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CrackWISE® goes probabilistic

TWI has developed a new, probabilistic version of its popular CrackWISE® software, which supports engineers inspecting the integrity of welds on pipelines, pressure equipment and other structures.

Establishing the probability of failure

BS 7910 (“Guide to methods for assessing the acceptability of flaws in metallic structures”) has long been used to assess the structural integrity of welded joints in safety-critical structures. The fracture mechanics equations given in the standard are numerous and complex, so TWI developed special software – CrackWISE – to automate the process.  

BS 7910 and CrackWISE assume deterministic inputs, ie single values of the important variables such as applied stress, fracture toughness and flaw size. In reality, these variables are statistically distributed and should be treated using probabilistic fracture mechanics (PFM). In principle, the probability of failure (Pf) can be calculated and used as part of a quantitative risk assessment (QRA) of safety-critical structures. 

Annex K of BS 7910 contains a simplified method of quantifying Pf, using partial safety factors (PSFs) derived from PFM calculations, but in practice it is rarely used. 

TWI has now developed a probabilistic module for CrackWISE, to expand its functionality. The new module is currently undergoing alpha testing to assess its robustness and user-friendliness, and to validate it against results from earlier PFM work. 

Project objective

In this project TWI applied the CrackWISE probabilistic module to a known problem, with a view to validating the software.

Verifying the new approach

Whereas the deterministic version of CrackWISE can be validated by comparison against hand calculations (the answer for a given set of inputs is unique), the same considerations do not apply to PFM calculations, which produce a different set of input data each time the software is run. Consequently, the programme consisted of two tasks:

  • Reproducing the PFM calculations (based on simple cracked plate models) that were used to develop the PSFs in Annex K of BS 7910:2013 and comparing the CrackWISE results with those of the earlier work. 
  • Replicating an earlier probabilistic assessment of a more complex welded joint that better represents a welded structure.

Encouraging results

This project demonstrated the ability of the new software to reproduce selected models used in the derivation of the PSFs in Annex K of BS 7910:2013 (see Figure 1). However, it also revealed that the agreement between Annex K and a direct PFM approach is weak, except for certain specific cases against which the model was originally calibrated. A direct PFM approach is therefore preferable to the use of PSFs to achieve a particular target Pf.

There are a number of outstanding challenges in the application of PFM (including the availability of high-quality data and the skill required of the analyst) to real engineering structures. Nevertheless, this work showed the ability of the software to calculate the relative effects of important variables, such as the scatter in tensile properties and fracture toughness, or the precision of the non-destructive testing technique (see Figures 2 and 3, overleaf).

The next step will be for TWI to investigate further the application of PFM to real engineering structures, such as pressure vessels and offshore structures, including wind turbine support towers.

See over for Figures 2 and 3.

Figure 1. Comparison of PSF calculations using CrackWISE with those implied in Annex K of BS 7910 (different symbols denote different models).
Figure 1. Comparison of PSF calculations using CrackWISE with those implied in Annex K of BS 7910 (different symbols denote different models).
Figure 2. Example of the output from the probabilistic version of CrackWISE; Pf (8x10-5 in this case) is the ratio of point outside the FAD (red) to the total number of assessment points.
Figure 2. Example of the output from the probabilistic version of CrackWISE; Pf (8x10-5 in this case) is the ratio of point outside the FAD (red) to the total number of assessment points.
Figure 3. Example of the effects of NDT precision on reliability; by increasing the scatter in flaw size (with no other changes to inputs), Pf has risen by a factor of 16 relative to the previous diagram.
Figure 3. Example of the effects of NDT precision on reliability; by increasing the scatter in flaw size (with no other changes to inputs), Pf has risen by a factor of 16 relative to the previous diagram.

References and credits

The support of past and present members of the TWI CrackWISE team, in particular Paul Brown, is gratefully acknowledged.

Contact us

For further information about Integrity Management, including probabilistic fracture mechanics, please email contactus@twi.co.uk. To find out more about CrackWISE, and TWI’s other industry software, visit  www.twisoftware.com.

Avatar Dr Isabel Hadley CEng, Eur Ing, FWeldI Technology Fellow – Materials and Structural Integrity Technology Group, TWI

Isabel is a Technology Fellow, recently semi-retired from TWI. Her work focuses on the development of analytical flaw assessment techniques, and their application to safety-critical welded structures and pressure equipment. She has a particular involvement in the development and validation of structural integrity procedures and standards such as:

  • BS 7910 (UK flaw assessment procedure); Chair of the committee, 2008-2021
  • R6 (UK nuclear assessment procedure); member of the panel
  • FITNET (European fitness-for-service procedure); co-author.

She is also Royal Academy of Engineering Visiting Professor in Integrity Management at the University of Bristol, as part of an initiative to transfer industrial know-how to engineering undergraduates.

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