied stress levels and strain fields associated with cracking events pre- and posthealing.
1.1 Introduction
Glass-fiber and carbon-fiber reinforced polymers (GFRPs and CFRPs) are used widely in high-performance applications for
their high stiffness, specific strength, chemical resistance, and low thermal sensitivity. However, FRPs are prone to fatigueinduced failure, which is often due to transverse cracking or interfacial de-bonding. As a result, FRPs often fail catastrophically under stresses significantly lower than their quasi-static strengths, motivating the need for self-reporting and self-healing
strategies to both detect and repair cracks before failure. This dual functionality has powerful implications in extending the
safe and usable lifespan of vehicles and load-bearing structures for aerospace and defense applications. While self-healing can
provide in-situ damage control, additional and simultaneous self-reporting functionality can indicate where the healing
functionality has already been spent, thereby informing users of the most vulnerable regions before impending failure occurs.
1.2 Background
Biologically inspired self-healing polymers and FRP composites have the ability to autonomously detect and repair damage
[1–3]. In capsule-based systems, an autonomous healing reaction is triggered by rupturing an embedded microencapsulated
healing agent, which reacts with the matrix to heal the damaged volume. This approach to self-healing is effective for damage
at the microscale [4]. Furthermore, it has been shown that incorporating solvent-filled microcapsules (polydopamine-coated
polyurethane/urea-formaldehyde triple-walled shell wall with ethyl phenylacetate core (PDA(PU/UF(EPA))) slowed the
fatigue-induced decay of stiffness in GFRP and CFRP composites [3]. These PDA(PU/UF(EPA)) capsules were successfully
embedded into fiber bundles via the traditional prepreg method used in industry [3].
K. M. Chang (*) · N. R. Sottos
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA
Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, USA
e-mail: kellymc2@illinois.edu; n-sottos@illinois.edu
© The Society for Experimental Mechanics, Inc. 2021
R. P. Singh, V. Chalivendra (eds.), Mechanics of Composite, Hybrid and Multifunctional Materials, Volume 6,
Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59868-6_1
1
Autonomous Healing and Indication of Transverse Crack Damage
in Carbon Fiber Composite Laminates
Kelly M. Chang and Nancy R. Sottos
Abstract The performance of fiber-reinforced polymer (FRP) composites is limited by susceptibility to transverse
microcracking and interfacial debonding. In this work, we introduce a microcapsule-based self-reporting and self-healing
strategy for simultaneous detection and repair of cracks in FRPs. This dual functionality is achieved via the microencapsulation of a solvent-based healing agent doped with aggregation induced emission (AIE) luminogens and the subsequent
dispersion of such microcapsules in carbon prepreg containing a thermoplastic-toughened epoxy matrix. Composite specimens are fabricated with a [0/90/0] stacking sequence from self-healing prepreg tapes and loaded in transverse tension until
crack saturation is achieved. The transverse cracks rupture the microcapsules and release of the encapsulated solvent into the
crack plane. Crack healing is achieved by the dissolution and redistribution of thermoplastic-rich regions into the damage
volume. Evaporation of the solvent leaves solid thermoplastic in place of the crack and allowing AIE luminogens to aggregate
and fluoresce. Using a small load frame that mounts under an optical microscope, we measure full-field surface strains during
loading of the composite specimens via digital image correlation (DIC). The self-reporting functionality is evaluated by
correlating the presence of microcracks with regions of damage-induced fluorescence. The healing efficiency of the composite
specimens is assessed by comparing the applied stress levels and strain fields associated with cracking events pre- and posthealing.
1.1 Introduction
Glass-fiber and carbon-fiber reinforced polymers (GFRPs and CFRPs) are used widely in high-performance applications for
their high stiffness, specific strength, chemical resistance, and low thermal sensitivity. However, FRPs are prone to fatigueinduced failure, which is often due to transverse cracking or interfacial de-bonding. As a result, FRPs often fail catastrophically under stresses significantly lower than their quasi-static strengths, motivating the need for self-reporting and self-healing
strategies to both detect and repair cracks before failure. This dual functionality has powerful implications in extending the
safe and usable lifespan of vehicles and load-bearing structures for aerospace and defense applications. While self-healing can
provide in-situ damage control, additional and simultaneous self-reporting functionality can indicate where the healing
functionality has already been spent, thereby informing users of the most vulnerable regions before impending failure occurs.
1.2 Background
Biologically inspired self-healing polymers and FRP composites have the ability to autonomously detect and repair damage
[1–3]. In capsule-based systems, an autonomous healing reaction is triggered by rupturing an embedded microencapsulated
healing agent, which reacts with the matrix to heal the damaged volume. This approach to self-healing is effective for damage
at the microscale [4]. Furthermore, it has been shown that incorporating solvent-filled microcapsules (polydopamine-coated
polyurethane/urea-formaldehyde triple-walled shell wall with ethyl phenylacetate core (PDA(PU/UF(EPA))) slowed the
fatigue-induced decay of stiffness in GFRP and CFRP composites [3]. These PDA(PU/UF(EPA)) capsules were successfully
embedded into fiber bundles via the traditional prepreg method used in industry [3].
K. M. Chang (*) · N. R. Sottos
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA
Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, USA
e-mail: kellymc2@illinois.edu; n-sottos@illinois.edu
© The Society for Experimental Mechanics, Inc. 2021
R. P. Singh, V. Chalivendra (eds.), Mechanics of Composite, Hybrid and Multifunctional Materials, Volume 6,
Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59868-6_1
1