What is an ECA?
Most welding fabrication codes specify maximum tolerable flaw sizes and minimum tolerable Charpy energy, based on good workmanship, i.e. what can reasonably be expected within normal working practices. These requirements tend to be somewhat arbitrary, and failure to achieve them does not necessarily mean that the structure is at risk of failure. An Engineering Critical Assessment (ECA) is an analysis, based on fracture mechanics principles, of whether or not a given flaw is safe from brittle fracture, fatigue, creep or plastic collapse under specified loading conditions. An ECA can therefore be used:
- During design, to assist in the choice of welding procedure and/or inspection techniques.
- During fabrication, to assess the significance of:
- a) known defects which are unacceptable to a given fabrication code, or
- b) a failure to meet the toughness requirements of a fabrication code.
- During operation, to assess flaws found in service and to make decisions as to whether they can safely remain, or whether down-rating/repair are necessary.
The ECA concept (also termed 'fitness-for-purpose analysis') is widely accepted by a range of engineering industries. TWI has been deeply involved in the development of methods for conducting ECAs for more than 40 years, and can offer this service to its members on a consultancy basis.
What information is needed?
For an analysis of a known flaw (cases 2a and 3 above), the following information is needed (see Fig.1):
- size, position and orientation of flaw,
- stresses acting on the region containing the flaw,
- toughness and tensile properties of the region containing the flaw,
The fact that knowledge of all these three aspects is necessary implies a multidisciplinary approach, involving stress analysis, NDT expertise and materials engineering. All these disciplines are available at TWI.
The analysis is carried out in accordance with the British Standard procedure BS 7910 ('Guide to methods for assessing the acceptability of flaws in metallic structures') or any other procedure specified by the client. Although simplified analyses can be carried out based on code values of Charpy energy and maximum allowable stresses, it is usually necessary to carry out fracture-mechanics testing (critical K, CTOD or J) in order to obtain an accurate measurement of the material toughness. Additional stress analysis (eg by hand calculation or Finite Element Analysis) may also be required.
For design purposes, or for analysis of weldments which fail to meet a toughness requirement (cases 1 and 2b above), the ECA is based on a hypothetical 'reference flaw' which is highly unlikely to be missed during inspection.
Although the example cited above refers to prevention of brittle fracture, i.e. to the analysis of static flaws, an ECA can also be used to assess the significance of growing flaws, eg fatigue, creep or stress corrosion cracks, in order to make decisions on life extension and safe inspection intervals. Such analyses require additional information and are outside the scope of this document.