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BS 7910: the UK guide on methods for assessing the acceptability of flaws in metallic structures

Rohit Rastogi

Introduction   



Significance of flaws in terms of structural integrity PD6493 BS 7910 “Guide on methods for assessing the acceptability of flaws in metallic structures” Provides methods for:   

Fracture assessment procedures Fatigue assessment procedures Assessment of flaws operating at high temperatures

Fracture Assessment  

   

Based on CEGB-R6 method The basic assumption is that the flawed body could fail by one of two extreme failure modes fracture or plastic collapse (overload). 3 Levels of assessment Level 1: “Simplified assessment” Level 2: “Normal Assessment” Level 3: “Ductile Tearing Instability”

Sequence of Operation      

Identify the flaw type Establish the essential data Determine the size of the flaw Assess possible material damage mechanisms and damage rate Determine limiting size of the flaw Based on the damage rate, assess whether the flaw will grow to this final size within the remaining life of the structure or in-service inspection interval, by sub-critical flaw growth

Sequence of operation   

Assess the consequence of failure Carry out sensitivity analysis If the flaw could not grow to the limiting size, including appropriate factor of safety, it is acceptable. Ideally, the safety factors should take account of both the confidence in the assessment and the consequence of failure

Essential Data        

Nature, position of the flaw Structural and weld geometry, fabrication procedure Stresses (pressure, thermal, residual, transients) Tensile properties Fatigue and corrosion data Fracture toughness Creep data Stress corrosion cracking data

Information from NDE     

Flaw length Flaw height Flaw position Flaw orientation Planar or non-planar cross-section

Assessment of fracture 

  

Level 1: Simple, used when limited information is available on material properties Level 2: Normal assessment route Level 3: Tearing analysis permitted for ductile materials In general, the analysis is first performed using the Level 1 analysis. If the flaw is unacceptable then the analysis is done using higher levels.

Failure Assessment Diagram KI K  K mat ' r

 ref L   ys ' r

K r  f  Lr 

Advantages of FAD 

Double criteria approach: 

Fracture LEFM  EPFM 





Collapse

Elasto-Plastic Fracture Mechanics: 

J-Integral calculation not required

Other features    

Flaw re-characterization rules LBB Procedures Calculation of reserve factors Sensitivity analysis

Step 1: Define Stresses  

Primary and Secondary Guidance for residual stresses due to welding 

Level 1 Assessment 

PWHT:  





30% room temperature σy , parallel to the weld 20% room temperature σy , transverse to the weld

No PWHT: residual stress = σy at room temperature

Level 2 and 3 Assessment 

Annex Q gives residual stress profiles

Residual stress profiles

Step 2: Define Fracture toughness 



Level 1 and 2 

Kmat is required



Can be estimated from Charpy energy

Level 3 

Ductile tearing curve is necessary

Step 3: Define tensile properties  

Level 1: Yield stress required Level 2 and 3: Analysis based on  

Yield stress and Ultimate Stress only Stress strain curve

Step 4: Characterize flaw 

Flaw from inspection   

 

Semi-elliptical (surface flaw) Elliptical (embedded flaw) Rectangular (through thickness flaw)

In planes normal to max. principal stresses Worst combination to be chosen

Step 5: Nearness to collapse 

Level 1:

 ref S   flow



Level 2 and 3

 ref L   ys

 



' r

' r

σref is the stress at the cracked section that will lead to plastic collapse Formulations for a variety of cracked configurations are listed in the Annexure P of the code Secondary stresses not considered

Step 6: Nearness to fracture 

All levels



Secondary Stresses also considered KI due to primary and secondary stresses



KI K  K mat ' r

Level 1 Assessment 



 



Based on Conservative failure assessment diagram Kr : ratio of applied crack driving force to fracture toughness Sr : ratio of applied stress to flow strength Single-point value of fracture toughness (sometimes Charpy energy) FAD is a rectangle: Sr_max = 0.8, Kr_max = 0.7

Step 7: Construct FAD Level 1 

Kr < 0.707 and Sr < 0.8

UNSAFE

1

Kr



0.5

SAFE

0 0

0.5 Sr

1

Level 2 Assessment       

2 types: Level 2A and 2B Depends on the type of stress-strain data 2A: Full curve not available 2B: Full curve available Lr is used in place of Sr in FAD Guidance for materials with discontinuous yield point Single point value of Fracture toughness is required

Step 7: Construct FAD 

Level 2A FAD: Only σys and σuts is known



K r   1  0.14 L2r  0.3  0.7 exp  0.65 L6r 

 for L

r

 Lr (max)

for Lr  Lr (max)

=0

Lr (max)

  

ys

  uts 

2 ys

Step 7: Construct FAD 

Level 2B FAD: Full Stress curve known  E ref

L  ys  3 r

1 2

Kr     Lr ys 2 E ref     =0

for 0.0  Lr  Lr (max) for Lr  Lr (max)

εref is the true strain corresponding to true stress Lr. σys The Level 3 definition for FAD is similar to Level 2 FAD, but it permits increased margins by using unstable crack growth as failure mode.

Constructing Level 2B FAD 

A number of points are taken on the Lr axis in between 0 and Lrmax.



For each Lr reference determined from Lrσy



Corresponding true strain εref is read from the true-stress strain curve of the material of the component. Now for each chosen Lr point, The FAL is 1 2 3 plotted using  E refequation Lr ys 



Kr     Lr ys 2 E ref     =0

stress

σref

for 0.0  Lr  Lr (max)

for Lr  Lr (max)

is

Level 3 Assessment     

3 types: Level 3A, 3B and 3C Depends on the type of stress-strain data 3A: Full curve not available 3B: Full curve available 3C: Detailed J-Integral calculations

Level 3 analysis 1.2 FAL

1

L3 L1 L2

Kr

0.8

B

L3'

0.6 L1' L2'

0.4 A

0.2 0O

0

0.2

0.4

0.6

0.8 Lr

1

1.2

1.4

1.6

Factor of Safety  

Level 1 FAD: 2 on crack size Level 2 and 3 FAD: Use partial safety factors on:    



Applied stress Flaw size Toughness Yield stress

These correspond to probability of failure of    

2.3x10-1 1.0x10-3 7.0x10-5 1.0x10-5

Step 8: Assess the component 



If the assessed point is in the safe region the flaw is acceptable. The code recommends a sensitivity analysis on the results with respect to the flaw sizes, loads and material properties before the decision is made.

API 579 vs. BS 7910 





API 579 is intended for equipment designed using the ASME code and materials and gives results consistent with the original ASME design safety margins. API 579 may be used for equipment designed to other codes but users should be prepared to interpret the procedures in an appropriate manner. BS 7910 is applicable to all metallic structures and materials and is written in a more generalized manner without reference to a particular industry, design code or material thereby allowing users to decide safety margins.

API 579 vs. BS 7910 





API 579 covers a wide range of damage types typically found in refining and petrochemicals application, and gives procedures for different types of metal loss, physical damage, low and high temperatures, and crack like defects. BS 7910 deals comprehensively with fatigue and fracture of defects in and around welded joints and gives annexes covering advanced aspects such as mismatch, mixed mode loading , residual stress effects and leak before break. API 579 is designed at level 1 for use by plant inspectors and plant engineering personnel with the minimum amount of information from inspection and about the component.

API 579 vs. BS 7910 





BS 7910 requires some technical expertise in fracture mechanics and access to fracture parameter solutions and toughness data at all levels. API 579 is supported by a number of organizations based in the USA where most experience resides. BS 7910 was developed in the UK where TWI is the main source of expertise, training and software.

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