NDE, DEsign and Engineering Critical Assessments (NDECA): Bridging the gaps

The NDECA project aims to extend the applicability of fracture mechanics methods for predicting the behaviour of structures/components containing non-sharp flaws.

Many defects formed during manufacture or in service (e.g. porosity, dents or corrosion pits, weld defects, etc.) and certain design features (e.g. crevices in partial penetration welds) are not sharp i.e, have non-zero crack tip radius.
Common structural integrity assessment procedures- such as R6 [1] and BS7910 [2]- use fracture mechanics principles for the assessment of flaws that are assumed to be infinitely sharp. While this approach is appropriate for planar (2D) flaws, such as fatigue cracks, it can be excessively conservative for non-sharp defects, leading to erroneous decisions (replace/repair/re-inspect), thus reducing assets cost-effectiveness (through increasing operating costs and/or reducing service life).

Several assessment methods have been proposed in the literature to quantify the additional margins of safety of non-sharps defects compared to the margins that would be calculated if the defects were assumed to be sharp cracks. Unfortunately, the validation and application of these methods is currently limited by the lack of both credible non-destructive evaluation (NDE) techniques to distinguish between sharp and non-sharp flaws and; representative and reproducible effective fracture toughness testing procedures.

Therefore, this proposal will focus in the:
-Development and validation of novel NDE methodologies for accurate notch-tip acuity characterisation;
-Development of recommendations for future fracture mechanics-based test methods to account for the defect topology on the resistance to failure.

This multidisciplinary effort - it cuts across multiple academic fields, i.e. ultrasonics and NDT, mathematical modelling, engineering structural integrity, finite element analysis, and engineering design - will produce a step change improvement in damage tolerance methods for the next generation of design (by analysis) and structural integrity procedures of high integrity structures, allowing enhanced efficiency of assets. The project is supported by TWI, Wood, RCNDE, University of Cantabria and BP.

NDECA takes advantage of the UK's leadership and experience on the development of structural integrity assessment procedures (TWI, Wood, BP), the application of NDE methods for defect characterisation (University of Bristol Ultrasonics and NDT group, TWI and RCNDE members) and experience with FEA damage simulation and testing of non-sharp defects (Larrosa [PI], Wood, University of Cantabria).
The timeliness and critical needs for this project are reflected in requirement for more precise methods in life extension programmes for in-service nuclear power plants and Oil and Gas high integrity assets which are currently at (or close to) the end of the design life.

References
1. R6 - Revision 4, Assessment of the Integrity of Structures Containing Defects, Latest Updates: March 2015. EDF Energy, Gloucester, UK.
2. BS7910: 2013+A1:2015, incorporating Corrigenda Nos.1 and 2, 2016. Guide to Methods for Assessing the Acceptability of Flaws in MetalliMetallic Structures. British Standards Institution, London.

The major ambition of this research is to introduce a paradigm shift in fracture mechanics principles. The novelty in this proposal is the development of non-destructive evaluation (NDE) methods for accurate flaw acuity measurement and fracture mechanics testing protocols for non-sharp defects.

The mechanistic understanding gained through the research programme will facilitate the development and widespread adoption of less conservative (but safe) methods for non-sharp defects and modifications to UK fracture assessment methods (R6, BS7910) to be made, reducing the number of unplanned outages and the amount of material to be used through less repairs/replacements of in-service structures and through more efficient designs.

The project outcomes will be of benefit to a range of industrial sectors - oil and gas, nuclear, aerospace and wind energy sectors- that will make significant savings in maintenance activities and also to manufacturers in learning how their design of products could be improved. Results and new understanding obtained during the work programme will be used to influence the planned 2020 version of BS7910 through links with TWI and the new R6 section on blunt defect assessment through the membership of TWI and Wood on the R6 Panel.

The project will also have positive impacts on the environment, through reduced use of natural resources and energy; and the elimination of carbon dioxide and pollutant emissions resulting from less manufacturing processes. The new procedures will assist in reducing reliance on foreign sources of energy (nuclear and oil and gas mainly) by maintaining current power plants in operation and through the design of more efficient plants with higher design lives (70+ years).

The project aims to be a platform to inspire the next generation of researchers and engineers with new skills across NDECA fields. The development of new expertise in these fields would have demonstrable positive impacts on the economy, through contributing to extending the life of high integrity assets, continued energy supply, maintaining jobs in operating plants and creating new job opportunities.

The project offers the ability to further strengthen UK's position by extending the applicability of current procedures with a prospect of far-reaching consequences. In recognition of this, this project is supported by key UK players in NDE and structural integrity, such as RCNDE, TWI, Wood (previously AMEC Foster Wheeler) and BP.