Material characterization in nano-mechanical tests may provide information on the potential heterogeneity of mechanical properties. Here, we develop a robust neural-network interatomic potential ...(NNIP), and we provide a test for the example of molecular dynamics (MD) nanoindentation, and the case of body-centered cubic crystalline molybdenum (Mo). We employ a similarity measurement protocol, using standard local environment descriptors, to select ab initio configurations for the training dataset that capture the behavior of the indented sample. We find that it is critical to include generalized stacking fault (GSF) configurations, featuring a dumbbell self-interstitial on the surface, to capture dislocation cores, and also high-temperature configurations with frozen atom layers for the indenter tip contact. We develop a NNIP with distinct dislocation nucleation mechanisms, realistic generalized stacking fault energy (GSFE) curves, and an informative energy landscape for the atoms on the sample surface during nanoindentation. We compare our NNIP results with nanoindentation simulations, performed with three existing potentials – an embedded atom method (EAM) potential, a gaussian approximation potential (GAP), and a tabulated GAP (tabGAP) potential – that predict different dislocation nucleation mechanisms, and display the absence of essential information on the shear stress at the sample surface in the elastic region. Finally, we compared our NNIP nanoindentation results with experiments, showing reliable predictions for reduced Young’s modulus and observable slip traces.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Advances in the application of polymers for electrochemical cells require an understanding of their viscous deformation mechanisms and their interaction with moisture. Nanoindentation offers a ...localized, microscale testing alternative to traditional tensile testing. However, the viscoelastic nature of the polymers, combined with their increased compliance, presents challenges in the analysis of nanoindentation results. In addition, the dependence on moisture results in significant scatter and low repeatability. This study combines nanoindentation and tensile testing as a verification method and compares different correction protocols for static nanoindentation to investigate the mechanical behavior of polymer electrolyte membranes. Comparisons of different indentation devices, analysis methods, and indentation protocols show a significant overestimation of Young’s modulus using the classical Oliver–Pharr method compared to values determined from tensile tests. Nanoindentation at different humidity levels revealed different mechanisms leading to a decrease in Young’s modulus and hardness with increasing humidity.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
4.
Couple stress theory for solids Hadjesfandiari, Ali R.; Dargush, Gary F.
International journal of solids and structures,
09/2011, Volume:
48, Issue:
18
Journal Article
Peer reviewed
Open access
By relying on the definition of admissible boundary conditions, the principle of virtual work and some kinematical considerations, we establish the skew-symmetric character of the couple-stress ...tensor in size-dependent continuum representations of matter. This fundamental result, which is independent of the material behavior, resolves all difficulties in developing a consistent couple stress theory. We then develop the corresponding size-dependent theory of small deformations in elastic bodies, including the energy and constitutive relations, displacement formulations, the uniqueness theorem for the corresponding boundary value problem and the reciprocal theorem for linear elasticity theory. Next, we consider the more restrictive case of isotropic materials and present general solutions for two-dimensional problems based on stress functions and for problems of anti-plane deformation. Finally, we examine several boundary value problems within this consistent size-dependent theory of elasticity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
This study concerns that the high-silicon alloy steel treated by the laser hardening with an in-situ thermal treatment (LHITT) between Ms and Md temperature. The shear band, such as nano-twin and ...ε-martensite is expected to be obtained by the aid of thermal stress during laser hardening process to strengthen the steel. The in-situ thermal treatment temperature of the steel was confirmed by thermal mechanical simulator and thermodynamic model. The residual stress of the hardening layer was measured by X-ray stress analyzer. The phase structures were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Moreover, the nano-hardness was evaluated by the nano-mechanical tester. The experimental results demonstrate that the stacking faults were generated by the laser thermal stress which drives high density dislocations motion on {111}γ planes. The shear band is formed by the overlapping of the stacking faults in austenite during the LHITT process. Besides, the high surface hardness (7.9GPa) can be achieved by the LHITT. The hard points (9.5GPa) in LHITT sample are related with the shear band as the analysis of the elastic modulus.
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IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SBCE, SBJE, UPUK
Most of the research to date has focused on tailoring the interphase adhesion by controlling the degree of chemical bonding between fiber and resin. However, recent studies suggest that apart from ...chemical bonding, mechanical interlocking plays a crucial role to enhance the interphase thickness and hence the interfacial shear strength. In this study, the effect of textured carbon fiber surface on the interphase thickness, and interfacial shear strength has been studied and analyzed. The hot water was used to remove the epoxy-based sizing agent from the carbon fiber (sized CF) surface and thereby to create a textured surface morphology in de-sized one (De-sized CF). To study the distinctions of the interphase between sized and de-sized CF/PP composites, the Derjaguin-Muller-Toporov (DMT) modulus by Peak Force Quantitative Nano-Mechanics (PF-QNM) was applied to determine the interphase thickness and its nanomechanical properties. It was found that no chemical changes occurred after hot water treatment albeit the surface morphology of de-sized carbon fiber became textured. The average interphase thickness in de-sized CF/PP composites was found to be 84.17% higher (212.9 ± 21 nm) as compared to 115.6 ± 21 nm in sized CF/PP composites. This study shows the potential for using mechanical interlocking effects without significantly affecting the chemical bonding, to improve the interphase thickness in fiber reinforced composites.
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NUK, OILJ, SAZU, UKNU, UL, UM, UPUK
Micro- and nanomechanical testing techniques have become an integral part of today’s materials research portfolio. Contrary to well-studied and majorly standardized nanoindentation testing, in situ ...testing of various geometries, such as pillar compression, dog bone tension, or cantilever bending, remains rather unique given differences in experimental equipment and sample processing route. The quantification of such experiments is oftentimes limited to load-displacement data, while the gathered in situ images are considered a qualitative information channel only. However, by utilizing modern computer-aided support in the form of the recently developed Segment Anything Model (SAM), quantitative mechanical information from images can be evaluated in a high-throughput manner and adds to the data fidelity and accuracy of every individual experiment. In the present work, we showcase image-assisted mechanical evaluation of compression, tension and bending experiments on micron-scaled resin specimens, produced via two-photon lithography. The present framework allows for a determination of an accurate sample strain, which further enables determination of quantities such as the elastic modulus, Poisson’s ratio or viscoelastic relaxation after fracture.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Improvement of contact resonance atomic force microscopy (CR-AFM) frequency sensitivity at a high contact stiffness condition and subsequent enhancement of its subsurface imaging capability were ...demonstrated by adding an additional mass on the cantilever backside near the free end. The influence of mass and its position on the CR-AFM performances was analytically investigated by using a simplified cantilever vibration model and further compared with finite element analysis. Theoretical results and FEA simulations showed good agreement and they could provide a general guide on the cantilever's modification by attaching a proper mass to improve the CR-AFM capability. As a prototype, a Pt micro-disc with precisely controlled size and position on the cantilever, which served as the attached mass, was deposited by utilizing focused ion beam. The improvement of frequency sensitivity was verified through CR spectroscopy and subsurface imaging on cavities having pre-determined diameters and covered with highly oriented pyrolytic graphite flakes. Compared with the original unmodified cantilever, obvious enhancement of CR-AFM subsurface nano-imaging was obtained. The mass-attached cantilever can benefit characterizing sample's mechanical properties and enhancing subsurface imaging based on tip-generated stress.
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