Yielding and irreversible deformation below the microscale: Surface effects and non-mean-field plastic avalanches

Nanoindentation techniques recently developed to measure the mechanical response of crystals under external loading conditions reveal new phenomena upon decreasing sample size below the microscale. At small length scales, material resistance to irreversible deformation depends on sample morphology. Here we study the mechanisms of yield and plastic flow in inherently small crystals under uniaxial compression. Discrete structural rearrangements emerge as series of abrupt discontinuities in stress-strain curves. We obtain the theoretical dependence of the yield stress on system size and geometry and elucidate the statistical properties of plastic deformation at such scales. Our results show that the absence of dislocation storage leads to crucial effects on the statistics of plastic events, ultimately affecting the universal scaling behavior observed at larger scales.Comment: Supporting Videos available at http://dx.plos.org/10.1371/journal.pone.002041

Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 μm. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200 nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale

Energy dissipation via acoustic emission in ductile crack initiation

The final publication is available at Springer via http://dx.doi.org/10.1007/s10704-016-0096-8.This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture

Thermodynamic investigation of yield-stress models for amorphous polymers

International audienceA thermodynamic study of the yield process of amorphous polymers is proposed to investigate four yield theories: the Eyring's model and its linearized form, the cooperative model and the Argon's model. For a poly(methyl methacrylate) (PMMA) and a polycarbonate (PC), the corresponding apparent activation volumes and apparent activation energies are calculated and compared for a wide range of temperatures and strain rates. In the case of the cooperative model, we show that the secondary molecular relaxation is a key parameter in the explanation of the specific mechanical behavior of glassy polymers at low temperatures and at high strain rates. For the three other models thermodynamic inconstancies were found and discussed

Modeling of strain rates and temperature effects on the yield behavior of amorphous polymers

Three molecular theories are used to predict the yield behavior of amorphous polymers for a wide range of temperatures and strain rates. These include the state transition theory of Ree-Eyring, the conformational change theory of Robertson and the disclinations theory of Argon. For each of these models, the yield stress behavior of polymethylmethacrylate (PMMA) is described over a wide range of strain rates and temperatures. According to experimental values for polycarbonate (PC), the Ree-Eyring model seems to be the suitable theory at high strain rates for the prediction of the yield stress of amorphous polymers

Constitutive modeling of polymer materials at impact loading rates

Starting from physical basis, a robust three-dimensional constitutive model for the finite strain response of amorphous polymers is briefly presented. This model accounts for the high strain rate and temperature effect. Intramolecular as well as intermolecular interactions under large elastic-inelastic behavior are considered for the mechanisms of deformation and hardening. In particular, it is found that the secondary relaxations of polymer chains play an important role in the deformation process for the high strain rates and low temperatures. For a wide range of temperature and strain rate, the proposed constitutive model has been validated in compression for three amorphous polymers: polymethylmethacrylate (PMMA), polycarbonate (PC) and polyamideimide (PAI)