Investigation of the electrical conductivity of propylene glycol-based ZnO nanofluids

Electrical conductivity is an important property for technological applications of nanofluids that has not been widely studied. Conventional descriptions such as the Maxwell model do not account for surface charge effects that play an important role in electrical conductivity, particularly at higher nanoparticle volume fractions. Here, we perform electrical characterizations of propylene glycol-based ZnO nanofluids with volume fractions as high as 7%, measuring up to a 100-fold increase in electrical conductivity over the base fluid. We observe a large increase in electrical conductivity with increasing volume fraction and decreasing particle size as well as a leveling off of the increase at high volume fractions. These experimental trends are shown to be consistent with an electrical conductivity model previously developed for colloidal suspensions in salt-free media. In particular, the leveling off of electrical conductivity at high volume fractions, which we attribute to counter-ion condensation, represents a significant departure from the "linear fit" models previously used to describe the electrical conductivity of nanofluids

Experimental and finite element simulation methods for metal forming and cutting processes

Experimental procedures and finite element simulation methods for metal forming and cutting processes are developed. The development includes the formulation of a tangential stiffness matrix for plane-strain and axisymmetric solid finite elements with four node, eight degree-of-freedom, and quadrilateral cross-section. The effects of elasticity, viscoplasticity, temperature, friction, strain rate, and large strain are included in this formulation. The solution procedure is based on a Newton-Raphson incremental-iterative method which solves the nonlinear equilibrium equations and gives temperatures and incremental stresses and strains. For the metal forming processes, finite element simulation for the upsetting of a cylindrical workpiece between two perfectly rough dies is first performed and the results are compared with alternative finite element solutions. Both experimental and finite element methods are then conducted for the upsetting of a cylindrical billet and the forging of a ball. The orthogonal metal cutting experiments are set-up on a shaper, and the distributions of residual stresses of the annealed 1020 carbon steel sample are measured using the x-ray diffraction method. Under nominally the same cutting conditions are the experiment, the cutting processes are also simulated using the finite element method. Comparisons of the results for the forming forces, deformed configurations, and the distributions of residual stresses between the experimental and finite element methods indicate a fairly reasonable level of agreement. The versatility of the present finite element simulation method allows for displaying detailed results and knowledge during the metal forming and cutting processes, such as the distributions of temperature, yield stress, effective stress, plastic strain, plastic strain rate, hydrostatic stress, and deformed configuration, etc. Such knowledge is useful to provide physical insights into the metal forming and cutting processes as well as to better design the processes for engineering components with improved performance