Page 422 - Marine Structural Design

398 Part III Fatigue and Fracture

21.7 Fracture Mechanics Applied in Aerospace, Power Generation Industries
Fracture control in the aerospace industry is based on the fracture mechanics analysis of the
growth of assumed preexisting cracks of a size related to inspection detection capabilities
(Harris, 1997). For space structures, the NASA (1988) requirements are applied to all pay
loads in space shuttle, as well as life/mission-control items in space applications, such as space
station. A fracture mechanics analysis of the component is conducted using an initial flaw size
that is referred to as the nondestructive examination (NDE) size. Smaller size can be assumed
in the analysis if a better detection capability can be demonstrated for the particular
examination method applied. Median material properties are used in the crack growth
calculations. Commercial software is available to calculate crack-growth based on fracture
mechanics. The requirement is that the flaw size should demonstrate to survive four lifetimes.
Fracture mechanics has been applied to aircraft structures because of the high-required
reliability and severe weight penalties for overly conservative design. Probabilistic methods
have been applied to deal with the randomness of initial flaws and load spectra. Provan (1987)
described the military aircraft approach known as "damage tolerance" and "fail safe", see Part
III Section 22.4. The purpose of a damage tolerance analysis is to ensure structural safety
throughout the life of a structure. The analysis evaluates the effects of accidental damage that
might occur during the service life and verify that the structure can withstand this damage until
the next inspection or until the current mission is completed with a safety factors of two.
Harris (1997) also reviewed applications of fracture mechanics in the electric power generation
industry, such as nuclear pressure vessels, steam turbine rotors, and the like. The requirement
for extreme reliability and the prohibitive cost of full-scale testing (as used in the aircraft
industry) led to extensive use of fracture mechanics to predict behavior of defected
components. The ASME (1989) Boiler and Pressure Vessel Code Section XI was developed
for in-service nondestructive inspection intended to detect cracks before they grow to lead a
failure. The code defined locations to be inspected, procedures to be used, and procedures for
analyzing its future behavior if a crack is found. As the codes used in airspace and aircraft
industries, the ASME code also gives procedures for defining initial crack size, material
(fatigue crack-growth) properties, and stress intensity factors to be used in the fracture
mechanics analysis. Tables of crack size are also given to define the crack sizes that need not
be further analyzed if the detected size is smaller. Cracks larger than these tabulated values can
still be left in service if a more detailed analysis shows them not to grow beyond a specified
fraction of the critical crack size in the remaining desired lifetime. The ASME (1991, 1992,
1994) provides guidelines for risk-based inspection of the most risk-prone locations, and
consequently provide a greater risk reduction for given number of inspections or the same risk
reduction for fewer inspections.
The probabilistic fracture mechanics developed in these industries have been applied and
further developed by the shipping, bridge and oiVgas industries for the design and operation of
marine structures. In particular, the defect control criteria for pipeline installation, the
damagddefect tolerance criteria and inspection planning methods applied in operation of
tubular joints and pipelines have been benefiting the research efforts of the airspace and
aircraft industries.
Fracture mechanics also plays a major role in the analysis nd control of failure in the chemical
and petroleum industries, where the "fitness-for-service" is employed.