Effect of pH and Al Cations on Chromate Inhibition of Galvanic-Induced Corrosion of AA7050-T7451 Macro-Coupled to 316SS

The performance of chromate in protecting AA7050-T7451 coupled to 316SS in simulated fastener environments, including those representative of the boldly exposed surfaces and downhole conditions, was investigated utilizing a number of electrochemical and surface characterization techniques. The influence of pH and Al3+ on the galvanic coupling behavior and damage evolution on AA7050 as a function of chromate concentration were assessed. The degree of chromate inhibition was observed to decrease as pH decreased, owing to chromate speciation and reduced capacity to suppress the hydrogen evolution reaction (HER) compared to the oxygen reduction reaction (ORR). The addition of 0.1 M Al3+ significantly increased HER kinetics and produced a large buffer effect which overwhelmed the ability of chromate to slow damage propagation on AA7050. Assessment of cathodes indicated that Cu was more important than 316SS in driving damage initiation, but less active than 316SS in supporting high-rate damage propagation in simulated crevice environments. The implications of this study for actual bimetallic systems are discussed.

HYPERVELOCITY IMPACT RESPONSE OF STITCHED CFRP LAMINATES

In this study, hypervelocity impact experiments were performed on both unstitched and through-thickness Vectran™-stitched laminates. Both laminate types were fabricated from DMS-2436 class-72 warp-knit multiaxial carbon fabric, infused with API-1078 resin using a Controlled Atmospheric Pressure Resin Infusion (CAPRI) process. The laminates were impacted by 4 mm diameter, spherical, Nylon 6/6 projectiles at nominal velocities of 4 km/s using a two-stage light gas gun. The primary measures of the performance of the composite at protecting against impact were in plane hole damage areal comparisons and the comparison of the target back-face debris cloud (BFDC) velocities relative to the incoming projectile velocities. Additional post-shot forensics include characterization of damage morphology and analysis of high-speed videos. Initial inferences about the damage produced in the laminate indicate that the Vectran™ stitching can effectively arrest in-plane damage propagation; impacts at or near a stitchline resulted in no damage propagation across the stitchline boundaries.

Damage-tolerant 3D-printed ceramics via conformal coating

Ceramic materials, despite their high strength and modulus, are limited in many structural applications due to inherent brittleness and low toughness. Nevertheless, ceramic-based structures, in nature, overcome this limitation using bottom-up complex hierarchical assembly of hard ceramic and soft polymer, where ceramics are packaged with tiny fraction of polymers in an internalized fashion. Here, we propose a far simpler approach of entirely externalizing the soft phase via conformal polymer coating over architected ceramic structures, leading to damage tolerance. Architected structures are printed using silica-filled preceramic polymer, pyrolyzed to stabilize the ceramic scaffolds, and then dip-coated conformally with a thin, flexible epoxy polymer. The polymer-coated architected structures show multifold improvement in compressive strength and toughness while resisting catastrophic failure through a considerable delay of the damage propagation. This surface modification approach allows a simple strategy to build complex ceramic parts that are far more damage-tolerant than their traditional counterparts.

Different Flavors of Astrocytes: Revising the Origins of Astrocyte Diversity and Epigenetic Signatures to Understand Heterogeneity after Injury

Astrocytes are a specific type of neuroglial cells that confer metabolic and structural support to neurons. Astrocytes populate all regions of the nervous system and adopt a variety of phenotypes depending on their location and their respective functions, which are also pleiotropic in nature. For example, astrocytes adapt to pathological conditions with a specific cellular response known as reactive astrogliosis, which includes extensive phenotypic and transcriptional changes. Reactive astrocytes may lose some of their homeostatic functions and gain protective or detrimental properties with great impact on damage propagation. Different astrocyte subpopulations seemingly coexist in reactive astrogliosis, however, the source of such heterogeneity is not completely understood. Altered cellular signaling in pathological compared to healthy conditions might be one source fueling astrocyte heterogeneity. Moreover, diversity might also be encoded cell-autonomously, for example as a result of astrocyte subtype specification during development. We hypothesize and propose here that elucidating the epigenetic signature underlying the phenotype of each astrocyte subtype is of high relevance to understand another regulative layer of astrocyte heterogeneity, in general as well as after injury or as a result of other pathological conditions. High resolution methods should allow enlightening diverse cell states and subtypes of astrocyte, their adaptation to pathological conditions and ultimately allow controlling and manipulating astrocyte functions in disease states. Here, we review novel literature reporting on astrocyte diversity from a developmental perspective and we focus on epigenetic signatures that might account for cell type specification.

A Robust Numerical Methodology for Fatigue Damage Evolution Simulation in Composites

Composite materials, like metals, are subject to fatigue effects, representing one of the main causes for component collapse in carbon fiber-reinforced polymers. Indeed, when subject to low stress cyclic loading, carbon fiber-reinforced polymers exhibit gradual degradation of the mechanical properties. The numerical simulation of this phenomenon, which can strongly reduce time and costs to market, can be extremely expensive in terms of computational effort since a very high number of static analyses need to be run to take into account the real damage propagation due the fatigue effects. In this paper, a novel cycle jump strategy, named Smart Cycle strategy, is introduced in the numerical model to avoid the simulation of every single cycle and save computational resources. This cycle jump strategy can be seen as an enhancement of the empirical model proposed by Shokrieh and Lessard for the evaluation of the fatigue-induced strength and stiffness degradation. Indeed, the Smart Cycle allows quickly obtaining a preliminary assessment of the fatigue behavior of composite structures. It is based on the hypothesis that the stress redistribution, due to the fatigue-induced gradual degradation of the material properties, can be neglected until sudden fiber and/or matrix damage is verified at element/lamina level. The numerical procedure has been implemented in the commercial finite element code ANSYS MECHANICAL, by means of Ansys Parametric Design Languages (APDL). Briefly, the Smart Cycle routine is able to predict cycles where fatigue failure criteria are likely to be satisfied and to limit the numerical simulation to these cycles where a consistent damage propagation in terms of fiber and matrix breakage is expected. The proposed numerical strategy was preliminarily validated, in the frame of this research study, on 30° fiber-oriented unidirectional coupons subjected to tensile–tensile fatigue loading conditions. The numerical results were compared with literature experimental data in terms of number of cycles at failure for different percentage of the static strength. Lastly, in order to assess its potential in terms of computational time saving on more complex structures and different loading conditions, the proposed numerical approach was used to investigate the fatigue behavior of a cross-ply open-hole composite panel under tension–tension fatigue loading conditions.

Advances in the Stochastic Modeling of Constitutive Laws at Small and Finite Strains

The characterization and identification of uncertainties in the physical properties of complex materials have been the subjects of longstanding interest in both research and engineering. These efforts were supported by the growing interest in Uncertainty Quantification (UQ) where predominating system-parameter and model-form uncertainties are integrated in a unified mathematical treatment to endow predictions with some statistical measure of uncertainty (fidelity) [7] (see also [23, 30]). Once properly modeled and calibrated, these uncertainties can then be propagated to the structural response, following for instance the spectral approach introduced in the celebrated monograph by Ghanem and Spanos [8] (see also [13]). Such stochastic simulations are then purposely used in order to increase the robustness of the computational models and design procedures, especially when the mechanical models are highly nonlinear (in which case small variations in the inputs can have dramatic effects on the predicted outputs and thus, on design procedures). They also enable a deeper understanding of the critical mechanisms governing the physics (associated with damage propagation, for instance) at relevant scales.