Fabrication of lotus-type porous carbon steel by the continuous zone melting method and its mechanical properties

A lotus-type porous carbon steel with cylindrical pores was fabricated by the continuous zone melting method in a pressurized mixture of hydrogen and helium gases. The porosity increases with increasing partial pressure of the hydrogen gas, while the pore diameter remains almost constant, independent of the pressure. The ultimate tensile strength of the specimen with cylindrical pores parallel to the tensile direction is lower than the estimated value, assuming that the strength is decreased in proportion to the decrease of the cross-sectional area of the specimen, while the yield strength is higher than the estimated value. The compressive yield strength is also higher than the estimated value. The increase in yield strength is considered to be due to precipitation strengthening. The tensile strength is increased by quenching and tempering, while the elongation decreases. Such mechanical properties are discussed in terms of the microstructural analysis.     

Numerical evaluation of three-dimensional water impact by the displacement potential formulation

The numerical accuracy of the displacement potential approach for a three-dimensional water-impact problem is investigated. An examination of the elliptic paraboloid shows that the boundary-value problem for the displacement potential, as well as the velocity potential, can be solved accurately, even if the panel size of the numerical scheme is no small enough. The numerical accuracy of the position of the contact line is poor. However, the accuracy of the virtual mass is good because of the averaging effect along the contact line. A comparison with Scolan and Korobkin's designed body confirms the accuracy of the drop speed and the penetration displacement. The displacement potential approach is extended to the case of water impact with trapped air. The numerical accuracy of the calculated air pressure in the cavity is confirmed by comparison with an experimental result. With these results, the suitability of the displacement potential approach to the ship-slamming problem is demonstrated.     

Breakdown of oil films and formation of residual films

The breakdown processes of oil films under quasi-static loading have been investigated by using a newly developed steel-oil-mercury system. The relationship between the thickness and breakdown ratio of a hexadecane film is represented by a single master curve independently of the indentation speed, indentation load, and temperature. The master curve shows that the breakdown process of hexadecane includes two stages; one is the decrement of the thickness without breakdown and the other is the decrement of the thickness with a drastic progress of breakdown. By solving a small amount of fatty acid in hexadecane, the thickness increases and the breakdown ratio decreases noticeably; a multilayer residual film supporting normal load is formed between two metal surfaces. Experiments at different temperatures reveal a negative relationship between the temperature and thickness of residual film, which indicates that the residual film is organized by physical interaction rather than chemical interaction. At least under a lower concentration, the residual film appears to consist of not only fatty acid molecules but also hexadecane molecules.     

Establishment time of liquid flow in a bath agitated by bottom gas injection

The establishment time of gas-liquid two-phase flows in a cylindrical bath agitated by bottom gas injection through a central single-hole bottom nozzle was investigated. Because the turbulence intensity in the bath was comparable to or larger than the unity, the conventional definition of the flow establishment time based on the history of mean velocity was not suitable for the present case. In fact, it was difficult to determine the flow establishment time based on the well-known 90 or 99 pct criterion for the mean velocity. Accordingly, two methods of determining the flow establishment time by focusing on the turbulence components instead of the mean velocity components were proposed. Velocity measurements were made with a two-channel laser Doppler velocimeter. The flow establishment time was correlated as a function of gas flow rate. Close agreement was obtained by the two methods.