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The procedure is based on the principle that failure is deemed to occur when the
applied driving force acting to extend a crack exceeds the material's ability to resist the
extension of that crack. This material property is called the material's fracture toughness
or fracture resistance.
The procedure can be applied in the design, fabrication or operational stages.
1.1.1. Design stage
The method can be used for assessing hypothetical planar discontinuities at the design
phase in order to specify the material properties, design stresses, inspection procedures,
acceptance criteria and inspection intervals.
1.1.2. Fabrication stage
The method can be used for fitness-for-purpose assessment during the fabrication.
However, this procedure shall not be used to justify shoddy workmanship and any flaws
occurring should be considered on a case by case basis with respect to fabrication stan-
dards. If non-conforming discontinuities are detected, which cannot be accepted to the
present procedure, the normal response shall be:
(i) correcting the fault in the fabrication process causing the discontinuities, and
(ii) repairing or replacing the faulty product.
1.1.3.Operational stage
The method can be used to decide whether continued use of a structure or component
is possible and safe despite detected discontinuities or modified operational conditions. If
during in-service inspection discontinuities are found which have been induced by load
fluctuations and/or environmental effects, these effects must be considered using suitable
methods which may not be described in the present procedure. The procedure may be
used to show that it is safe to continue operation till a repair can be carried out in a
controlled manner. Further applications of the method described are the provision of a
rationale for modifying potentially harmful practices and the justification of prolonged
service life (life extension).
1.2. Approach
The approach used in this application is suitable for the assessment of structures and
components containing, or postulated to contain, flaws. The failure mechanisms conside-
red are fracture and plastic collapse, together with combinations of these failure modes.
The philosophy of the approach is that the quality of all input data is reflected in the
sophistication and accuracy of the resulting analysis. A series of levels is available, each
of increasing complexity and each being less conservative than the next lower level;
consequently 'penalties' and 'rewards' accrue from the use of poor and high quality data,
respectively. This procedural structure means that an unacceptable result at any level can
become acceptable at a higher one. The user should perform the work necessary to reach
an acceptable level and need not invest in unnecessarily complicated tests or analysis.
Due to the hierarchical structure of data and assessment levels, path selection through
the procedure is made based on the relative levels of contribution of brittle fracture and
plastic collapse in overall failure. Qualitative and quantitative guidance is provided for
the user in the direction that will yield most benefit in terms of data improvement.
The basis for this is the location of the initial analysis point in terms of brittle fracture
and plastic collapse. This can be assessed by either the Failure Assessment Diagram
(FAD) or the Cracking Driving Force (CDF) curve. The methods can be applied to