Hiroshi Shimanuki

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.

Fracture assessment of wide plate component subjected to biaxial loading

This study investigated the effect of biaxial loading on the fracture performance of a center through cracked panel and a center surface cracked panel of a structural steel of 25mm thick. The experiment revealed that the critical CTOD of the biaxially loaded specimen was smaller than that of the uniaxially loaded specimen, due to the constraint effect of the biaxial loading. This constraint effect was remarkable in the case of the through thickness notched specimen. The stress distributions near the cracks and crack tip deformation behaviors were simulated by FE-analyses, in order to discuss brittle fracture initiation conditions of the cracked wide plate specimens. All fracture initiation points were located in the large CTOD part of the crack front, as well as in the widely extended high opening stress region. However, the initiation points did not always correspond to the highest opening stress point along the crack front. These results indicated that the existence of a extensive high stress region accompanied by a large CTOD, is significant in the brittle fracture initiation condition. These results also suggested that fracture assessment based on the Weibull stress criterion is reasonable, because the Weibull stress takes the volume effect of the high stress area into account.

Fracture Assessment Procedure for Steel Structures Subjected to Large Cyclic and Dynamic Strain – Based on WES TR 2808

With a lesson leaned from the Kobe Great Earthquake, an engineering method, WES TR 2808, has been developed in Japan for evaluating the structural integrity under the seismic condition. This paper presents a main story of WES TR 2808, which is characterized by two key ideas. One is a reference temperature concept for fracture toughness evaluation. The material fracture toughness under the seismic condition is replaced by the static fracture toughness without prestrain at a reference temperature of T − ΔTPD , where T and ΔTPD are the service temperature and the temperature shift of fracture toughness by prestraining and dynamic loading, respectively. WES TR 2808 gives ΔTPD as a function of the elevation of flow stress in the seismic condition. Another idea is a correction of constraint loss in structural components. A large amount of plastic deformation causes a loss of constraint in structural components, which leads to a significant difference in the fracture driving force of the components and fracture toughness specimen. In WES TR 2808, the CTOD ratio β = δ3P.Equiv /δStruc was defined on the basis of the Weibull stress criterion, where δStruc is the CTOD of the structural component and δ3P.Equiv is an equivalent CTOD of the fracture toughness specimen at which the toughness specimen presents a compatible Weibull stress with the structural component. The CTOD ratio β is apparently less than 1 in a large-scale yielding condition, and roughly 0.4 for the wide plate component with a surface crack. It was shown that WES TR 2808 is applicable to the fracture performance assessment of column-to-beam connections subjected to large cyclic and dynamic strain. The discussion was given on the engineering judgement in WES TR 2808, which was adopted to make the procedures as simple as possible.Copyright