YIELD BEHAVIOR OF SOLID FOAMS UNDER MULTIAXIAL STATES OF STRESS

Growing use of solid foams as core material in load bearing composite sandwich structures requires the development as well as validation of predictive yield criteria for these extremely porous and, hence, lightweight engineering materials. It has been well established that solid foams, unlike the majority of bulk materials, have a pressure dependent yield behavior. This makes the validation of various yield criteria proposed in literature a challenging task because of the need for complex biaxial and triaxial experiments, which is the main reason for both the scarcity of data and the lack of predictive power in existing models. Although there have been a limited number of studies recently in literature to generate data beyond uniaxial compression and tension experiments by introducing new loading apparatuses such as Arcan apparatus and universal biaxial testing device (UBTD), they have been limited to probe shear-compression and shear-tension states of stresses without independent control of the shear component. Therefore, the data extracted from these experiments cover only a small fraction of stress space. An example of a truly biaxial testing machine with independent load control is biaxial crushing machine (BICURMA) which, however, is restricted to only compressive loads. One must recognize that biaxial testing, even with full tension/compression control, doesn’t expand the fraction of yield surface that can be probed in any significant proportion beyond shear/compression techniques. Extremely pressure dependent mechanical behavior of solid foams requires triaxial experiments for a full description of their yield surface, hence the challenge. This work addresses this challenge by using a multiaxial testing apparatus (MATA) developed at IIT to comprehensively probe the yield surface of Divinycell H100 and H130 PVC foams through uniaxial, biaxial and triaxial experiments where loads are independently controlled in each orthogonal direction. Data reduction relies on new scalar measures of characteristic stress and strain proposed by Ayyagari and Vural (2011), and yield data is compared with their newly developed pressure dependent yield criterion. Experimental results reveal that solid foams exhibit both quadratic and linear pressure dependence. Although quadratic dependence on pressure has been previously shown in literature with a limited number of experimental yield data, extensive yield data of the current study suggests a significant linear dependence as well. The final portion of this study outlines a set of new micro-scale experiments designed to test mechanical behavior of individual ligaments that make up Duocell aluminum foam, and some preliminary results.M.S. in Mechanical, Materials, and Aerospace Engineering, December 201