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

The effect of different spectro-temporal representations of an input auditory stimulus on the fitting of a point process model of auditory neurons

We compare the effect of the use of three different spectro-temporal representations of an input auditory stimulus on the fitting of a point process model of auditory neuron firing. The three spectro-temporal representations considered are the spectrogram, a gammatone filterbank and the Hilbert spectrum. We firstly investigate how the model fits the recorded neuronal data when using either one of the three representations and secondly how well do the estimated parameters of the model correspond to their experimentally measured counterparts. It is observed that all three representations yield a model that fits well the recorded data. However, the characteristic frequencies obtained with the spectro-temporal parameters of the model using the gammatone filterbank corresponds better to the experimentally measured characteristic frequency than the characteristic frequency obtained with the models using the other two spectro-temporal representations. Therefore, it is concluded that the quality of the fitted parameters can be affected by the choice of the spectro-temporal representation and that, as could have been expected, the gammatone filterbank seems to more accurately extract the relevant spectro-temporal characteristics of the input auditory stimulus.Fonds quebecois de la recherche sur la nature et les technologiesNational Institutes of Health (U.S.) (Grant DP1-OD003646

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A Point Process Model for Auditory Neurons Considering Both Their Intrinsic Dynamics and the Spectrotemporal Properties of an Extrinsic Signal

We propose a point process model of spiking activity from auditory neurons. The model takes account of the neuron's intrinsic dynamics as well as the spectrotemporal properties of an input stimulus. A discrete Volterra expansion is used to derive the form of the conditional intensity function. The Volterra expansion models the neuron's baseline spike rate, its intrinsic dynamics-spiking history-and the stimulus effect which in this case is the analog of the spectrotemporal receptive field (STRF). We performed the model fitting efficiently in a generalized linear model framework using ridge regression to address properly this ill-posed maximum likelihood estimation problem. The model provides an excellent fit to spiking activity from 55 auditory nerve neurons. The STRF-like representation estimated jointly with the neuron's intrinsic dynamics may offer more accurate characterizations of neural activity in the auditory system than current ones based solely on the STRF

Speed and Terrain Impact Ground Reaction Forces During Load Carriage

Load carriage leads to larger, faster vertical ground reaction forces (GRFs), and greater risk of musculoskeletal injury. Although increased lower limb flexion can help dissipate the vertical GRF, reducing injury risk, it is unclear if terrain affects limb flexion and vertical GRFs during load carriage. This study quantified lower limb biomechanics as participants walked, jogged and ran with heavy body borne load over different terrain. We hypothesize that participants will decrease lower limb flexion, but increase peak vertical GRFs as speed increases, but these changes will vary with terrain. Each participant walked (1.3 m/s), jogged (3.0 m/s) and ran (4.5 m/s) with body borne load (15 kg) over a rocky, firm, soft, and flat terrain. During each locomotor task, peak vertical GRF and range of hip, knee and ankle motion (ROM) were calculated, and submitted to statistical analysis. Significant speed by surface interactions were evident for peak vertical GRF (p \u3c 0.001), and ankle and knee ROM (p=0.030 and p \u3c 0.001). Speed impacted peak vertical GRF, and ankle and knee ROM (all: p \u3c 0.001), where GRF and ankle were larger, but knee motion smaller during the run. Surface impacted peak vertical GRF and ankle ROM (p \u3c 0.001 and p=0.008). Unexpectedly, participants decreased peak vertical GRF on the rocky compared to all other surfaces. Through the use of this study, training programs can be adapted to help athletes and military personnel decrease their risk of injury while training

Quantifying the spatiotemporal dynamics in a chorus frog (Pseudacris) hybrid zone over 30 years

© 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. Although theory suggests that hybrid zones can move or change structure over time, studies supported by direct empirical evidence for these changes are relatively limited. We present a spatiotemporal genetic study of a hybrid zone between Pseudacris nigrita and P. fouquettei across the Pearl River between Louisiana and Mississippi. This hybrid zone was initially characterized in 1980 as a narrow and steep “tension zone,” in which hybrid populations were inferior to parentals and were maintained through a balance between selection and dispersal. We reanalyzed historical tissue samples and compared them to samples of recently collected individuals using microsatellites. Clinal analyses indicate that the cline has not shifted in roughly 30 years but has widened significantly. Anthropogenic and natural changes may have affected selective pressure or dispersal, and our results suggest that the zone may no longer best be described as a tension zone. To the best of our knowledge, this study provides the first evidence of significant widening of a hybrid cline but stasis of its center. Continued empirical study of dynamic hybrid zones will provide insight into the forces shaping their structure and the evolutionary potential they possess for the elimination or generation of species

Expressive probabilistic sampling in recurrent neural networks

In sampling-based Bayesian models of brain function, neural activities are assumed to be samples from probability distributions that the brain uses for probabilistic computation. However, a comprehensive understanding of how mechanistic models of neural dynamics can sample from arbitrary distributions is still lacking. We use tools from functional analysis and stochastic differential equations to explore the minimum architectural requirements for $\textit{recurrent}$ neural circuits to sample from complex distributions. We first consider the traditional sampling model consisting of a network of neurons whose outputs directly represent the samples (sampler-only network). We argue that synaptic current and firing-rate dynamics in the traditional model have limited capacity to sample from a complex probability distribution. We show that the firing rate dynamics of a recurrent neural circuit with a separate set of output units can sample from an arbitrary probability distribution. We call such circuits reservoir-sampler networks (RSNs). We propose an efficient training procedure based on denoising score matching that finds recurrent and output weights such that the RSN implements Langevin sampling. We empirically demonstrate our model's ability to sample from several complex data distributions using the proposed neural dynamics and discuss its applicability to developing the next generation of sampling-based brain models

Spectropolarimetry and Modeling of the Eclipsing T Tauri Star KH 15D

KH 15D is a strongly variable T Tauri star in the young star cluster NGC 2264 that shows a decrease in flux of 3.5 magnitudes lasting for 18 days and repeating every 48 days. The eclipsing material is likely due to orbiting dust or rocky bodies in a partial ring or warped disk that periodically occults the star. We measured the polarized spectrum in and out of eclipse at the Keck and Palomar observatories. Outside of the eclipse, the star exhibited low polarization consistent with zero. During eclipse, the polarization increased dramatically to ~2% across the optical spectrum, while the spectrum had the same continuum shape as outside of eclipse and exhibited emission lines of much larger equivalent width, as previously seen. From the data, we conclude that (a) the scattering region is uneclipsed; (b) the scattering is nearly achromatic; (c) the star is likely completely eclipsed so that the flux during eclipse is entirely due to scattered light, a conclusion also argued for by the shape of the ingress and egress. We argue that the scattering is not due to electrons, but may be due to large dust grains of size ~10 micron, similar to the interplanetary grains which scatter the zodiacal light. We construct a warped-disk model with an extended dusty atmosphere which reproduces the main features of the lightcurve, namely (a) a gradual decrease before ingress due to extinction in the atmosphere (similar for egress); (b) a sharper decrease within ingress due to the optically-thick base of the atmosphere; (c) a polarized flux during eclipse which is 0.1% of the total flux outside of eclipse, which requires no fine-tuning of the model. (abridged)Comment: 9 pages, 7 figures, accepted for publication in ApJ, MPEG simulation available at http://www.astro.washington.edu/agol/scatter2.mp