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

Simulation of the microlevel damage evolution in polymer matrix composites

A 3D Isogeometric Interface-Enriched Generalized Finite Element Method (IIGFEM) is developed to analyze problems with complex, discontinuous gradient fields commonly observed in the structural analysis of heterogeneous materials including polymer matrix composites [1]. In the proposed approach, the mesh generation process is significantly simplified by utilizing simple structured meshes that do not conform to the complex microstructure of the heterogeneous media. Non-Uniform Rational B-Splines, commonly used in computer-aided design, are adopted in the IIGFEM to augment the finite element approximation space and capture the weak discontinuity present along material interfaces. The IIGFEM offers many advantages, such as the simplicity and accuracy of numerical integration, the straightforward implementation of essential boundary conditions, and the flexibility in the choice of the local solution refinement The ability to model complex material interfaces and the mesh independence are two of key features of the IIGFEM that enable it to tackle problems with evolving material response, such as computational study of damage in solids. Here, we utilize the IIGFEM scheme to study the impact of microstructural details on the initiation and evolution of the damage in polymer matrix composites. For this purpose, in this study, we incorporate a three-parameter isotropic damage model [2] into our IIGFEM solver to capture the fracture response of the matrix in a unidirectional composite layer. To bypass numerical issues associated with mesh bias, we use a viscous regularization scheme proposed by Simo and Ju [3]. The numerical stability of the proposed approach is studied and its advantages and limitations are discussed in detail. Finally, a number of numerical examples are presented to demonstrate the effect of RVE size and filler volume fraction on the damage behavior of fiber-reinforced polymer matrix composites. REFERENCES [1] Safdari, M., Najafi, A.R., Sottos, N.R., Geubelle, P.H. An Isogeometric Interface-Enriched Generalized Finite Element Method (IGFEM) for problems with complex discontinuous gradient field. Submitted (2014). [2] Matous, K., Kulkarni, M.G., Geubelle, P.H. Multiscale cohesive failure modeling of heterogeneous adhesives. Journal of the Mechanics and Physics of Solids. 2008, 56, 1511–1533. [3] Simo, J.C., Ju, J.W. Strain- and stress-based continuum damage models—ii. computational aspects. International Journal of Solids and Structures. 1987, 23(7), 841–869

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

Modulus variation of composite graphite electrodes in lithium-ion batteries during electrochemical cycling

Graphite is currently the most common anode material used in commercial lithium-ion batteries. During battery charging and discharging processes, lithium ions intercalate into and deintercalate from graphite, forming several distinct stages of graphite-lithium intercalation compounds (G-LICs). Each stage of G-LIC has a unique spacing between graphene layers, with the spacing increasing for increasing lithium content. In graphite-based composite electrodes (graphite particles in a porous polymer matrix), the changing layer spacing leads to stress and strain evolution on the composite length scale. In two separate experiments, we use substrate-curvature measurements to monitor stress changes in a thin electrode constrained on an inert, rigid substrate, and we use digital image correlation to track strain changes in a free-standing, unconstrained electrode. Combining the in-situ stress and strain analyses enables us to extract the change in the apparent modulus of the composite graphite electrode as a function of electrode potential and lithium content. As expected, we found that constrained electrodes develop compressive stress during lithiation (~10 MPa) and that unconstrained electrodes undergo free expansion (~1.5% linear strain). Interestingly, the apparent modulus of the electrode increases the most significantly during the formation of the dilute stage I compound, increases slightly with the formation of the stage IV, dilute stage II, and stage II compounds, and then decreases with the formation of the stage I compound (LiC 6). During delithiation, unconstrained electrodes contract, recovering nearly their original size. In constrained electrodes during delithation, however, the compressive stress is first relaxed, and then a tensile stress develops and is subsequently relaxed. The tensile stress leads to an apparent softening of the composite electrode over a broad range of electrode potential and capacity. At the end of one complete lithiation/delithiation cycle, the apparent modulus returns to approximately its original value. The evolution of stress, strain, and modulus data provides quantitative information on the coupled electro-chemo-mechanical response of battery electrodes and insight on material strategies to increase battery reliability

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

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