Page 78 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaVMultiaxial Fatigue and Fracture
Andrea Carpinteri, Manuel de Freitas and Andrea Spagnoli Ws.)
Q Elsevier Science Ltd. and =IS. All rights reserved. 63

MICRO-CRACK GROWTH BEHAVIOR IN WELDMENTS OF A NICKEL-BASE
SUPERALLOY UNDER BIAXIAL LOW-CYCLE FATIGUE AT HIGH
TEMPERATURE

Nobuhiro ISOBE* and Shigeo SAKURAI**
*Mechanical Engineering Research Laboratory, Hitachi Ltd.,
** Thermal & Hydroelectric Systems Division, Power & Industrial Systems, Hitachi, Ltd.,
3-1-1, Saiwai, Hitachi, Ibaraki, Japan

ABSTRACT

Tensile-torsional-combined biaxial low-cycle fatigue tests on welded tubular specimens of the
Ni-base superalloy Hastelloy-X were carried out and the micro-crack growth behavior in the
weldments was investigated with the aim of improving life assessment methods for high-
temperature components. Welded hollow cylindrical specimens of two types were prepared:
one welded in the axial direction and the other welded in the circumferential direction. Fatigue
lives of the welded specimens were about half that of the base metal. In both weld and base
metal, the initiation of micro-cracks was observed in the early stage of life, but the initiated
length of the micro-cracks in the weld metal was about 0.5 mm while the equivalent figure for
base metal was about 0.1 mm. The crack growth life from 0.5 mm to failure in the base metal
specimen almost coincided with the failure life in the welded specimen. The maximum
principal strain was confirmed to be a good parameter for evaluating crack growth rates for
both weldments and base metal. These results show that the reduction in fatigue strength is not
due to the strain concentration at the weld. The fatigue life of weldments of Hastelloy-X is
affected by the initiated lengths of micro-cracks affect.

KEYWORDS

Biaxial low-cycle fatigue, Micro-crack, Weldment, Crack growth rate, Principal strain

INTRODUCTION

In recent years, the demands placed on fossil power plants, such as for high efficiency and
frequent start-stop cycles, have harshened their service conditions. More advanced methods of
life assessment and maintenance for reliability of such power plants have thus become more
important.
A suitable way of assessing reduction in the strength of weldments is necessary in the
fatigue assessment of components, and a fatigue strength reduction factor (FSRF) is, in many
cases, introduced to achieve this. For example, ASME Boiler and Pressure Vessel Code