BS 7910-2013+A1-2015 Guide to methods for assessing the acceptability of flaws in metallic structures

Publisher: BSI Standards Publication
Publish Date: 2013 (Ammendment 2015)
Pages: 492
Language: English
Format: PDF
ICS 25.160.40
ISBN: 978-0-580-91721-9
Size: 13 397 kBScopeThe scope of the BS 7910 procedure is to guide the user in assessing the acceptability of flaws in metallic structures. The procedure can be used in two ways:- to perform an assessment of an existing flaw and support continued operation/repair
/replacement decisions
- to determine a limiting flaw size for a component subject to a given operating environment
and determine the time to reach the limiting state.The engineering critical assessment is performed in both cases. Essential to the ECA is reliable information on possible flaw size and shape and flaw location. In this regard the non-destructive examination (NDE) is essential to the process. Guidelines on performing NDE are however not the subject of BS 7910 and can be found in BS EN 571-1, 1289, 1290, 1291, 1435, 1712, 1714, 12517 and BS EN ISO 6520-1.The next key ingredient in performing an ECA is information about the material properties:- tensile data to BS EN 10002-1 and 10002-5
- fatigue data to BS ISO 12108
- fracture toughness data to BS 7448, BS EN ISO 15653 and BS EN ISO 12737
- stress corrosion data to BS EN ISO 7539BS 7910 addresses common failure mechanisms - fracture, plastic collapse, fatigue, corrosion and creep as well as more application specific issues, such as buckling or leak before break. It is imperative to an ECA to establish the cause of cracking/material degradation which may involve review of the design parameters, operating environment, material in-service degradation, etc.The essential steps in performing an assessment are:- identify the flaw type
- establish the essential data for the structure (material, stresses, operating environment,
etc.)
- determine the size (and shape) of the flaw and distribution if multiple flaws are present
- assess material damage mechanisms and damage rates
- determine the limiting flaw size for the final failure mode
- assess the subcritical crack growth based on the damage mechanisms
- evaluate the consequences of failure
- perform sensitivity analyses.The flaw is deemed acceptable if it does not grow to the limiting state during the inspection interval, including the appropriate safety factors.The stresses to be used in the assessment are those calculated for the unflawed structure, across the entire section thickness. The actual stress distribution may be used or stresses can be linearised. Stresses are classified into primary and secondary and each sub-divided into membrane and bending components. Bending stresses are defined across the section thickness containing the flaw and not across the component.The assessment for fracture is performed using several analysis options of increasing complexity/reduced in-built conservatism. Failure assessment diagrams (FADs) are used as a basis for assessment. Detailed guidance can be found in BS
7910.[1] The key steps are outlined here to set the remainder of the paper into context:- Define stresses
- Define fracture toughness
- Determine material properties
- Characterise the flaw
- Select the failure assessment diagram
- Calculate Lr (plastic collapse axis of FAD)and Kr (fracture axis of FAD) and plot the
- assessment point in the diagram
- Assess the significance of the resultThese steps in the assessment were set by the PD 6493:1991 and have remained largely unchanged. Some changes are being made to the specifics of these steps as the current standard develops into the new standard (2012 revision of BS7910). This paper aims to summarise and explain the evolution of the flaw characterisation step central to which are the interaction criteria.