1、raising standards worldwideNO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBSI Standards PublicationBS ISO 12108:2012Metallic materials Fatigue testing Fatigue crack growth methodBS ISO 12108:2012 BRITISH STANDARDNational forewordThis British Standard is the UK implementation o
2、f ISO 12108:2012. It supersedes BS ISO 12108:2002, which is withdrawn.The UK participation in its preparation was entrusted to T e c h n i c a l C o m m i t t e e I S E / 1 0 1 / 6 , F a t i g u e t e s t i n g o f m e t a l s a n d metal matrix composites.A list of organizations represented on this
3、 committee can be obtained on request to its secretary.This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. The British Standards Institution 2012Published by BSI Standards Limited 2012ISBN 978 0 580 66453 3 ICS 7
4、7.040.10 Compliance with a British Standard cannot confer immunity from legal obligations.This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 August 2012.Amendments issued since publicationDate T e x t a f f e c t e dBS ISO 12108:2012 ISO 2012
5、Metallic materials Fatigue testing Fatigue crack growth methodMatriaux mtalliques Essais de fatigue Mthode dessai de propagation de fissure en fatigueINTERNATIONAL STANDARDISO12108Second edition2012-08-15Reference numberISO 12108:2012(E)BS ISO 12108:2012ISO 12108:2012(E)ii ISO 2012 All rights reserv
6、edCOPYRIGHT PROTECTED DOCUMENT ISO 2012All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address
7、below or ISOs member body in the country of the requester.ISO copyright officeCase postale 56 CH-1211 Geneva 20Tel. + 41 22 749 01 11Fax + 41 22 749 09 47E-mail copyrightiso.orgWeb www.iso.orgPublished in SwitzerlandBS ISO 12108:2012ISO 12108:2012(E) ISO 2012 All rights reserved iiiContents PageFore
8、word . vIntroduction . vi1 Scope 12 Normative references . 13 Terms and definitions . 14 Symbols and abbreviated terms . 34.1 Symbols . 34.2 Abbreviated terms for specimen identification . 45 Apparatus 55.1 Testing machine . 55.2 Cycle-counter . 55.3 Grips and fixtures for CT specimens . 55.4 Grips
9、and fixtures for CCT/SENT specimens 75.5 Grips and fixtures for the SENB specimens 115.6 Crack length measurement apparatus 116 Specimens .126.1 General .126.2 Crack plane orientation .166.3 Starter notch precracking details .186.4 Stress-intensity factor 206.5 Specimen size 216.6 Specimen thickness
10、 .226.7 Residual stresses 237 Procedure 237.1 Fatigue precracking 237.2 Crack length measurement .237.3 Constant-force-amplitude, -increasing, test procedure for da/dN 105mm/cycle247.4 K-decreasing procedure for da/dN 0 conditions, it may be beneficial to plot the da/dN data versus Kmax.3.12fatigue
11、crack growth threshold stress-intensity factor rangeKthasymptotic value of K for which da/dN approaches zeroNOTE For most materials, the threshold is defined as the stress-intensity factor range corresponding to 108mm/cycle. When reporting Kth, the corresponding lowest decade of da/dN data used in i
12、ts determination should also be included.2 ISO 2012 All rights reservedBS ISO 12108:2012ISO 12108:2012(E)3.13normalized K-gradientC = (1/K) dK/dafractional rate of change of K with increased crack length, aC = 1/K (dK/da) = 1/Kmax(dKmax/da) = 1/Kmin(dKmin/da) = 1/K (dK/da)3.14K-decreasing testtest i
13、n which the value of the normalized K-gradient, C, is negativeNOTE A K-decreasing test is conducted by reducing the stress-intensity factor either by continuously shedding or by a series of steps, as the crack grows.3.15K-increasing testtest in which the value of C is positiveNOTE For standard speci
14、mens, a constant force amplitude results in a K-increasing test where the value of C is positive and increasing.3.16stress-intensity factor geometry functiong (a/W)mathematical expression, based on experimental, numerical or analytical results, that relates the stress-intensity factor to force and c
15、rack length for a specific specimen configuration3.17crack-front curvature correction lengthacordifference between the average through-thickness crack length and the corresponding crack length at the specimen faces during the test3.18fatigue crack lengthafatlength of the fatigue crack, as measured f
16、rom the root of the machined notchNOTE See Figure 12.3.19notch lengthanlength of the machined notch, as measured from the load line to the notch root NOTE See Figure 12.4 Symbols and abbreviated terms4.1 SymbolsSee Table 1. ISO 2012 All rights reserved 3BS ISO 12108:2012ISO 12108:2012(E)Table 1 Symb
17、ols and their designationsSymbol Designation UnitLoadingC Normalized K-gradient mm1E Tensile modulus of elasticity MPaF Force kNFmaxMaximum force kNFminMinimum force kNF Force range kNK Stress-intensity factor MPam1/2KmaxMaximum stress-intensity factor MPam1/2KminMinimum stress-intensity factor MPam
18、1/2K Stress-intensity factor range MPam1/2KiInitial stress-intensity factor range MPam1/2KthFatigue crack growth threshold stress-intensity factor range MPam1/2N Number of cycles 1R Force ratio or stress ratio 1RmUltimate tensile strength at the test temperature MPaRp0,20,2 % proof strength at the t
19、est temperature MPaGeometrya Crack length or size measured from the reference plane to the crack tip mmacorCrack-front curvature correction length mmafatFatigue crack length measured from the notch root mmanMachined notch length mmapPrecrack length mmB Specimen thickness mmDHole diameter for CT, SEN
20、T or CCT specimen, loading tup diameter for bend specimensmmg(a/W) Stress-intensity factor geometry function 1h Notch height mmW Specimen width, distance from reference plane to edge of specimen mm(W a) Minimum uncracked ligament mmCrack growthda/dN Fatigue crack growth rate mm/cyclea Change in crac
21、k length, crack extension mm4.2 Abbreviated terms for specimen identificationCT Compact tensionCCT Centre cracked tensionSENT Single edge notch tensionSEN B3 Three-point single edge notch bend4 ISO 2012 All rights reservedBS ISO 12108:2012ISO 12108:2012(E)SEN B4 Four-point single edge notch bendSEN
22、B8 Eight-point single edge notch bend5 Apparatus5.1 Testing machine5.1.1 GeneralThe testing machine shall have smooth start-up and a backlash-free force train if passing through zero force. See ISO 4965-1. Cycle to cycle variation of the peak force during precracking shall be less than 5 % and shall
23、 be held to within 2 % of the desired peak force during the test. F shall also be maintained to within 2 % of the desired range during test. A practical overview of test machines and instrumentation is available 33, 34.5.1.2 Testing machine alignmentIt is important that adequate attention be given t
24、o alignment of the testing machine and during machining and installation of the grips in the testing machine.For tension-compression testing, the length of the force train should be as short and stiff as practical. Non-rotating joints should be used to minimize off-axis motion.Asymmetry of the crack
25、 front is an indication of misalignment; a strain gauged specimen similar to the test article under investigation can be used in aligning the force train and to minimize nonsymmetrical stress distribution and/or bending strain to less than 5 %.5.1.3 Force measuring systemAccuracy of the force measur
26、ing system shall be verified periodically in the testing machine. The calibration for the force transducer shall be traceable to a national organization of metrology. The force measuring system shall be designed for tension and compression fatigue testing and possess great axial and lateral rigidity
27、. The indicated force, as recorded as the output from the computer in an automated system or from the final output recording device in a noncomputer system, shall be within the permissible variation from the actual force. The force transducers capacity shall be sufficient to cover the range of force
28、 measured during a test. Errors greater than 1 % of the difference between minimum and maximum measured test force are not acceptable.The force measuring system shall be temperature compensated, not have zero drift greater than 0,002 % of full scale, nor have a sensitivity variation greater than 0,0
29、02 % of full scale over a 1 C change. During elevated and cryogenic temperature testing, suitable thermal shielding/compensation shall be provided to the force measuring system so it is maintained within its compensation range.5.2 Cycle-counterAn accurate digital device is required to count elapsed
30、force cycles. A timer is to be used only as a verification check on the accuracy of the counter. It is preferred that individual force cycles be counted. However, when the crack velocity is below 105mm/cycle, counting in increments of 10 cycles is acceptable.5.3 Grips and fixtures for CT specimensFo
31、rce is applied to a CT specimen through pinned joints. The choice of this specimen and gripping arrangement necessitates tension-tension test conditions only. Figure 1 shows the clevis and mating pin assembly used at both the top and bottom of a CT specimen to apply the force perpendicular to the ma
32、chined starter notch and crack plane. Suggested dimensions are expressed as a proportion of specimen width, W, or thickness, B, since these dimensions can vary independently within the limits specified in Clause 6. The pin holes have a generous clearance over the pin diameter, 0,2W minimum, to minim
33、ize resistance to specimen and pin in-plane rotation which has been shown to cause nonlinearity in the force versus displacement response 35. A surface finish, ISO 2012 All rights reserved 5BS ISO 12108:2012ISO 12108:2012(E)Ra, range of 0,8 m to 1,6 m is suggested for grip surfaces. With this grip-a
34、nd-pin arrangement, materials with low proof strength may sustain plastic deformation at the specimen pin hole; similarly, when testing high strength materials and/or when the clevis displacement exceeds 1,05B, a stiffer force pin, i.e. a diameter greater than 0,225W, may be required. As an alternat
35、ive approach to circumvent plastic deformation, a flat bottom clevis hole may be used along with a pin diameter equaling 0,24W. Any heat treatable steel thermally processed to a 0,2 % proof strength of 1 000 MPa used in fabricating the clevises will usually provide adequate strength and resistance t
36、o fretting, galling and fatigue.In addition to the generous pin hole clearance, the mating surfaces shall be prepared to minimize friction which could invalidate the provided K-calibration expression. The use of high viscosity lubricants and greases has been shown to cause hysteresis in the force ve
37、rsus displacement response and is not recommended if compliance measurements are required.Key1 clevis2 pinNOTE For high strength materials or large pin displacements, the pin may be stiffened by increasing the diameter to 0,24W along with using D-shaped flat bottom holes.aLoading rod thread.bThrough
38、 diameter.cThese surfaces are perpendicular and parallel as applicable to within 0,05W.Figure 1 Clevis and pin assembly for gripping a CT specimen6 ISO 2012 All rights reservedBS ISO 12108:2012ISO 12108:2012(E)5.4 Grips and fixtures for CCT/SENT specimens5.4.1 GeneralForce can be applied to CCT and
39、SENT specimens through pinned joints and/or through frictional clamping grips. Gripping for the CCT and SENT specimens depends on specimen width and whether the test condition is to be tension-tension or tension-compression. The minimum CCT specimen gauge length varies with gripping arrangement and
40、shall provide a uniform stress distribution in the gauge length during the test.Under certain conditions, the CCT specimen can be prone to general and localized buckling. The use of buckling constraints is recommended.49Formula (6) is applicable only for a single pinned end SENT specimen, as shown i
41、n Figure 2. The SENT pinned end specimen (Figure 2) is appropriate for tension-tension test conditions only.Formula (7) is applicable for a SENT specimen with clamped ends and is appropriate for both tension and compression force conditions. For the clamped-end SENT specimen, the grips must be suffi
42、ciently stiff to circumvent any rotation of the specimen ends or any lateral movement of the crack plane; the presence of either condition introduces errors into the stress-intensity factor calculation 29.Surface roughness values in micrometresNOTE 1 The machined notch is centred to within 0,005W (T
43、IRe). NOTE 2 The surfaces are parallel and perpendicular to within 0,002W.NOTE 3 The crack length is measured from the reference loading plane containing the starter V-notch.NOTE 4 This specimen is recommended for notch root tension at a force ratio R 0 only.aD = W / 3.bSee Figure 12 for notch detai
44、l.cReference plane.dRecommended thickness: B 0,5W.eTotal indicated reference value.Figure 2 Standard single edge notch tension, SENT, specimen5.4.2 Tension-tension testing of a CCT specimenFor tension-tension testing of a specimen with a width 2W, less than 75 mm, as shown in Figure 3, a clevis with
45、 single force pin is acceptable for gripping provided the specimen gauge length, defined here as the distance between the pin hole centrelines, be at least 6W. Shims may be helpful in circumventing fretting fatigue at the specimens pin hole. Another step that can be taken to prevent crack initiation
46、 at the pin holes is the welding or adhesive bonding of reinforcement plates or tabs to the gripping area, especially when testing very thin materials. Cutting the test section down in width to form a “dog bone” shaped specimen design is another ISO 2012 All rights reserved 7BS ISO 12108:2012ISO 121
47、08:2012(E)measure that can be adopted to circumvent failure at the pin holes; here the gauge length is defined as the uniform width section and it shall be at least 3,4W in length.For tension-tension testing of a specimen with a width greater than 75 mm, distributing the force across the specimen wi
48、dth with multiple pin holes is recommended. A serrated grip surface at the specimen-grip interface increases the force that can be transferred. With this force application arrangement, the gauge length between the innermost rows of pin holes must be at least 3W.Surface roughness values in micrometre
49、sNOTE 1 The machined notch is centred to within 0,002W.NOTE 2 The faces are parallel to 0,05 mm/mm.NOTE 3 The two faces are not out-of-plane more than 0,05 mm.NOTE 4 The crack length is measured from the reference plane of the longitudinal centreline.NOTE 5 The clevis and pin loading system is not suitable for a force ratio R 40 HRC) steel 23.5.6 Crack length measurement apparatus5.6.1 GeneralAccurate measurement of crack length during the test is very important. There are a num
