A method is described for measuring the wall shear stress on the hull of a ship model using a three-hole Preston tube. Integrating along the direction of the wall shear stress, the limiting stream lines are obtained.
The velocity distributions in the boundary layer are also measured using the three-hole Pitot tube. Some of the cross flow profiles measured at the after stations are of the type of the so-called reverse cross flow.
Further, the author proposes a new prediction method for the development of the three-dimensional turbulent boundary layer on a ship hull on the basis of assumption of the reverse cross flow profile and the employment of the crosswise moment of momentum integral as an additional auxiliary equation. The results of the calculation agree fairly well with the measured.

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The paper deals with a method to measure and analyse hull surface pressure and with interpretation of the results for application to hull form design.
A new and simple method to measure pressure in the proximity of free surface is introduced at.first, and then, integration of the measured pressure is introduced to examine a pressure drag. This new method of integration does not require any conventional correction term due to the sinkage and trim of a ship.
Further, some application examples of the above methods for studying the following data are introduced.
Wave profiles and pressure distribution on hull surface in breaking wave at the bow
Pressure distribution over the hull surface in the fore and aft body
Skin friction resistance derived by deducting the pressure resistance from the total resistance
Viscous pressure resistance derived by deducting the wave pattern resistance (obtained througth the wave analysis) from the pressure resistance
Pressure components of thrust deduction obtained from the pressures measured with and without a propeller
The measurements for the above application were conducted at the Towing Tank of IHI (Ishikawajima-Harima Heavy Industries Co., Ltd.).

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Side wall effects of towing tank on the responses of a ship in regular head waves are examined theoretically.
Ship motions in regular head waves are assumed to be solved by 0. S. M. and the waves reflected by the tank wall are assumed to act on ship's surface as the secondary exciting force and moment. The velocity potential of the reflected waves initially generated by the ship motions are obtained 3-dimensionally by introducing the thin ship assumption.
Some examples of numerically calculated results are shown and the quantitative characteristics of the side wall effects on ship motions are explained clearly.

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In this paper, the ship's course keeping motion is represented by an Auto Regressive model (A. R. model) in the time series analysis and it is shown that the Akaike's FPE method is effective to the determination of the order of the model. This method is immediately applied to the estimation of spectra of some ship's motions and we designed one optimal controller using the Dynamic Programming.
According to our digital simulation, our controller shows some remarkable characteristics and in particular, through a smoother and a lower rudder motion than the manual or the present auto-pilot steering, ship's motions (especially Yawing and Rolling) are considerably reduced. This last point is expected to contribute to the development of the Direct Digital Control.

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Considering a three dimensional disk with a circular or ringed or rectangular waterplane as a ship of shallow draft, hydrodynamic forces in shallow water in heaving mode are investigated.
The boundary value problem is formulated by the use of the source technique so that integral equations for the source densities are obtained. Series expansion has been applied as the Green's function.
In this paper, two kinds of numerical schemes are investigated, one restricted to axi-symmetric bodies such as a circular disk and a ringed plate and another as general source technique applied to a rectangular plate. The numerical calculations of added mass and damping coefficient by both methods give fairly good agreement with experimental results by forced oscillation technique. The authors also show the negative added mass and the wave free frequency for heaving ringed plate.

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To study the longitudinal motion of the circular type submarine, the hydrodynamic coefficients of the equation motion of the 16series models of that type of ship were obtained experimentally by the forced oscillation method in a towing tank..
And as a first step, the relationships between the ship forms and the inherent stability of the longitudinal motion at fully submerged condition were clarified by use of the concept of the classical control theory.

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To get better understanding of the interference between surface waves and a frictional wake of a ship, a simple mathematical model is introduced. The wake is assumed as a current of uniform width and uniform velocity in the surrounding still water. Two infinite vortex sheets are employed in constructing the uniform current. Numerical studies show that the incident waves are considerably attenuated through the process of reflection and transmission across the uniform current. Experimental studies on a ship model of roughened surface show an encouraging result to verify the present theoretical approach.

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The authors showed a method of calculating the hydrodynamic characteristics of the Axisymetrical Nozzle Propeller (hereinafter to abreviated to A. N. P.) in an Axisymetrical Flow (A. F.) and made calculations about the open characteristics and the velocity field in a Uniform Flow (U. F.). From some comparison with experimental results, they showed that the method is considerably effective for grasping open characteristics of the Nozzle Propeller (N. P.).
Until now, the number of actual ships installed with N. P. increased and so the comparison with N. P. ships and C. P. (Conventional Propeller) ships has been made on both model and actual ships. From these results it has been made clearer that the difference of characteristics of open and behind condition in case of N. P. is larger than that in case of C. P.. The factors may be as follows; At first, in case of C. P., thrust is distsibuted over the propeller disc and so the performance is mainly decided by the mean flow velocity. On the other hand, in case of N. P., thrust is divided into two parts due to the impeller and the ring and the performance of nozzle is strongly affected by the local wake distribution in the vicinity of the nozzle. As the second reason, suction effect of N. P. being stronger than that of C.P., N.P. characteristics, especially Nozzle Thrust is affected by the deformation of nominal wake. From the abovementioned points of view, it is very important to investigate about N. P. characteristics in each step of Uniform, Axisymetrical and Non-Axisymetrical Flow, taking some consideration about the deformation of nominal wake.
In this second report, they showed a method of calculation of a Non-Axisymetical Nozzle Propeller (N. A. N. P.) in a Non-Axisymetical Flow (N. A. F.) and made som study about flow field and pressure around N.P. in U.F., A.F. and N.A.F., using the calculating method based on both the 1st and the 2 nd reports and experimental method.

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With larger and higher-powered ships, “Wide Bladed Propeller” has recently come into use along with multiple shafts as a countermeasure to cavitation erosion due to increased propeller loads. As a result, shaft alignments including propeller shaft bearings in bossings also have become a important design subject for marine engineers.
In order to evaluate the propeller bearing forces, a computer program has been developed at the Ship Research Institute of Japan using non-steady lifitng-surface propeller theory. Although the results computed by the program showed good agreement with measured model propeller forces, they have not been verified for operating full-scale propellers.
It is hardly possible to measure the propeller bearing forces directly. By virtue of the above mentioned program, however, the bearing forces can be checked by measuring the bending stresses on a full-scale propeller blade according to the process shown in Fig. 1.
In this report the statical stress analysis for the propeller blade of a twin screw container ship of 80, 000 SHP, supposed to be operating in a circumferentially mean wake, is presented along with stress measurements obtained on a 114 scale propeller.
In the second report, these data are to be compared with the actual fluctuating blade stresses measured on full scale.

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The fluctuating pressures were measured on a flat plate above the cavitating propeller and the behavior of unsteady cavity was observed in detail at the same time, in order to make clear what is essential to estimating the pressure signals in full scale from model experiments.
As a result, the shape of pressure signals became very close to that in full scale when the pressure impulses induced by collapse of unsteady cavity were superposed on the fluctuating pressure. Accordingly, collapse of unsteady cavity should be taken into account in model experiments in order to simulate the fluctuating pressure in full scale. Further studies are necessary on simulation of collapse of unsteady cavity or simulation of flow field in full scale.

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The supposed wave condition has considerable effects on the theoretically estimated extreme value of wave induced random variable of a ship. Author showed that the severity of a given wave condition is evaluated by the maximum wave height and its frequency of occurrence in each wave period interval, in the previous paper¹⁾ . In this paper, applying the evaluation method, author clarifies the severity of wave condition in winter which is considered as the roughest one in all seasonal wave conditions.
It is found out by observation of wave statistics in the North Atlantic by Walden³⁾ , those by Hogben and Lumb⁴⁾ (sea area 1, 2, 6 and 7), and those in the North Pacific by 80 th Committee of the Japan Ship Research Association⁵⁾ (sea zone 3), that at longer wave period intervals the highest wave in year is observed in winter in almost cases but at shorter wave period intervals the one is observed in other season than winter in some cases. Accordingly, though the extrem value of the variable estimated by supposing wave condition in winter instead of that in all seasons is larger at most about 10% than the one in all seasons generally, sometimes the value is less than the latter.
The population of the variable in all seasons is total sum of the population of it in each season. As it is expected that the population in winter includes the extreme value in all seasons which is to be regarded as the exact value for a ship, the value in winter is compared with that in all seasons. The relation between these two values varies according to the wave period T_max at which the maximum standard deviation of short-term distribution of the variable is observed. In the case where T_max is over 5 sec the former is nearly equal to the latter in most cases, but sometimes the former is less than the latter considerably where T_max is from 5 to 9 sec.
Similar relation must be found between the extreme value predicted by the result of long-term full scale measuement of the variable in winter and the exact value. The effect of sample size is also to be considered in this case, because the period of full scale test is usually shorter than the observation period of such wave statistics described above. It is concluded by my study that the value may be nearly equal to the exact value where T_max is from 7 to 13 sec and may be considerably less than the exact one in other range of T_max. This means that the correct extreme value of wave bending moment may be predicted by full scale measurement where length of ship is from 120 to 400 m (cargo ship) or from 100 to 320 m (tanker), but to predict the value by this method may be under estimation where ship length is out of the ranges.
The method of seasonal wave observation to get more correct data for estimation of the extreme value than the wave statistics in winter is studied. It is made clear by analysis of above wave data, that the value of the variable in winter and autumn in the North Atlantic or in winter and spring in the North Pacific agrees with that in all seasons accurately even if T_max is in the range from 5 to 9 sec. This is an useful guide of wave observation when continuous measurement is not allowed.

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When structures are constructed by welding, structural members are usually accompanied by welding distortions. In this paper, the ultimate strength of transverse girders in ship structures with initial deflections under bending are investigated.
First, the entire behavior, elastic-plastic large deflection, of these girders are theoretically analysed by the strip element method. A series of experiments are carried out on model girders subjected to bending moment with respect to their strong axes. Comparison is made between the test results and theoretical one and good agreement is observed between them. By these studies, a great deal of valuable information is obtained on the ultimate strength of the girders.
An approximate model is also proposed to exhibit the influence of initial deflection on the ultimate strength of girders. It is found that the proposed approximate method of analysis is very useful for prediction of the ultimate strength.

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The function of corner bracket is considered to reduce the stress concentration at the corner of the main members, to increase the rigidity of the conjunction, etc. In the case where the size of the bracket is comparative to the main members, its contribution to the strength can be expected to a certain extent. In this paper, the buckling, and ultimate strength and the rigidity of triangular corner bracket is theoretically and experimentally investigated with consideration of the effects of welding residual stresses and deflection, and reinforcement by stiffener.
For the theoretical analysis, such as elastic-plastic large deflection analysis, thermal elastic-plastic analysis, etc., the methods which were developed by the authors with the aid of the finite element method are applied. Initial deflection of the bracket reduecs the rigidity and ultimate strength as observed in the case of square plates under uniaxial compression. Welding residual stresses in the bracket are not selfbalanced due to the existence of the main members, and are mostly tension. This increases the strength of the bracket. Stiffener provided along the free edge also increases the strength approximately to the yield strength.

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The greater part of curved ship-hull is consisted of assembly of thin shallow shells reinforced with stiffeners.
Static strength of shells and stiffened shells is analyzed by means of the finite strip method¹⁾ proposed by author in addition to the Fourier's analysis and the effects of stiffeners fitted for the purpose of stress reduction on shell plate are clarified. Therewith, the practical formulas for determining optimum stiffener for given shell are obtained.
As results of investigation on stiffened shells, it is found that the less stiffeness of shell itself, the more effective of stiffener for strength of shell structure for the case of fitting a few stiffners.

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The purpose of this work is to examine in some detail the feasibility of a collision protection method which utilises square panels, placed between the hull plating and an inner shell. The method would appear to be applicable to protective structures against a minor collision, as an alternative to deck structures which are used for protection of nuclear powered ships against a major collision.
The experiments were carried out on double-hull models penetrated statically by a rigid bow model, in order to make a reasonable estimation of the amount of energy absorbed in the hull plating which is neglected in the design procedure due to Minorsky. Two typical humps are seen quite clearly in the load-penetration curves ; the first one is mainly due to the membrane force in the hull plating and the second due to the ultimate strength of vertical girders. The humps have an important influence on the capacity for absorbing the energy in a collision. Considerable amount of energy was found to be absorbed by the hull plating in a minor collision.

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Wave forces exciting springing of ships are investigated theoretically and experimentally in relation to ship-side waves herein. Ship-side waves around a ship with zero forward velocity in regular waves are calculated by applying the finite element method extended on the basis of Chen and Mei's theory for long waves, assuming that the effect of ship's bottoms can be disregarded in case of springing. Ship-side waves thus calculated become very large near the bow part compared with the incident wave, and go along the ship with the same velocity changing the amplitude which can be regarded as a point function.
With the use of the ship-side wave pattern obtained, the bottom pressure of ship is calculated on the basis of the strip theory, and the results are compared with the experments performed with a wooden model of rectangular cross sections. In the frame work of the linearized theory, only resonace of the first order appears, and resonance of the higher order is presumably caused by the nonlinearity of water pressure caused at the bow parts. This conclusion is completely different from Kurnai's, which is based on the frequency modulated ship-side waves.

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The technical method of analysis on the strength of ship structure has been improved and become more precise by the recent development of the electronic computer and the technique of its usage.
As an example of such analysis, the statistical approach to the fluctuating stresses, caused by 'dynamical loads on the structural members of ships navigating in irregular waves, has been employed by us. Such approach is very important in examining fatigue strength, ultimate strength, reliability on strength, etc. of the structure of ships.
In these circumstances we had reported a paper of “Total System of Analysis on the Longitudinal Strength of Ships” and in this paper we present the outline of the system for analyzing transverse strength of ships and the calculated results by this system on a large tanker.

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The authors present the results of statistical predictions on the horizontal wave shearing force for a gigantic oil tanker of 310 meters length and similar ships of different lengths.
In the first place, the response functions of horizontal wave shearing force are calculated by the aid of the modified strip theory. Then, the standard deviations of horizontal wave shearing force are evaluated based upon the linear superposition theory, by using the caluculated response functions and the formulated wave spectra for given sea conditions. Finally, the longterm predictions of horizontal wave shearing force are determined by the obtained standard deviations and the wave statistics of the North Atlantic Ocean.
According to the systematic prediction analysis, the longitudinal distributions of horizontal wave shearing force, which are predicted with the exceeding probability of 10^-6, are investigated in relation to the ship speed, heading angle and ship length.

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The recent outstanding development of computer, accompanied with such powerfull methods of analysis as FEM, has yielded the design concept “design by analysis” where the structures are not designed by “rules”, but almost entirely by “analysis”. In the application of the design based on the concept, the designers are required to analyse the each physical quantities as the stochastic valuables, and to prove the safety of the objective structures statistically.
Then it is the problems how to prove the rationality of the analysed values or, if the analysis have some difficulties how to correct those, because the stochastic valuables are too difficult, though not impossible, to be simulated artificially. And it can be said that the experiments under the natural conditions are the most reliable way to solve those problems.
In the sense above, Hitachi Zosen have built up the 1, 000m³ experimental LNG ship, where two types of tanks prismatic independent tank and spherical tank, were equiped. The several kinds of quantities such as ship motion, pressure and stress in the tanks and so on, were measured under the conditions where the ship ran on the actual sea. Those measured values were compiled to obtain the statistical distributions or the various kinds of the statistical quantities. And the method of analysis of the safety assessment which are being developed by Hitachi Zosen were checked by those experimental values and the problems which requires further research were pointed out.
In this report, as the 2nd report, the results of the experiments as to the pressure and stress response to the ship motion, and the several kinds of the statistical distribution of stress are indicated and are investigated by the analysis.

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It is more than 10 years since various computerizing works have been applied in the field of curved shell plates processing.
In this period, many kinds of these programs have been developed, and the become to one of the most effective tools of mold loft works. However most of these programs were developed separately, so it can be said that there should be some problems in inter communication of data between each programs.
In order to solve above problems, the improved integrated system for curved shell plate “SHELL” (SHell Expansion and Logical Layout system) has been newly developed and applied to actual ships. The system is featured by many characteristics as follows ;
1) The offset data such as frame lines, water lines, longitudinals, seams and butts are to be organized by the most suitable co-ordinate which corresponds to the jig at the plate assembly stage. Above process should bring the better result in accuracy of the calculation of the shell plate developments, roll sets for the plate bending, jig tables and final marking plans for the assembly stage.
2) The theory of fabrication of double curved plate such as ships shell is so complicated that not only geometrical calculation but also experimental considerations have taken in account. Then the means of accumulation of feed back data from field should be effective in the application of next sister ship.
3) Many kinds of outputs can be utilized with various usages, such as steel material size list for purchase order, negative film for photo-project marking, punched tape for numerical controlled cutting machine and so on, in accordance with yard facilities.

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Utilization of a numerically controlled machine for marking and cutting steel plates involves the necessity of adopting some system of “nesting”, that is, of geometrically arranging the various pieces to be cut out from a given material plate, and to present the resulting arrangement in a form suitable as input data for manipulating the particular NC machine to be used.
Two kinds of nesting system are in current use- (a) by card and (b) by graphic display apparatus. With the card system, the designer works out the nesting plan manually, using cutout card patterns. that he arranges by eye on a framed sheet representing the edges of the material plate. The resulting arrangement is then translated into digital input data by visually reading out the coordinates that represent the patterns in their nested positions. The output from the machine is produced by drafter or plotter in visible form for purposes of checking, as well as in the form of taped digital data for use in the NC machine. The graphic system, on the other hand, utilizes an optical screen for both input and graphic display. Both systems have their merits and shortcomings.
It is an established fact that digital computers are not suited to treating analog information. The human eye is far more adapted to this kind of operation. On the other hand, these computers possess a large capability for calculation, once the information has been digitalized.
The basic principle underlying the CADS NESTING SYSTEM is to divide the functions between man and machine in such manner that each performs the operations to which each is best suited, i. e., the designer entrusted with the decisions involving patterns and pattern arrangement, and the machine with the calculations required to evalutate the arrangement worked out by the designer. In line with this principle, the system has been devised to permit the designer to “converse” with the machine by means of a variety of “commands” each embodying a simple instruction for the computer to perform a certain function.

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The detail pipe fitting design method is presented here utilizing versatile CADS which is the man-machine system conversational with mini-computer and large table of analyzing and drafting. By this method pipe fitting detail plan can be designed. So far computerization in the outfitting arrangement design field has been fallen behind.
The characteristics of this system are as follows :
1) Input Data can be generated by this system for the production design system, such as pipe piece drawing, pipe fabrication schedules, material lists etc., and all these data are connected to the large scale computer system in form of magnetic tape.
2) This system is connected to the automatic piping arrangement design system (PPAC-IHI Program) to design pipe outline arrangement plan by the use of large scale computer, and also generated the data for practising pipe design process from outline pipe arranging to detail pipe fitting and piece. drawings.
3) Several arbitrary ship's sections can be projected at on time on the wide table of CADS.
4) The ship's design data can be generated from this system by in-putting minimal data, using the master data files.

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It was found that ductility of steel plate under statical bending was affected by shape of specimen, especially ratio of width to thickness of plate by our experimental investigation and a new method for statical bending test of steel plate was proposed¹⁾.
Generally, stiffend panels in ship structure has large aspect ratio and many cracks were found in plane strain state of panels under bending loads.
Therfore, in this report fatigue tests of steel plates of various width to thickness ratio were carried out by Schenk type fatigue machine and the effect of shape of the plates on the strength was discussed.
From these test results, the follwing conclusions were obtained.
1) The constraint to deformation of transvers section of plate increases with increase of width to thickness ratio. Strain distribution approaches to pure plane strain state and fatigue life decreased with increase of the ratio.
2) Ductility of plate under statical bending is affected mainly by strain conditions on surface of the plate and secondarily by the strain gradient in cross section of the plate.
3) Fatigue strength of plate is much more affected by the ratio than in case of statical test and therfore shape of the speciemen should be decided in considaration of the above results.

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This paper presents a finite element method to simulate two-dimensional crack growth based on the analysis of stress intensity factor. Two criteria are available to determine the direction of crack growth path, maximum stress criterion and strain energy density criterion. In the present calculation, maximum stress criterion was used, as it is reported that difference between two criteria is practically small in a material with Poisson's ratio near 0.3 such as steels. A good coincidence was obtained between numerical calculation and fatigue crack growth experiments on some examples such as inclined slit problems and two-cracks interaction.

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Large scale model tests have been now increasingly attempted for the study on fatigue strength of welded structures. Main advantages of the use of large scale model are the possibility to confirm directly the fatigue behaviour of total structure.
The authors carried out a series of full scale model fatigue tests on a slotted part of ship hull structure. Much efforts are exerted to detect the crack initiation and to pursue the crack propagation during the test. An approximate analysis of crack growth was attempted by a fracture mechanics approach. It is noted that the crack growth behaviours are much influenced by various factors such as defects or distortion of structure induced by the fabrication, redistribution of stress among adjoining structural members, etc. Empirical formulas are obtained to estimate crack initiation life and failure life of total structure.

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Fatigue crack growth from surface planar defects is one of the most important subjects for the evaluation and the assurance of safety in pressure vessels, piping systems, ship hulls and other various structures. The authors reported previously a preliminary analysis on fatigue crack growth from a single surface flaw⁷⁾ . The present study attempts to apply the same approach to some practical or general problems such as the estimation of crack growth life to penetrate the plate thickness, analysis on the interaction of multiple cracks etc.
An experiment on the coalescence of two cracks was carried out under cyclic plane bending condition as a first attempt to the analysis of multiple crack problems. It is noted that the crack shape change is well characterized by the analogy to that in a single crack growth. A simplified formulation is proposed to characterize the interaction effect of adjoining cracks on the crack growth rate.

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In this paper, brittle fracture initiation characteristics of plate with an inclined crack which is subjected to bi-axial tensile load is investigated by using cruciform specimens made of polymethylmethacrylate (PMMA) which is perfectly brittle material, and mild steel which is accompanied with small scale yielding. The brittle fracture tests of the cruciform specimen under proportional loading ratio of 1/ 2, 1/1 and 0/1, and the non-cruciform or straight edge specimen under uni-axial loading for comparison are conducted.
The test results on direction of fracture initiation and load at fracture are analyzed by applying the following concepts : (1) maximum tangential stress, σ_{θ, max} (2) minimum strain energy density factor, S_min (3) maximum decreasing rate of total potential energy, G_{θ, max}.
The test results can be explained fairly in terms of σ_{θ, max} in which a certain ambiguity remains. The experimental values except the case of direction of fracture initiation for load ratio 1/2 agree with the calculated ones by S_min concept. However, the physical meaning of S_min is not understandable. G_{θ, max} concept is based on the Griffith-Orowan energy condition. Practically, G_{θ, max} is evaluated by finite element method instead of analytical method. Therefore, the experimental results can be explained most clearly by applying G_{θ, max} concept.
Test results lead to the following conclusions :
(1) G_{θ, max} concept is the most suitable one in discussing the brittle fracture behaviors under mixed mode condition. The mixed mode means the combination of tensile and in-plane shear ones.
(2) In case of load ratio 1/2, as the inclined angle of the notch to the direction of the main load, β, decreases, the load at fracture increases at maximum up to 2. 4 times as that for β=90°.
(3) In case of load ratio 1/1, that is, equi-axial load, the load at fracture does not vary with β, and the direction of fracture initiation coincides with the line of notch.
(4) In case of load ratio 0/1, that is, uni-axial load, the load at fracture increases as β decreases. However, the direction of fracture initiation is almost normal to the load irrespective of β.

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In large welded structures such as ships, pressure vesseles, etc., the initial imperfections including misalignments and angular distortions are inevitable in course of assembling and welding workmanship. Some of such initial imperfections are liable to produce unexpectedly high local stresses especially due to bending.
The authors have investigated experimentally as well as analytically the effect of out-of-plane bending on brittle fracture initiation from a surface flaw, using surface notched plate specimens subjected to eccntric tension specifically designed for the present purpose. As an analytical tool, the approximate elastic analysis of a surface crack developed by J. R. Rice et al. was utilized on its strain basis and the experimental results were discussed in terms of the COD concept.
This report gives a proposal on an engineering method to apply the COD concept to the critical assessment of a surface flaw under combined tension and bending on the basis of critical COD data obtained from the conventional COD testings.

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Several criteria to predict fracture initiation from cracks subjected to combined modes of loading are discussed. In conjunction with the discussion, a strip yield model for an elasto-plastic crack under combined modes of loading is proposed, which is an extension of the model for single mode of loading developed by D. S. Dugdale and B. A. Bilby et al.
Experimental work was also performed to investigate the brittle fracture initiation under combined mode I and II or mode I and III. Tension and bend tests were carried out at low temperature using mild steel and 80 kg/mm² class high tensile steel. Notches in the specimen are introduced in planes inclined to that of principal stress to give a required state of stress in the vicinity of the crack tip. The results of these tests and those by other researchers were analysed using various criteria proposed-hitherto. It was observed that the small scale yielding fracture under combined modes of loading can be predicted reasonably well using the maximum hoop stress criterion or the crack opening displacement criterion based on the proposed model. For practical application of the proposed model, further investigation is need as how to define the stress condition within the yielded zone and also the crack opening displacement.

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In the process of the fracture initiation from a precrack, plastic blunting or, metallographically, stretched zone is formed at the notch tip. Although this plastic blunting is considered to be favorable because of its effect of relaxation of stress concentration, large plastic strain to be formed around the blunted notch tip will cause strain aging and embrittlement of the notch tip. The effect of this type of strain aging on toughness of steels has been investigated.
Such test conditions are chosen as to predict the embrittlement which may occur in actual service and the typical test conditions employed are as follows : preloading to induce 3040μm stretched zone depth (which is 3040% of S. Z. D. at fibrous fracture initiation), aging temperature : 50°C, aging. time : 01000^h. Some tests on other conditions have also been made additionally.
The test results have shown that SS 41 is most susceptible to embrittlement by strain aging and increase in T_i (the lowest temperature at which fracture occurs by fibrous crack) due to strain aging is 27°C. SM 41 A is also susceptible to strain aging but has shown smaller increase in T_i (increase in T_i due to strain aging ≅17°C) than SS 41. SM 50 C has not shown any indications of embrittlement by strain aging.
COD-criterion of preloaded and aged steels has also been investigated and it is shown that COD at fibrous crack initiation, δ_i, is almost constant regardless of preloading and strain aging.

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Robertson type test method such as double tension test or ESSO test is very popular for the assessment of propagating and arresting properties of brittle fracture of steel plates with fracture mechanics approach. This is a so-called large scale test, requiring large amount of material, the test pieces being very expensive. Thus it is not suitable as a material test method.
Recently, double cantilever beam (DCB) test has been proposed as a small scale fracture mechanics test for this purpose. However, it is necessary to machine very sharp grooves on the specimen surfaces in order to guide the crack path on the symmetric axis of specimen. Because of grooves, specimens tend to be expensive, and full thickness properties of plates can not be obtained.
Therefore, it is required to develop a new test method which has following advantages ;
(1) Specimen is small and not expensive.
(2) Test method is simple and convenient.
(3) Full thickness properties can be obtained ; i. e. crack propagates straightly on the symmetric axis without any surface grooves.
(4) Test result can be evaluated with the fracture mechanics.
In this paper, triangle specimens which were subjected to three point bending load were examined. Then, it was found that full thickness properties of crack arresting can be determined with the triangle bend specimens which have a pressed notch and gradient temperature distribution.
Furthermore, using finite element method, crack paths of DCB specimen and triangle bend specimen were analyzed with a criterion of maximum strain energy release rate. As a result, it was found that the crack path of DCB specimen is unstable. Namely, in case of ideally symmetric specimen, the crack would propagate along the symmetric axis. However, if any small disturbances exist, the crack deviates from the symmetric axis. On the other hand, the crack path of triangle bend specimen is stable, the crack propagating along the axis even if small disturbances exist.

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In the welded joint having a pre-existing defect, the crack tip is subjected to thermal-strain and stress cycles by successive welding. Hot straining embrittlement and residual stress due to successive welding will influence the brittle fracture initiation of weldment.
Two-dimensional finite element method analyse are carried out for obtaining thermal stress-strain cycles during welding and distribution of residual stresses-strains near the tip of a pre-existing crack.
Appreciable difference in residual stress distribution is not observed between Before-Weld-Notch (BWN) and After-Weld-Notch (AWN) specimens. Much difference in the residual equivalent plastic strain is observed between BWN and AWN specimen. In BWN-specimen, there is a large concentration of plastic strain near crack tip during welding. This hot-straining causes considerable decrease in fracture toughness. In the welded joint of steel, the residual stress-strain distributions near crack tip is decided by the parameter T^C_max/σ_Y0, where T^C_max is the maximum heating temperature at the crack tip and σ_Y0 is the yield stress of materials at room temperature. The conventional formulae on residual stress distribution and residual equivalent plastic strain in the vicinity of a pre-existing crack are shown using the parameter T^C_max/σ_Y0.

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The uncertainties in the defect-detecting methods and the information on the probability distribution function (P. D. F.) of defects in a structure are taken into account to make use of the results of inspection.
It is shown that Bayesian approach is effective even if very few data on the probability of successful inspection and the P. D. F. of defects are given.

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