NB/T 20013-2010Structural integrity assessment for nuclear pressure-retaining components containing defects (English PDF)

Foreword

This document was issued on 1 May 2010 by the National Energy Administration of the PRC and takes effect on 1 October 2010.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 23.020.30, Chinese classification J 74.

This standard is drafted in accordance with the rules given in GB/T 1.1-2009.

This standard was proposed by the National Energy Administration.

This standard is under the jurisdiction of the Nuclear Industry Standardization Research Institute.

This standard was drafted under the responsibility of the Research Institute of Nuclear Power Operation. The organizations that took part in the drafting are: CNNC Wuhan Nuclear Power Operation Technology Co., Ltd., East China University of Science and Technology, Shanghai Nuclear Engineering Research and Design Institute, Hefei General Machinery Research Institute, Nanjing University of Technology, China Institute of Atomic Energy and Wuhan University.

The main drafters of this standard are: Nie Yong, Liu Hongyun, Li Peining, Tang Yi, Li Siyuan, Hui Hu, He Yinbiao, Bao Zhanggen, Chen Xuedong, Yang Tiecheng, Shen Shiming, Zhao Jianping, Yang Hongyi, Yu Huajin, Zhang Jianqiang, Cao Ming, Guan Weihe, Fan Zhichao, Wang Yueying, Qi Min and Hu Rong.

1 Scope

NB/T 20013-2010 sets out how a nuclear pressure-retaining component found to contain a defect is assessed for continued service. When an inspection reveals a crack, a lack of fusion or a wall loss in a reactor coolant pipe, a steam generator or a vessel nozzle, the question is not simply whether the flaw is acceptable against a workmanship standard, but whether the component still has adequate margin against fracture, plastic collapse and fatigue growth for the life ahead of it. That is a fitness-for-service calculation, and this standard is the Chinese procedure for it. It gives the scope of application and the full set of defined terms and symbols - the notation alone runs to about a hundred entries - then the general requirements: the assessment levels, the data needed on geometry, loading, material properties and defect size, and the safety factors applied at each level. Characterisation of planar defects comes next: how an indication reported by non-destructive examination is idealised into an analysable flaw, how flaws close together are combined, and how surface, embedded and through-wall cases are treated. The assessment methods follow, covering brittle fracture through the failure assessment diagram, plastic collapse, fatigue crack growth, stress corrosion cracking, creep where temperature demands it, and the treatment of residual stresses. Material property determination, the appendices of stress intensity and reference stress solutions, and the reporting of the assessment close the document. It applies to nuclear pressure equipment in China.

This standard specifies the methods for the structural integrity assessment of in-service nuclear pressure-retaining components containing defects, including linear elastic fracture assessment, elastic-plastic fracture assessment, plastic collapse assessment and fatigue crack growth assessment.

This standard applies to the integrity assessment of in-service steel nuclear safety class 1 pressure-retaining components of pressurized water reactors. The integrity assessment of nuclear safety class 2 and class 3 pressure-retaining components may be carried out with reference to this standard.

For non-repairable defects found during pre-service inspection, assessment may be carried out with reference to this standard after the agreement of the owner and of the relevant departments.

This standard does not apply to the assessment of pumps and valves among nuclear pressure-retaining components, nor to newly manufactured nuclear pressure-retaining components accepted against manufacturing quality control standards.

2 Normative references

The following documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

GB/T 228 Metallic materials - Tensile testing at ambient temperature.

GB/T 229-2007 Metallic materials - Charpy pendulum impact test method.

GB/T 4161 Metallic materials - Test method for plane-strain fracture toughness KIc.

GB/T 4338 Metallic materials - Tensile testing at elevated temperature.

GB/T 6398 Metallic materials - Test method for fatigue crack growth rate.

GB/T 6803 Test method for the nil-ductility transition temperature of ferritic steels - Drop weight test.

GB/T 12778 Method for the determination of the Charpy impact notch of metals.

GB/T 19744 Test method for the crack arrest fracture toughness KIa of ferritic steels under plane strain.

GB/T 21143 Metallic materials - Unified test method for quasi-static fracture toughness.

EJ/T 560 Surveillance of the irradiation of light-water-cooled reactor pressure vessels.

3 Terms and definitions

The following terms and definitions apply to this document.

3.1 Nuclear pressure-retaining component: a pressure-retaining component among nuclear safety mechanical equipment, including nuclear pressure vessels and piping.

3.2 Structural integrity: the degree to which the defects contained in a nuclear pressure-retaining component affect the satisfaction of safety and reliability requirements.

3.3 Structural integrity assessment: the evaluation of whether a nuclear pressure-retaining component containing defects can continue to be used safely.

3.4 Defect characterization: the simplification of an actual defect, according to specified rules, into a defect of a prescribed geometric shape.

3.5 Defect characterization size: the defect size determined after the characterization or idealization of the actual defect.

3.6 Fracture assessment: the evaluation, by means of fracture mechanics methods, of whether fracture failure occurs in a nuclear pressure-retaining component containing defects.

3.7 Plastic collapse assessment: the evaluation, by means of plastic limit analysis methods, of whether plastic collapse failure occurs in a nuclear pressure-retaining component containing defects.

3.8 Plastic collapse load: the maximum load that a structure made of an ideal elastic-plastic material can withstand, calculated by limit analysis methods using the flow stress of the component material.

3.9 Service limits: the stress limits and stress intensity limits defined for all the service loadings specified in the design specification. Note: service limits are divided into service level A, level B, level C and level D limits.

3.10 Service level A limits: the limits that shall be satisfied by all the level A service loadings specified in the design specification. Note: these loadings are those that the equipment or the support may experience while performing its specified service function.

3.11 Service level B limits: the limits that shall be satisfied by all the level B service loadings specified in the design specification. Note: the equipment or the support shall be able to withstand these loadings without damage requiring inspection or repair.

3.12 Service level C limits: the limits that shall be satisfied by all the level C service loadings specified in the design specification. Note: these limits allow large deformations to occur in structural discontinuity regions; such deformations may require the equipment or the support to be replaced after inspection, or the damage to the equipment or the support to be repaired before service can be continued.

3.13 Service level D limits: the limits that shall be satisfied by all the level D service loadings specified in the design specification. Note: these limits allow significant gross deformation to occur, leading to the loss of dimensional stability and to damage requiring inspection or repair; such damage may require the equipment or the support to be replaced before service can be continued.

3.14 Irradiation effect: the change in the properties of a material caused by the neutron fluence.

3.15 Primary stress: the normal stress or shear stress caused by the applied loading and required to satisfy the simple laws of equilibrium of internal and external forces and moments.

3.16 Secondary stress: the normal stress or shear stress caused by the constraint of adjacent parts or by the self-constraint of the structure.

4 Symbols

The following symbols apply to this document.

a: the characterization crack size of a planar defect after idealization (for a through-wall crack, its half length; for a surface crack, the crack height; for an embedded crack, one half of its own height; for a corner crack, the height along the pipe wall), in millimetres (mm).

a with subscript aw: the maximum allowable defect height corresponding to the defect length l at the end of the assessment period, in millimetres (mm).

a with subscript c: the minimum critical defect height under normal operating conditions, in millimetres (mm).

a with subscript f: the final size of a after fatigue crack growth, in millimetres (mm).

a with subscript i: the minimum critical initial defect size under emergency and faulted conditions, in millimetres (mm).

a with subscript 0: the value of a of the initial crack used in the fatigue analysis, in millimetres (mm).

CL: the orientation of the specimen; for a specimen containing a longitudinal planar crack, the loading is applied in the circumferential direction.

c: the half length, along the direction of the shell surface, of an idealized elliptical embedded crack or of a semi-elliptical surface crack, in millimetres (mm).

c with subscript f: the final size of c after fatigue crack growth, in millimetres (mm).

c with subscript 0: the value of c of the initial crack used in the fatigue analysis, in millimetres (mm).

D: the outside diameter of the pipe, in millimetres (mm).

D with subscript i: the inside diameter of the vessel, that is, the inside diameter of a cylindrical vessel or of a pipe, or of a spherical shell or spherical head, in millimetres (mm).

D with subscript in: the inside diameter of the nozzle, in millimetres (mm).

D with subscript n: the mean diameter of the nozzle, in millimetres (mm).

D with subscript on: the outside diameter of the nozzle, in millimetres (mm).

da over dN: the fatigue crack growth rate, in millimetres per cycle.

E: the elastic modulus of the material at the assessment temperature, in megapascals (MPa).

e: the eccentricity of the centre of an idealized elliptical embedded crack from the mid-thickness of the wall, in millimetres (mm).

h: the measured maximum height of the defect in the through-thickness direction of the wall, in millimetres (mm).

h with subscript w: the leg size of a fillet weld, in millimetres (mm).

I: the moment of inertia of the pipe, in millimetres to the fourth power.

J: the value of the J-integral, in megajoules per square metre.

J with subscript c: the J-integral fracture toughness of the material corresponding to the brittle point or pop-in point at which cleavage or pop-in occurs when the stable crack extension is less than 0.2 mm, in megajoules per square metre.

J with subscript Ic: when the stable crack extension is greater than 0.2 mm, the J-integral fracture toughness of the material corresponding to a crack extension of 0.2 mm, in megajoules per square metre.

K with subscript c: the fracture toughness of the material expressed by the stress intensity factor, or the fracture toughness of the material expressed by the stress intensity factor converted from the J-integral fracture toughness or from the CTOD (crack tip opening displacement) fracture toughness, in megapascals times the square root of a metre.

K with subscript I: the mode I stress intensity factor, in megapascals times the square root of a metre.

K with subscript I and superscript max: the maximum stress intensity factor, in megapascals times the square root of a metre.

K with subscript I and superscript min: the minimum stress intensity factor, in megapascals times the square root of a metre.

K with subscript Ic: the plane-strain fracture toughness of the material, in megapascals times the square root of a metre.

K with subscript Jc: the stress intensity factor fracture toughness converted from the J-integral fracture toughness, in megapascals times the square root of a metre.

K with subscript p: the fracture toughness of the material expressed by the stress intensity factor after the safety factors have been taken into account in the planar defect assessment, in megapascals times the square root of a metre.

K with subscript r: the fracture ratio, that is, the ratio of the stress intensity factor under the applied load to the fracture toughness of the material expressed by the stress intensity factor, dimensionless.

K with subscript I and superscript P: the stress intensity factor caused by the primary stress, in megapascals times the square root of a metre.

K with subscript I and superscript S: the stress intensity factor caused by the secondary stress, in megapascals times the square root of a metre.

KV2: the impact absorbed energy of a V-notch specimen under a 2 mm pendulum striker, in joules (J).

L: one half of the length of the plate, in millimetres (mm).

LE: the lateral expansion value, in millimetres (mm).

L with subscript msd: the distance from the edge of the local thinned area to the nearest major structural discontinuity, in millimetres (mm).

L with subscript r: the load ratio, that is, the ratio of the applied load causing the primary stress to the plastic yield limit load; it indicates how close the load is to the plastic yield limit load of the material, dimensionless.

L with subscript W: the distance between two adjacent symmetrical weld toes on a welded joint containing a weld toe crack or a fillet weld root crack, in millimetres (mm).

L with subscript r and superscript max: the allowable limit of the load ratio, dimensionless.

l: the measured maximum length of the planar defect along the free surface of the shell, in millimetres (mm).

l with subscript aw: the allowable length limit of an axial through-wall defect, in millimetres (mm).

M with subscript b: the resultant bending moment produced by the combination of the primary loads, in newton millimetres.

M with subscript e: the combined secondary bending moment, including the bending moment produced by thermal expansion loads and by seismic anchor movement, in newton millimetres.

N: the total number of constant-amplitude fatigue stress cycles, in cycles.

n: the exponent in the relation between the fatigue crack growth rate and the stress intensity factor range, dimensionless.

P: the primary stress, in megapascals (MPa), or the total axial load of the pipe including the pressure, in newtons (N).

P with subscript b: the primary bending stress, in megapascals (MPa).

P with subscript m: the general primary membrane stress, in megapascals (MPa).

p: the pressure of the nuclear pressure-retaining component under the assessment operating condition, in megapascals (MPa).

p1 and p2: the distances from the embedded defect to the two surfaces of the shell wall, in millimetres (mm).

Q: the secondary stress, in megapascals (MPa).

Q with subscript b: the secondary bending stress, in megapascals (MPa).

Q with subscript m: the secondary membrane stress, in megapascals (MPa).

R with subscript c: the mean radius of the pipe, in millimetres (mm).

R with subscript i: the inside radius of the vessel, in millimetres (mm).

R with subscript m: the tensile strength of the material, in megapascals (MPa).

R with subscript n: the mean radius of the nozzle, in millimetres (mm).

R with subscript o: the outside radius of the vessel, in millimetres (mm).

R with subscript on: the outside radius of the nozzle, in millimetres (mm).

R with subscript p and a superscript star: the lower of the two values of the yield strength of the base metal and the yield strength of the weld metal, in megapascals (MPa).

R with subscript p and a superscript plus sign: the greater of the two values of the yield strength of the base metal and the yield strength of the weld metal, in megapascals (MPa).

R with subscript pp: the yield strength of the base metal (the yield stress or the 0.2 percent elastic limit stress; for austenitic steel, the 1 percent elastic limit stress), in megapascals (MPa).

R with subscript pw: the yield strength of the weld metal (the yield stress or the 0.2 percent elastic limit stress; for austenitic steel, the 1 percent elastic limit stress), in megapascals (MPa).

R with subscript p0.2: the yield strength, that is, the stress at a non-proportional elongation of 0.2 percent, in megapascals (MPa).

RSF with subscript a: the allowable remaining strength factor, taken as 0.9, dimensionless.

RT with subscript NDT: the reference nil-ductility transition temperature, in degrees Celsius.

R with subscript e: the stress intensity factor ratio, equal to the minimum mode I stress intensity factor divided by the maximum mode I stress intensity factor, dimensionless.

r with subscript 0: the size of the yield zone of a thick plate, in millimetres (mm).

SR: the stress ratio, dimensionless.

s1: the shortest distance between two coplanar cracks along the direction of the shell surface, in millimetres (mm).

s2: the shortest distance between two coplanar cracks along the through-thickness direction of the wall, in millimetres (mm).

T: the temperature, in degrees Celsius.

T with subscript NDT: the nil-ductility transition temperature, in degrees Celsius.

t: the nominal wall thickness, or the actual wall thickness measured by ultrasonic examination, in millimetres (mm).

t with subscript FCA: the wall thinning of the local thinned area over a future inspection period, in millimetres (mm).

t with subscript mm: the minimum measured wall thickness, in millimetres (mm).

t with subscript no: the measured wall thickness of the nozzle in the vicinity of the defect, in millimetres (mm).

W: one half of the width of the specimen, in millimetres (mm).

x: the position variable in the thickness direction, that is, the distance from the inner surface of the pipe, in millimetres (mm).

y with subscript 0: the size of the yield zone of a thin plate, in millimetres (mm).

z: the position variable in the thickness direction, that is, the distance from the surface of the last weld pass, in millimetres (mm).

delta K: the range of the stress intensity factor at the crack tip, in megapascals times the square root of a metre.

delta K with subscript a: the stress intensity factor range at the crack tip in the a direction, in megapascals times the square root of a metre.

delta K with subscript c: the stress intensity factor range at the crack tip in the c direction, in megapascals times the square root of a metre.

delta K with subscript th: the threshold value of the stress intensity factor range, in megapascals times the square root of a metre.

delta T: the temperature difference between the inner and the outer wall, in degrees Celsius.

delta (lower case): the crack tip opening displacement (CTOD) value, in millimetres (mm).

theta and phi: angles, in radians (rad).

nu: the Poisson ratio, dimensionless.

rho: the plasticity correction factor used for the secondary stress in the planar defect assessment, dimensionless.

sigma: the stress, in megapascals (MPa).

sigma with subscript b: the bending stress obtained by the linearization of the stress distribution (the one caused by the primary stress is called the primary bending stress, the one caused by the secondary stress is called the secondary bending stress); in Clause 12, on the assessment of planar defects in nuclear pressure piping, it is the primary bending stress, in megapascals (MPa).

sigma with subscript b and a prime: the bending stress of any combination of primary stresses under the limit load, in megapascals (MPa).

sigma with subscript e: the secondary bending stress, including the stress caused by thermal expansion and by seismic anchor movement, in megapascals (MPa).

sigma with subscript f: the flow stress, equal to one half of the sum of the yield strength at 0.2 percent non-proportional elongation and the tensile strength, using the values of yield strength and tensile strength measured at the service temperature; where measured values cannot be obtained, the yield strength and tensile strength values specified in the standard may be used, in megapascals (MPa).

sigma with subscript R: the residual stress, in megapascals (MPa).

sigma with subscript m: the membrane stress, or, in Clause 12 on the assessment of planar defects in nuclear pressure piping, the primary membrane stress, in megapascals (MPa).

sigma with subscript m and a prime: the membrane stress under the limit load, in megapascals (MPa).

sigma with subscript R and superscript L: the longitudinal residual stress, in megapascals (MPa).

sigma with subscript R and superscript T: the transverse residual stress, in megapascals (MPa).

sigma with subscript R and superscript T,B: the transverse residual stress at the inner surface, in megapascals (MPa).

sigma with subscript R and superscript T,O: the transverse residual stress at the outer surface, in megapascals (MPa).

5 General

5.1 General principles of integrity assessment. Integrity assessment shall include the investigation of the condition of the object assessed (history, operating conditions, environment and so on), defect detection, analysis of the causes of the defect, identification of the failure mode, material examination (properties, damage and degradation and so on), stress analysis, and the necessary experiments and calculations; on this basis, a comprehensive analysis and evaluation of the safety of the object assessed shall be made in accordance with the provisions of this standard.

5.2 Qualification. The organization and the personnel carrying out the integrity assessment of nuclear pressure-retaining components shall have the corresponding design experience and non-destructive examination experience, and shall have adequate structural integrity assessment capability.

5.3 Identification of failure modes. This standard considers the following types of failure mode: a) fracture failure; b) plastic collapse failure; c) fatigue failure.

The failure mode of the equipment shall be identified on the basis of the failure analyses of nuclear pressure-retaining components of the same type, of integrity assessment cases and experience, of manufacturing and inspection records, of the operating conditions, and of the results of the physical and chemical examination and physical diagnosis of the defect; the environmental influence factors that may be present and their effect on the failure mode and on the integrity assessment shall be fully taken into account.

The integrity assessment shall consider the three failure modes stated above that may occur in the nuclear pressure-retaining component under the specified operating conditions (including the hydrostatic test) within the assessment period. One assessment method may evaluate the corresponding failure mode; after each possible failure mode has been identified or evaluated, a conclusion may be drawn as to whether the nuclear pressure-retaining component is safe.

5.4.1 Reference documents required for the integrity assessment. The reference documents for the integrity assessment shall include: the equipment design specification; the as-built drawings and the strength calculation report of the nuclear pressure-retaining component; the documents relating to the acceptance of the nuclear pressure-retaining component, including material data, welding records, repair records, non-destructive examination records, heat treatment reports and pressure test reports; the documents relating to the operating condition of the nuclear pressure-retaining component, including transient operating records, the treatment of non-conformities, in-service inspection records, failure records and maintenance records; the reference loads to be considered for the design basis, including the cyclic operating loads and their number of occurrences and the various postulated accidents.

5.4.2 Basic data required for the integrity assessment. The basic data for the integrity assessment shall include: the type, size and location of the defect; the geometric shape and dimensions of the structure and of the welds; the physical properties, chemical composition and mechanical properties of the material (including the fracture toughness and so on); the loads; the residual stresses; the effect of the environment on the properties of the material, such as irradiation and corrosion.

5.5.1 Defect examination. In accordance with the requirements of the integrity assessment and with the structural and material characteristics of the object assessed, non-destructive examination methods shall be used to detect the various defects that may be present; the examination results shall be accurate and reliable.

5.5.2 Stress analysis. All the possible loads shall be considered, and the stresses required for the integrity assessment shall be calculated, according to the specific failure mode, by mature and reliable calculation methods.

5.5.3 Testing and selection of the mechanical properties of the material. The mechanical properties of the material shall be selected in accordance with the provisions of Clause 8.

5.5.4 Assessment conclusion and report. After the defect assessment has been completed, the assessing organization shall promptly issue a complete assessment report and shall give a clear assessment conclusion and the conditions for continued service.

The assessment report shall in general include the following contents: the basic situation and data on the design, manufacture, installation and operation of the object assessed; the defect examination data; the testing and selection of the material property data; the stress analysis; the comprehensive safety evaluation and the assessment conclusion.

6 Characterization of planar defects

6.1.1 Defects are classified into the following types: planar defects, meaning cracks, lack of fusion, lack of penetration, undercut and so on; volumetric defects, meaning porosity, slag inclusions, local thinning and so on. Note: for the assessment of local thinning defects see Annex A.

6.1.2 In the integrity assessment, the measured planar defect shall be idealized and characterized in accordance with the provisions of this clause, and shall be characterized as one of a surface defect, an embedded defect or a through-wall defect. The characterization size of the defect shall be determined from the length l and the height h of the circumscribed rectangle of the defect, and the long side of the circumscribed rectangle shall be parallel to the adjacent shell surface (Figure 1). The dimensions and shapes of the idealized cracks are as follows: a surface crack is a semi-elliptical shape of height a and length 2c; an embedded crack is an elliptical shape of height 2a and major axis 2c; a through-wall crack is a rectangular shape of height t and length 2a; a corner crack at a hole edge is a quarter-elliptical shape of height a and length c.

6.1.3 For a component having a weld overlay (cladding), the wall thickness used in the characterization of the defect shall include both the thickness of the base metal and the thickness of the overlay. However, a defect located entirely within the overlay layer need not be assessed.

6.1.4 Where it cannot be established that a volumetric defect will not crack and grow, the volumetric defect shall be characterized and assessed as a planar defect.

6.2 Idealization of surface defects. For a surface defect of length l and height h, the characterization is as follows: when h is greater than 0.75 t, it is idealized as a through-wall crack of length 2a equal to l plus h (Figure 2 a)); when h is less than or equal to 0.75 t, then: if h is less than 0.5 l, it is idealized as a semi-elliptical surface crack with a equal to h and 2c equal to l (Figure 2 b)); if h is greater than or equal to 0.5 l, it is idealized as a semi-circular surface crack with a equal to h and 2c equal to 2h (Figure 2 c)).

6.3 Idealization of embedded defects. If the shortest distances from an embedded defect of length l and height h to the inner and outer surfaces of the shell are respectively p1 and p2, with p1 less than or equal to p2, then the following applies.

When p1 is less than or equal to p2 and p2 is less than 0.2 h, the defect is idealized as a through-wall crack with 2a equal to l plus h (Figure 3 a)).

When p1 is less than 0.2 h and 0.2 h is less than or equal to p2: if h plus p1 is less than 0.5 l, the defect is idealized as a semi-elliptical surface crack with a equal to h plus p1 and 2c equal to l (Figure 3 b)); if h plus p1 is greater than or equal to 0.5 l, the defect is idealized as a semi-circular surface crack with a equal to h plus p1 and 2c equal to twice the sum of h and p1 (Figure 3 c)).

In the same case, a surface crack for which a, equal to h plus p1, is less than or equal to 0.8 t need no longer be idealized as a through-wall crack; a surface crack for which a, equal to h plus p1, is greater than 0.8 t shall be idealized as a through-wall crack with 2a equal to l plus h plus p1.

When 0.2 h is less than or equal to p1 and p1 is less than or equal to p2: if h is less than or equal to l, the defect is idealized as an elliptical embedded crack with 2a equal to h and 2c equal to l (Figure 3 d)); if h is greater than l, the defect is idealized as a circular embedded crack with 2a equal to h and 2c equal to h (Figure 3 e)).

6.4 Idealization of corner defects at hole edges. A corner defect at a hole edge of height h and length l is idealized as a quarter-elliptical corner surface crack with a equal to h and c equal to l (Figure 4).

6.5 Idealization of inclined cracks. When the plane of the crack is not perpendicular to the direction of the principal stress, the crack shall be projected onto the plane perpendicular to the direction of the principal stress, and the idealized crack shall be determined within that plane from the projected dimensions.

6.6.1 If the two surface defects in Figure 5 a) satisfy one of the following conditions, they shall be idealized as a surface crack with a equal to the greater of a1 and a2 and with 2c equal to 2c1 plus 2c2 plus s1: the greater of the ratios a1 over c1 and a2 over c2 is greater than or equal to 1, and s1 is less than or equal to the smaller of 2c1 and 2c2; or a1 over c1 is less than 1, a2 over c2 is less than 1, and s1 equals 0.

6.6.2 If the spacing s2 of the two defects in Figure 5 b) is less than or equal to a1 plus a2, they shall be idealized as an embedded crack with 2a equal to 2a1 plus 2a2 plus s2 and with 2c equal to the greater of 2c1 and 2c2.

6.6.3 If the two defects in Figure 5 c) satisfy one of the following conditions, they shall be idealized as an embedded crack with 2a equal to twice the greater of a1 and a2 and with 2c equal to 2c1 plus 2c2 plus s1: the greater of the ratios a1 over c1 and a2 over c2 is greater than or equal to 1, and s1 is less than or equal to the smaller of 2c1 and 2c2; or a1 over c1 is less than 1, a2 over c2 is less than 1, and s1 equals 0.

6.6.4 If the spacing s2 of the two defects in Figure 5 d) is less than or equal to a1 plus a2, they shall be idealized as a surface crack with a equal to 2a1 plus a2 plus s2 and with 2c equal to the greater of 2c1 and 2c2.

6.6.5 If the two embedded defects in Figure 5 e) satisfy one of the following conditions, they shall be idealized as an embedded crack with 2a equal to 2a1 plus 2a2 plus s2 and with 2c equal to 2c1 plus 2c2 plus s1: the greater of the ratios a1 over c1 and a2 over c2 is greater than or equal to 1, with s1 less than or equal to the smaller of 2c1 and 2c2 and s2 less than or equal to a1 plus a2; or a1 over c1 is less than 1, a2 over c2 is less than 1, s1 is less than or equal to 0, and s2 is less than or equal to a1 plus a2.

6.6.6 If the two defects in Figure 5 f) satisfy one of the following conditions, they shall be idealized as a surface crack with a equal to 2a1 plus a2 plus s2 and with 2c equal to 2c1 plus 2c2 plus s1: the greater of the ratios a1 over c1 and a2 over c2 is greater than or equal to 1, with s1 less than or equal to the smaller of 2c1 and 2c2 and s2 less than or equal to a1 plus a2; or a1 over c1 is less than 1, a2 over c2 is less than 1, s1 is less than or equal to 0, and s2 is less than or equal to a1 plus a2.

6.7 Idealization of non-coplanar defects. The rules for the idealization of non-coplanar planar defects are given in this subclause of the standard.