Hierarchically structured bioinspired nanocomposites.

Next-generation structural materials are expected to be lightweight, high-strength and tough composites with embedded functionalities to sense, adapt, self-repair, morph and restore. This Review highlights recent developments and concepts in bioinspired nanocomposites, emphasizing tailoring of the architecture, interphases and confinement to achieve dynamic and synergetic responses. We highlight cornerstone examples from natural materials with unique mechanical property combinations based on relatively simple building blocks produced in aqueous environments under ambient conditions. A particular focus is on structural hierarchies across multiple length scales to achieve multifunctionality and robustness. We further discuss recent advances, trends and emerging opportunities for combining biological and synthetic components, state-of-the-art characterization and modelling approaches to assess the physical principles underlying nature-inspired design and mechanical responses at multiple length scales. These multidisciplinary approaches promote the synergetic enhancement of individual materials properties and an improved predictive and prescriptive design of the next era of structural materials at multilength scales for a wide range of applications.

From Molecular to Multiasperity Contacts: How Roughness Bridges the Friction Scale Gap.

The tangential force required to observe slip across a whole frictional interface can increase over time under a constant load, due to any combination of creep, chemical, or structural changes of the interface. In macroscopic rate-and-state models, these frictional aging processes are lumped into an ad hoc state variable. Here we explain, for a frictional system exclusively undergoing structural aging, how the macroscopic friction response emerges from the interplay between the surface roughness and the molecular motion within adsorbed monolayers. The existence of contact junctions and their friction dynamics are studied through coupled experimental and computational approaches. The former provides detailed measurements of how the friction force decays, after the stiction peak, to a steady-state value over a few nanometers of sliding distance, while the latter demonstrates how this memory distance is related to the evolution of the number of cross-surface attractive physical links, within contact junctions, between the molecules adsorbed on the rough surfaces. We also show that roughness is a sufficient condition for the appearance of structural aging. Using a unified model for friction between rough adsorbed monolayers, we show how contact junctions are a key component in structural aging and how the infrajunction molecular motion can control the macroscopic response.

Foreseeing metal failure from its inception.

The life span of metals can be inferred from early microscopic deformation events.

Strain rate dependency of dislocation plasticity.

Dislocation glide is a general deformation mode, governing the strength of metals. Via discrete dislocation dynamics and molecular dynamics simulations, we investigate the strain rate and dislocation density dependence of the strength of bulk copper and aluminum single crystals. An analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which agrees well with current simulations and published experiments. Results show that material strength displays a decreasing regime (strain rate hardening) and then increasing regime (classical forest hardening) as the dislocation density increases. Accordingly, the strength displays universally, as the strain rate increases, a strain rate-independent regime followed by a strain rate hardening regime. All results are captured by a single scaling function, which relates the scaled strength to a coupling parameter between dislocation density and strain rate. Such coupling parameter also controls the localization of plasticity, fluctuations of dislocation flow and distribution of dislocation velocity.

Hierarchical Multiscale Approach for Modeling the Deformation and Failure of Epoxy-Based Polymer Matrix Composites.

Coarse-grained molecular dynamics (CG-MD) simulations were conducted to characterize the molecular structure and mechanical properties in the epoxy matrix around fibers in polymer matrix composites (PMCs). From these simulations, the molecular structure was quantified by measuring the free-volume hole radius distribution as a function of position from the matrix-fiber interface. Additionally, correlations between the epoxy mechanical properties and the average free-volume hole radius were established for different degrees of cross-linking. These results were then upscaled into a finite element model (FEM) of a PMC representative volume. The results from the CG-MD-informed FEM model were compared to conventional FEM simulations that assume uniform epoxy mechanical properties, and the results indicate that conventional FEM simulations overestimate the strength of PMCs and predict a symmetric damage evolution in the matrix. On the other hand, the CG-MD-informed FEM simulations predict a more realistic localization of damage around the fiber-matrix interface.

The heterogeneity of persistent slip band nucleation and evolution in metals at the micrometer scale.

Fatigue damage in metals manifests itself as irreversible dislocation motion followed by crack initiation and propagation. Characterizing the transition from a crack-free to a cracked metal remains one of the most challenging problems in fatigue. Persistent slip bands (PSBs) form in metals during cyclic loading and are one of the most important aspects of this transition. We used in situ microfatigue experiments to investigate PSB formation and evolution mechanisms, and we discovered that PSBs are prevalent at the micrometer scale. Dislocation accumulation rates at this scale are smaller than those in bulk samples, which delays PSB nucleation. Our results suggest the need to refine PSB and crack-initiation models in metals to account for gradual and heterogeneous evolution. These findings also connect micrometer-scale deformation mechanisms with fatigue failure at the bulk scale in metals.

Scanning transmission electron microscopy image simulations of complex dislocation structures generated by discrete dislocation dynamics.

Scanning Transmission Electron Microscopy Diffraction Contrast Imaging (STEM-DCI) has been gaining popularity for the identification and analysis of dislocations in crystalline materials due to its ability to supress undesirable image features that are often present in conventional TEM images. However, there does not yet exist a robust body of work demonstrating expected contrast in these imaging conditions. A novel approach for the simulation of STEM-DCI images was developed using a modified form of the scattering matrix formalism. This algorithm was used to simulate a variety of dislocation configurations generated using three-dimensional discrete dislocation dynamics.

Correlating Free-Volume Hole Distribution to the Glass Transition Temperature of Epoxy Polymers.

A new algorithm is developed to quantify the free-volume hole distribution and its evolution in coarse-grained molecular dynamics simulations of polymeric networks. This is achieved by analyzing the geometry of the network rather than a voxelized image of the structure to accurately and efficiently find and quantify free-volume hole distributions within large scale simulations of polymer networks. The free-volume holes are quantified by fitting the largest ellipsoids and spheres in the free-volumes between polymer chains. The free-volume hole distributions calculated from this algorithm are shown to be in excellent agreement with those measured from positron annihilation lifetime spectroscopy (PALS) experiments at different temperature and pressures. Based on the results predicted using this algorithm, an evolution model is proposed for the thermal behavior of an individual free-volume hole. This model is calibrated such that the average radius of free-volumes holes mimics the one predicted from the simulations. The model is then employed to predict the glass-transition temperature of epoxy polymers with different degrees of cross-linking and lengths of prepolymers. Comparison between the predicted glass-transition temperatures and those measured from simulations or experiments implies that this model is capable of successfully predicting the glass-transition temperature of the material using only a PDF of the initial free-volume holes radii of each microstructure. This provides an effective approach for the optimized design of polymeric systems on the basis of the glass-transition temperature, degree of cross-linking, and average length of prepolymers.

Coarse-Grained Molecular Dynamics Study of the Curing and Properties of Highly Cross-Linked Epoxy Polymers.

In this work, a coarse-grained model is developed for highly cross-linked bisphenol A diglycidyl ether epoxy resin with diaminobutane hardener. In this model, all conformationally relevant coarse-grained degrees of freedom are accounted for by sampling over the free-energy surfaces of the atomic structures using quantum mechanical simulations. The interaction potentials between nonbonded coarse-grained particles are optimized to accurately predict the experimentally measured density and glass-transition temperature of the system. In addition, a new curing algorithm is also developed to model the creation of highly cross-linked epoxy networks. In this algorithm, to create a highly cross-linked network, the reactants are redistributed from regions with an excessive number of reactive molecules to regions with a lower number of reactants to increase the chances of cross-linking. This new algorithm also dynamically controls the rate of cross-linking at each local region to ensure uniformity of the resulting network. The curing simulation conducted using this algorithm is able to develop polymeric networks having a higher average degree of cross-linking, which is more uniform throughout the simulation cell as compared to that in the networks cured using other curing algorithms. The predicted gel point from the current curing algorithm is in the acceptable theoretical and experimental range of measured values. Also, the resulting cross-linked microstructure shows a volume shrinkage of 5%, which is close to the experimentally measured volume shrinkage of the cured epoxy. Finally, the thermal expansion coefficients of materials in the glassy and rubbery states show good agreement with the experimental values.

Unravelling the physics of size-dependent dislocation-mediated plasticity.

Size-affected dislocation-mediated plasticity is important in a wide range of materials and technologies. Here we develop a generalized size-dependent dislocation-based model that predicts strength as a function of crystal/grain size and the dislocation density. Three-dimensional (3D) discrete dislocation dynamics (DDD) simulations reveal the existence of a well-defined relationship between strength and dislocation microstructure at all length scales for both single crystals and polycrystalline materials. The results predict a transition from dislocation-source strengthening to forest-dominated strengthening at a size-dependent critical dislocation density. It is also shown that the Hall-Petch relationship can be physically interpreted by coupling with an appropriate kinetic equation of the evolution of the dislocation density in polycrystals. The model is shown to be in remarkable agreement with experiments. This work presents a micro-mechanistic framework to predict and interpret strength size-scale effects, and provides an avenue towards performing multiscale simulations without ad hoc assumptions.