Defect Size Margin 

DSM – Defect Size Margin is the margin between a defect that may be present at the end of life of a component and the End of Life Limiting Defect Size (ELLDS – see last month’s What the Hec?!).

LFCG – Lifetime Fatigue Crack Growth, is the amount a fatigue crack will grow over the lifetime of a component.

So, if manufacturing inspections are capable of detecting and sizing defects equal to or greater than the Qualified Examination Defect Size (QEDS – see April’s What the Hec?!), then at the start of life a component will have no defect greater than the QEDS.

At the end of life, any such defect present at the start will have grown to a size of QEDS + LFCG. The DSM is the ratio of the ELLDS to the sum of this calculated defect size (QEDS + LFCG), ie DSM = ELLDS / (QEDS + LFCG).

The objective for safety cases in the UK is to show that components have a DSM with a value of two or more. This ensures that if a component starts service with a defect present, the defect will not cause failure during the life of the component.

For more information on the use of LFCG, DSM, QEDS and ELLDS see: www.onr.org.uk/new-reactors/ap1000/reports/assessment-reports/onr-nr-ar-16-009.pdf

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org