Studies Of Stress Concentrations And Tolerances Fdr Weld Defects In Full-Scale Tubular Joints'
Authors: Yamasaki Tokuya , Yamamoto Noboru 
Published: October 2008

Tests were conducted on Full-scale tubular joints (1) to investigate the effects of joint scale and steel strength on the stress concentration and ultimate failure characteristics of the joints and (2) to evaluate weld defect tolerances for the prevention of brittle fracture. Simple T and cross joints, made of mild steel and high strength steel, were used for the test models. They consisted of chord pipes of 1400 mm OD × 26 mm and 36 mm and brace pipes of 800 mm OD × 12 mm.

First, the T and cross joints were subjected to statis compressive and tensile loads, respectively, through the braces. Stress concentration factors and ultimate strengths obtained from the tests were compared with each other and also with previous test results upon the effects of joint scale and steel strength. Furthermore, the applicability of previously proposed design formulas was examined.

As a second step, crack opening displacement (COD) tests were performed on small specimens taken from the welded joints of the cross type models. Allowable defect sizes for the prevention of brittle fracture at the design stress level were estimated from these COD values and local plastic strains in the joints.

A method of applying fracture mechanics technique to material selection and fabrication is presented and its reliability is discussed.

INTRODUCTION

When used in offshore structural members, steel pipes exhibit greater efficiency than other materials because they have a higher strength-to-weight ratio as well as uniform flexural section properties in all directions. But since pipe-to-pipe joints are highly complex in both shape and stress propagation mechanism, they are inevitably subject to local deformation and stress concentration. In designing tubular joints, therefore, careful attention must be paid to these drawbacks.

Many studies have been conducted on the ultimate strength and stress concentration of tubular joints, and prediction formulas have been proposed on the basis of these studies. Most of these formulas, however, were derived from the results of small-scale or mildsteel model tests. This makes it necessary to verify the applicability of the existing formulas to fullscale and high-strength tubular joints. For stress concentration, in particular, more accurate evaluation methods are required because this factor greatly influences both fatigue life and brittle fracture of tubular joints.

In recent years, offshore structures have been constructed in deeper water and in severer environmental conditions -- a trend that requires the designer to give serious consideration to brittle fracture because increased section-thickness, higher material strength and lower operating temperature lead to a reduced fracture toughness of steel itself, resulting in an even greater reduction in the fracture toughness of welded joints. On the other hand, the design method of preventing brittle fracture has not been sufficiently developed, especially for locally yielding weldments such as tubular joints. Further research must be conducted to permit an appropriate material selection and to ensure the reliable acceptance levels of weld defects for avoiding brittle fracture in tubular joints.

To obtain the necessary data, the authors conducted static loading tests using full-scale tubular joints of mild and high-strength steels.