Yuzuru Fujita

Studies on collision-protective structures in nuclear powered ships

Abstract For the purpose of obtaining design formulae of collision-protective side structures installed in nuclear powered ships, various series of tests and theoretical analyses were carried out. Fracture types of side structures could be grouped into two types such as deformation and crack ones. Discrimination of fracture, estimation of load, and absorbed energy against penetration depth were formulated. The dynamic effects could be explained well with the rise of yield stress of steel. Relationship between added mass of water and elapsed time of collision was analyzed. The influence of the strength of a stem of striking ship could be examined with strength ratio of stem to side structure and semi-experimental formulae of the energy absorption were obtained. The influence of the stem angle and penetration angle to the energy absorption was clarified to be small. The condition of the side structures required to have large absorbed energy was discussed using the model tests of various side structures.

Plane Elastic-Plastic Analysis of Perforated Plates based on Complex Stress Functions

The problem of stress distribution around a hole or a crack belongs to the very important one in the field of fracture mechanics. In this paper, the problem of an infinite plate with a circular or elliptic hole under simple tension was investigated.Basic equations were derived from complex stress functions introduced by E. Goursat and developed by N. I. Muskhelishvili and S. Moriguchi. As a first step, stress function FX (x, y;x0, y0) for a unit force in x-direction valid for a infinite plate without any discontinuity was derived. Then, by the use of principle of reflection (proposed by S. Moriguchi), the stress function FXH (x, y;x0, y0) for a plate with a hole was deduced.Two dimensional elastic-plastic analysis was conducted by using the “Modified Initial Strain Method” introduced by T. Fujimoto in the field of welding dynamics.In order to derive stress function Fex* (x, y;x0, y0) for unit initial strain e*x atpoint (x0, y0), the concept of thermo-elastic theory was used.The calculated results show that the applied analytical method is more useful in stress calculation than F. E. M.

Studies on Prevention of End Cracking in One Side Automatic Welding (2nd Report)

Recently one sided automatic welding has widely been prevailed in the assemble stage of Japanese shipyards. As one of the problems of welding procedure, as well as any other welding techniques, cracking which sometimes occurs near the end of weld, has become an important problem.The authors have proposed the calculation technique in the seam welding. The most significant feature of the method differing from the ordinary welding thermal stress analysis is that the conditions of calculation are simulated to be those of more realistic welding procedures.Then, the following assumptions are imposed in order to calculate stress and strain during welding.(1) Before welding, specimens are connected by tack welds and prepared edges are free boundaries.(2) Molten region containing arc tip is free from stress.(3) After the arc passed by, the plates are connected by solidifying of the molten pool, that is, free prepared edge changes to supported one as temperature is cooled down.(4) When the arc passed through a tack weld, it should be melted away and free from stress.In order to calculate the thermal stress analysis during welding, firstly we solved un-stationary heat conduction problems and obtained temperature distribution during welding. Based on the calculated temperature distribution, we analysed uncoupled thermal stress problems by using the finite element method, and conclusively showed that one of the main causes of end cracking is the transient tensile stresses generated near the end part of weldment when the stress re-distribution is taken place due to the melting of the last tack weld.To endorse the numerical calculation by the proposed method, experiments are carried out and it is confirmed that the method could be applicable to this kind of transient welding stress-strain state with time depending varying boundary conditions.

On the Corrosion of Ship Structural Steel

In order to study the effects of prestrain and welding on the corrosion of ship structural rimmed steel, we have performed some experiments to decide the degree of corrosion by measuring the electrical resistance increment of test pieces.The results obtained are as follows:(1) Steel becomes more sensitive to corrosion as the amount of pre-strain increases.(2) Particularly, the corrosion of steel which contains weldment is seriously precipitated.(3) The effects of pre-strain on the corrosiveness of steel have been reduced by welding which produces a large quantity of heat such as union melt welding.(4) The rim part of steel in more anticorrosive than the core part.

On the Slamming Test of a Wooden Model Ship

The authors performed towing test of a wooden model ship under regular waves and measured the pressure distributions at foreward bottom of the model when she was slamming, by using 19 specially designed pressure gauges.The maximum pressure observed reached about 4 to 5 times of static pressure under regular waves and position of maximum pressure located about 0.1L from F. P., and dynamical pressure became practically zero at about 0.25L from F. P..According to the experimental results, maximum pressure seemed to be related to the relative normal velocity of ship to wave rather than pitching or heaving velocity. And this result is expected from rough theoretical consideration written in Appendix and this maximum pressure can also be estimated from the pressure measured by falling test of flat or wedge shaped body on the surface of water.Slamming appeared when the ratio of length of wave to that of ship became about 1.1 to 1.2 in this test.