Crucial factors in the reliable application of fitness-for-service methods are the ability to detect flaws and knowledge of the flaw location and dimensions. It is therefore important to quantify the effectiveness of NDT techniques both to detect and size flaws.
TWI has an active role in developing new guidance on these issues for BS 7910, the UK procedure for structural assessment of welded structures.
The concept of Probability of Detection (POD) is used in various industry sectors to establish the capability of an inspection to detect flaws. This is generally expressed as a POD curve, which relates the likelihood of detection to a characteristic parameter of the flaw, usually its size.
Estimation of the POD typically relies on the manufacture of large numbers of realistic defect specimens, followed by practical trials of the inspection procedure. These can be costly and time consuming activities.
TWI is therefore also developing methods, called simulation-supported PODs, for reducing this reliance on practical trials through simulation of NDT techniques.
TWI Resources
TWI uses the MINITAB general-purpose statistical software for estimation of PODs from experimental data and two other software packages for simulation-supported PODs:
- CIVA (CEA) – POD module
- PODPEDGE – in-house software (based on previously validated EDF Energy code PEDGE)
Benefits
- Quantitative measure for inspection effectiveness versus key parameters (eg flaw height) used for deterministic Engineering Critical Assessment (ECA)
- Flaw acceptance criteria based on fitness-for-service principles
Applications
POD information is used:
- To demonstrate compliance with standard requirements for inspection qualification e.g ‘90% probability of detection (POD) with 95% confidence’
- As input to probabilistic safety assessment
- As input to risk-based inspection (RBI)
For more information about TWI's NDT services, please email contactus@twi.co.uk.