1、ASD STANDARD NORME ASD ASD NORM prEN 3873 Edition P 1 January 2010 PUBLISHED BY THE AEROSPACE AND DEFENCE INDUSTRIES ASSOCIATION OF EUROPE - STANDARDIZATIONAvenue de Tervuren, 270 - B-1150 Brussels - Tel. 32 2 775 8126 - Fax. 32 2 775 8131 - www.asd-stan.orgICS: Descriptors: ENGLISH VERSION Aerospac
2、e series Test methods for metallic materials Determination of fatigue crack growth rates using Corner-Cracked (CC) test pieces Srie arospatiale Mthodes dessais applicables aux matriaux mtalliques Dtermination de la vitesse de propagation de fissure en fatigue laide dprouvettes avec fissure en coin L
3、uft- und Raumfahrt Prfverfahren fr metallische Werkstoffe Ermittlung der Rifortschritts- Geschwindigkeit an Cornercrackproben (Eckanris) This “Aerospace Series“ Prestandard has been drawn up under the responsibility of ASD-STAN (The AeroSpace and Defence Industries Association of Europe - Standardiz
4、ation). It is published for the needs of the European Aerospace Industry. It has been technically approved by the experts of the concerned Domain following member comments. Subsequent to the publication of this Prestandard, the technical content shall not be changed to an extent that interchangeabil
5、ity is affected, physically or functionally, without re-identification of the standard. After examination and review by users and formal agreement of ASD-STAN, it will be submitted as a draft European Standard (prEN) to CEN (European Committee for Standardization) for formal vote and transformation
6、to full European Standard (EN). The CEN national members have then to implement the EN at national level by giving the EN the status of a national standard and by withdrawing any national standards conflicting with the EN. Edition approved for publication 31 January 2010 Comments should be sent with
7、in six months after the date of publication to ASD-STAN Metallic Material Domain Copyright 2010 by ASD-STAN prEN 3873:2010 (E) Contents Page Foreword2 Introduction.3 1 Scope 3 2 Normative references3 3 Symbols and abbreviations3 4 General5 5 Resources.6 6 Test pieces .7 7 Procedures .9 8 Health and
8、Safety.12 9 Evaluation of results .12 10 Test record .14 Annex A (normative) Information on measuring crack depths in corner-crack test pieces with the direct-current Potential-drop method.16 Annex B (normative) Stress-intensity function for corner-crack test pieces 20 Annex C (normative) Guidelines
9、 on test piece handling and degreasing 22 Foreword This standard was reviewed by the Domain Technical Coordinator of ASD-STANs Metallic Material Domain. After inquiries and votes carried out in accordance with the rules of ASD-STAN defined in ASD-STANs General Process Manual, this standard has recei
10、ved approval for Publication. prEN 3873:2010 (E) 3 Introduction This standard and its parts belong to the general organization of the ASD collection of metallic material standards for aerospace applications 1 Scope This standard specifies the requirements for determining fatigue crack growth rates u
11、sing 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 crac
12、k 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 amendment
13、s) applies. EN 2002-2, 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-measurin
14、g system EN ISO 3785, Metallic materials Designation of test specimen axes in relation to product texture ISO 3785. ASTM E 1012-2005, Verification of test frame and specimen alignment under tensile and compressive axial force application. 1)ASTM E 647-2008, Standard test method for measurement of fa
15、tigue crack growth rates. 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 center of the crack front (45 position). For the calculation of stress-intensity factor, the crack length must be given in metres
16、 (m) aefinal crack depth (in millimetres) aiInitial crack depth (in millimetres) amMeasured crack depth (optical, post-test fracture surface micrography or with SEM) 1) Published by: American Society for Testing and Materials (ASTM), 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-29
17、59 USA. prEN 3873:2010 (E) avCalculated (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 measuremen
18、ts (in millimetres). a Crack growth increment (in millimetres) da/dN Fatigue crack growth rate (FCGR) (in m/cycle) W Test piece width (in millimetres) a/W Normalized crack depth (in millimetres/millimetres) f Frequency (in Hz) E Youngs modulus (in GPa) K Stress-intensity factor (general) The stress-
19、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
20、 K during a loading cycle, (in MPa m) K Range of K during a loading cycle Kmax. Kmin. (1 R)*Kmax. (in MPa m) KeffEffective range of K, due to crack closure-induced reduction applied K, (in MPa m) KthFatigue crack growth threshold The asymptotic value of K for which da/dN approaches zero. For most ma
21、terials the operational threshold is defined as the K corresponding to 10-7m/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 th
22、e 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 dK /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 o
23、f stress cycles between two marker cycles NmNumber of stress cycles in a marker cycle N Stress cycle difference r Notch radius (expressed in mm) FmMean force (expressed in kN) FmaxMaximum tensile force applied to the test piece during a cycle (expressed in kN) FminMinimum tensile force (in kN) F For
24、ce range (in kN), F Fmax. Fmin.q Resolution of crack depth measuring system (expressed in mm) R Force ratio ( Fmin./Fmax. Kmin./Kmax.) R*mRatio Fmin./Fmax.during a marker cycle prEN 3873:2010 (E) 5 Rp0,2 % offset yield strength (Proof Stress Rp0,2) at test temperature (expressed in MPa) RmTensile st
25、rength at test temperature (expressed in MPa) f Flow stress here defined as the arithmetic mean of Rpand Rm(expressed in MPa) Z Axial distance from crack plane to each wire used for potential measurement (expressed in mm) (2Z wire separation distance) 4 General 4.1 The corner-crack (CC) test piece i
26、s 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 negativ
27、e 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 mo
28、nitored 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 th
29、e 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 o
30、f 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)
31、 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-int
32、ensity factor K is: K = Kmax. Kmin.(1) where the ratio R R = min.max.KK(2) applies. From eq. (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 t
33、he 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
34、point and the rounded corner where the measurement wire is welded, which is used in the calibration of the potential-drop measurements. prEN 3873:2010 (E) 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 point
35、s, 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 func
36、tion 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 environm
37、ent 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 parameter
38、s, 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 affec
39、t 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) Investiga
40、ting 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, microstructu
41、re, manufacture, etc.). 5 Resources 5.1 Test machine 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
42、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. 5.1.1 Load control The apparatus must satisfy the foll
43、owing requirements in accordance with ISO EN 7500: a) Accuracy of electronic force measurement: 0,5 % and 0,25 % of nominal range respectively, b) Accuracy of control throughout testing: better than 0,5 % of specified value of F, prEN 3873:2010 (E) 7 c) Recording instrument voltage requirements (upp
44、er and lower stress range, cycles): digital recorder is recommended, d) Recording accuracy throughout testing: better than 0,25 %. 5.1.2 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 ri
45、gid, 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
46、 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 6 months in accordance with ISO EN 3785. 5.3 Temper
47、ature 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 - 4 mm above or below the crack plane
48、. 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 d
49、eviate from the specified values by more than the following: For elevated temperature tests up to 1 000 C 3 C 1 000 C 4 C to 1 100 C 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
