Although metamaterials have shown their superior multiphysics responses compared to their solid counterparts, designing and deploying them requires that their durability and damage tolerance properties be well understood and characterized. The current work investigates low cycle fatigue properties of octet-based metamaterials by conducting an experimental fatigue study and identifying the failure modes and sequence within the strut-based lattices. A geometry consisting of octet cells was manufactured by selective laser sintering (SLS) using a PA2200- Polyamide 12 material. The fatigue life was experimentally measured and compared for samples made with octets of different strut thicknesses in terms of the normalized specimen effective stiffness as a function of cycles. The progression of strut failures showed that the breaking of a strut within the main center volume of the sample was the most significant event in the fatigue life of the sample. A novel visualization technique to better analyze the finite element modeling results of the tested samples was developed and additional tests for samples with an initial state of damage demonstrated the validity of the approach.

In this work, we develop a structural model for the fracturing of an aircraft coating system applied to a complex airframe structure that includes aluminum panels and stainless-steel fasteners. The mechanical properties of the coating system, which consisted of an MIL-PRF-85582E, Type II, Class C1, two-part epoxy primer and an MIL-PRF-85285 Rev E, Type IV, Class H, two-part polyurethane topcoat, were measured before and after 8 months of atmospheric exposure. The loads applied to the coating occurred from local deformations of the fastener-panel system in response to flight stresses. Two types of flight stresses, compression dominated and tension dominated, were modeled. The degradation of the mechanical properties of the coating after atmospheric exposure increases the severity of cracking of the coating at a critical fastener–skin interface.

Environmentally assisted cracking can significantly affect the performance of high strength alloys and limit material selection to minimize the risk of subcritical crack growth in service. UNS N07718 is widely used in marine service applications and under a variety of conditions, such as: alternate immersion, different levels of cathodic protection, and freely corroding galvanic couples, because of its demonstrated corrosion and fracture resistance in these environments. In this work we developed a representative model of the material microstructure including the metal grains, the material texture, and the precipitates along the grain boundaries and within the grains. The microstructural model was subjected to the boundary conditions identified at the notch root of a fracture mechanics sample and the results are used as input for a simulation of hydrogen diffusion from the surface of the notch, assuming the material has been introduced to a hydrogen producing environment. The diffusion of hydrogen was modeled by Fick's law and included both hydrostatic stress and mobile dislocation velocity as driving forces. The influence of immobile dislocations was also modeled to account for the irreversible trapping. The results show that hydrostatic stress and immobile dislocation trapping can significantly alter the highest concentration of hydrogen and its location within the microstructure towards the fracture process zone. Mobile dislocation velocity has a small influence in determining the hydrogen distribution near the fracture process zone.

We report the crack growth on peak aged Al 7075 under tensile loading in a saltwater environment to investigate whether hydrogen plays a role in the initiation and the subsequent growth of the crack. As the hydrostatic stress is at a maximum ahead of the notch, it has been speculated that hydrogen would diffuse to region of maximum hydrostatic stress and initiate the crack. An elastic-plastic finite element (FE) model is developed to determine the stress distribution around the circular notch at the edge of the plate. The crack growth was monitored using a camera attached with an optical microscope, and various regimes corresponding to hydrogen evolution, pit formation and growth, crack nucleation and initial growth and rapid crack growth leading to failure have been identified. We observe the crack was nucleated in a region associated with the highest hoop stress instead of the maximum hydrostatic stress from the base of a pit at the circular notch. We demonstrated the crack growth rate in a salt water environment is governed mostly by the dissolution of grain boundary MgZn2 precipitate, rather than the hydrogen induced decohesion mechanism.

Following pull-off coating adhesion tests such as ASTM D4541, visual examination of the fracture surfaces often reveals intriguing fracture patterns where similar (and seemingly predictable) features are observed across like coating systems. Nonetheless, adhesion test stubs are typically discarded after a brief visual inspection or no inspection at all—suggesting a missed opportunity to gain further insight into fracture processes. This paper examines the pull-off adhesion test from a fracture mechanics perspective. We present two microscopy-based methods for extracting analytically useful fracture data from pull-off adhesion tests. First, we develop an energy-based method for obtaining mode mixity from observed crack kink angles, providing the powerful capability to disambiguate mode I and mode II fracture. We demonstrate agreement between experimental results and theoretical predictions in a set of 1,305 crack path measurements for 111 pull-off adhesion test stubs, representing both single- and multi-layered coating systems. Second, we deploy a machine-learning based image segmentation technique (Trainable WEKA) to rapidly quantify the fractional area coverage of materials removed with the pull stub, enabling further insight into the surfaces most vulnerable to delamination in a layered coating stack. Beyond the immediate utility of these techniques for data enhancement in pull-off adhesion tests, these techniques also have long-term potential as tools for failure analysis in coated systems.

Water uptake by organic coating systems used for corrosion prevention on airframes is one of the principal contributors to the loss of barrier properties of the coating. We used equivalent circuit analyses of electrochemical impedance spectroscopy (EIS) data to track changes in coating layer capacitance for a two-layer coating system consisting of an epoxy primer and polyurethane topcoat immersed in NaCl solutions with different concentrations and temperatures. The capacitance curve exhibited two different response regions, consistent with the "two-stage kinetics" mechanisms for water uptake by the polymers. We tested several numerical diffusion models of water sorption and found the most successful to be one that varied the diffusion coefficient as a function of polymer type and immersion time and accounted for physical aging processes in the polymer. We employed the Brasher mixing law along with the water sorption model to estimate the coating capacitance as a function of water uptake. The predicted capacitance of the coating was found to be consistent with the capacitance obtained from the EIS data, which is consistent with theories that water uptake occurs via initial rapid transport followed by a much slower aging process. Thus, both these water uptake processes need to be considered when making EIS measurements to assess the state of a coating system.

Electrochemical impedance spectroscopy (EIS) is a widely used method for monitoring coatings because it can be done in situ and causes little damage to the coating. However, interpreting the impedance data from coatings in order to determine the state of the coating and its protective abilities is challenging. A modified version of the rapid electrochemical assessment of paint (REAP) equivalent circuit is developed here, along with a method to calculate the impedance of a circuit using matrix algebra. This new equivalent circuit and the calculation method are used to analyze EIS data obtained from a two-layer commercial organic coating system immersed in NaCl solutions with different concentrations and at different temperatures. The matrix calculation method is validated by comparing results obtained from commercial analysis software to this method for two different equivalent circuits, and the parameter values are nearly equal. Physics-based models of the equivalent circuit elements are derived and used to obtain both initial estimates for the regressions and physics-based constraints on the model parameters. These models are integrated into the regression procedure, and the corrected Akaike information criterion (AICc) is used to compare fits between the new circuit and classic equivalent circuits. The AICc values indicate the new circuit results in better fits than classic equivalent circuits used for coatings analysis.

Grain and precipitate morphologies, orientations, and distributions in precipitation hardened nickel alloy 718 are directly affected by material processing, thermal and mechanical history, and tailored to optimize its thermo-mechanical behavior in service. A computational approach based on a dislocation-density crystalline plasticity formulation, was used to investigate and identify dominant microstructural mechanisms and defects, such as perfect and partial dislocation-densities, in an experimentally characterized specification of 718 alloy. The role of perfect and partial dislocation densities and their interaction with the material microstructure, affecting the mechanical behavior of the alloy were investigated. Different precipitate volume fractions were used to characterize and identify these interactions and behavior. Using an integrated experimental and modeling approach, the δ phase precipitated along grain boundaries in the form of elongated rods is shown to be a source of dislocation-density accumulations. Interactions include strengthening, achieved by impeding the motion of dislocations by the coherent precipitates, and shear deformation competition, associated with shear slip or plasticity accumulation between the preferentially oriented slip systems of the precipitates and the matrix.

The advent of additive manufacturing (AM) has enabled the prototyping of periodic and non-periodic metamaterials (a.k.a. lattice or cellular structures) that could be deployed in a variety of engineering applications where certain combinations of performance features are desirable. For example, these structures could be used in a variety of naval engineering applications where light-weight, large surface area, energy absorption, heat dissipation, and acoustic bandgaps are critical. Furthermore, combining the multifunctional design optimization of these structures with progressive degradation due to cyclic fatigue would create attritable systems with tailorable performances not yet in reach by current conventional systems. Nevertheless, in order to deploy these complex geometry structures their multiphysics response has to be well understood and characterized. The objective of the current effort is to describe an initial approach for designing a uniaxial fatigue specimen as the first step toward the design of a multiaxial fatigue test coupon. In order to compare bending- and stretching-dominated structures, two strut-based lattices made of Ti-6Al-4V alloy consisting of the octet and tetrakaidecahedron (or Kelvin) cells are examined. The specimens are designed to fail in the central gauge area where edge effects are minimized. Finite element results of the relevant structural mechanics are used to compare the performance of the four geometries and to evaluate the effect of relative density on fatigue life.

UNS N07718 is widely used in marine service applications under a variety of conditions: alternate immersionAlternate immersion, different levels of cathodic protection, and freely corroding galvanic couples. Environmentally assisted cracking can significantly affect the performance of this alloy and constrains design as it needs to account for subcritical crack growth in service. We measured subcritical crack growth rates and thresholds in different environmental conditions for two different heat treatmentsHeat treatment of UNS N07718. The first heat treatmentHeat treatment, following AMS 5664 is widely used in the aircraft industry and for marine fasteners, and the second, following API 6A, is principally used in the marine and oil and gas industries. The material environmentally assisted cracking was studied under alternate immersionAlternate immersion to natural seawaterNatural seawater and under cathodic protection in natural seawaterNatural seawater. Microstructural modeling is presented to understand and predict how precipitates, their volume fraction, morphology, and properties, affect the evolution and accumulation of dislocation densities within the microstructureMicrostructures, influencing the fracture process at different physical scales.

Electrochemical impedance spectroscopy (EIS) is frequently employed to nondestructively evaluate coating systems. However, there are several challenges associated with this approach. One of these challenges is associated with isolating the physical processes that are contributing to the measured impedance. We attempted to address this concern by measuring several of the coating system components before starting the exposures and attempted in-situ measurements of some properties during the course of the exposures. In this work, we analyze impedance data obtained from a two-layer coating system that was applied to AA 2024 substrates that were subjected to either immersion and simulated outdoor exposure conditions. The impedance data from the full coating system is compared with data obtained from coating systems in which gold electrodes were inserted between the layers of the coating system that could be used to monitor changes in each coating layer. Several equivalent circuit models were evaluated before settling on a modified coatings equivalent model. Using this approach, we were able to estimate several coating system parameters, including electrolyte penetration depth as a function of temperature, and solution concentration from the values obtained from the equivalent circuit model. Acknowledgments The Department of Defense Strategic Environmental Research and Development Program (SERDP) sponsored this project under work unit 5399. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government.

Organic coating systems are used to manage the corrosion of metal surfaces subjected to atmospheric degradation. These coating systems are frequently complex and multilayered and capable of providing several functions, including: substrate adhesion, corrosion inhibition and electrolytic barriers. In addition, the service environment can be equally complex with exposure to UV radiation, wet/dry cycling, and application of thermal and physical stresses on the substrate and coating system alike. Refining our ability to predict service life and explain how these coatings evolve over the course of their lifetimes provides an opportunity to decrease maintenance costs and aid in future coating development. These objectives can be achieved by the development of predictive models that incorporate the fundamental effects of relevant service environment parameters. This would provide reliable damage predictions based upon the measured environmental data thereby decreasing our dependence on proxy corrosion sensors. To that end, the first phase of this project has focused on testing and monitoring an aircraft coating system that is comprised of a surface pretreatment, an epoxy primer with inhibitors, and a polyurethane topcoat exposed to a variety of degradation conditions that simulate the extreme ranges of environmental stresses expected to be seen in-service. Coating condition was monitored using electrochemical impedance spectroscopy (EIS) and characterized with equivalent-circuit models so that changes to the coating system properties could be quantified and tracked over time. Regression models for the changes in the coating system properties are being developed and will be validated by follow-on outdoor exposure testing in the next phase of the program. Acknowledgments The Department of Defense Strategic Environmental Research and Development Program (SERDP) sponsored this work under program WP19-1017. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, or the U.S. government.

Stress corrosion cracking susceptibility of selected high strength alloys was evaluated during exposure in alternate immersion conditions using bolt loaded compact test geometries with a strain gauge positioned at the back-face to allow a large sample throughput while including sensitive monitoring of crack progression during exposure. This approach combines advantages of a traditional KISCC exposure testing and laboratory-based instrumentation-intensive test methods. Crack length was continuously monitored by specimen compliance during up to 2 years of alternate immersion exposure in unfiltered natural seawater. These wet and dry cycles alternated 6 times a day, and the wet cycle was approximately 15 minutes long. The back-face strain gauge test method proved reliable for up to 20 months of exposure with a crack growth resolution of approximately 25-50 μm/day. Alternate immersion SCC testing of high strength materials, steels and nickel-based alloys: UNS K91973, ASTM(1) A722, UNS S66286, UNS N05500, UNS N07718 API(2) 6A and AMS(3) 5664, and UNS R30035 is presented. A key finding was that alternate immersion significantly decreased SCC resistance of some alloys, while other alloys proved to be resistant. Incubation times alternated with periods of fast crack growth were observed in most cases.

In order to delay the onset of corrosion structures subjected to atmospheric degradation, such as aircraft, exterior metal surfaces are frequently covered in a corrosion prevention organic coating system. Organic coating systems, in many cases, provide a reservoir of corrosion inhibitors along with establishing a critical electrolyte barrier between the atmosphere and the structure. These coating systems include a surface pretreatment, a primer barrier layer that can contain a sacrificial inhibitor additive, and a topcoat that also provides corrosion barrier properties and other additives for color. Improving our understanding, and modeling how these coating properties change, as a function of exposure to a variety of service environments can decrease maintenance costs associated with paint removal, and re-painting. A model that can incorporate the fundamental effects of various environmental parameters can provide damage predictions based upon the measured environmental data.To that end, the initial phase of this program has focused on testing and monitoring a US Navy aircraft coating system, exposed to a variety of degradation conditions that simulate the extreme ranges of environmental stresses expected to be seen in-service. Coating condition was monitored using electrochemical impedance spectroscopy (EIS) and characterized using equivalent-circuit models so that changes to the coating system properties could be quantified and tracked over time.

Specimen blanks of additive-manufactured nickel Alloy 625 and Ti-6AI-4V were produced by the laser powder-bed-fusion process (L-PBF) with the principal test axis in both the Z direction (parallel to the build direction) and the X-Y direction (perpendicular to the build direction). The high cycle fatigue and corrosion fatigue properties of these metals were measured using R. R. Moore rotating cantilever fatigue tests, both in air and with a salt water drip on the test sections. Testing was conducted in order to determine the fatigue and corrosion fatigue limits of these materials at 10(8) cycles. The fatigue limit for L-PBF Alloy 625 material at 10(8) cycles in air was roughly 48 ksi (331 MPa), independent of build orientation. This is similar to the air fatigue limit of wrought material. The corrosion fatigue limit for L-PBF Alloy 625 material at 10(8) cycles in salt water was roughly 39 ksi (269 MPa), which was also independent of build orientation and slightly below the values for wrought material and values obtained by other investigators. The fatigue limit for hot isostatic pressed L-PBF Ti-6AI-4V material at 10(8) cycles in air was roughly 90 ksi (620 MPa), independent of build orientation. This is significantly better than the air fatigue limit of wrought material. The corrosion fatigue limit for L-PBF Ti-6AI-4V material at 10(8) cycles in salt water was roughly 78 ksi (540 MPa), which was also independent of build orientation, better than values for wrought material, and comparable to values obtained by other investigators. The fatigue crack growth rate behavior in air was characterized and compared with information available in the literature.

•A generalization of the Castillo-Canteli probabilistic fatigue model is proposed.•Generalized fatigue damage variables are proposed keeping original model structure.•Fatigue damage parameters for uniaxial and multiaxial loading conditions are discussed.•Experimental fatigue data demonstrated appropriateness of proposed generalization.

Conditions for propagation or non-propagation of an incipient crack from stress concentrations such as notches, holes, etc., are evaluated using elastic-plastic fracture mechanics methods and the Kitagawa-Takahashi diagram, hence linking smooth specimen behavior to fracture mechanics. The analysis differs from all other previous models which are either empirically based or assume that short crack growth behavior differs from that of long cracks due to crack closure or lack of it. It is shown that the stress intensity factor of an incipient crack decreases and then increases as it moves away from the internal stress field of a notch. Crack propagation is ensured only when the minimum is equal to or greater than the unique fatigue threshold for crack growth. Otherwise non-propagation conditions prevail. Equations are developed to establish the conditions for the minimum stress required for continuous propagation of the originated crack. It is shown that by analyzing published results on notch-fatigue that specimen failure occurs only if the applied stresses exceed the minimum stress required for crack propagation. Otherwise crack arrest occurs resulting in non-propagating cracks. The analysis can be readily used to determine the conditions for failure of a notched specimen if the elastic stress concentration factor, k(t), the notch-tip radius, rho, and fatigue crack threshold, K-th, for the material are known. The analysis, in principle, is applicable to all subcritical crack growth processes. (C) 2017 Published by Elsevier Ltd.