Kunihiko Satoh

A probabilistic analysis on thickness effect in fracture toughness

Abstract Fracture toughness values are often influenced by specimen thickness and they indicate generally decreasing toughness with increasing thickness. In the present paper, a probabilistic analysis has been carried out by using various kinds of toughness data in order to clarify the applicability of the weakest link model to thickness effect in fracture toughness. Moreover, a new statistical method is proposed for determining fracture toughness distribution, which is necessary for the above analysis, with taking the temperature dependency of fracture toughness into account. Thickness effect in fracture toughness is brought about by its statistical nature and the weakest link model can be applied to evaluate the thickness effect for both steel plate and its welds with heterogeneity in toughness. This thickness effect is considerably affected by Weibull shape parameter and the probability of cleavage fracture for the material. The statistical method proposed newly in this paper is sufficiently applicable and superior to the conventional method. By using this new method, Weibull parameters at a temperature of interest can be determined with much the same reliability as in the conventional method, and also Weibull parameters at lower and higher temperatures can be obtained with a certain confidence depending on the number of specimens tested.

PREHEAT TEMPERATURE FOR PREVENTING ROOT CRACKING IN CONTINUOUS MULTI-PASS WELDING

There are instances where employing the preheat temperature for preventing root cracking as determined from single-pass welding cracking test results for the continuous multi-pass welding errs on the side of safety.The authors devised a preheat temperature selection procedure and successfully developed a handy formula to prevent the root cracking in continuous multi-pass welding.Parameters employed in the preheat temperature selection procedure and the formula thereby obtained are summarized as follows : (1) The susceptibility to weld root cracking in continuous multi-pass welding is less than in single-pass welding, mainly because of decrease in hydrogen at the root due to post weld heating effect. This effect is expressed by thermal factor (ΣDδt) which is determined by thermal cycle.(2) Nearly the same critical thermal factor [(ΣDδt) cr] for root cracking applies to both continuous multi-pass welding and in single-pass welding. (ΣDδt) cr is given by the following weldment cracking parameter (Cw) (ΣDδt) cr=7.94×10-4Cw-0.123Cw=0.32HVmax.+30.5 log H0+F1 × σwwhere, F1 × σw is a local stress parameter for root cracking. (3) Critical cooling time to 100°C [(t100) cr] for root cracking is given as : (t100) cr=exp [1.22 {ln1/α (7.94×10-4Cw-0.123) +9.79}] α= 1 : Single-pass welding2 : Continuous multi-pass welding

Study on Mechanism of Stable Fibrous Crack Initiation and Growth Under Constant Loading

Constant loading tests for double edge-cracked tensile specimens and smooth round bar tensile specimens of mild steel have been carried out at room temperature. And according to these results and visco-elasticity theory, time-dependent deformation behavior has been revealed. Moreover, monotonic loading with extremely low loading rate of cracked tensile specimens and smooth round bar specimens have been carried out. Comparing with these results and constant loading tests, consideration has been conducted on the mechanism of stable fibrous crack growth behavior under constant loading.Under constant loading, time-dependent strain-that is creep strain-appears; and plastic strain in the vicinity of crack tip increases with time. This time-dependent strain contributes fibrous crack initiation and crack growth under constant loading. The crack growth occurs until time-dependent strain is saturated due to strain-hardening in the vicinity of crack tip, that is, fibrous crack extension under constant loading reaches that obtained at the same load level under monotonic loading with extremely low loading rate. Crack growth is arrested when time-dependent strain is saturated, but unstable fracture occurs when holding load level is higher than maximum load under monotonic loading with extremely low loading rate. Accordingly, the time-dependent strain and strain-hardening behavior plays an important role on the mechanism of stable fibrous crack growth under constant loading.

Practical Application of Locally Drying Underwater Welding for Steel Pipes

This paper deals with a new technique called “Locally Drying Underwater Welding Process”, and can be summarized as follows: The cooling characteristic of welds made by this method was analyzed and the satisfactory mechanical properties of the welds were confirmed. After those fundamental tests, an automatic welding system for this process was fabricated and by means of it, steel pipes were welded in the sea with satisfactory results. Further, the appendix shows methods developed for improving the mechanical properties of underwater welds in high tensile steel.

Fracture Toughness of Explosive Stainless-Clad HT80 High Strength Steel

Fracture toughness of explosive clad composed of SUS 304 stainless steel and HT80 high strength steel is investigated. To clarify the effect of metallurgical properties and composite structures on the fracture toughness, both V-notch Charpy test and bending COD test are carried out by using homogeneous and heterogenous specimens which are cut from various portions in thickness direction of the clad plate and have notch root or precrack leading edge parallel to the plate surface. The energy absorbed for fracturing the specimens and its component energy for crack initiation from notch root in V-notch Charpy test are affected by the material involving notch root and by the thickness ratio of two different steels in net section of specimen.In static bending test of the heterogeneous specimens with pre-crack tip in stainless steel, fracture occurs in the high strength steel adjacent to weld interface before the pre-crack extends although ductile fracture occurs at pre-crack tip over all temperature range. In relation to crack initiation, the critical COD, if the first fracture initiation in interface zone can be measured, and the stretched zone width, if fracture firstly occurs at pre-crack tip, can be a parameter of fracture toughness respectively.In the heterogeneous specimens with pre-crack tip in high strength steel, both the critical COD at low temperature where ductile fracture has not yet initiated and the stretched zone width over all tem-perature range can be fracture toughness parameters since the fracture toughness values depend largely on the material involving pre-crack tip, namely, high strength stecl.

Matching between Base Metal and Weld Metal on viewpoint of Reliability on Welded Joints on Static Tensile Strength

Mechanical properties of steels and welding rods are usually random variables, so mechanical properties of welded joints which are made using those materials are also them. In this report, it is pointed out that the reliability of welded joints on static tensile strength must be considered in discussion about matching between base metal and weld metal. The average tensile strength σWu of weld metal is dependent upon the reliability of welded joints, the form of frequency histogram of their strength and the parameter r which decides the lowest required strength of weld metal. A few calculations about the average tensile strength σWu of weld metal were done and the effects of the reliability, r-value and frequency distributions on strength were considered. On these calculation results, the problem of matching was discussed in relation to the reliability.

A Quality Assessment of Welded Joints of High Strength Steels in Relation to Brittle Fracture Initiation

From critical COD δc of HT 80 welded bonds, brittle fracture initiation temperatures at the residual stress field are calculated using COD concept. These temperatures are compared with the temperature Ty, which is calculated from Kc using the equation Kc =√ Eσ yδc· Ty indicates the temperature at which the brittle fracture initiation stress reaches to the yield stress of the material. It is clarified that the temperature Ty can be substituted approximately for the brittle fracture initiation temperature of welded joints with the residual stress of the longitudinally welded bead. The validity of Ty is certified by an example of cross-welded notch tests.This paper proposes to employ this temperature Ty for the quality assessment of welded joints of high strength steels. Ty can be obtained from bending COD tests and tensile tests at low temperature.An experimental example on HT 80 welded bonds shows a good correlation between Tyand Charpy transition temperature v Ts, consequently giving the relation between the Charpy requirement and the defect size at HT 80 welded bonds.

Thermal elasto-plastic analysis on stress and strain induced in weld metal during multi-pass welding

This paper is concerned with the elasto-plastic analysis of thermal stresses and strains in weld metal of multi-pass butt weld. Analysis is performed based on the incremental theory of thermo-elasto-plasticity under one dimensionaly distributed temperature. Numerical calculations are made for two kinds of weld joint as shown in Figs. 1(b), (c), each of which corresponds to the behavior of the transverse and longitudinal section of weld joint respectively.Assumptions are made for heat flow that a rectangular distribution of initial temperature is given at the start of each pass and heat flow occurs only in y-direction. Another initial condition is also applied to the calculations that a line heat source is given at the start of each pass as shown in Fig.9.From the calculations are estimated successfully transient thermal stress and plastic strain distributions. Effects of weld conditions on residual stress and strain are clarified. Relations between plastic strain in back surface and angular distortion under several weld conditions are also obtained.

Intensity of Restraint of Weld Joints Restrained by Shearing Force Through Fillet Welds (Part 1)

Measurements of the reaction stress and the intensity of restraint were undertaken and the results were compared with the analytical results in Part 1.Experiments were made for two types of specimens, H-type and T-type (see Figs. 14 (a) (b) ), in which bending moment by reaction forces may or may not be neglected respectively.Some analyses showed good agreement between test results and calculations (see Figs. 17 (a) (b)). It was found that the occurrence of weld joints might be estimated by comparison of the Kcr-value with the K-value. The necessary value of uncompleted fillet-weld length 2Δ to prevent weld cracking may be obtained from equation (14) for given values of joint geometry, size of fillet and welding heat input.