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Case Studies -
Small Scale Fracture Toughness Specimens for Structural Integrity
Assessment
 In
many industrial situations ageing equipment must be assessed to
demonstrate its continued integrity. Examples include onshore and
offshore process plant. This assessment is made difficult, in many
cases, by a lack of adequate data.
Over a period of time many structures develop crack-like defects
and an engineering-critical assessment may be needed to ensure the
continued safety of the cracked structure. To do this, data are
required relating to the applied stresses, both static and cyclic;
the size and nature of cracks; and the behaviour of the material
in the presence of cracks. The last of these is defined by the material's
fracture toughness and can be measured using standard laboratory
specimens.
For many existing structures, fracture toughness data do not exist
and cutting out large sections of material from an existing structure
to obtain fracture toughness specimens would itself reduce the integrity
of the structure. It is possible to extract small amounts of material
from, say, a vessel wall without compromising its integrity. However,
the fracture toughness specimens that are manufactured from such
a sample are very much smaller than those allowed by standard test
methodologies.
In a project funded jointly by HSE and industry, HSL explored the
possibility of obtaining useful data from specimens as small as
5mm by 5mm in cross section. The analytical technique used is based
on obtaining a reference temperature which characterises the fracture
toughness of materials undergoing a ductile to brittle transition.
Experimental results were used to define a master curve which described
the shape and position of the ductile to brittle transition. Fracture
toughness of the large, conventional specimens could thus be predicted
from the small ones.
The results of this work, on more than 350 specimens, have clearly
demonstrated the validity of the approach to homogeneous material.
The findings have been widely publicised in the UK and Europe. Further
research is currently underway to assess the method on more structurally
relevant and statistically variable weld materials.
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