Aerospace series - Test methods for metallic materials - Determination of fatigue crack growth rates using Corner-Cracked (CC) test pieces

This standard specifies the requirements for determining fatigue crack growth rates using the corner-crack (CC) test piece. Crack development is measured using a potential-drop system, and the calculated crack depths can be corrected via marker bands created on the fracture surface during the test. Results are expressed in terms of the crack-tip stress-intensity range (K), with crack depths and test stress level noted.

Luft- und Raumfahrt - Prüfverfahren für metallische Werkstoffe - Ermittlung der Rissfortschritts- Geschwindigkeit an Cornercrackproben (Eckanris)

Diese Norm legt die Anforderungen für die Bestimmung der Geschwindigkeit des Ermüdungswachstums unter
Verwendung von Eckanriss-Proben (Corner-Crack, CC) fest. Die Rissentwicklung wird mit einem
Potentialabfallsystem gemessen, und die berechneten Risstiefen können über Markierungsbänder, die im
Verlauf der Prüfung an der Bruchfläche erzeugt wurden, korrigiert werden. Die Ergebnisse werden in Form
der Schwingbreite der Rissspitzen-Spannungsintensität (ΔK) dargestellt, wobei die Risstiefen und das
Prüfspannungsniveau aufgezeichnet werden.

Série aérospatiale - Méthodes d'essais applicables aux matériaux métalliques - Détermination de la vitesse de propagation de fissure en fatigue à l'aide d'éprouvettes avec fissure en coin

Aeronavtika - Preskusne metode za kovinske materiale - Ugotavljanje stopnje rasti razpok, ki nastanejo zaradi utrujenosti, z uporabo preskusnih kosov z razpokami na robovih (CC)

Ta standard določa zahteve za ugotavljanje stopnje rasti razpok, ki nastanejo zaradi utrujenosti, z uporabo preskusnega kosa z razpokami na robovih (CC). Razvoj razpok je merjen z uporabo sistema za potencialni padec, izračunana globina razpoke pa je lahko popravljena z označevalnimi pasovi, narejenimi na razpokani površini med preskusom. Rezultati so izraženi v izrazih za razpon intenzivnosti napetosti konice razpoke (K), z globino razpoke in upoštevano stopnjo preskusne napetosti.

General Information

Status
Published
Publication Date
02-Nov-2010
Technical Committee
Drafting Committee
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
03-Nov-2010
Completion Date
03-Nov-2010

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Aeronavtika - Preskusne metode za kovinske materiale - Ugotavljanje stopnje rasti razpok, ki nastanejo zaradi utrujenosti, z uporabo preskusnih kosov z razpokami na robovih (CC)Luft- und Raumfahrt - Prüfverfahren für metallische Werkstoffe - Ermittlung der Rißfortschritts- Geschwindigkeit an Cornercrackproben (Eckanris)Série aérospatiale - Méthode d'essais applicables aux matériaux métalliques - Détermination de la vitesse de propagation de fissure en fatigue à l'aide d'éprouvettes avec fissure en coinAerospace series - Test methods for metallic materials - Determination of fatigue crack growth rates using Corner-Cracked (CC) test pieces49.025.05Železove zlitine na splošnoFerrous alloys in generalICS:Ta slovenski standard je istoveten z:EN 3873:2010SIST EN 3873:2011en01-maj-2011SIST EN 3873:2011SLOVENSKI

STANDARD
SIST EN 3873:2011
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 3873
November 2010 ICS 49.025.05; 49.025.15 English Version

Aerospace series - Test methods for metallic materials - Determination of fatigue crack growth rates using Corner-Cracked (CC) test pieces

Série aérospatiale - Méthodes d'essais applicables aux matériaux métalliques - Détermination de la vitesse de propagation de fissure en fatigue à l'aide d'éprouvettes avec fissure en coin

Luft- und Raumfahrt - Prüfverfahren für metallische Werkstoffe - Ermittlung der Rißfortschritts-Geschwindigkeit an Cornercrackproben (Eckanris) This European Standard was approved by CEN on 30 July 2010.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre:
Avenue Marnix 17,

B-1000 Brussels © 2010 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 3873:2010: ESIST EN 3873:2011

EN 3873:2010 (E) 2 Contents Page Foreword .............................................................................................................................................................. 3Introduction ......................................................................................................................................................... 41Scope ...................................................................................................................................................... 52Normative references ............................................................................................................................ 53Symbols and abbreviations .................................................................................................................. 54General .................................................................................................................................................... 75Resources............................................................................................................................................... 86Test pieces ............................................................................................................................................. 97Procedures ........................................................................................................................................... 118Health and safety ................................................................................................................................. 149Evaluation of results ........................................................................................................................... 1410Test record ........................................................................................................................................... 16Annex A (normative)

Information on measuring crack depths in corner-crack test pieces with the direct-current − Potential-drop method ...................................................................................... 18Annex B (normative)

Stress-intensity function for corner-crack test pieces
............................................ 22Annex C (normative)

Guidelines on test piece handling and degreasing .................................................. 24SIST EN 3873:2011

EN 3873:2010 (E) 3 Foreword This document (EN 3873:2010) has been prepared by the Aerospace and Defence Industries Association of Europe - Standardization (ASD-STAN). After enquiries and votes carried out in accordance with the rules of this Association, this Standard has received the approval of the National Associations and the Official Services of the member countries of ASD, prior to its presentation to CEN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by May 2011, and conflicting national standards shall be withdrawn at the latest by May 2011. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.

SIST EN 3873:2011

EN 3873:2010 (E) 4 Introduction This standard and its parts belong to the general organization of the ASD collection of metallic material standards for aerospace applications SIST EN 3873:2011

EN 3873:2010 (E) 5 1 Scope This standard specifies the requirements for determining fatigue crack growth rates using the corner-crack (CC) test piece. Crack development is measured using a potential-drop system, and the calculated crack depths can be corrected via marker bands created on the fracture surface during the test. Results are expressed in terms of the crack-tip stress-intensity range (∆K), with crack depths and test stress level noted. 2 Normative references The following referenced 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. EN 2002-002, Aerospace series — Metallic materials — Test methods — Part 002: Tensile testing at elevated temperature EN ISO 7500-1, Metallic materials — Verification of static uniaxial testing machines — Part 1: Tension/compression testing machines — Verification and calibration of the force-measuring system (ISO 7500-1:2004) EN ISO 3785, Metallic materials — Designation of test specimen axes in relation to product texture (ISO 3785:2006) ASTM E 647-2008, Standard test method for measurement of fatigue crack growth rates 1) ASTM E 1012-2005, Verification of test frame and specimen alignment under tensile and compressive axial force application 1) 3 Symbols and abbreviations a Crack depth. The crack depth a is the distance from the extrapolated original corner containing the notch to the centre of the crack front (45° position). For the calculation of stress-intensity factor, the crack length must be given in metres ae Final crack depth (in millimetres) ai Initial crack depth (in millimetres) am Measured crack depth (optical, post-test fracture surface micrography or with SEM) av Calculated (Potential) crack depth is the average crack depth due to the averaging nature of the potential measurement method. Calculation involving average lengths measured at several positions along the crack front are best for correlation with the potential measurements (in millimetres) ∆a Crack growth increment (in millimetres) da/dN Fatigue crack growth rate (FCGR) (in metres per cycle) W Test piece width (in millimetres)

1) Published by: American Society for Testing and Materials (ASTM), http://www.astm.org/. SIST EN 3873:2011

EN 3873:2010 (E) 6 a/W Normalized crack depth (in millimetres/millimetres) f Frequency (in Hertz) E Young's modulus (in gigapascals) K Stress-intensity factor (general) The stress-intensity factor K is a load parameter which characterises the stress field at the crack tip. It is a function of load, crack depth and test piece geometry (in MPa √m) Kmax. Maximum value of K during a loading cycle, corresponding to the maximum tensile force applied (in MPa √m) Kmin. Minimum value of K during a loading cycle (in MPa √m) ∆K Range of K during a loading cycle = Kmax. − Kmin. = (1 − R)*Kmax. (in MPa √m) ∆Keff

Effective range of K, due to crack closure-induced reduction applied ∆K (in MPa √m) ∆Kth Fatigue crack growth threshold

The asymptotic value of ∆K for which da/dN approaches zero. For most materials the operational threshold is defined as the ∆K corresponding to 10-7 m/cycle. When reporting ∆Kth, the corresponding lowest decade of near threshold data used in its determination must be given C Normalized K-gradient C = (1/K × dK/da). For load-shedding to attain a desired initial ∆K, C defines the fractional rate of change of K with increasing crack depth a.

C = 1/K × dK/da = 1/Kmax. × dKmax./da = 1/Kmin. × dKmin./da = 1/∆K × d∆K /da, (in mm-1) N Number of loading cycles Stress cycle (fatigue cycle, load cycle) is the smallest segment of the loading waveform spectrum which is repeated periodically N'

Number of stress cycles between two marker cycles Nm Number of stress cycles in a marker cycle ∆N Stress cycle difference r Notch radius (expressed in millimetres) Fm Mean force (expressed in kilonewtons) Fmax. Maximum tensile force applied to the test piece during a cycle (expressed in kilonewtons) Fmin. Minimum tensile force (in kilonewtons) ∆F Force range (in kilonewtons), ∆F = Fmax. − Fmin. q Resolution of crack depth measuring system (expressed in millimetres) R Force ratio (= Fmin./Fmax. = Kmin./Kmax.) R*m Ratio Fmin./Fmax. during a marker cycle Rp 0,2 % offset yield strength (Proof Stress Rp0,2) at test temperature (expressed in megapascals) SIST EN 3873:2011

EN 3873:2010 (E) 7 Rm Tensile strength at test temperature (expressed in megapascals) σf Flow stress – here defined as the arithmetic mean of Rp and Rm (expressed in megapascals) Z Axial distance from crack plane to each wire used for potential measurement (expressed in millimetres) − (2Z = wire separation distance) 4 General 4.1 The corner-crack (CC) test piece is useful in determining da/dN for components where the cracks usually appear at a corner, such as in holes in turbine disks. The determination involves the use of an axially-loaded test piece of square or rectangular cross-section. It may be loaded in tension and compression for positive and negative stress ratio testing if suitable end designs permit backlash-free loading. A carefully defined and produced notch or a small arc strike enables cracking to be initiated at the centre of the reduced section. A fatigue crack is induced at the root of the notch by cyclic loading, and its growth is monitored by a suitable method, e.g. potential drop techniques. As the crack grows, the force range applied to the test piece is maintained or reduced in a controlled manner until the cracks are of sufficient depth for the influence of the notch and the crack initiation method to be negligible, and the ∆K has reached the lowest level of interest. The test is then carried out. The force range is maintained constant and the crack depth recorded as a function of elapsed cycles. These data are then subjected to numerical analysis, enabling da/dN to be determined as a function of ∆K. 4.2 The majority of metallic materials can be tested using the method described here, provided that the force applied is such as to ensure that the plastic zone in front of the crack tip is small in relation to the remaining cross section (linear-elastic criterion). 4.3 The test piece used here is a corner-crack (CC) test piece. See Figure C.1. 4.4 In the standard crack-growth test the load amplitude is assumed to be constant throughout the test, after the required ∆K level and R-ratio is reached. Another load range can be added if certain transient effects are to be investigated. 4.5 The range of the stress-intensity factor ∆K is: ∆K = Kmax. − Kmin. (1) where the ratio R R = min.max.KK (2) applies. From Equations (1) and (2) it follows that, for R > 0, ∆K = (1 − R) Kmax.

(3) 4.6 The reference point for measuring the crack depth with CC test pieces is the original corner of the test piece, determined by the projections of the sides of the test piece on the fracture surface adjacent to the notch. Possible rounding of the corner during test piece manufacture will result in this reference point being no longer on the fracture surface. This rounding must be determined to obtain a “Zero-point offset” between the reference point and the rounded corner where the measurement wire is welded, which is used in the calibration of the potential-drop measurements. SIST EN 3873:2011

EN 3873:2010 (E) 8 4.7 The purpose of the crack propagation measurements is to allocate the relevant load cycles N to the crack depth a. The measurements (a-N points, see Figure C.2) are normally evaluated in the form of a da/dN versus ∆K curve (see Figure C.3). It is not always the case that the crack propagation can be described by the range of stress-intensity factor ∆K. If it cannot be so described, other laws can be applied, e.g. crack growth rate as a function of Kmax. 4.8 The crack growth behaviour depends on a number of parameters. The framework within which the test is to be carried out needs to be precisely defined in order to avoid undesired effects on the results. The most important factors affecting the results are: a) temperature and environment; b) load spectrum. The test parameters R, dwell time and loading frequency must be defined and recorded before testing commences. The results can also be affected by the loading history, including interruption times, e.g. stop and restart of cycling to check surface crack length or other parameters, work stoppage at weekends, etc. These should be recorded. c) Residual stresses Residual stresses are usually ignored, as they are difficult to determine, and a duplication of the residual stresses in a component is very difficult to obtain in a test piece. Their presence in a component will affect the life of the component, and should be regarded in the use of the crack growth data. The presence of unexpected residual stresses in the test piece may be witnessed in an asymmetry of the crack front. 4.9 Applicability of results The crack-growth measurements are generally used for: a) investigating the influence of fatigue-crack growth on the predicted life of a component, or for evaluating the crack-growth resistance of a material or heat-treat condition; b) defining the requirements of NDT testing; and c) macroscopic quantitative determination of various factors (e.g. load, microstructure, manufacture, etc.). 5 Resources 5.1 Test machine 5.1.1 General Tests shall be performed with a feed-back load-controlled servohydraulic or electromechanical test system designed for smooth loading from first load cycle without exceeding the desired Fmax. The system should be capable of halting the cycling at desired intervals of cycles or crack depth, at a desired potential level, or at will, to enable measurements of the optical crack depth, potential or thermal potential, without stopping the test or causing overloads during the following restart. 5.1.2 Load control The apparatus must satisfy the following requirements in accordance with EN ISO 7500-1: a) accuracy of electronic force measurement: ± 0,5 % and ± 0,25 % of nominal range respectively; SIST EN 3873:2011

EN 3873:2010 (E) 9 b) accuracy of control throughout testing: better than 0,5 % of specified value of ∆F; c) recording instrument voltage requirements (upper and lower stress range, cycles): digital recorder is recommended; d) recording accuracy throughout testing: better than 0,25 %. 5.1.3 Load alignment Good alignment in the load train is essential for ensuring loading symmetry. An alignment test should be carried out. The loading train should be rigid, to avoid loading eccentricity as the crack grows, which would influence the applied stress-intensity factor at the crack tip. Alignment should be carried out in accordance with ASTM E 1012. 5.2 Calibration All instruments shall be calibrated at least once a year, as well as after every incident that may have affected the calibration accuracy. Multiplication factors (e.g. ×10 or ×100) shall not be used when counting the cycles, unless the factor is less than 10×-2, where 10-× mm/cycle is to be measured. Thermocouples should be calibrated every six months in accordance with EN ISO 3785. 5.3 Temperature measurement and control Temperature of the test piece shall be measured by a calibrated Platinum/ Rh (Type R) or Chromel-Alumel (Type K) thermocouple in adequate thermal contact with the test piece, at the centreline of one face adjacent to the notch, 2 mm to 4 mm above or below the crack plane. Shielding of the junction from radiation is not necessary if the difference in indicated temperature from an unshielded bead and a bead inserted in a hole in the test piece has been shown to be less than one-half the permitted variation shown below. Throughout the test, the temperature shall not deviate from the specified values by more than the following: For elevated temperature tests up to (1 000 ± 3) °C: (1 000 ± 4) °C to (1 100 ± 4) °C. Temperatures shall be recorded and monitored, irrespective of the accuracy of temperature control, by each change of 1 °C. The recording accuracy shall be better than 0,25 % of the specified value. Room temperature variations; i.e. at night, over weekends, should be known and limited to ± 5 °C. For tests at elevated temperatures, a three-zone furnace featuring electronic PID control shall be used. The grips must also be heated. 5.4 Gripping of test pieces To compensate for the weight of the grips and fixtures, the load cell must be adjusted to ensure that the test piece is not under stress at zero indicated force. 6 Test pieces 6.1 Corner-crack (CC) test piece The corner-crack (CC) test piece is illustrated in Figure C.1. CC test pieces may be used with positive or negative R-ratio loading, assuming the gripping system can transmit loads without backlash. SIST EN 3873:2011

EN 3873:2010 (E) 10 6.2 Stress-intensity − Factor calculation The stress-intensity factor for the CC test piece is calculated as follows (Annex B describes this in more detail): First, the geometry factor for the 45° position is calculated: Y45° = 0,574 + 1,199 (1 − a/W) − 1,324 (1 − a/W)2 + 0,4845 (1 − a/W)3 (4) Then, for crack depths a ≤ 0,2 W: Y = 1,143 (1 + 0,6 a/W) (1 + 0,7 a/W)2 (2/π) Y45° (5) and for crack depths a > 0,2 W: Y = [0,1 (a/W)2 + 0,29 (a/W) + 1,081] × [0,75 (a/W)2 − 0,185 (a/W) + 1,019] × [( 0,9 (a/W)2 − 0,21 (a/W) + 1,02 )] 2/π ×Y45° (6) and

K = 102aWYF×××π (7) where a is the crack depth (expressed in centimetres); W
is the test piece width (expressed in centimetres); F
is the force (expressed in kilonewtons); K

is the stress-intensity (expressed in MPa√m). A check for this calculation may be made with the following input and results:

1 mm (0,1 cm);
0,8 cm;

32 kN. here Y 0,691; K 19,365. 6.3 Test piece size requirements For the results to be valid, the test pieces are to be subjected to a stress within the elasticity range of the material for all values of the applied load.

SIST EN 3873:2011

EN 3873:2010 (E) 11 6.4 Crack plane orientation The crack plane orientation, as related to the characteristic direction of the product is identified with a hyphenated letter code as in Figure C.4. The letter(s) preceding the hyphen represent the loading direction normal to the crack plane; the letter(s) following the hyphen represent the expected direction of crack extension. For wrought metals the letter L always denotes the direction of principal processing deformation, T denotes the direction of least deformation and the letter S is the third orthogonal direction. C denotes the circumferential direction and R the radial direction in a disk, while L denotes the direction along the longitudinal axis of the disk. 6.5 Residual stresses In test pieces where stress relief has not been applied, or where forging may have introduced residual stresses which cannot be adequately relieved, the crack growth rate and/or crack symmetry may be affected, particularly at lower ∆K levels. 7 Procedures 7.1 Condition of test pieces The test pieces shall be cleaned and measured before testing commences. Test pieces should be degreased and cleaned in accordance with the guidelines in Annex C. The notch depth and measurement wire spacing must be measured before the test, since the wire spacing cannot be determined after the test, and the notch depth is necessary to determine the initial stress-intensity levels. 7.2 Heating The maximum heating rate shall be 1 K/s. After the specimen temperature reaches that specified, at least 30 min shall be allowed for stabilisation. 7.3 Number of tests The number of tests depends on the use to which the data are to be put. In any case, at least two tests should be carried out for each set of tested parameters. As far as possible, the tests should be identical so that the scatter can be attributed to material effects. If this is not possible, all tests should be carried out within the same ∆K range, with similar number of measurement points and similar measurement intervals. 7.4 Testing, general 7.4.1 General The test should be conducted at constant load amplitude (∆F). However, crack growth measurements under variable load amplitude may be desired, especially when obtaining specific information from a limited number of test pieces. In this case the procedure must be such as to exclude undesired transient effects. 7.4.2 Measurement of crack depth Crack-depth measurements are to be made using the potential-drop method: The DC (direct current) potential-drop method determines the crack depth through the increase in potential (V), measured across the mouth of the crack, from an initial reference potential Vo measured at a known or estimated initial crack depth at test temperature, induced by a constant current (~ 10 A) passing through the plane of the crack. For a constant current flow, the electric potential or voltage drop across the crack plane will increase with increasing crack size due to modification of the electrical field and associated perturbation of the current streamlines. The relationship between potential and crack depth depends on the arrangement SIST EN 3873:2011

EN 3873:2010 (E) 12 of the current- and measurement leads on the test piece. Appendix A gives information on the use of this method. An AC (alternating current) potential-drop method may instead be used, and also requires calibration. 7.5 Notch preparation 7.5.1 General To facilitate crack initiation at low stress ratios, the notch root radius should be on the order of < 0,05 mm. The notch depth used depends on the initial K required for initiation. For nickel-based alloys, either the arc-strike technique with an effective notch depth of 0,1 mm or a diamond-sawed notch of 0,25 mm depth have proven effective. An EDM notch of 0,1 mm width or smaller and 0,1 mm depth can be effectively used for the initiation of the fatigue crack with titanium alloys. 7.5.2 Precracking The condition of the test piece (e.g. heat treatment) when initiating the precrack shall be the same as that with which testing is carried out. No intermediate heat treatments between precracking and testing are allowed. The purpose of precracking is to provide sharp fatigue cracks of sufficient depth so that the K-calibration expression is no longer influenced by the starter notches and that the subsequent fatigue crack growth rate is not influenced by the precracking force history. Frequently a ∆K is required to initiate the crack that is larger than the ∆K desired as the starting point for the test. In this case, the forces must be stepped down to meet the desired starting criteria (see Figure C.9). If Fmax., j and aj are the maximum load and crack depth in one step j, and Fmax., j + 1 and aj + 1 are the corresponding values in the next step j + 1, the following conditions must be met: 1,2max.,1max.,FFjj≥+ (8) (a 10 % change is a good initial recommendation; later steps of only 5 % may be necessary to avoid excessive delays before crack growth resumes after each step) 2p max.,131≥−π+RKaajjj (9) The best initial Kmax. should be determined for each material. But if this is not known, a value of 0,000 08*E √m may be initially used until experience is gained. A net section stress of 500 MPa to 600 MPa is recommended for high-strength nickel alloys. Procedure a) For the DC method, apply ~ 0,1.W2 amperes direct current to produce an initial potential of ~ 1 mV. b) A fatigue precrack of 0,03 mm should be produced using a stress intensity range of ~ 10 MPa√m to 15 MPa√m until a potential change of 0,005 mV to 0,01 mV is noted. This change represents approx. 0,01 mm to 0,04 mm crack extension, and usually indicates the crack is growing steadily. For some materials, small incremental increases in ∆K will be necessary to initiate a crack. Here it is useful to drop the minimum load used into the compressive range, as well as increasing the tensile maximum load, to avoid net section stresses too near to yield. c) The loading should then be adjusted for the desired minimum ∆K for the test, while the precrack extends to a = ~ 0,3 mm. Figure C.6 shows a schematic of the load-shedding process typically recommended (ASTM E 647); the shedding process must be accelerated for CC test pieces due to the small test piece dimensions. SIST EN 3873:2011

EN 3873:2010 (E) 13 The temperature during precracking should be the same as during testing to avoid transient effects which tend to retard the initial crack growth after precracking. Precracking may be performed at room temperature, to enable monitoring the precrack length on the specimen sides. To save time during load shedding, higher frequencies than during testing may be used initially (e.g. 5 Hz to 20 Hz), but the final 0,05 mm of precrack growth should be performed using a waveform having similar loading rates as the waveform used during testing.

If hold times at maximum load are used during CGR testing at elevated temperatures, the initial 0,05 mm to 0,1 mm of growth data may be influenced by such transient effects, and should be considered as suspect, especially if a gradual transition to higher rates is evident after switching to a hold time. NOTE If reduction of the frequency from precracking to test conditions allows Fmax. to increase, due to test machine control characteristics, then all frequency changes, including stops and restarts, should be immediately preceded by a precautionary 10 % reduction in Fmax. and Fmin. to avoid Fmax. overshoot, and later increased. d) The potential at this reference crack depth and Fmin. is recorded and, if precracking was performed at RT the test piece is heated, still at Fmin., to the test temperature, if elevated temperature tests are required. If so, the potential at the test temperature is recorded and used to obtain the temperature correction coefficient

Temp.)(Test (RT)VVYooTC= (10) This coefficient may be used to correct the potentials measured, for use in the crack depth equation (see Annex A). Optimally, the test piece should be precracked at the test temperature. The reference potential is taken at RT in the notched condition, or after heating, to obtain YTC. A different potential-drop calibration will be necessary, however, due to differences between potentials for notches and cracks of the same depths. If the desired R-ratio is higher than 0,1, further cycling must now be performed to reach the desired R-ratio, reducing the loading range by 10 % after each 0,1 mm of crack extension by increasing the minimum load. For materials with Rp ~ 1 000 MPa, and for low R-ratios, reductions may be made after each 0,05 mm of extension, permitting the use of higher initial precracking stress-intensities, or enabling the possibility to reach lower stress-intensities for the initial testing conditions without excessive precrack depths. This can also be achieved by initially cycling at the R-ratio desired for the test, with a ∆K of ~ 16 MPa √m, but the resulting higher Kmax. may require larger crack extension increments to avoid retardation, due to the larger crack tip plastic zone at elevated temperature. The final transition

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