1、 CEN N*IS0*?539- 5 95 3404589 0109308 L87 Corrosion of metals and alloys - Stress corrosion testing Part 5. Preparation and use of C-ring specimens The European Standard EN 7539-5 : 1995 has the status of a British Standard BS EN IS0 7539-5 : 1995 - CEN EN*IS0*7539- Amd.No. =?%- 5 95 m Date Text aff
2、ected *-M”S s. b.sp-e-T BS 6980 : Part 5 : 1990 Committees responsible for this British Standard The piqmratioii of this I3i-itish Standard was entiiistetl by the Iron and Steel Staiidrtiuls Policy Coiniiiit t er (ISMi-) and Non-cwous Metals Standards Policy Coininittce (NFM -) to Technical Coininit
3、tce ISM NFJI 8. upon which the following twdies were represented: Aluminium Federatior? British Gas pic British Steel Industry Department of Trade and Industry (National Physical Laboratory) Department of Transport (Transport and Rtmd Research Laboratory) Electricity Supply industry in England and W
4、ales Institution of Carrosion Science and Technology Institution of Structural Engineers Society of Chemical Industry United Kingdom Atomic Energy Authority Welding Institute This British Standard, having been prepared under the direction of the Iron and Steel Standards Poiicy Cornmittee and the Non
5、-Ferrous Metals Standards Policy Committee, was published under the authority of the .Paration and use of bent-beam specimens Part 3 Prepamtion and use of U-bend speciw Part 4 that is, transverse as well as circumferential stresses are developed. The transverse axial stress varies from a maximum at
6、the mid-width to zero at the edges, and has the same sign as the Circumferential stress. In general, the transverse stress decreases with decreasing width-to-thickness and increasing diameter-to-thickness ratios. 5.2.3 In the case of the notched C-ring a triaxial stress state is present adjacent to
7、the root of the notch. In addition, the cir- cumferential stress at the root of the notch will be greater than the nominal stress and generally may be expected to be in the plastic range. 5.2.4 When C-rings are machined from products that contain appreciable residual stress or are subjected to heat
8、treatment involving quenching after being machined, internal stresses may be present. These may introduce errors in the calculated stress. It is necessary to measure the tubing diameter before and after the axial cut is made and to use these measurements to calcu- late the residual stresses in the t
9、ube. 5.2.5 The possibility of relaxation during the exposure period should be considered, especially when specimens are exposed at elevated temperatures. Relaxation can be estimated if creep data are available for both the ring and the stressing bolt. NOTE - If the ring and bolt have different coeff
10、icients of thermai expansion, the applied stress may be significantly changed when testing at elevated temperatures Alm, if plastic insulators are used to Dimensions in millimetres ext. 10,05 II 9int. 1o,o5 II 1,6 min. Figure 2 - Example of C-ring specimen 3 - CEN EN*ISO*7539- 5 95 M 3404589 OLO93L7
11、 L9T IS0 7!539-5 : 1989 (E) avoid galvanic corrosion, the possibility of stress relaxation should be anticipated. 5.3 Stressing methods metre; where E is the modulus of elasticity, in newtons per square 5.3.1 C-ring specimens are usually loaded under constant p is Poissons ratio: displacement condit
12、ions with tensile stress produced on the exterior of the ring by tightening a bolt centred on the diameter of the ring see figure 3a)l. 5.3.2 C-rings can alternatively be stressed in the reverse direc- E, is the circumferential strain; ct is the transverse strain. When using electrical strain gauges
13、 on thin walled C-rings, a , correction should be allowed for the displacement of the gauge adhesive must be removed from the C-ring before it is exposed. tion by spreading the ring and creating a tensile stress on the in- side by the use Of a in figure 4. In the latter case the necessary displaceme
14、nt is pro- vided by inserting an accurately machined wedge of the same as in figure 3c Or wedge Opening technique to the arms Of the C-rings as from the of the ring. All traces ?f the gauge and material as the C-ring, CO avoiding galvanic effects. A suitable jig for inserting the wedge is shown in f
15、igure 4. Calculation of stresses above the limit of elasticity may be car- ried out on the basis of an electric-plastic analysis. 5.3.3 The C-ring test can be modified for approximately con- 5.3.5 When several rings of the same alloy and dimensions are to be loaded, it is convenient to determine a c
16、alibration stant load conditions by the use of a suitably calibrated spring placed on the loading bolt see figure 3b)l. 5.3.4 The most accurate stressing procedure is to attach cir- cumferential and transverse electrical strain gauges to the sur- face stressed in tension and to tighten the bolt unti
17、l the strain measurements indicate the desired circumferential stress. The circumferential oc and transverse ot stresses are calculated as follows provided that they are within the elastic range : E 1-112 ff, = - (E, + PEt) curve of circumferential stress versus ring deflection to avoid the inconven
18、ience of strain-gauging each ring. 5.3.6 The amount of compression required on the C-ring to produce elastic straining only, and the degree of elastic strain can be predicted theoretically. Therefore, C-rings may be stressed by calculating the deflection required to develop a desired elastic stress
19、by using the individual ring dimensions in a modified curved beam formula as shown in annex A. Experience shows good agreement between the stresses calculated in this way and those measured by fixing strain gauges to specimens. a) Constant strain bl Constant load Figure 3 - Methods of stressing C-ri
20、ngs ci Constant strain 4 IS0 7539-5 : 1989 (E) 5.3.7 Alternatively the stress-strain distribution throughout the specimen, for various applied displacements of the C-ring, may be calculated using the finite element method of stress analysis. Such analyses should be done using well established finite
21、 element programmes and by personnel fully conversant with the finite element technique. The method would normally be used for specimens with more complex geometries or loading configurations for which simple theoretical analysis is not applicable. 5.3.8 For notched specimens (see 5.2.3) a nominal s
22、tress is assumed using the ring outside diameter measured at the root of the notch. The maximum stress at the notch is then calculated from the product of the nominal stress and the stress concentration factor K, for the specific notch. 5.4 Machining and surface preparation 5.4.1 A high quality mach
23、ined surface is the most desirable for corrosion test purposes unless it is desired to test the as- manufactured surface of a tube or bar. When rings are machined from solid stock, precautions should be taken to avoid practices that overheat, plastically deform, or develop residual stress in the met
24、al surface. Machining should be done in stages so that the final cut leaves the principal surface with a clean finish of 1 pm rms or better. Lapping, mechanical polishing, and similar operations that pro- duce flow of the metal should be avoided. 5.4.2 The surface of the specimen should be degreased
25、 before exposure. Chemical or electrochemical treatments may be used to remove oxide films or thin layers of surface metal which have become distorted during machining. If chemical or electrochemical treatments are employed, care must be taken to ensure that the conditions used do not result in sele
26、ctive phase attack on the metal or leave a deposit of undesirable residues on the surface. Treatments that generate hydrogen on the specimen surface must not be used for materials that are susceptible to hydrogen-induced damage. 5.4.3 The surface preparation should be completed before the C-ring is
27、stressed except for a possible final degreasing of the stressed area. 5.4.4 Every precaution should be taken to avoid finger- printing or any rough handling which could mar the finish of the surface after its final preparation. Figure 4 - Suitable jig for the placement of a wedge opening insert CEN
28、EN*IS0*7539- 5 95 m 3404589 OLO93L9 Tb2 m IS0 7539-5 : 1989 (E) 5.5 Specimen identification 5.5.1 Specimen numbers may be scribed on one of the ends adjacent to the cut away segment of the C-ring. No markings of any kind should be made on the critically stressed arc between the bolt holes. Non-metal
29、lic tapes may be attached to the stressing bolt by means of a second nut. 5.5.2 Numbers for wedge-opening loaded specimens may be scribed on the outer surface of the C-ring adjacent to the wedge insert. 6 Procedure 6.1 The C-ring, because of its small size and the simple methods of stressing, can be
30、 exposed to almost any kind of corrosive environment. The specimens should be supported so that nothing except the corrosive medium contacts the critically stressed area. 6.2 Care should be exercised to avoid galvanic effects between the C-ring, the stressing bolt, nuts or wedge insert and the xposu
31、re racks. Protection can be provided by in- sulating bushing as shown in figure 5a) and b) or by coatings as shown in figure 5c). It is also essential to prevent crevice corrosion that could develop corrosion products between the ring and its stressing assembly and thus alter the stress in the C-rin
32、g; coating as shown in figure 5c) is suitable for this pur- pose. The coatings or insulators selected should not con- taminate the corrosive environment nor be deteriorated by it. 6.3 Specimens should be exposed to the test environment immediately after being stressed or should be stored, in such a
33、way as to avoid contamination or deterioration, until they can be exposed. 7 Assessment of results 7.1 The time required for cracks to appear after exposure of stressed specimens to the test environment or the threshold stress below which cracks do not appear can be used as a measure of stress corro
34、sion resistance of the material in the test environment at the stress level employed. 7.2 Cracking is usually obvious in highly stressed C-rings of alloys that are susceptible to stress corroson cracking. 7.3 Cracking may be much less obvious in C-rings exposed to lower stress, or in more resistant
35、alloys, especially if corrosion products obscure the cracks. 7.4 If a Ging does not fracture, some arbitrary criterion of failure must be adopted, based on a recognizable degree of cracking. It is common practice to inspect for cracks with the naked eye or at a low magnification. 7.5 If there are in
36、dications noted that cannot be established definitely as a crack by this examination, the investigator should either ai note the time and date of this first suspicion of cracking and continue the exposure of the specimen, watching for further growth to confirm the first indication as the failure tim
37、e; or bi discontinue exposure of the specimen and perform a metallographic examination of a cross-section taken through the suspected crack to establish whether there is cracking. 7.6 Metallographic examination of fractured or cracked C-rings can also be helpful in determining whether failure was ca
38、used by stress corrosion cracking or by some other form of corrosion. 8 Test report 8.1 In addition to reporting the number of specimens failed and the time to failure of each specimen. particulars should be reported concerning the following : a) the method of stressing; ab Stopped insulating bushin
39、g b) Spherical insulating bushing CI Coating Figure 5 - Protection against crevice corrosion and galvanic effects 6 IS0 7539-5 : 1989 (E) b) c) d) e) the test environment; f) g) the criterion of failure. the magnitude of the applied stress; the orientation of the specimen; the dimensions and surface
40、 preparation: the duration of the test; 8.2 metal being tested, including the following : Complete information should also be reported about the a) alloy designation or specification number: b) the composition of the material under test; c) the fabrication history; d) the heat treatment; e) the mech
41、anical properties. 7 Annex A (normative) Formula for calculating stressing of C-ring specimens The final diameter D, required to give the desired stress can be calculated using the following equations : D,= DfAD and AD = ond2/4EtZ where D is the outside diameter, in millimetres, of the C-ring before
42、 stressing; D, is the outside diameter of stressed C-ring, in millimetres, measured at right angles to a centre line passing through the point of maximum stress; o is the desired stress, in meganewtons per square metre, within the limit of proportionality; AD is the change of D, in millimetres, givi
43、ng the desired stress; d is the mean diameter (D - I), in millimetres; 1 is the wall thickness, in millimetres; E is the modulus of elasticity, in meganewtons per square metre: 2 is a correction factor for curved beams (see figure A.1). Tables such as table A.l can be developed to avoid repetitive c
44、alculations for investigations involving many tests of a given size of C-ring. The main source of error in this procedure is in the measure- ment of the C-ring dimensions. If in a typical example of a C-ring of 19 mm outside diameter and 1.50 mm wall thickness the measurements are made to the neares
45、t 0,03 mm, the ran- dom error in the calculated value should not exceed 3 %; and the error would be less for longer and thicker rings. An error of 0,03 mm in measuring to the outside diameter before and after stress, however, will have a variable effect upon the stress actually developed, depending
46、upon the magnitudes of the desired stress and outside diameter of the rings. For the size of rings mentioned the percentage error in applying AD would be f 3 % for o = 350 MN/m2 ranging to k 30 % for o = 35 MN/m? 8 Table A.l - Deflection AD for a C-ring of nominal 19 mm outside diameter and 1.50 mm
47、wall thickness of alloy with a modulus of elasticity of 100 o00 MN/m2 for stressing to 500 MN/m2 Wall thickness 1.41 1.42 1.43 1 .a 1.45 1.46 1.47 1,48 1.49 1.50 1,51 1.52 1.53 1.54 1.55 1.56 1,57 158 1.59 1 .a NOTES Dimensions in millimetres Outside diameter 18.88 0.899 0,892 0,885 0,878 0,871 0,86
48、5 0,858 0,851 0,845 0.839 0,833 0,827 0.821 0,815 0.808 0,802 0,797 0,791 0,785 0,780 8.91 0.902 0,894 0.887 0,881 0,874 0.868 0,861 0,854 0,848 0,842 0,835 0,829 0,823 0,817 0,811 0,805 0,800 0,794 0,788 0,783 1834 0,905 0,897 0.890 0,884 0.877 0,871 0.864 0,851 0.844 0.838 0,832 O, 826 0,820 0,814
49、 0,808 0,802 0,797 0,791 0,785 0,857 18.97 0,907 0,900 0,893 0,887 0.880 0,874 0,867 0.860 0.853 0,847 0,841 0,835 0,829 0,823 0,817 0,811 0.805 0,799 0,793 0,788 19.00 0,910 0,903 0.896 0,890 0.883 0,876 0,870 0,863 0.856 0,850 0.844 0.838 0,832 0,825 0,820 0,814 0.808 0,802 0,796 0,791 -. 19.03 0,913 0,906 0,899 0,892 0.886 0,879 0,873 0.866 0,859 0,853 0,847 0,841 0,835 0.829 0,823 0,817 0.81 1 0,805 0,799 0,793 19.06 0,916 0,910 0,902 0,895 0.889 0,882 0.876 0,869 0.862 0.8% 0,849 0.843 0,837 0.831 0.825 0,819 0,813 0,807 0,802 0,796 _ - 19.09 0,919 0,912 0.905 0.8
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1