Micro-scale mechanical characterization of materials is a useful technique for understanding local material behavior, particularly where inhomogeneous materials or microstructures are present. In this work, a custom microsample testing system that utilizes digital image correlation (DIC) is used to measure the response of samples that have a 3 mm x 1 mm footprint and a cross sectional gage area of 250 mu m x 250 mu m. The mechanical behaviors of Ti-5111 base metal and friction stir weld refined microstructures were examined. In a novel use of strain contour mapping, microstructural analysis and measurements were performed directly on microsamples and these results were linked to the fractography and measured properties using the DIC data. Larger variations, in several mechanical properties, were measured in the base metal Ti-5111 than in the weld material. For example, the yield base metal stress was 771 +/- 41 MPa versus 828 +/- 9 MPa for the weld. These variations were found to be due to the larger colony structure of the wrought plate. The colonies in wrought plate Ti-5111 can be as large as 250 mu m and were found to promote the formation of shear bands which resulted in reduced ductility. (C) 2019 The Authors. Published by Elsevier Ltd.

The base plate of the acorn barnacle Amphibalanus amphitrite (equivalent to Balanus amphitrite) is composed of hierarchically scaled, mutually aligned calcite grains, adhered to the substratum via layered cuticular tissue and protein. Acorn barnacles grow by expanding and lengthening their side plates, under which the cuticle is stretched, and adhesive proteins are secreted. In barnacles with mineralized base plates, such as A. amphitrite, a mineralization front follows behind, radially expanding the base plate at the periphery. In this study, we show that the new mineralization develops above the adhesion layers in a unique trilayered structure. Calcite crystallites in each of the layers have distinct sizes, varying from coarse-grained (>1 μm across) in the upper layer, to fine-grained (∼1 μm) in the middle layer, to nanoparticulate (∼40 nm) in the basal layer. The fine-grained crystallites dominate the growth front, comprising the bulk of the shell at the periphery, with later coarse grain development on the top of the base plate (toward the barnacle interior) and nanocrystalline calcite templating underneath in contact with the cuticle/protein layer. While the coarse-grained calcite on the upper surface contains a range of crystal orientations, the underlying fine-grained and nanocrystalline calcite are mutually oriented to within a few degrees of each other. Electron diffraction and X-ray absorption spectroscopy confirm that all of the crystallites are calcite, and metastable aragonite or amorphous calcium carbonate (ACC) phases are not observed. The complex morphology of the leading edge of the base plate suggests that crystallization initiates with the emplacement of mutually aligned fine-grained calcite, followed by the accumulation of coarser grains above and nucleation of highly oriented nanocrystalline grains below.

A response is provided to comments by Krell concerning the validity of the Hall-Petch relationship and the optical transmission in nanocrystalline ceramics discussed in a recent Acta Materialia paper. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Mechanical strengthening by grain refinement is a method whereby a material’s strength and hardness can be increased by decreasing the average crystallite grain size. The empirical Hall–Petch relationship mathematically describes grain boundary strengthening and provides guidance for a straightforward way to produce stronger materials. While the phenomenon has been widely explored in nanocrystalline metals, the difficulty associated with fabricating high-quality dense nanocrystalline ceramics has left unanswered the question of the validity and extent of the relationship in ceramics. Prior studies suggest the occurrence of an inverse Hall–Petch response in ceramics with grain sizes <100nm. This paper demonstrates a novel integrated approach, comprised of nanopowder processing and high-pressure, low-temperature sintering to fabricate bulk, fully dense and high-purity nanocrystalline ceramics with unprecedentedly small nanometer-sized grains. Using magnesium aluminate spinel as an archetypal hard ceramic, the hardness of this transparent ceramic armor is shown to rigorously follow the Hall–Petch relationship down to grain sizes of 28nm. Consequentially, the nanocrystalline spinel ceramics are shown to exhibit a 50% increase in hardness over a corresponding order of magnitude reduction in grain size without a decline in density or fracture resistance. Additionally, the produced nanocrystalline ceramics have an optical transparency near theoretical. Reaching an exceptional hardness of 20.2GPa at 28nm, the behavior shows no evidence supporting an inverse Hall–Petch effect.

The radial growth and advancement of the adhesive interface to the substratum of many species of acorn barnacles occurs underwater and beneath an opaque, calcified shell. Here, the time-dependent growth processes involving various autofluorescent materials within the interface of live barnacles are imaged for the first time using 3D time-lapse confocal microscopy. Key features of the interface development in the striped barnacle, Amphibalanus (= Balanus) amphitrite were resolved in situ and include advancement of the barnacle/substratum interface, epicuticle membrane development, protein secretion, and calcification. Microscopic and spectroscopic techniques provide ex situ material identification of regions imaged by confocal microscopy. In situ and ex situ analysis of the interface support the hypothesis that barnacle interface development is a complex process coupling sequential, timed secretory events and morphological changes. This results in a multi-layered interface that concomitantly fulfills the roles of strongly adhering to a substratum while permitting continuous molting and radial growth at the periphery.

Recent experimental measurements, involving small additions of Ni foil to CP-Ti friction stir welds (FSWs), have motivated a re-evaluation of issues concerning material flow and the resulting microstructure in these welds. One issue concerns the fact that the Ni additions improved weld microstructure. This implies a need for better understanding the basic mechanisms whereby the introduction of small amounts of this fast diffuser into the stirred region results in improved microstructure and associated improvements in weld performance. Another issue concerns the fact that a relatively high level of detailed information related to material flow patterns occurring in FS welds of Ti can be experimentally measured by x-ray microtomography using the introduced Ni as a flow tracer. This potential availability of large amounts of flow pattern information for Ti FSWs informs our need for a predictive methodology that incorporates large amounts of flow pattern information.

This work presents novel mapping of the structure and crystallography of the shell of the barnacle Balanus amphitrite. Calcium carbonate in the form of hexagonal calcite was observed, with fine crystallites on the order of 1 μm in diameter forming clusters of similar orientation. While no apparent preferred orientation was measured in the parietal shell cross section, the base plate shell cross section revealed a preference for the alignment of the 〈0001〉 direction at the exterior of the shell, rotating by 90° toward the center of the base plate cross-section. These maps will be used to construct image-based models of the barnacle shell for further study of mechanical response.

Barnacles adhere permanently to surfaces by secreting and curing a thin interfacial adhesive underwater. Here, we show that the acorn barnacle Balanus amphitrite adheres by a two-step fluid secretion process, both contributing to adhesion. We found that, as barnacles grow, the first barnacle cement secretion (BCS1) is released at the periphery of the expanding base plate. Subsequently, a second, autofluorescent fluid (BCS2) is released. We show that secretion of BCS2 into the interface results, on average, in a 2-fold increase in adhesive strength over adhesion by BCS1 alone. The two secretions are distinguishable both spatially and temporally, and differ in morphology, protein conformation, and chemical functionality. The short time window for BCS2 secretion relative to the overall area increase demonstrates that it has a disproportionate, surprisingly powerful, impact on adhesion. The dramatic change in adhesion occurs without measurable changes in interface thickness and total protein content. A fracture mechanics analysis suggests the interfacial material's modulus or work of adhesion, or both, were substantially increased after BCS2 secretion. Addition of BCS2 into the interface generates highly networked amyloid-like fibrils and enhanced phenolic content. Both intertwined fibers and phenolic chemistries may contribute to mechanical stability of the interface through physically or chemically anchoring interface proteins to the substrate and intermolecular interactions. Our experiments point to the need to reexamine the role of phenolic components in barnacle adhesion, long discounted despite their prevalence in structural membranes of arthropods and crustaceans, as they may contribute to chemical processes that strengthen adhesion through intermolecular cross-linking.

The capability of using recoverable martensitic transformation to modify the residual stress-state of hybrid Shape Memory Alloy (SMA) composites is explored. It is shown that through careful selection of a thermomechanical loading path the composite can be "processed" such that the constituent phases have a beneficial residual stress-state. Specifically, for materials which have preferred loading conditions (i.e., compression versus tension) resulting in improved material properties, such processing places the considered phase into a preferred stress state. This processing is explored here by considering composites with an SMA phase whose constititutive behavior is described by a recent phenomenological model and an elasto-plastic second phase. To consider realistic microstructural effects, a 3D numerical representation of the composite is generated using microtomography. It is shown that through an actuation (isobaric) loading path, the martensitic transformation of the SMA phase generates irrecoverable strains in the elasto-plastic phase which, upon unloading, results in a favorable residual stress-state. To consider the applicability of this methodology for a variety of composites, the effect of thermal residual stresses due to thermal expansion mismatch is identified and matrix phases with different elastic moduli and plastic hardenings are considered. Specifically, it is shown that martensitic transformation is the driving force behind the generation of the new composite residual stressstate. Through computational simulation, it is shown that increased elastic moduli or plastic hardening coefficients of the elasto-plastic phase yield small increases in residual stresses.

NiCoMnSn porous metamagnetic shape memory alloys were successfully fabricated using solid-state replication of ammonium bicarbonate powders via sintering at 1273 K. Around 50% porosity of 300-500 mu m interconnected pores was achieved. Oxygen contamination from the space holder and binder, which negatively influences the magneto-thermal coupling, was prevented using a titanium oxygen getter during sintering. The resulting porous samples demonstrate a metamagnetic response similar to the homogenized powder, offering possible utilization in magnetic refrigeration and actuation. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Soft elastomeric materials that mimic real soft human tissues are sought to provide realistic experimental devices to simulate the human body's response to blast loading to aid the development of more effective protective equipment. The dynamic mechanical behavior of these materials is often measured using a Kolsky bar because it can achieve both the high strain rates (>100 s−1) and the large strains (>20%) that prevail in blast scenarios. Obtaining valid results is challenging, however, due to poor dynamic equilibrium, friction, and inertial effects. To avoid these difficulties, an inverse method was employed to determine the dynamic response of a soft, prospective biomimetic elastomer using Kolsky bar tests coupled with high-speed 3D digital image correlation. Individual tests were modeled using finite elements, and the dynamic stiffness of the elastomer was identified by matching the simulation results with test data using numerical optimization. Using this method, the average dynamic response was found to be nearly equivalent to the quasi-static response measured with stress–strain curves at compressive strains up to 60%, with an uncertainty of ±18%. Moreover, the behavior was consistent with the results in stress relaxation experiments and oscillatory tests although the latter were performed at lower strain levels.

Abstract : Multifunctional material systems (MfMS) are material/systems that can perform more than one primary function simultaneously or sequentially in time to enhance system-level performance through the elimination of redundancy between sub-system materials and functions. A system-level performance metric of considerable importance is energy performance and efficiency. Total energy usage in 2009 by US Department of Defense (DoD) has been estimated at 900 trillion BTU's with approximately 75% of that being consumed for mobility related applications (e.g., aircraft, ships, vehicles, etc.). This paper describes the development of a new class of multifunctional laminate composites with controllable surface morphology at NRL. The current focus is on two-phase (solid-fluid), thin, flexible, skin-like laminates with surface pore arrays that are connected to an internal vascular network. These new materials with structural (skin) + surface (morphology control) multifunctionality are being called poro-vascular composites or PV composites for short.

The uniformity and reaction kinetics of ion-beam deposited Al/Ni multilayer samples with the same composition, Al81.8Ni18.2, and modulation wavelength, Λ = 20 nm, but with different total film thicknesses were investigated by x-ray diffraction and differential scanning calorimetry measurements. The total film thicknesses varied between approximately 0.5 and 2.0 μm. It was found that the interface widths were approximately 1 nm and the Ni layers are much more disordered than the Al layers. The thicker samples show an increase in disorder on a length scale comparable to Λ. In other experiments, a change was observed with increasing modulation wavelength from semicoherent interfaces with a low density of misfit dislocations to semicoherent interfaces with a high density of misfit dislocations. The reaction kinetics for forming the Al9Ni2 phase is independent of the sample thickness.

Stereoscopic digital image correlation (DIC) is used to measure the shape evolution of a soft, transparent thermoplastic elastomer subject to a high strain rate compression test performed using a Kolsky bar. Rather than using the usual Kolsky bar wave analysis methods to determine the specimen response, however, the response is instead determined by an inverse method. The test is modeled using finite elements, and the elastomer stiffness giving the best match with the shape and force history data is identified by performing iterative simulations. The advantage of this approach is that force equilibrium in the specimen is not required, and friction effects, which are difficult to eliminate experimentally, can be accounted for. The thermoplastic is modeled as a hyperelastic material, and the identified dynamic compressive (non-linear) stiffness is compared to its quasi-static compressive (non-linear) stiffness to determine rate sensitivity.

We study the mechanics of pull-off of a barnacle adhering to a thin elastic layer which is bonded to a rigid substrate. We address the case of barnacles having acorn shell geometry and hard, calcarious base plates. Pull-off is initiated by the propagation of an interface edge crack between the base plate and the layer. We compute the energy release rate of this crack as it grows along the interface using a finite element method. We also develop an approximate analytical model to interpret our numerical results and to give a closed-form expression for the energy release rate. Our result shows that the resistance of barnacles to interfacial failure arises from a crack-trapping mechanism.

SUMMARY Enzymes and biochemical mechanisms essential to survival are under extreme selective pressure and are highly conserved through evolutionary time. We applied this evolutionary concept to barnacle cement polymerization, a process critical to barnacle fitness that involves aggregation and cross-linking of proteins. The biochemical mechanisms of cement polymerization remain largely unknown. We hypothesized that this process is biochemically similar to blood clotting, a critical physiological response that is also based on aggregation and cross-linking of proteins. Like key elements of vertebrate and invertebrate blood clotting, barnacle cement polymerization was shown to involve proteolytic activation of enzymes and structural precursors,transglutaminase cross-linking and assembly of fibrous proteins. Proteolytic activation of structural proteins maximizes the potential for bonding interactions with other proteins and with the surface. Transglutaminase cross-linking reinforces cement integrity. Remarkably, epitopes and sequences homologous to bovine trypsin and human transglutaminase were identified in barnacle cement with tandem mass spectrometry and/or western blotting. Akin to blood clotting, the peptides generated during proteolytic activation functioned as signal molecules, linking a molecular level event (protein aggregation) to a behavioral response (barnacle larval settlement). Our results draw attention to a highly conserved protein polymerization mechanism and shed light on a long-standing biochemical puzzle. We suggest that barnacle cement polymerization is a specialized form of wound healing. The polymerization mechanism common between barnacle cement and blood may be a theme for many marine animal glues.

Measuring the response of soft materials to high strain rate deformation is extremely challenging because of the difficulty of achieving dynamic equilibrium during high strain rate mechanical testing such as Kolsky bar testing. Digital image correlation (DIC) using high speed cameras is well suited for improving the ability to characterize the non-equilibrium deformation of soft samples subject to dynamic loading conditions, providing a rich data set that can be used to better deduce the dynamic constitutive response of the material. A prospective tissue simulant material is tested in compression at high strain rate using a Kolsky bar. The non-equilibrium deformation of the specimen, including the surface (Rayleigh) wave motion, is captured using a high speed stereo camera pair recording at 180,000 frames per second. The image correlation results are used in conjunction with finite element modeling to deduce the dynamic constitutive response of the material in this high strain rate test.