Marisol Koslowski - Purdue University Numerical simulations can be exploited to understand the sensitivity of the mechanical properties of materials to microstructural variability and hence improve their mechanical performance and reliability. In this talk I will present results that show the effect of initial defects on strength and toughness of two materials systems: high entropy alloys and polymer composites. High entropy alloys are solid-solution alloys with five or more combined elements in nearly equiatomic concentrations. Their name derives from the concept that entropy of mixing is responsible for stabilizing the phases in a many components system. Although the behavior of the random alloys has been a major topic in metallurgy for over fifty years, the strengthening mechanism underneath is still an open question especially for alloys with high elemental concentration. Predictive models of flow, strengthening, ductility, fatigue and other macroscopic quantities are fundamental to advance the design of alloy families by selecting elements to achieve a specific purpose. In high entropy alloys the stacking fault energy varies depending on the local composition. Our work reveals the value and the characteristic length of the regions over which the stacking fault energy varies are of key importance on the strength of the alloy. The most predominant failure mechanism in carbon fiber reinforced composites is delamination. In laminated composites dispersed second phase particles have been added to the interlayer region to reduce delamination. Simulations of crack propagation in particle-toughened interlayers show several damage mechanisms observed in experiments and the processes that produce crack growth. In particular, failure involves multiple micro-cracks forming ahead of the main crack tip that coalesce and lead to crack branching, deflection and the formation of ligaments. Additionally, varying the particle stiffness and the surface energy of the matrix-particle interface leads to improvement of the interlaminate toughness as the fiber-matrix interface becomes a preferential path for the crack through the nucleation of extended delamination zones ahead of the crack tip.