Guide to methods for assessing the acceptability of flaws in metallic structures

BS 7910:2005

(Summary)

In many circumstances it is necessary to examine critically the integrity of new or existing

structures by the use of NDT methods and establish the acceptance levels for the flaws

revealed. The derivation of acceptance levels for flaws is based on the concept of 'fitness for

purpose'. By this principle, a particular fabrication (or structure) is considered to be adequate

for its purpose, provided the conditions to cause failure are not reached.

Quality control levels are usually both arbitrary and conservative but are of considerable

value in the monitoring and maintenance of quality during production. Flaws that are less

severe than such quality control levels as given, for example, in current application standards,

are acceptable without further consideration. If flaws more severe than the quality control

levels are revealed, rejection is not necessarily automatic. Decisions on whether rejection

and/or repairs are justified may be based on fitness for purpose, either in the light of

previously documented experience with similar material, stress and environmental

combinations or on the basis of an “engineering critical assessment” (ECA). BS 7910

document is concerned with ECA.

The following stages in the assessment of flaws need to be followed.

a) If the flaws do not exceed the quality control levels in the appropriate application standard,

no further action is required.

b) If acceptance limits have already been established on the basis of an ECA for the

appropriate combination of materials, fabrication procedure, welding consumables, stress and

environmental factors, flaws need to be assessed on that basis.

c) If no relevant documented experience exists, then an ECA based on the guidance given in

BS 7910 needs to be carried out.

The document has 10 sections, 21 annexes, and numerous figures, graphs and tables.

1. Scope

BS 7910 document outlines methods for assessing the acceptability of flaws in all types of

structures and components. Although emphasis is placed on welded fabrications in ferritic

and austenitic steels and aluminium alloys, the procedures developed can be used for

analysing flaws in structures made from other metallic materials and in non-welded

components or structures. The methods described can be applied at the design, fabrication

and operational phases of a structure’s life.

2. Other codes (References)

These include amongst many others:

BS 7448 Fracture mechanics toughness tests

BS 7608 Code of practice for fatigue design and assessment of steel structures

Others are related to Welded Structures, NDT, Corrosion, and Tensile Testing etc.

MSc (MED) - Engineering Design 1 [MACE 61061]

Fracture, Fatigue & Creep in Design - Dr M A Sheikh

3. Symbols and Definitions

As used by BS 7910.

4. Types of Flaw

(a) Planar flaws such as cracks.

(b) Non-planar flaws such as cavities and inclusions.

(c) Shape imperfections such as misalignment

5. Modes of Failure and Material Damage Mechanisms

5.1 Failure modes/damage mechanisms

Fracture

Plastic collapse

Fatigue

Creep

Corrosion

and others..

5.2 Sequence of operations

(i) Identify flaw type

(ii) Establish data

(iii) Determine flaw size

(iv) Assess possible damage mechanisms

(v) Determine limiting size for the final mode of failure

(vi) Assess whether flaw would grow to this final size (within remaining

life or inspection interval)

(vii) Assess consequences of failure

(viii) Carry out sensitivity analysis

(ix) Determine acceptability (Factors of Safety)

5.3 Treatment of flaws

This is carried out at three levels. The choice of level depends upon available

material data and application.

Level 1 Simple, conservative

Level 2 Normal procedure (has many options)

Level 3 Advanced, includes the effects of ductile tearing, greater

accuracy.

6. Information Required for Assessment

6.1 General

It is inevitable that the ECA will require assumptions to be made about input parameters.

Therefore, if there is any likelihood that an ECA will be required during the life of a

structure, it is advisable to generate relevant material properties at the construction stage, or

to retain appropriate materials for later testing. In particular, the desirability of having

accurate fracture toughness data cannot be emphasized too strongly and tests on weld

procedure test samples are advisable. Similarly, fatigue crack growth, creep and stress

corrosion cracking data may be obtained from the actual materials of construction. Any such

tests should be performed in accordance with the appropriate standards from the list in

Section 2.

6.2 Essential data

(i) Nature, position and orientation of flaw

(ii) Structural, weld geometry

(iii) Stresses

(iv) σY (or σ0.2), σuts, E, σ/ε data

(v) Fatigue, S-N data, Fatigue crack propagation

(vi) KIC, J, CTOD

(vii) Creep rupture, propagation

etc..

6.3 NDT

Flaw - length, height, position, orientation, planar or non-planar etc.

[Ultrasonic, Radiography, P.D method etc.]

6.4 Stresses

Consider actual stresses without flaw.

Primary [Membrane (Pm), Bending (Pb)]

Secondary [Q (Qm & Qb): Thermal, Residual etc.]

Stresses at discontinuities (welds, holes, notches etc) - SCF

7. Assessment of Fracture Resistance

7.1 Background

7.1.1 General

Level 1 Simplified, used when material data is limited. (Section 7.2)

Level 2 Normal (Section 7.3)

Level 3 Advanced, more accurate, conservative, suitable for ductile materials,

includes ductile tearing. (Section 7.4)

The choice of level depends upon material data, input data, and required accuracy.

Figures 4, 5, 6 & 7 give the flowcharts for the general methods, Level 1, Level 2, and

Level 3 respectively.

FAD: Assessment is generally made by means of a failure assessment diagram (FAD)

based on the principles of fracture mechanics. The vertical axis of the FAD is a ratio

(Kr) of the applied conditions, in fracture mechanics terms, to the conditions required

to cause fracture, measured in the same terms. The horizontal axis is the ratio (Lr) of

the applied load to that required to cause plastic collapse. An assessment line is

plotted on the diagram. Calculations for a flaw provide either the co-ordinates of an

assessment point or points. The positions of these are compared with the assessment

line to determine the acceptability of the flaw

Kr

UNSAFE

1.0

SAFE

Assessment Point

1.0 Lr

�� = ������ �� �� ���� ��� ����

�� = ������ ������ �� ���� ������ �������

7.1.2 Flaw Dimensions & Interactions

Planar flaws should be characterized by the height and length of their containment rectangles.

These dimensions are as follows: 2a for through thickness flaws; a and 2c for surface flaws;

and 2a and 2c for embedded flaws.

Multiple flaws on the same cross-section may lead to an interaction and to more severe

effects than single flaws alone. Simple criteria for interaction is given to obtain the

dimensions of the effective flaws after interaction.

7.1.3 Tensile data

σY (or σ0.2), σuts, E are required at the appropriate temperature. HAZ data for welds is

also required if softening is present.

[BS EN10002-1 & BS EN10002-5]

7.1.4 SIF (K) & CTOD (δ)

FAD: The ordinate of FAD (fracture ratio) is either K or √δ - thereby giving two

routes for the assessment at each level.

K-route requires: KIC from Plane Strain Fracture Toughness test, or

Kmat obtained from Charpy V-notch impact test data, or

conversion from Jmat using the following equation:

����� = � �������

KIC, J, δ - BS 7448

7.1.8 Sr and Lr

The parameter for plastic collapse appears as the abscissa on the FAD. In Level 1, it is Sr and

is the reference stress divided by the flow strength (arithmetic mean of σY and σuts, up to a

maximum of 1.2σY). In Levels 2 and 3, it is Lr and is the reference stress divided by yield

strength. The reason for the difference is that the assessment line for Level 1 is based on the

assumption of an elastic-perfectly plastic stress-strain curve with no strain hardening. This is

conservative and to compensate for this conservatism, it is permissible to use in the

denominator of Sr the flow strength rather than the yield strength. Levels 2 and 3 allow more

accurately for the actual shape of the material stress-strain curve and so no such concession

can be made.

7.2 Level 1 - Simplified Assessment

This is a simplified assessment route applicable where there is limited information on

material properties or applied stresses. It contains two methods, Levels 1A and 1B.

Level 1A: A single FAD (see Fig. 10) is used with

Kr or √δr < 0.707 and Sr < 0.8

If the assessment point lies in the area within the assessment

line, the flaw is acceptable; if it lies on or outside the line, the

flaw is not acceptable.

Level 1B: Manual estimation - does not involve FAD (Annex N)

Maximum Stress:

The stress used is the maximum tensile stress, σmax, which is taken to be equal to the

sum of the values of the stress components. If only nominal membrane stresses, Snom,

are known,

σmax = kt Snom + (km – 1) Snom + Q.

Q : Secondary stresses

If membrane and bending components are known,

σmax = ktm Pm + ktb [Pb + (km – 1) Pm] + Q

Pb : Primary bending stress

Pm : Primary membrane stress

kt : SCF due to discontinuity

km: SCF due to misalignment

Fracture Ratio:

�� = !"!��� ; �$ = (&')√*�

+,� = - .".��� ; ,$ = !"

/0�

for all steels and aluminium alloys when /��1/0 ≤ 0.5,

and all ratios of (/��1/0 ) for other materials.

For all steels and aluminium alloys when /��1/0 ≥ 0.5,

,$ = !" /0� 7 /0

/��18� 7/��1/0 − 0.258

Load Ratio: ;� = /<=>/>

σref is defined in Annex P,

'? = /0@/A� �� �� � B�C�B�B �� 1.2 'E.

7.3 Level 2 - Normal Assessment

It has two methods; 2A, and 2B.

Each method has an assessment line given by the equation of a curve and a cut-off. If

the assessment point lies within the area bounded by the axes and the assessment line,

the flaw is acceptable; if it lies on or outside the line, the flaw is unacceptable.

The cut-off is to prevent localized plastic collapse and it is set at the point at which Lr

= Lrmax where:

����F = /0@/A�/0

Level 2A - generalized FAD, not requiring stress/strain data

The equations describing the assessment line are:

For Lr ≤ Lrmax ,

+,� �� �� = (1 − 0.14���)H0.3 + 0.7exp (−0.65��P)Q

For Lr > Lrmax ,

+,� �� �� = 0

[See Figure 11a with different cut-offs for different materials]

Level 2B - material specific curve

This method is suitable for parent material and weld metal of all types. It will

generally give more accurate results than Level 2A but requires significantly

more data. A typical FAD and the associated stress-strain curve are shown in

Figure 11b and 11c respectively.

Stress Components:

The assessments take account of the actual distributions of stress in the vicinity of the

flaws, where they are known. The stresses required are the membrane and bending

components of the primary and secondary stresses, i.e. Pm, Pb, Qm and Qb. They

should be multiplied by the appropriate partial safety factors, if required.

Fracture Ratio: For levels 2 & 3, KI has the general form,

�$ = (&')√*�

where (Yσ) is given by:

(Yσ) = (Yσ)p + (Yσ)s

where (Yσ)p and (Yσ)s represents contributions from primary and secondary stresses,

respectively.

(Yσ)p = M fw [ ktm Mkm Mm Pm + ktb Mkb Mb {Pb + (km – 1) Pm}]

(Yσ)s = Mm Qm + Mb Qb

In the above equations, expressions for M, fw, Mm and Mb are given in Tables M.2a to

M.4 and Table M.6 for different types of flaw in different configurations. Mkm and Mkb

apply when the flaw or crack is in a region of local stress concentration and are given

in section M.5. For ktm, ktb and km, reference should be made to section 6.4 and Annex

D.

Kr is calculated from the equation:

�� = !"!���

Where secondary stresses are present, a plasticity correction factor, ρ, is necessary to

allow for interaction of the primary (Yσ)p and secondary (Yσ)s stress contributions,

such that:

�� = !"!��� + R [ ρ is defined in Annex R]

Load Ratio: For levels 2 & 3, the load ratio is calculated from the equation:

�� = /<=>/0

where σref is obtained from an appropriate reference stress solution as outlined in

Annex P, with partial safety factors applied as appropriate.

Level 3 - Ductile Tearing Assessment

This is appropriate for ductile materials that exhibit stable tearing (e.g. austenitic

steels and ferritic steels on the upper shelf). There are three assessment methods:

Levels 3A, 3B and 3C.

Each method uses a different assessment line and applies a ductile tearing analysis.

The analysis results in a plot of either a single assessment point or a locus of

assessment points. If either the point or any part of the locus lies within the area

bounded by the axes and the assessment line, the flaw is acceptable; if it does not, the

flaw is not acceptable.

For the ductile tearing analysis, the fracture toughness is required in the form of a J

resistance curve.

Level 3A: generalized FAD of Level 2A (not requiring stress-strain data)

The FAD (Figure 12) is the same as that for Level 2A described by the same

equations.

Level 3B: material-specific curve

The material-specific FAD is derived as for Level 2B.

Level 3C: J-integral

A FAD specific to a particular material and geometry is obtained by

determining the J integral using both elastic and elastic-plastic analyses of the

flawed structure under the loads of interest. Determination of the respective

values, Je and J, for a range of loads (i.e. a range of values of Lr) leads to the

assessment line being described by the following equations:

Kr = (Je/J)½

for Lr ≤ Lrmax

Kr = 0 for Lr > Lrmax

where Je and J are values corresponding to the same load (same Lr) and Kr is

plotted as a function of Lr.

All analyses to determine Je or J should be performed using validated

computer codes. An accurate description of the true uniaxial stress-strain

curve should be used in the analysis.

8. Assessment of Fatigue

In this section, procedures are given for assessing the acceptability of flaws found in service

in relation to their effects on fatigue strength, both in welded or unwelded parts, or for the

estimation of tolerable flaw sizes based on fitness for purpose. Planar and non planar flaws

are considered in a fatigue assessment. Fracture mechanics principles are used to describe the

behaviour of planar flaws whilst the assessment of non-planar flaws is based on experimental

S-N data.

8.1 Fracture Mechanics Analysis of Planar Flaws:

Paris Law: �S = �(∆�)�

where A and m are constants which depend on the material and the applied

conditions, including environment and cyclic frequency.

∆Ko is the threshold stress intensity factor range below which crack growth is

insignificant.

For ∆K < ∆Ko, da/dN is assumed to be zero.

The stress intensity factor range, ∆K, is a function of structural geometry,

stress range and instantaneous crack size and is calculated from the following

equation:

∆K = Y(∆σ) (πa)1/2

Procedure:

Select A, m, ∆Ko

Calculate ∆K for ∆σ, flaw height, shape.

Calculate ∆a (and ∆c) for one cycle.

Calculate (a + ∆a) ... and continue until design life is reached.

[See Tables 4, 5, 6 and Figures 14, 15]

8.5 Procedures for assessing flaws using quality 'Categories'

The quality categories refer to particular fatigue design requirements or

the actual fatigue strengths of flaws and are defined in terms of the ten

S-N curves (Figure 16) labelled Q1 to Q10. These are described by the

following equation:

(∆σ)3 N = constant

Values of the constant are given in Table 7. It is convenient to characterize each curve in

terms of the stress range, S, corresponding to a particular fatigue life and Table 7 includes

values of S corresponding to a life of 2 × 106 cycles.

Procedure for establishing the Quality Category:

(i) Specify ãmax to which fatigue crack growth is permitted

[Figures N1and N2, Annex N]

(ii) Obtain ãi - effective initial flaw parameter [Use Figs. 17a - 21a]

(iii) Use ãmax for given B - Read off Sm

Figures 17b - 21b

(iv) Use ãi for given B - Read off Si

(v) Calculate S = (Si3 + Sm

3)1/3

Quality Category is the next below 'S' in Table 7. If this is the same as or

higher than the required quality category, the flaw is acceptable.

9. Assessment of Flaws under Creep Conditions

9.1 General

The information required for making creep assessments is described in Annex T. A worked

example which takes the user through the procedure step by step, is provided in Annex U.

This clause provides a method of assessing the significance of flaws when time dependent

creep effects have to be taken into account. It is based on the 'R5 Procedure' developed at

Nuclear Electric, but simplified where appropriate. It is intended for use when assessing

components made in ferritic and austenitic steels since most information is available on these

materials. The same principles are applicable to other metallic components, provided the user

can obtain the relevant materials data.

The assessment procedure may be applied at the design stage to components containing

planar flaws. It may be applied, subject to some restrictions, to hypothetical flaws, in order to

set inspection sensitivity or to check that a proposed component is tolerant to flaws. In the

design assessment of components that may be allowed to enter service with permissible pre-

existing flaws, it may be appropriate to allow for any creep crack incubation time in the

determination of flaw tolerance.

The procedure may also be applied, subject to the same restrictions to flaws that are actually

discovered during pre-service or in-service inspection. The objective is to decide whether the

flaw is innocuous and will never affect the integrity of the plant, whether remedial action can

be deferred until some time in the future or whether repairs are needed immediately. It would

normally be inappropriate to allow for a creep crack incubation time when determining the

tolerance of flaws discovered during in-service inspection.

9.2 Creep Exemption Criteria

(a) T < Tc

For materials with uniaxial creep rupture ductility > 10% in the

time/temperature régime of interest, Tc is the temperature at which 0.2%

creep strain is accumulated at a stress level equal to the proof strength over the

period of operation of the component (see Figure 22).

[Example values of Tc given in Table 13]

(b) The total operating period/temperature history of the component satisfies the

life-fraction rule based inequality: U V��(W) < 1

where t(T) for materials with creep rupture ductility > 10 %, is the time

required, at a constant temperature, T, to achieve an accumulated creep strain

of 0.2% at a stress level equal to the proof strength (see Figure 23). The time,

t(T), should be derived using relevant creep rupture properties for the material,

and published data may be considered suitable for this purpose.

9.5 Assessment Procedures

[Figure 26, P 91]

Summary:

� Establish stresses

� Characterise flaws

� Establish material data

� Check on fatigue

� Perform flaw assessment

� Sensitivity analysis

� Remedial action

Worked Example: Annex U