Abstracts - Conference Planning and Management - Iowa State ...

increases in the healing agent, healing efficiency also increases. However, healing efficiency of 33% is
observed when maleimides are not present in the healing agent, meaning that mechanical interlocking
of crack surfaces contributes to healing. Mechanical interlocking results from swelling and subsequent
deswelling of the polymer network with the solvent‐based healing agent. These healing mechanisms
occur on different length scales; mechanical interlocking occurs across micrometer scale cracks, while
cracks must be smaller than 1.3 nm in order for the bismaleimide molecule to react across a crack. As a
result, swelling and mechanical interlocking appears to enhance the amount of covalent bonding, and
therefore healing efficiency. The overall healing mechanism is displayed in Figure 1. The healing
system described is designed for use in fiber‐reinforced composites. Three composite geometries were
investigated for their ability to selfheal. In all cases, specimens were loaded to failure, healed, and
loaded to failure again. An alternate healing efficiency was devised for the composite studies in order to
account for the residual mechanical properties of a failed composite specimen:
In Equation 1 α is the property measured for each geometry, i.e. maximum load or fracture toughness.
Glass fiber‐reinforced flexural specimens healed with a bismaleimide solution demonstrated 48%
healing efficiency, while short beam shear specimens gave 37% healing efficiency. Healing efficiencies
were low because failure of the composite resulted in significant irreversible failure of glass fibers.
Double cantilever beam specimens were tested to focus on failure of the resin phase. However, 50%
healing efficiency was obtained. Evaluation of the fracture surface revealed that only 30‐35% of failure
was cohesive in nature, with the remainder
being adhesive (irreversible) failure. The optimal amount of healing agent is a significant consideration.
Too little healing agent limits mechanical interlocking. However, more healing agent increases the time
for total diffusion of solvent from the composite piece. Another challenge is optimizing the amount of
bismaleimide in solution. Less bismaleimide results in fewer bonds bridging crack surfaces, but too
much bismaleimide limits the number of molecules that will react across the crack, with most only
reacting with one surface. Surface conditions also play a role in healing. Since covalent bonding across
a crack surface is limited by the areas where the distance between surfaces is less than the size of the
bismaleimide molecule, a smoother surface should result in more covalent bonding. However, another
consequence of a smoother surface is less mechanical interlocking. The competing effects of surface
condition on healing efficiency are under investigation. Solution‐induced healing of glass‐reinforced
composites has been investigated. Although this study focuses on di‐functional maleimide solutions as
healing agents, higher functionality maleimides have been studied and their use results in improved
load recovery. Systems with other furan‐functionalized polymers are being investigated and have also
demonstrated the ability to heal.
Society of Engineering Science ▪ 47 th Annual Technical Meeting 26