Fatigue crack propagation in microcapsule toughened epoxy

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 doublecantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 micron 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, 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.published or submitted for publicationis peer reviewe

In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene

Microencapsulated healing agents that possess adequate strength, long shelf-life, and excellent bonding to the host material are required for self-healing materials. Ureaformaldehyde microcapsules containing dicyclopentadiene were prepared by in situ polymerization in an oil-in-water emulsion that meet these requirements for self-healing epoxy. Microcapsules of 10-1000 ??m in diameter were produced by appropriate selection of agitation rate in the range of 200-2000 rpm. A linear relation exists between log(mean diameter) and log(agitation rate). Surface morphology and shell wall thickness were investigated by optical and electron microscopy. Microcapsules are composed of a smooth 160-220 nm inner membrane and a rough, porous outer surface of agglomerated urea-formaldehyde nanoparticles. Surface morphology is influenced by pH of the reacting emulsion and interfacial surface area at the core-water interface. High yields (80-90%) of a free flowing powder of spherical microcapsules were produced with a fill content of 83-92 wt% as determined by CHN analysis.published or submitted for publicationis peer reviewe

Self-healing elastomer system

A composite material includes an elastomer matrix, a set of first capsules containing a polymerizer, and a set of second capsules containing a corresponding activator for the polymerizer. The polymerizer may be a polymerizer for an elastomer. The composite material may be prepared by combining a first set of capsules containing a polymerizer, a second set of capsules containing a corresponding activator for the polymerizer, and a matrix precursor, and then solidifying the matrix precursor to form an elastomeric matrix

Interfacial Mechanophore Activation Using Laser-Induced Stress Waves

A new methodology is developed to activate and characterize mechanochemical transformations at a solid interface. Maleimide–anthracene mechanophores covalently anchored at a fused silica–polymer interface are activated using laser-induced stress waves. Spallation-induced mechanophore activation is observed above a threshold activation stress of 149 MPa. The retro [4+2] cycloaddition reaction is confirmed by fluorescence microscopy, XPS, and ToF-SIMS measurements. Control experiments with specimens in which the mechanophore is not covalently attached to the polymer layer exhibit no activation. In contrast to activation in solution or bulk polymers, whereby a proportional increase in mechanophore activity is observed with applied stress, interfacial activation occurs collectively with spallation of the polymer film

Spatially Selective and Density-Controlled Activation of Interfacial Mechanophores

Mechanically sensitive molecules known as mechanophores have recently attracted much interest due to the need for mechanoresponsive materials. Maleimide–anthracene mechanophores located at the interface between poly(glycidyl methacrylate) (PGMA) polymer brushes and Si wafer surfaces were activated locally using atomic force microscopy (AFM) probes to deliver mechanical stimulation. Each individual maleimide–anthracene mechanophore exhibits binary behavior: undergoing a retro-[4 + 2] cycloaddition reaction under high load to form a surface-bound anthracene moiety and free PGMA or remaining unchanged if the load falls below the activation threshold. In the context of nanolithography, this behavior allows the high spatial selectivity required for the design and production of complex and hierarchical patterns with nanometer precision. The high spatial precision and control reported in this work brings us closer to molecular level programming of surface chemistry, with promising applications such as 3D nanoprinting, production of coatings, and composite materials that require nanopatterning or texture control as well as nanodevices and sensors for measuring mechanical stress and damage in situ

Damage Detection and Self-Repair in Inflatable/Deployable Structures

Inflatable/deployable structures are under consideration for applications as varied as expansion modules for the International Space Station to destinations for space tourism to habitats for the lunar surface. Monitoring and maintaining the integrity of the physical structure is critical, particularly since these structures rely on non-traditional engineering materials such as fabrics, foams, and elastomeric polymers to provide the primary protection for the human crew. The closely related prior concept of monitoring structural integrity by use of built-in or permanently attached sensors has been applied to structures made of such standard engineering materials as metals, alloys, and rigid composites. To effect monitoring of flexible structures comprised mainly of soft goods, however, it will be necessary to solve a different set of problems - especially those of integrating power and data-transfer cabling that can withstand, and not unduly interfere with, stowage and subsequent deployment of the structures. By incorporating capabilities for self-repair along with capabilities for structural health monitoring, successful implementation of these technologies would be a significant step toward semi-autonomous structures, which need little human intervention to maintain. This would not only increase the safety of these structures, but also reduce the inspection and maintenance costs associated with more conventional structures

Spin tunnelling in mesoscopic systems

We study spin tunnelling in molecular magnets as an instance of a mesoscopic phenomenon, with special emphasis on the molecule Fe8. We show that the tunnel splitting between various pairs of Zeeman levels in this molecule oscillates as a function of applied magnetic field, vanishing completely at special points in the space of magnetic fields, known as diabolical points. This phenomena is explained in terms of two approaches, one based on spin-coherent-state path integrals, and the other on a generalization of the phase integral (or WKB) method to difference equations. Explicit formulas for the diabolical points are obtained for a model Hamiltonian.Comment: 13 pages, 5 figures, uses Pramana style files; conference proceedings articl

Thermomechanical characterization of actively cooled vascularized composites

Fiber-reinforced polymer matrix composites (PMC) possess outstanding structural properties, including high strength and stiffness, low density, and highly tunable properties. However, their application is limited where elevated temperature stability is required, e.g., during high-speed flight, in engine exhaust systems, or as support for high-power electronics. Mechanical properties are greatly reduced above the glass transition temperature of the matrix and thermal decomposition will occur as the temperature is further increased. Damage may form in PMCs after only short exposure to high temperature including delamination, matrix cracking, and plastic deformation. Service temperatures for typical aerospace grade epoxies are ~150°C, whereas even the best high temperature structural polymers, bismaleimides and polyimides, typically have service temperatures \u3c250°C. Active cooling through a vascular network provides a platform for thermal regulation, allowing for on-demand, adaptive heat removal and preservation of material properties. In actively cooled composites, coolant is pumped through a microvascular channel network to remove heat and maintain structural performance. In this article we present results from thermomechanical testing of actively cooled PMCs. Microchannels are formed in PMCs using Vaporization of Sacrificial Components (VaSC). In the VaSC process, sacrificial fibers composed of poly(lactic acid) treated with tin(II) oxalate catalyst are embedded into the PMC during normal processing, then removed during a postcure at 200°C for 24 h under vacuum. Stress, strain, material temperature, and heat removal by the cooling network are monitored during thermomechanical loading. Actively cooled specimens are compared to nonactively cooled control specimens

Molecular tailoring of thin film/substrate interfaces using self-assembled monolayers

Due to the flexibility they offer in the selection of the end groups attached to the substrate and film materials, self-assembled monolayers (SAMs) composed of very short (nanometer-long) aligned polymer chains have been proposed as a unique way to tailor the electrical, thermal and mechanical properties of interfaces. This combined experimental and computational study aims at shedding some light on the impact of the SAMs on the failure properties of a gold film/silicon substrate interface. In this study, we investigate SAMs with methyl (‑CH3) and mercapto (‑SH) terminated functional groups. In the experimental component of the project, we adopt a noncontact laser-based spallation technique to measure the failure strength of a silicon/SAM/gold system. This method consists in converting the thermal energy imparted by a very short (ten nanoseconds-long) Yag laser pulse to an absorbing layer placed on the back side of the substrate into a compressive acoustic pressure pulse that propagates through the substrate toward the interface of interest. Upon reflection from the free surface of the film, the pulse loads the film/substrate in tension, leading to its spallation failure. Preliminary results show a strong dependence of the failure strength of the interface on the choice of SAM. Detailed AFM and XPS analyses performed on the postspallation surfaces provide information on the roughness profile and chemical composition of the failure surface. On the modeling side, molecular dynamics (MD) simulations, based on the ReaxFF model, are used to investigate the separation characteristics and interfacial mechanical behavior between SAM and a thin gold film on a silicon substrate. Although these MD simulations predict the experimentally observed four-fold increase in the failure strength of SH-terminated SAMs compared with their CH3-terminated counterpart, the predicted values are between one and two orders of magnitude higher than those observed experimentally. To address this gap between experimental and simulated strength values, we investigate the key role played by the roughness of the substrate and of the film, using a multiscale approach that combines the cohesive model derived from the MD simulations and a continuum model of the bending response of the film. A substantial drop in the effective strength of the SAM-enhanced film/substrate interface is predicted for relatively small values of the roughness