Page 415 - Marine Structural Design

Part I11

Fatigue and Fracture

Chapter 21 Application of Fracture Mechanics

21.1 Introduction

21.1.1 General
Applications of the fracture mechanics in marine structural design include:
Assessment of final fracture,
Determination of crack propagation to plan in-service inspection and determine remaining
life of an existing structure,
Fatigue assessment in case S-N based fatigue assessment is inappropriate,
Calibration of fatigue design S-N Curves
In this Chapter, three levels of fracture assessment are outlined, Paris equation is applied to
predict crack propagation and the comparison is made between S-N curve based fatigue
assessment and fracture mechanics-based fatigue assessment.

21.1.2 Fracture Mechanics Design Check
The Fracture Mechanics Design Check of Ultimate Limit-State can be applied in three
alternative ways. These are evaluation of:
Maximum allowable stress
Minimum required fracture toughness
Maximum tolerable defect size

Maximum Allowable Stress
The fracture mechanics strength criteria can be applied to the derivation of the maximum
allowable stress at a given cross section. This value is obtained when the material fracture
toughness and the defect size are specified. If the actual local stress exceeds the maximum
allowable stress derived through this procedure, a different local design should be undertaken
in order to reduce the local stress level and fulfill the fracture mechanics criteria.
Minimum Required Fracture Toughness
The minimum required fracture toughness should be derived through the fracture mechanics
design check when the design geometry is established and a defect tolerance parameter is
specified. The derived fracture toughness then allows designers to select a suitable material for
any particular structure of concern.