An application of the theory of minimum wave resistance to the practical design of high speed ship forms is introduced. Use is made of the theory, indicated by Krein and Bessho, of the variation of hull forms without change in wave resistance. Tank experiments are conducted with a series of high speed container ship models, of which the hull forms are derived from the theoretical form of minimum wave resistance. The result can well prove the feasibility of the design method proposed here.

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Before the calculation of velocity distributions in ship wake, three kinds of experiments are carried out in order to get theoretical models as to the separation phenomena and the diffusivity coefficient; (a) observation of the limiting streamlines by the oil film method, (b) the velocity measurement near the separation position and (c) the vorticity measurement in ship wake. The separation position is determined experimentally which can be predicted roughly using some suitable parameters of the boundary layer calculation.
It has been made clear from the velocity measurements that the velocity near the seperation position makes remarkable changes and the newly-generated-vorticity affects very much the down stream pattern.
It has also been made clear that not only the x-axis vorticity but also the y- and z-axis vorticity compose the stern vortex.
Using the measured velocity and vorticity distributions the diffusivity coefficient is determined, which amounts to about 300 times of the molecular one.
The velocity calculation of a tanker model is carried out after these experimental considerations, which shows a fairly well agreements with the measured one.

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In this paper the authors study the improvement method of cavitation characteristics of MAU type propeller.
The “two-dimensional cavitation bucket chart” which was developed by Gutsche, Nakajima, Ito, etc., is insufficient to predict cavitation of a propeller because it is originally based on two-dimensional sections. The authors proposed a new chart, “three-dimensional cavitation bucket chart” by using Koyama's lifting surface theory of a propeller and the concept of two-dimensional corresponding section.
The theoretical pressure distribution calculated by the above mentioned theory, agrees with experimental results, measured on a model blade in uniform flow.
Calculations in non-uniform flow were also carried out by using Koyama's unsteady lifting surface theory. The results were compared with the three dimensional cavitation bucket chart. It was found out that the unsteady effect was not large and the three dimensional cavitation bucket chart is also useful for the prediction of cavitation of a propeller in ship wake.

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A simple procedure for predicting stability derivatives of a ship equipped with a propeller and rudder from those for the bare hull was examined. A procedure of this kind is very important as it enables us to easily examine an influence of rudder area on stability derivatives and scale effect in maneuvering characteristics.
This paper indicates that the lateral force exerted by rudder deflection on a hull must be taken into consideration when making a presumption of this kind, and the influence of a propeller operating on stability derivatives is, in comparison with the influence of the rudder force, not so negligibly small that it cannot be disregarded. Further, through the above mentioned studying process, lateral force acting on the propeller installed on the obliquely moving ship was directly measured, and it was made clear that the force delivered to the propeller itself (the so-called fin effect of the propeller) would not be a significant component of the lateral forces produced by the action of the propeller on the hull and propeller.

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Analysing correlations among resistance at Froude number F_∇ (= V/√∇^1/3g) =3.5, trim angles at F_∇=2.5 through 3.5, by using model test data in still water of the high speed craft, the clear correlation was found to exist between resistance at F_∇=3.5 and trim angle at F_∇=2.5.
Therefore, the sequential unconstrained minimization technique was first applied to get the smallest total resistance hull form for each 30 minutes interval of trim angle change, and then followed the discussion concerning deviations arisen and resistance performance due to obtained hull form parameters. Using diagrams related to the total resistance at F_∇=3.5 and the trim angle at F_∇=2.5, we clarified finally the influence of the unit trim angle on the total resistance coefficient within high speed range.

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With the progress of the ocean development, the estimation of the hydrodynamic forces acting on floating and/or submerged bodies in shallow water, has come into an important and urgent problem for the prediction of the motion of the structure and, in addition to model tests, some theoretical approaches have been made by several authors using the eigenfunction, expansion or the singularity distribution methods. Moreover, the introduction of the finite element method to flow problems has made possible to analyse the flow field around the complicated body configuration and the bottom topography with ease.
In the present paper, firstly the author analysed various kinds of the two-dimensional problems in shallow water by means of the finite element technique called the method of superposition proposed by Yamamoto and the author in the first report, taking advantage of the linearity of the governing equations and the asymptotic behaviors of the solution at infinity. The results showed good agreement with previous authors'.
Secondly, the author formulated a new solution method based on the extended three-dimensional Haskind's relation between the radiation and the diffraction potentials corresponding to Bessho's theory, and conducted a few numerical calculations for an axi-symmetric body.

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This paper provides numerical and experimental results for the first-, second- and third-order hydrodynamic forces acting upon cylinders oscillating vertically with large amplitude on a free surface. The procedure of calculations is based on the theory in which it is assumed that the thickness of the cylinder and the free surface elevation are small enough but the amplitude of the oscillation is so large that the second-order contribution to the forces could not be neglected.
Numerical calculations are made for two elliptic cylinders, a circular cylinder, a stern-section cylinder, whose section is similar to the one of a ship at the stern, and a wedge, and their results are compared with those obtained through forced oscillation tests. It is observed that the calculated values for the second-order hydrodynamic forces, in particular, are in fairly good agreement with the measured ones.

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This paper presents an investigation of the drifting force acting on a sphere which is fixed or freely floating in regular waves. The theoretical drifting force coefficient C_d of a fixed sphere increases monotonically with the increasing non-dimensional wave number Ka, and agrees well with experimental data. This makes the author sure to conclude that the theory can predict correctly the drifting force acting on a fixed body.
On the other hand, in a freely floating case, the theoretical C_d curve has a peak at Ka= 1.15 and this tendency is also confirmed by the experimental results. It is interesting that the peak value of C_d is greater than that value when the oncoming waves are completely reflected by the sphere. As yet the author cannot be sure to say whether this excess is still due to the part of the waves in the width of the diameter of the sphere, or to any other part. But if the latter is true, a three dimensional body for which C_d is 1.0 at a complete reflection of the oncoming waves, will presumably have a C_d value greater than 1.0, when it is freely floating in the waves.

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Extending the theoretical work of Biésel, a preliminary study is made of dynamic characteristics of an articulated double-flap wavemaker. The underlying motivation of utilizing a double-flap waveboard is based on its effectiveness in reducing the local disturbances in the immediate vicinity of the waveboard, which is demonstrated through decreasing inertia pressures. The additional degree of freedom in the double-flap waveboard movements is shown to be capable of encompassing a wider range of wavelength that can be satisfactorily generated in a given tank. The inertia pressure is reduced significantly by use of double-flap wavemakers, especially when short waves are generated. The usual wavemaker performance curves are computed for double-flap wavemakers. Comparison with the results of a single-flap wavemaker characteristics reveals that considerable improvement in performance curves is feasible by selecting a suitable combination of design parameters.

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The deck wetness and the impulsive water pressures induced by shipping water on the deck are serious problems of a ship operating in rough seas. For these problems, model experiments were conducted in extreme head seas in a towing tank.
In this study, relations between the probability of deck wetness and the following matters are discussed :
1. significant wave height
2. mean wave period
3. ship speed
4. significant values of ship motions
5. relative bow motion.
Relations between the peak values of impulsive pressures, relative bow motion and impact velocity of shipping water are also discussed.
Moreover, the prediction of the probability of deck wetness was made, applying the empirical expression of the dynamic swell-up proposed by R. Tasaki, and compared with the experimental results. Finally some recorded patterns of impulsive water pressures are discussed in detail.

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At present, both the coordinate axis fixed in the space and that fixed on a moving ship are used to almost the same extent when the sway and yaw motions of a ship are described. The former is preferred rather in the seakeeping study to treat ship motion in waves, and the latter is preferred in the study of ship manoeuvre in calm sea. So far, these two branches of ship dynamics have been developed independently of each other. In the near furture, however, we shall have to treat more frequently problems related to both seakeeping and manoeuvring. On the occasion, we shall be forced to make a due choice of the coordinate system, by which the equations of motion will be described. For this purpose, the author tries to throw light on the essential difference between the above-mentioned coordinate systems. In section 2, are compared the body-fixed coordinate axis and the space-fixed coordinate axis with each other by assuming that neither sway force nor yaw moment generate due to surging, heaving and pitching of a ship, and that sway force and yaw moment are represented as linear functionals of sway and yaw velocities of a ship. Furthermore in section 3, the same discussion is succeeded by attaching the condition of causality to the hydrodynamic force generated on a ship by its motion. As a result, it is ascertained that the body-fixed coordinate system is more suitable than the space-fixed coordinate system for clear understanding of physical nature of the hydrodynamic force.

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Recently some ships have been constructed with a vertical well which have the bottom opening for the purpose of raising and lowering the equipment. Some examples are vessel-type drilling rigs, vessels carrying small submersibles and so on.
This paper shows the behavior of the water in the well, and its effects on the motion of the ship, when she is running.
Experimental studies were performed under two different sets of conditions.
(1) The motion of the free water surface in the well was studied, when the ship-model was fixed stationary in the stream.
(2) The effect of the motion of the water in the well on the motion of the ship-model was studied, when the model was towed.
The following conclusions were obtained.
1) When the ship-model with a well is fixed stationary in the stream, it can be observed that the water surface moves regularly in it. This phenomenon is similar in many respects to the motion of a cylinder which is elastically supported by springs in the stream.
2) The type of water motion, whether up and down, right and left or forwards and backwards, depends on the velocity of the stream and the shape of the well.
3) The water motion can be seen at the dimensionless stream velocity of 0.2<2U/lω₀<1.6, where U is the stream velocity, lis the length of the bottom opening and ω₀ is the natural circular frequency of the water moving in the well.
4) The experimental values agree well with the results of the theoretical calculation on the heaving amplitude of the free water surface in the well, and on the increased drag of the ship-model by the motion of water in it.
5) In order to reduce the motion of water in the well, it is very effective to fix short flanges on its vertical walls a little below the water surface.

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In the previous paper, it has been clarified that motion characteristics of a submersible can be divided into idealized controllability, stability, response behavior, and control forces, and that the relationship between motion characteristics and design parameters are easily examined if a specification of the submersible design satisfies some conditions for improvement of idealized controllability.
The present paper deals with the investigation of the relationship between design parameters and the motion characteristics in case that the theory of idealized controllability does not always hold. In order to obtain the physical relations the stability, the response behavior and the control forces are respectively analyzed with both eigenvalues and zeros of the characteristics equations of motion. Finally, the criterion to discriminate whether the idealized controllability is actually able to be satisfied or not, is discussed. Thus the authors believe that the designing method of a submersible with idealized controllability and motion stability is made clear.

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The acoustic position measuring systems for the unmanned research submersible are used in severe environment conditions : thruster noise and disturbances, signal reflections, etc. In this paper the detection probability of position based on statistical decision theory are examined using stochastic model for the acoustic channel, and some useful results are extracted in order to design high reliable system even though in such severe environmental conditions :
1) time window is effective, but do not need in case of high signal level.
2) redundancy of one hydrophone is equivalent to 1 dB in S/N.
3) threshold level should be higher than half of received signal.

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In the first report [1] the present authors investigated the stability of mooring anchors. The present paper deals with the maximum holding power of a dragged anchor.
The maximum holding power of a mooring anchor depends upon forces and moments acting on the shank, which are measured experimentally and are given in empirical formulas. A set of equilibrium equations of a mooring anchor in sea bed is formulated by considering forces acting on all the structural elements of the anchor. By solving these equations, configurations of a dragged anchor and its maximum holding power are analyzed.
Forces acting on rectangular plates in wet sand are investigated in relation to those in dry sand, and as the results, behaviors of an anchor in wet sand can be derived from those in dry sand by modifying its specific weight.
It is important from the practical point of view to ensure that the anchor will always enter into the sea bed with a required attitude ; it can be achieved by choosing the position of hinge-pin properly.

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Weather Routing is a technique of optimizing the entire route depending on the estimated weather and sea states. But, the estimation can not always be perfect, that the local optimization will also be necessary and of worth.
This paper is an approach from the latter aspect to the problem of planning ship's course, and the author desires that design work and operation of ship will become tied more closely by applying the proposal made in the paper.

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The contents of this paper are concerned with a comparison of theoretical and full-scale responses for the 26, 800 dwt container ship “Pacific Arrow” when serving in the North Pacific Ocean.
The measurements were carried out on five voyages of the ship, and the test data were obtained on the main-hull stresses due to waves as well as vertical accelerations of ship's motion. In addition to these, the theoretical analysis was performed on the statistically estimated long-term stresses in the longitudinal strength members of the ship by means of the total system analysis computer programme developed by Nippon Kaiji Kyokai.
Comparisons between the theoretical predictions and the processed test data by the measurements indicate good agreement for the longitudinal stresses of the main-hull girder of the ship.

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Recently, a trend toward the safety analysis of ship structural design in which both the load on hull and the structural capability can in principle be treated as random variables, has been growing. The character of the strength capability as random variable taking the irregularity and variability of material strength and imperfection of workmanship into consideration, has been investigating. The stochastic nature of critical load assuming standard loading condition is studied by means of extreme theory.
The still water bending moment of a actual ship has a wide distribution. This has effects on the wave load. Moreover, the distribution is affected by accuracies of supposed wave and operating condition. So it is expected that the character of demand is not equal to that of conventional one.
Authors deals with study on the characteristic of the critical load. This paper is the first report and describes the long term distribution, expected extreme and distribution of critical value of wave bending moment under the influence of the variability of still water bending moment.
Supposing the distribution of still water bending moment is the normal distribution, the long term distribution of wave bending moment is a group of many exponential distributions, the distribution of parameters of which are described by normal distribution. This long term distribution for a usual ship can be approximated to Weibull distribution and its parameters are linear to coefficient of variation of λ.
By the investigation of a series of similar cargo ships it is clarified that the coefficient of variation of λ is usually less than 4% and the characteristic extreme of wave load increases about 4% in comparison with the value assuming standard loading. It may be concluded that the long term distribution for these ships is approximated by exponential distribution the parameter of which is the mean value of λ.
In usual the distribution of still water bending moment is expected to be more complicated. So the character of critical value of the wave load when the parameters showing distribution of still water bending moment are also stochastic variables, is shown. In this case, coefficient of variation of critical wave load increases with the variability of that parameters.

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A family of new element models in discrete structural analysis is proposed in this paper.
These models consist of finite number of small rigid bodies connected with springs distributed over the contact area of two neighbouring bodies. In general size of stiffness matrices of these elements are at most (6 × 6) which are equal to or even smaller than 1/2 of those of conventional finite elements so that considerable reduction of computing time can be expected.
Effectiveness of these elements in nonlinear structural analysis will be demonstrated by several numerical examples.

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Unstiffened spherical tanks supported at the equator are used in LNG-carriers. From among numerous other load conditions, the buckling load induced by partial filling with LNG plays an important role in determining the thickness of the tank. In this case, the region of the tank close to and above the liquid surface is the most critical area in terms of buckling requirements. In this area there are biaxial stresses consisting of membrane compressive stress in one direction and smaller or larger tensile stress in the opposite direction.
This paper describes experimental and theoretical studies of the buckling design of spherical tanks under biaxial stress conditions. The experiment on buckling was carried out with the use of an aluminum model tank of 1/75 scale. The result was compared with API standard and with an extension of Yao's theory to plastic region.
As a result of investigation of design criteria stipulated in API standard and ASME code, it became evident that design buckling load for tension-compression biaxial stress conditions. is too conservative due to neglect of the effect of tensile stress. Consequently a buckling design chart for arbitrary biaxial stress conditions has been proposed using the method of linear correlation between standard buckling loads.
To comfirm the validity of the above method, theoretical buckling loads for rectangular plates under biaxial stresses were calculated by using Galerkin's method, and the error resulting from the linear correlation was discussed.

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Ship and bridge structures mainly consist of stiffened plates. Studies on the ultimate strength of these stiffened plates were made by using the concept of the effective width. (Kármán, Yoshiki) Because of a recent great progress in application of finite element method in structural analysis, large-deformed elastic-plastic problems could be solved. The obtained results yield several significant informations.
Since there are, however, many types of stiffened plate structures, it is difficult to carry out direct calculations (such as F. E. M.) for each structure. In order to estimate the ultimate strength of stiffened plates, a new analytical method is presented in this paper.
The proposed method is compound of two parts. One is elastic analysis based on RayleighRitz method, the other is plastic analysis in which collapse mechanism is considered. Both analysis treat large deformation.
The following conclusions are obtained.
1) Post buckling behavior and the ultimate strength of a square plate subjected to compressive load can be obtained by using a newly proposed analytical method. These results show fairy good agreements with those based on the concept of effective width and those calcul ated by F. E. M.
2) Relationship between the ultimate strength of a stiffened plate and its stiffener height as well asγ^U min, (minimum stiffeness ratio of a stiffener against ultimate strength) can be also analysed by this method. These results fairy agree with those calculated by Ueda et al. using F. E. M.
3) This newly proposed method makes use of analytical solutions so that the ultimate strength of stiffened plates can be easily estimated comparing with other numerical methods.

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Buckling and collapse of the curved corner structure are investigated by model tests and FEM buckling analysis. And it is discussed how stiffener arrangement and load condition have effects on buckling and collapse load, and collapse mechanism.
The results are summarized as follows :
(1) Radial stresses have a significant effect on panel buckling in some cases.
(2) Local or overall collapse takes place according as a face bar is supported or not.
(3) P_cr /P_y and P_ult /P_y depend on load condition. P_y is a calculated load at which a face bar starts to yield.
(4) Buckling and collapse load for one stiffener arrangement may be 20% less than those for another arrangement with the same total length of stiffeners.

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In the near future, various huge off-shore structures will be constructed. It was concluded through preliminary studies that composite steel-concrete structures are best fitted for the strength members of the huge structures, due to the high strength and ductility of steel and due to the low ratio of the cost to the weight and reliability of concrete.
In the paper, the authors carried out both experimental and theoretical investigations into the strength of the composite steel-concrete structures where concrete is placed between steel plates. Experiments were carried out using the two-dimensional models of the composite structures under bending, shear and combined bending and shear. The fundamental properties of the composite structures were clarified through the experiments.
A new method for the ultimate strength analysis of the composite structure was developed. The calculated results were in good agreements with experimental ones. The effects of the depth of the composite structure, the sectional areas of compression and tension steel plates and the compressive strength of concrete upon the ultimate strength were investigated by the proposed method of analysis. It was clarified that the depth of the structure and the sectional area of the steel plate under tension exert decisive influences upon the ultimate strength.

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The coupled vibration of torsion and horizontal flexure of a container ship was studied theoretically and experimentary.
In the calculation method developed here a ship was treated as a thin-walled beam with non-uniform cross-section. The authors especially took note of the problems occurred in the calculation based on the non-uniformity of cross-section along the ship length.
Haslum and the other's method was applied to the continuity of warping and the equilibrium condition of bi-moment at the points where the shear center axis is not continuous. And the effects of the couples between the horizontal shear force and the torsional moment and between the horizontal displacement of the shear center and the twisting angle were also taken into consideration.
The vibration test was carried out on a ship model with non-prismatic form.
The results of this investigations led to the following conclusions.
The calculated results obtained through this method are in good agreement with experimental ones for lower mode vibration. And this calculation method can explain the phenomenon on a container ship in which the corresponding horizontal and torsional vibration modes have different number of nodes.

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As a continuation of the previous work, the study on wave-induced vibrations (springing) has been presented in this report. Model experiments in irregular waves have been carried out. Full scale experiments of springing on board large tanker have been performed. Using these results, characteristics of springing have been examined. Expressions for combined stress are shown. Short-term and long-term distributions of springing stress are also presented. It is shown that the combined stress is less than the sum of wave bending and springing stresses. The extreme value for 20 years of springing stress of 237 000 dwt tanker is estimated about 4 kg/mm².

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The paper presents a method of dynamic analysis of the three dimensional beam structure containing rigid bodies and mass particles, partialy immersed in water.
The governing equation is introduced into a stiffness matrix form by combining the solutions of four kinds of partial differential equations on vibration of beam, that is, those of two directions of bending, torsion and thrust.
The orthogonallity of the general mode function composed of the four kinds of shape functions containing rigid bodies and mass particles is proved, in order to analyse the dynamic response of the structure by using the mode method.
Assuming a fixed offshore structure as a structure constructed by a flat rigid deck with four legs, the theoretical results of the eigenvalues and dynamic stress distributions fairly agree with experimental ones.

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An application of Bayes theorem to the estimation of the probability distribution function of fatigue crack initiation life was discussed. Digital simulations and (simplified) experiments with multi-circular-hole specimen showed that this method could reduce the time needed for the experiments.
Though a Weibull distribution with no minimum life was assumed in this study, the general procedure is applicable to the any other possible distribution. The fact that it is not necessary to decide the time to the fatigue crack initiation but to identify the cracked locations after a certain number of load cycles made the experiment much easier.

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Surface defects in structural members are apt to be origins of fatigue crack growth, which may cause serious failure of whole structures. In real structures, defects are often found rather in multiple clustered configuration at some geometrically discontinuous parts of the structures, such as welded joints, corner parts, etc. Analysis on crack coalescence and growth from these multiple defects, therefore is one of the most important subjects for the safe-life estimation of structures.
The authors have performed an experimental analysis on fatigue crack propagation from two colinear surface defects. Mutual interaction effects of cracks are discussed using a simplified formula on crack propagation rate, together with a trial analysis on two-dimensional cracks. Changes in crack shape during propagation are well characterized by analogy with a single surface crack model. A set of formulas is presented on the prediction of crack length and life at the point of crack penetration of plate thickness.

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A set of load controlled fatigue tests of smooth and notched specimens was conducted in intermediate and high cycle fatigue ranges in order to re-evaluate the applicability of a predicting formula of fatigue strength reduction factor of crack initiation basis, that was proposed in a previous paper.
Materials tested were electro-gas welded metal of a mild steel and base metals of 50kg/mm² class and 70kg/mm² class high strength steels. Notch geometries investigated ranged over elastic stress concentration factors of approximately 2.2 to 8.6. An FEM analysis of cyclic elasto-plastic strains distributed in the notched net section was also made and discussed in comparison with experimental results measured by strain gauges during a fatigue test.
Results indicated that the fatigue strength reduction factor of crack initiation basis in high cycle range will become approximately 0.8 to 1.0 times as great as the elastic strees concentration factor of a notch. A formula presented in this paper will be available in estimating the fatigue strength reduction factor of crack initiation basis for any stress ratio. In case where the notch root may be subjected to cyclic yielding, the fatigue strength reduction factor is predicted by the proposed formula by applying the fatigue strength of smooth material for the stress ratio of -1. The application of the fatigue strength of smooth material for the stress ratio of zero will provide conservative estimation of the fatigue strength reduction factor.

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In fracture safe design of welded steel structure, it is one of the way to prevent the unstable brittle fracture beyond general yield, or under the case in which overall strain of welded member at fracture becomes beyond yield strain of material used. Generally, stable fibrous crack growth from the tip of pre-existing crack proceeds before unstable brittle fracture. Investigation has been carried out on the mechanical behaviors of stable fibrous crack to unstable fracture transition.
In the present paper, attention is focussed on the experimental study on the effects of various factors such as crack size, mode of loading and pre-straining on transition behaviors, and to the consideration of property of COD-criterion as the criterion which unstable brittle -fracture with stable fibrous crack starts to grow.
Main results are summarized as follows :
(1) In the unstable brittle fracture with stable fibrous crack, the length of stable fibrous crack at brittle fracture, l_F, is independent on the various factors such as crack size, mode of loading and pre-straining under a certain temperature. However, the relationship of fibrous crack length vs. crack opening displacement at initial crack tip is different between -tensile straining test and bending test.
(2) The COD-criterion is applicable to the criterion of starts of unstable brittle fracture in the engineering sense if using the value obtained by tensile test as the critical COD value.

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For better understanding of pressure vessel performance, it is necessary to clarify the effects of out-of-plane bending stress component and biaxial stresses on brittle fracture initiation. The authors carried out the burst tests of conical pressure vessel, the anticipated fracturing portion of which was subjected to biaxial stresses including an out-of-plane bending component. Every test vessel has a knuckled weld joint either convex or concave. A surface notch was provided circumferentially at the knuckled joint, in order that the tensile stress parallel to notch was not less than the perpendicular stress. Three-point bend tests and surface notched tension tests were also carried out and the results were compared with those of burst tests.
The effect of parallel component of stresses on brittle fracture initiation was not observed by data analysis. So, it may be concluded that the mechanism of brittle fracture initiation established for uniaxial stress state is always applicable. As to the small scale testing, it must be noted that a sufficient acuity of notch or plastic constraint is required for the safe-side estimation of service performance.

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It is known that pre-compressed bend specimen exhibits lower fracture toughness than that without pre-compression. In order to apply pre-compressed bend specimen for fracture toughness testing, fracture characteristics of such specimen have been studied. From this study, it is found that a crack is initiated at the tip of the notch and extends a short length when a certain amount of compression bending load has been applied and unloaded at a temperature exhibiting only small scale yielding. The crack front configuration is almost straight except the portions adjacent to the specimen surfaces. The applicability of compression induced crack to fracture toughness testing is verified by conducting a series of four-point bend fracture toughness tests for specimens with fatigue crack and campression induced crack. Simply prepared one described in the following is found to give practically the same result as compared with specimen with fatigue crack. The proposed notch tip preparation is ; firstly apply compression bening load to initiate and extend a crack during unloading stage, then apply tension bending load to extend the crack furthermore at liquified nitrogen gas temperature (-196°C).
The applicabilities of machined notch and fatigue cracks which are extended under large K_f (max) to fracture toughness testing have been also investigated experimentally and it is shown that such notch and cracks application is not so suitable.

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Within the limit of relatively short arrested crack, the static approximation appling linear fracture mechanics has yielded basis for theoretical investigation and practical application of currently used brittle fracture propagation and arrest test. But later experimental investigation using very wide specimens have revealed that the simple interpretation based on the arrest toughness concept fails to have a consistency with the case of long arrested crack.
It is considered that the inadequacy of the arrest theories comes from the fact that dynamics aspects associated with fast crack propagation have been disregarded.
In search of a more relevant theory and to see how the neglection of dynamic aspects affects the interpretation of unstable crack propagation arrest test and philosophy of crack arrest design, dynamic fracture mechanics analysis was made with the use of the finite-difference method to solve the equation of motion for the two dimensional elastic problem.
This report gives preliminary results of some numerical examples showing applicability of the numerical method as a useful tool to analyze experimental observations in terms of dynamic fracture mechanics with a focus on energetics aspect of the crack propagation and arrest processes.

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In general, most of the butt welded joints in ship hull structures are not removed of its weld reinforcements, so called as welded.
Even if the weld joint contains a certain defect, this joint should be rather assessed passable from the design basis when the failure occurs at the weld toe regardless of the included flaws.
According to the above mentioned consideration and to the basis of fatigue strength, the authors propose the method to determine the allowable limits of weld defects. Making use of this technique, some trial examinations are conducted as to two types of faults, blow holes and slag inclusions.

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The empirical formula are established to predict the size and fusion penetration of deep fillet welds on the assumption that a cross sectional form of fillet weld is a semi-ellipse. The data of experimental results are obtained from positioned one pass welding of mild steel Tee-joints with automatic submerged arc welding.
The size of a fillet weld, that is throat thickness or leg length, is determined as a function of welding current, welding speed, and deposition rate of filler metal. Where, the deposition rate is expressed as a function of electrode diameter, extention of electrode, arc voltage, welding current, and a material constant defined as the coefficient of deposition rate.
The depth of fusion penetration is determined as a function of leg length and theoretical fusion penetration. Where, the theoretical penetration is derived from the moving point heat source theory and expressed as a function of the deposition rate, welding current, welding speed, and a material constant defined as the coefficient of fusion penetration.
Appropriate quantitative correlation of the welding parameters stated above are investigated experimentally in order to hold sound profiles of fillet welds, and the empirical formulae to control proper welding condition are also established.
The utility of all empirical formulae are confirmed by practical welding tests.

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The effect of fillet size and fusion penetration on statical strength of end fillet welded joint is experimentally investigated. Tensile tests are conducted using idealized model specimens of the joints made of a mild steel plate to prevent the metallurgical effect of welding.
An empirical formula of breaking load per unit weld length is established as a function of leg length, effective fusion penetration, and strength of filler metal. The utility of the formula is confirmed by tensile tests of actual fillet welded joints.
Reduction of the leg length due to the effective fusion penetration is also investigated in accordance with the empirical formula stated above, and the formula of reduction rate is developed as a function of the ratio of effective penetration to leg length.

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