1、 Reference number ISO 27727:2008(E) ISO 2008INTERNATIONAL STANDARD ISO 27727 First edition 2008-09-01 Rubber, vulcanized Measurement of fatigue crack growth rate Caoutchouc vulcanis Mesurage du taux de croissance des craquelures de fatigue ISO 27727:2008(E) PDF disclaimer This PDF file may contain e
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6、equester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2008 All rights reservedISO 27727:2008(E) ISO 2008 All rights reserved iii Contents Page Foreword iv 1 Scope. 1 2 Norm
7、ative references. 1 3 Terms and definitions. 1 4 Principle. 2 5 Apparatus 2 5.1 Fatigue crack growth rate test machine. 2 5.2 Image-processing device. 2 5.3 Temperature-controlled chamber . 2 5.4 Thickness- and width-measurement instruments. 2 6 Test pieces 3 6.1 Dimensions 3 6.2 Number of test piec
8、es 3 6.3 Time interval between moulding and testing. 4 6.4 Conditioning 4 6.5 Preparation of test pieces for testing . 4 7 Test conditions . 5 7.1 Temperature 5 7.2 Cycle frequency 5 7.3 Strain amplitude 5 7.4 Testing under non-relaxed conditions . 5 8 Procedure 5 8.1 Determination of strain energy
9、density 5 8.2 Determination of tearing energy . 5 8.3 Measurement of crack growth. 6 9 Expression of results. 7 10 Precision 8 11 Test report. 8 Bibliography . 10 ISO 27727:2008(E) iv ISO 2008 All rights reservedForeword ISO (the International Organization for Standardization) is a worldwide federat
10、ion of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committe
11、e. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance
12、with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires app
13、roval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 27727 was prepared by Technical Commi
14、ttee ISO/TC 45, Rubber and rubber products, Subcommittee SC 2, Testing and analysis. INTERNATIONAL STANDARD ISO 27727:2008(E) ISO 2008 All rights reserved 1 Rubber, vulcanized Measurement of fatigue crack growth rate 1 Scope This International Standard specifies a method for the determination of the
15、 fatigue crack growth rate of vulcanized rubber under repeated loading over an extended period of time. The crack starts from the tip of a cut made in the test piece before the test, and grows progressively until it finally becomes large enough for complete fracture to occur. Using a pure-shear test
16、 piece, measurements are made to monitor the crack growth under the cyclic loading in order to obtain the crack growth rate, i.e. the increase in crack length per cycle. Tests are carried out at various tearing energies by varying the strain energy density in the test piece. This is done by changing
17、 the strain amplitude. 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. ISO
18、 23529, Rubber General procedures for preparing and conditioning test pieces for physical test methods 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 strain energy density W elastic energy stored in unit volume of the test piece when in a st
19、ate of strain and resulting from the work done in deforming the test piece NOTE It is measured in joules per cubic metre. 3.2 tearing energy T amount of energy required to propagate a tear/crack in a test piece NOTE The tearing energy is expressed as the ratio of the total work done to the surface a
20、rea of the crack. It is measured in joules per square metre. ISO 27727:2008(E) 2 ISO 2008 All rights reserved3.3 strain-range parameter P Rtest cycles in which the test piece is always in a strained state ratio of the minimum distance from the relaxed (zero strain) position to the maximum distance f
21、rom the relaxed position in each cycle NOTE The strain-range parameter is expressed as P R= d min /d max , where d minis the minimum distance from the relaxed position and d maxis the maximum distance from the relaxed position. 4 Principle A pure-shear geometry test piece with a deliberately introdu
22、ced cut is subjected to cyclic loading during which the length of the crack produced grows with the number of deformation cycles. The crack length is measured by means of an in situ monitoring device as a function of the number of cycles carried out, and the measured data are processed by digital an
23、alysis to establish the rate of crack growth. The crack growth rate is then interpreted in terms of the tearing energy of the test piece, determined from its strain energy density. 5 Apparatus 5.1 Fatigue crack growth rate test machine A suitable test machine for measuring fatigue crack growth rate,
24、 which operates with constant displacement cycles, is shown in Figure 1. The grips hold the test piece in a temperature-controlled chamber, with the upper grip connected to the load cell fixed to a crosshead, and the lower grip connected to a servo-controlled drive. The height of the crosshead is fi
25、xed, using stop rings, at a predetermined distance from the lower grip and, if required, a stress-strain curve can be obtained by varying the position of the lower grips. The test piece is periodically subjected to rectilinear deformation at a frequency and amplitude controlled by the servo mechanis
26、m. The strain range can be controlled by adjusting a pre-straining device positioned above the upper grip. 5.2 Image-processing device The growth of the crack in the test piece shall be monitored by means of an image-processing system equipped with a high-speed camera. Measurements shall be made to
27、track the tip of the crack, and the length of the crack shall be recorded as a function of the number of strain cycles, thus providing the rate of fatigue crack growth. 5.3 Temperature-controlled chamber When tests are made at a particular temperature, e.g. a standard laboratory temperature or anoth
28、er temperature specified in ISO 23529, a temperature-controlled chamber capable of maintaining the test piece at the temperature specified shall be used (see example in Figure 1). A temperature-sensing device shall be located within the chamber near or at the location of the test piece. 5.4 Thicknes
29、s- and width-measurement instruments Instruments for measuring the thickness and width of the test piece shall be in accordance with ISO 23529. ISO 27727:2008(E) ISO 2008 All rights reserved 3Key 1 upper and lower grips 4 cyclic shaft 7 temperature-controlled chamber 2 load cell 5 strain-range contr
30、oller 8 stop ring 3 crosshead 6 high-speed CCD camera 9 servo-controlled drive Figure 1 Example of a fatigue crack growth rate test machine 6 Test pieces 6.1 Shape and dimensions The test pieces shall be moulded strips of the shape shown in Figure 2. Each test piece shall have a smooth surface and b
31、e free from irregularities, and the thin section of the test piece shall be of uniform thickness. The recommended dimensions of the test piece are 200 mm long (L) and 20 mm wide (h 0 ). The length-to-width ratio L/h 0shall be at least 10 in order to minimize edge effects. The thickness t 0of the thi
32、n section shall be less than 2 mm and the thickness t of the thicker sections held in the upper and lower grips shall be greater than 5 mm. 6.2 Number of test pieces At least three test pieces shall be used for each set of test conditions. ISO 27727:2008(E) 4 ISO 2008 All rights reservedFigure 2 Sha
33、pe of pure-shear test piece for fatigue crack growth rate measurements 6.3 Time interval between moulding and testing Unless otherwise specified for technical reasons, the following requirements shall be observed (see ISO 23529): For all test purposes, the minimum time between moulding and testing s
34、hall be 16 h. For non-product tests, the maximum time between vulcanization and testing shall be four weeks and, for evaluations intended to be comparable, the tests shall, as far as possible, be carried out after the same time interval. For product tests, whenever possible, the time between vulcani
35、zation and testing shall not exceed three months. In other cases, tests shall be made within two months of the date of receipt of the product by the customer. 6.4 Conditioning Test pieces shall be protected from light as completely as possible during the interval between vulcanization and testing. T
36、he test pieces shall be conditioned at standard laboratory temperature for at least 3 h immediately before being measured and tested. If the test is to be carried out at a temperature other than a standard laboratory temperature, the test pieces shall, immediately prior to testing, be conditioned at
37、 the test temperature for a period sufficient to ensure that they have reached the test temperature (see ISO 23529). 6.5 Preparation of test pieces for testing Each test piece shall be prepared by making a cut in it in order to eliminate the random nature of the tear initiation process. Before makin
38、g the cut, strain the test piece three times to the highest strain to be applied in the test. Then, immediately before the test, make a cut about 30 mm long with a sharp razor blade at one end of the test piece. ISO 27727:2008(E) ISO 2008 All rights reserved 5 7 Test conditions 7.1 Temperature Measu
39、rements are normally carried out at a standard laboratory temperature as defined in ISO 23529. If tests are required at an elevated or subnormal temperature, the temperature shall be selected from those specified in ISO 23529. 7.2 Cycle frequency The test piece shall be subjected to rectilinear-defo
40、rmation cycles at a frequency normally in the range from 1 Hz to 10 Hz, but other frequencies may be used for special purposes. For tests intended to be comparable, the frequency used in each test shall be the same. 7.3 Strain amplitude The strain amplitude shall be changed in order to vary the stra
41、in energy density in the test piece up to a maximum of 200 % elongation. The test piece shall return to zero strain, which is the fully relaxed state, during each cycle (see, however, 7.4). 7.4 Testing under non-relaxed conditions If required, the test may be carried out under non-relaxed conditions
42、, in which the minimum strain in each cycle is not zero, by varying the strain-range parameter P R . 8 Procedure 8.1 Determination of strain energy density The strain energy density W, which is the elastic energy released during crack growth, is represented by the area under the stress-strain curve.
43、 With unfilled rubbers, it is obtained by testing a pure-shear test piece without first making a cut in it, as shown in Figure 3 a), cycling under exactly the same conditions as with a test piece with a crack. Due to the pronounced hysteretic behaviour of filled rubber compounds, the stress- strain
44、curve for such compounds is obtained from retractive-force/extension measurements, as shown in Figure 3 b). In either case, at least three test pieces shall be tested and the mean value of W calculated from the results. 8.2 Determination of tearing energy In a pure-shear test piece, the tearing ener
45、gy T is given by the equation: 0 TWh = where h 0is the unstrained width of the test piece. ISO 27727:2008(E) 6 ISO 2008 All rights reserveda) Stress-strain curve obtained from tensile measurements a) Stress-strain curve obtained from retractive- force measurements Key X strain Y stress Figure 3 Dete
46、rmination of strain energy density W from tensile and retractive-force measurements 8.3 Measurement of crack growth Place the test piece in the test machine and apply cyclic loading under the selected test conditions. Allow the machine to run for at least 3 min before making any measurements. Then m
47、onitor the length c of the crack in situ using a high-speed CCD camera as a function of the number of cycles n, measuring the crack length to an accuracy of 10 5m. Image control and calibration in advance are essential to ensure precise measurements of the crack growth. From the measurements, prepar
48、e a plot of crack length against number of cycles and read off the crack propagation rate dc/dn (the slope of the plot). An example of such a plot, obtained for a carbon black (N351) filled SBR compound using a strain amplitude of 35 % and a cycle frequency of 1 Hz at a temperature of 40 C, is shown
49、 in Figure 4. Repeat the procedure to give measurements at at least three different strain amplitudes. ISO 27727:2008(E) ISO 2008 All rights reserved 7Key X number of cycles, n Y crack length, c (m) Figure 4 Determination of the crack growth rate 9 Expression of results Using the results obtained at different strain amplitudes, plot the crack growth rate dc/dn against the tearing energy T using a log-log scale. At least three data points are required for the plot. Most rubber materials follow a power law relationship
