Strength and ductility with 10 11 10 12 double twinning in a magnesium alloy

Based on their high specific strength and stiffness, magnesium alloys are attractive for lightweight applications in aerospace and transportation, where weight saving is crucial for the reduction of carbon dioxide emissions. Unfortunately, the ductility of magnesium alloys is usually limited. It is thought that one reason for the lack of ductility is that the development of double twins DTW cause premature failure of magnesium alloys. Here we show with a magnesium alloy containing 4 amp; 8201;wt lithium, that the same impressively large compression failure strains can be achieved with DTWs as without. The DTWs form stably across the microstructure and continuously throughout straining, forming three dimensional intra granular networks, a potential strengthening mechanism. We rationalize that relatively easier lt;c a gt; slip characteristic of this alloy plastically relaxed the localized stress concentrations that DTWs can generate. This result may provide key insight and an alternative perspective towards designing formable and strong magnesium alloy

Fracture model with variable range of interaction

We introduce a fiber bundle model where the interaction among fibers is modeled by an adjustable stress-transfer function which can interpolate between the two limiting cases of load redistribution, the global and the local load sharing schemes. By varying the range of interaction several features of the model are numerically studied and a crossover from mean field to short range behavior is obtained. The properties of the two regimes and the emergence of the crossover in between are explored by numerically studying the dependence of the ultimate strength of the material on the system size, the distribution of avalanches of breakings, and of the cluster sizes of broken fibers. Finally, we analyze the moments of the cluster size distributions to accurately determine the value at which the crossover is observed.Comment: 8 pages, 8 figures. Two columns revtex format. Final version to be published in Phys. Rev.

Strength distribution and size effects for the fracture of fibrous composite materials

Random network models have recently been developed in the physics literature to explain the strength and size effect in heterogeneous materials. Applications have included the breakdown and brittle fracture. Unfortunately, conventional scaling approaches of statistical mechanics have yielded incorrect predictions, and new approaches have been proposed which build on field enhancement occurring near the tips of critical, random clusters together with the statistical theory of extremes. New distributions and size scalings for strength have been proposed and supported through Monte Carlo simulation. Here we consider an idealized, one-dimensional model for the failure of such networks where elements of constant strength may be initially present or absent at random. Our idealized rule for local stress redistribution near breaks reflects features we find in a discrete mechanics model that has limiting forms consistent with continuum theories for cracks. We obtain rigorous asymptotic results for the strength distribution and size effect with constants and exponents that are known. The validity of various analytical approximations in the literature is then discussed

Bounds on the Strength Distribution of Unidirectional Fiber Composites

Failure mechanisms under tensile loading of unidirectional fiber composites comprising of Weibull fibers embedded in a matrix are studied using Monte-Carlo simulations. Two fundamental mechanisms of failure are recognized--stress concentration driven failure and strength driven failure. It is shown that the cumulative distribution function for composite strength predicted by the stressconcentration-driven failure and strength-driven failure form apparent upper and lower bounds respectively and also that failure mechanism switches from one to the other as fiber strength variability changes