Zhenwen Zhou

NEW INSIGHTS INTO CONTROLLING SUB-MICRON FAILURE MECHANISMS IN COMPOSITES USING DISCRETE FUNCTIONALIZED MULTIWALL CARBON NANOTUBES

New technology has been developed that enables multiwall carbon nanotubes to be discrete, high aspect ratio and well bonded to the composite matrix of choice. Several composite types are examined using tubes of diameter about 12 nm and length about 700nm. Fully discrete, well-bonded tubes are shown to significantly enhance the matrix resistance to fracture and can be placed between fiber plies of composites. The challenges of maintaining the exfoliated state of discrete multiwall carbon nanotubes during composite part assembly from the liquid prepolymer to the cured part are discussed.© 2013 ASME

Application of Crack Layer in Modeling of Slow Crack Growth in High-Density Polyethylene

Crack layer model provides a comprehensive foundation for modeling of fracture growth, failure analysis, and lifetime prediction. During the past two decades, it has been widely applied for modeling various aspects of brittle fracture in general. This paper illustrates in details the procedure of implementation by an example of slow crack growth in a commercialized high-density polyethylene undergoing creep conditions. Firstly, we determine experimentally the basic parameters employed in constitutive equations of crack layer model such as draw ratio λ, the specific energy of transformation γtr, and drawing stress σdr, etc.. Secondly, we implement crack layer model numerically in lab-developed “Simulator”. The paper provides a paradigm for implementation of crack layer model in slow crack growth, and a blueprint for potential software development that can be used in ranking and the lifetime assessment of a large set of engineering polymers.© 2013 ASME

Modeling of stress corrosion cracking in plastic pipes

Stress corrosion cracking (SCC) is commonly observed in the form of a microcrack colony within a surface layer of degraded polymer exposed to mechanical stresses and a chemically aggressive environment. SCC results from strongly coupled mechano-chemical processes. We distinguish four stages of SCC: 1) localized material degradation leading to crack initiation, 2) individual stress corrosion (SC) crack propagation, 3) multiple crack interaction and crack clusters formation, and, 4) instability and dynamic growth of individual crack or a cluster of cracks leading to the ultimate failure. The duration of the last two stages of SCC is relatively short in comparison to the total time of SCC related failure process. Therefore, the duration of the first two stages of SCC serve as a conservative estimate of total time before failure. In this paper, we present a model with consistent results of the second stage of SCC. The proposed model improved the estimation of failure time. The scatter of crack initiation time presents the major part of uncertainty in failure time. Individual SC crack propagation is much more reproducible phenomenon than fracture initiation. Deterministic Crack Layer (CL) theory is employed to model the kinetics and duration of slow SC crack growth. SC crack growth is modeled by a system of nonlinear differential equations, which calls for a numerical solution. Comparison of cracks driven by SC and stress only is presented. Conventional plot of SC crack growth rate vs. the stress intensity factor is constructed and analyzed. In conclusion, an algorithm is discussed for a conservative estimate of the life of engineering thermoplastic submitted to a combine action of mechanical stresses and chemically aggressive environment.