Role of Mean Stress in Yield Behavior of Solid Foams
Ravi Ayyagari (Illinois Institute of Technology), Murat Vural (Illinois Institute of Technolo)
Eringen Medal Symposium in honor of G. Ravichandran
Tue 9:00 - 10:30
Salomon 001
Understanding and accurate modeling of the yield behavior of solid foams is essential to exploit full potential of these lightweight materials in growing engineering applications. In this study, we report both experimental and FE data to show the nature of pressure dependence in the yield surface of periodic as well as stochastic foams. To this end, a multitude of uniaxial, biaxial and triaxial stress paths were used to probe the yield surface for a wide range of mean stress. From a macroscopic viewpoint, it is observed that in addition to well-documented quadratic dependence of yielding on mean stress there also exists a linear pressure dependence which has not been properly addressed in the literature. Partition of strain energy into bending and stretch modes of deformation within struts provides a unique tool with which the dominant deformation mode that drives yielding can be identified as a function of the stress path followed. Analysis of FE results indicates that positive mean stress, as compared to negative mean stress, provides a strong configurational stability in deformation kinematics that limits bending mode and promotes the stretch mode of deformation in struts. Increasing the fraction of strain energy stored in stretch mode effectively increases the critical strain energy of yielding and, thereby, delays the onset of microscopic yielding. Negative mean stress, on the other hand, results in a weak configurational stability where bending mode is more prominent. Furthermore, it seems that the degree of this already weak stability quickly decays with the magnitude of negative mean stress and completely disappears when (and if) the stabilizing effect of joint stiffness is exceeded. We conclude that this contrasting behavior is the main source of (i) linear pressure dependence observed in the yield behavior as well as (ii) the stronger effect of linear pressure in low density foams.