BS 7448-4-1997 Fracture mechanics toughness tests Method for determination of fracture resistance curves and initiation values for stable crack extension in metallic materials《断裂机械.pdf

1、BRITISH STANDARD BS 7448-4:1997 Incorporating Corrigendum No. 1 Fracture mechanics toughness tests Part 4: Method for determination of fracture resistance curves and initiation values for stable crack extension in metallic materials ICS 77.040.10 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED

2、 BY COPYRIGHT LAWBS 7448-4:1997 This British Standard, having been prepared under the direction of the Engineering Sector Board, was published under the authority of the Standards Board and comes into effect on 15 December 1997 BSI 07-2001 The following BSI references relate to the work on this stan

3、dard: Committee reference ISE/NFE/4 Draft for comment 93/301852 DC ISBN 0 580 28630 4 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee ISE/NFE/4, Mechanical testing of metals, upon which the following bodies were represent

4、ed: British Non-ferrous Metals Federation British Railways Board Copper Development Association Electricity Association ERA Technology Ltd. GAMBICA (BEAMA Ltd.) Institute of Materials Ministry of Defence National Physical Laboratory Society of British Aerospace Companies Limited The Welding Institut

5、e UK Steel Association United Kingdom Accreditation Service University College London Co-opted members The following bodies were also represented in the drafting of the standard, through subcommittees and panels: BEAMA Ltd. Castings Development Centre Lloyds Register of Shipping Amendments issued si

6、nce publication Amd. No. Date Comments 12017 Corrigendum No. 1 July 2001 Corrections to Figure 9 and 9.3.1.BS 7448-4:1997 BSI 07-2001 i Contents Page Committees responsible Inside front cover Foreword iii 1S c o p e 1 2 References 1 3 Definitions 1 4 Symbols and designations 2 5 Principle 3 6 Test s

7、pecimens 4 7 Specimen preparation and fatigue precracking 12 8 Test equipment 15 9 Multiple specimen test procedure 19 10 Single specimen test procedure 24 11 Analysis of test data 25 12 Construction of R-curves 28 13 Determination of fracture parameters 31 14 Check lists for qualification of data 3

8、5 15 Test report 36 Annex A (informative) Bibliography 37 Annex B (normative) Crack plane identification 39 Annex C (informative) Measurement of load-line displacement, q, in a three-point bend test 41 Annex D (informative) Single specimen methods 43 Annex E (normative) Analytical methods for the de

9、termination of V pand U p 44 Annex F (normative) Offset power law fit to crack extension fracture resistance data 45 Figure 1 Flow chart for the test method 5 Figure 2 Proportional dimensions and tolerances for a single edge notch bend specimen 6 Figure 3 Proportional dimensions and tolerances for a

10、 straight notch compact specimen 7 Figure 4 Proportional dimensions and tolerances for a stepped notch compact specimen 8 Figure 5 Acceptable fatigue crack starter notches and fatigue crack configurations 9 Figure 6 Outward pointing knife edges and corresponding notch geometries 10 Figure 7 Inward p

11、ointing knife edges and corresponding notch geometries 11 Figure 8 Typical sidegrooved specimen 12 Figure 9 Fixture for three-point bend tests 17 Figure 10 Typical design of clevis for applying a tensile force to a compact specimen 18 Figure 11 Typical design of clevis for applying a tensile force t

12、o a compact specimen (suitable for the single specimen method employing the unloading compliance technique) 19 Figure 12 Measurement of crack lengths on single edge notch bend specimens 23BS 7448-4:1997 ii BSI 07-2001 Page Figure 13 Measurement of crack lengths on compact specimens 24 Figure 14 Sing

13、le specimen crack extension termination requirement 25 Figure 15 Graphical determination of V p 26 Figure 16 Determination of U p 28 Figure 17 Data spacing and curve fit 30 Figure 18 Limits to or J controlled crack extension 31 Figure 19 Derivation of 0.2BLand J 0.2BL 33 Figure 20 Derivation of 0.2a

14、nd J 0.2 34 Figure B.1 Basic fracture plane identification: rectangular section 39 Figure B.2 Basic fracture plane identification: cylindrical sections 40 Figure B.3 Non-basic fracture plane identification 40 Figure C.1 Schematic representation of the “comparator” bar 42 Figure C.2 Displacements ass

15、ociated with single edge notch bend specimens 42 Figure C.3 Determination of extraneous displacements for single edge notch bend specimens 43 Figure C.4 Location of two notch opening displacement measurements (V 1and V 2 ) for the determination of load-line displacement 43 Figure F.1 General form of

16、 the offset power law expression y = m + l( a) x 46 Table 1 Values of g 1(a o /W) for three-point bend specimens 14 Table 2 Values of g 2(a o /W) for compact specimens 15 List of references Inside back coverBS 7448-4:1997 BSI 07-2001 iii Foreword This Part of BS 7448 has been prepared by Technical C

17、ommittee ISE/NFE/4. It gives a method for the determination of fracture resistance curves and initiation values for stable crack extension in metallic materials. BS 7448 is being published in four Parts. Part 1 describes a method for determining K Ic , critical crack tip opening displacement (CTOD)

18、and critical J values of fracture toughness for metallic materials under displacement controlled monotonic loading at quasistatic rates. Part 2 describes a method for determining the fracture toughness of weldments in metallic materials. A further Part of BS 7448 is in preparation as follows. Part 3

19、 describes a method for determining dynamic fracture toughness. It has been assumed in the drafting of this standard that the execution of its provisions is entrusted to appropriately qualified and experienced people. CAUTION. It is important to note that tests of the type described involve the use

20、of large forces, and may involve the rapid movement of machine parts and fractured test specimens. Therefore it is important to consider the safety of machine operators. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsi

21、ble for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 46, an inside back cover and a back cover. The BSI copyright notic

22、e displayed in this document indicates when the document was last issued.iv blankBS 7448-4:1997 BSI 07-2001 1 1 Scope This Part of BS 7448 gives a method for determining the resistance to stable crack extension of metallic materials. The method uses fatigue precracked specimens. These are tested in

23、displacement controlled loading at a constant rate of increase in stress intensity factor within the range 0.5 MPam 0.5 s 1to 3.0 MPam 0.5 s 11)during the initial elastic deformation. Resistance to crack extension is characterized in terms of the variation in crack tip opening displacement (CTOD) an

24、d/or J with stable crack extension. These characterizations are referred to in this standard as and J R-curves, respectively. The method is only suitable for tests on specimens that exhibit stable crack extension. If unstable fracture or crack arrest occurs in any of the tests in an R-curve determin

25、ation it is necessary also to assess the test results in accordance with BS 7448-1. Testing requirements and analysis procedures are given which enable crack extension resistance curves to be calculated from both straight notch and stepped notch compact specimens and single edge notch bend specimens

26、. Individual fracture parameters are also defined for CTOD and J at a point where stable crack extension is close to the onset of initiation. The definition of fracture toughness values relevant to particular structural integrity assessments is outside the scope of this British Standard. NOTE Public

27、ations of relevance to the subject matter covered by this Part of BS 7448 are listed in the bibliography in Annex A. A system of crack plane identification is given in Annex B. 2 References 2.1 Normative references This Part of BS 7448 incorporates, by dated or undated reference, provisions from oth

28、er publications. These normative references are made at the appropriate places in the text and the cited publications are listed on the inside back cover. For dated references, only the edition cited applies; any subsequent amendments to or revisions of the cited publication apply to this Part of BS

29、 7448 only when incorporated in the reference by amendment or revision. For undated references, the latest edition of the cited publication applies, together with any amendments. 2.2 Informative references This Part of BS 7448 refers to other publications that provide information or guidance. Editio

30、ns of these publications current at the time of issue of this standard are listed on the inside back cover, but reference should be made to the latest editions. 3 Definitions For the purposes of this Part of BS 7448 the following definitions apply. 3.1 stress intensity factor (K) the magnitude of th

31、e elastic stress field singularity for a homogeneous, linear-elastic body NOTE It is a function of applied force, crack length and specimen geometry, and is expressed in units of MPam 0.5 . 3.2 crack tip opening displacement (CTOD) the relative displacement of the surfaces of a crack normal to the o

32、riginal (undeformed) crack plane at the tip of the fatigue precrack, expressed in millimetres 3.3 J-integral a line or surface integral that encloses the crack front from one crack surface to the other, which characterizes the local stress-strain field at the crack tip, expressed in MJ/m 22) 3.4 J a

33、n experimental equivalent of the J-integral as determined by the method given in the present standard 1) 0.0316 MPam 0.5= 1 Nmm 1.5= 0.0316 MNm 1.5 . 2) 1MJ/m 2= 1 MN/m.BS 7448-4:1997 2 BSI 07-2001 3.5 unstable crack extension an abrupt crack extension which occurs with or without prior stable crack

34、 extension 3.6 pop-in an abrupt discontinuity in the force versus displacement record, featured as a sudden increase in displacement and, generally, a decrease in force NOTE Subsequently displacement and force generally increase to above their values at pop-in. 3.7 stable crack extension crack exten

35、sion which, in displacement control, stops when the applied displacement is held constant 3.8 stretch zone width (SZW) the length of crack extension that occurs during crack tip blunting; that is, prior to the onset of slow stable crack extension, and which occurs in the same plane as the fatigue pr

36、ecrack 4 Symbols and designations For the purposes of this Part of BS 7448 the following symbols and designations apply. a nominal crack length (see Figure 2 to Figure 5) or, for the purposes of fatigue precracking (see 7.4.4 and 7.4.5), an assumed value a o , in millimetres a o measured original cr

37、ack length (see 9.9.2), in millimetres B thickness of test specimen, in millimetres B min minimum test specimen net thickness, in millimetres B N test specimen net thickness between sidegroove tips, in millimetres C specimen elastic compliance, in millimetres per kilonewton E Youngs modulus of elast

38、icity at the temperature of interest, in gigapascals F applied force, in kilonewtons F L collapse force, calculated using R m , in kilonewtons F f maximum fatigue precracking force during the final stages of fatigue crack extension (see 7.4.4 and 7.4.5), in kilonewtons J experimental equivalent of t

39、he J-integral, in megajoules per square metre J o resistance to crack extension expressed in terms of J, not allowing for stable crack extension, in megajoules per square metre J corr resistance to crack extension expressed in terms of J, corrected for stable crack extension, in megajoules per squar

40、e metre J 0.2BL resistance to crack extension expressed in terms of J, at 0.2 mm crack extension offset to the blunting line, in megajoules per square metre J 0.2 resistance to crack extension expressed in terms of J, at 0.2 mm crack extension including blunting, in megajoules per square metre J max

41、 resistance to crack extension expressed in terms of J, at the limit for J controlled crack extension, in megajoules per square metre K stress intensity factor, in MPam 0.5 rate of change of K with time, in MPam 0.5 s 1 q displacement of bend specimen or stepped notch compact specimen along the load

42、-line, in millimetres S span between outer loading points in a three point bend test, in millimetres T test temperature, in degrees celsius U p plastic component of area under plot of force (F) versus specimen displacement along the load-line (see Figure 16), in joulesBS 7448-4:1997 BSI 07-2001 3 3)

43、 5 Principle A fatigue crack is extended in a single edge notched bend or compact specimen by applying an alternating force within controlled limits. The specimen is then subjected to an increasing force. One of two techniques is used to determine crack extension: the multiple specimen method or the

44、 single specimen method. The multiple specimen method is regarded as the definitive approach. In the multiple specimen method a minimum of six nominally identical specimens are monotonically loaded to different displacements. NOTE If a multiple specimen R-curve is required in terms of both and J fra

45、cture toughness parameters, it may be necessary to test more than six specimens in order to satisfy the various spacing requirements (see 12.3). Measurements are made of forces and displacements up to the point of termination of the test. The specimen crack fronts are marked after testing thus enabl

46、ing measurement of stable crack extension after the specimens have been broken open. Alternatively, a single specimen method based on, for example, the unloading compliance or potential drop technique is used (see clause 10). The tests are then analysed to obtain resistance to crack extension curves

47、 (R-curves) i.e. a plot of fracture resistance, in terms of or J, against stable crack extension a. These can then be further analysed to obtain estimates of fracture toughness at the initiation of stable crack extension. This procedure is summarized in a flow chart in Figure 1. 3) At present no Bri

48、tish Standard exists for the measurement of tensile properties below ambient temperature. In these cases the values to be used are subject to agreement between interested parties. V notch opening displacement at or near to notch mouth, in millimetres V e elastic component of V, in millimetres V p pl

49、astic component of V, in millimetres W width of test specimen, in millimetres W t total width of compact specimen, in millimetres z for bend and straight notch compact specimens, distance of the notch opening gauge location above the surface of the specimen see Figure 6 b), or, in the case of a stepped notch compact specimen the distance of the notch opening gauge location above the load line, in millimetres crack tip opening displacement (CTOD) o resistance to crack extension exp