1、 - CEN EN*ISO*7539- 2 95 I 3404589 0107086 04T A BRITISH STANDARD Corrosion of metals and alloys - Stress corrosion testing Part 2. Preparation and use of bent-beam specimens The European Standad EN IS0 7539-2 : 1995 has the status of a British Standard BS EN IS0 7539-2 : 1995 -,_, .- pieces may fly
2、 off at high velocity and can be dangerous. Personnel installing and examining specimens must be made aware of this possiblity and be protected against injury. 1 Scope 1.1 This part of IS0 7539 covers procedures for designing, preparing and using bent-beam test specimens for investigating the suscep
3、tibility of a metal to stress corrosion. The term “metal“ as used in this part of IS0 7539 includes alloys. 5 1.2 Bent-beam specimens may be used to test a variety of product forms. They are used principally for sheet, plate or flat extruded material, which conveniently provides flat specimens of re
4、ctangular cross-section, but may also be employed for cast material, wire or rod, or for machined specimens of circular cross-section. They can also be used for parts joined by welding. 1.3 Since the preparation of the specimens and the apparatus used for stressing them are both simple and in- expen
5、sive, bent-beam specimens are especially suitable for multiple testing and for atmospheric stress corrosion tests. 1.4 Bent-beam specimens are usually tested under nominally constant strain conditions but nominally constant load con- ditions may be employed. In either case local change of curvature
6、in the specimen when cracking occurs results in changing conditions during crack propagation. The “test stress“ is taken as the highest surface tensile stress existing at the start of the test. 2 Normative references The following standards contain provisions which, through reference in this text, c
7、onstitute provisions of this part of IS0 7539. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of IS0 7539 are encouraged to investigate the possibility of applying the most recent editions of the stan
8、dards indicated below. Members of IEC and IS0 maintain registers of currently valid International Standards. IS0 7539-i : 1987, Corrosion of metals and alloys - Stress cor- rosion testing - Part I: General guidance on testing pro- cedures. IS0 7539-4 : 1989, Corrosion of metals and alloys - Stress c
9、orrosion testing - Part 4: Preparation and use of uniaxially loaded tension specimens. 3 Definitions For the purposes of this part of IS0 7539, the definitions given in IS0 7539-1 are applicable. 4 Principle 4.1 The test consists of applying a bending stress to a beam specimen of rectangular or circ
10、ular section and exposing the stressed specimen to a specified test environment. 4.2 The magnitude of the resultant applied tensile stress in the outer fibres of the bent-beam specimen is calculated from the dimensions and modulus of elasticity of the specimen and the bending deflection, as describe
11、d in 5.4. 4.3 Bent-beam specimens are used only for testing at stress levels below the elastic limit since the formulae used for calculating stress in bent beams apply only within the elastic range. CEN EN*IS0*7539- 2 95 m 3404589 0307093 28T m 4.4 The time required for cracks to appear after exposu
12、re of stressed specimens to the test environment or the threshold stress below which cracks do not appear can be used as a measure of the stress corrosion resistance of the material in the test environment at the stress level employed. 4.5 Wide variations in test results may be obtained for a given
13、metal and environment even when testing nominally identical specimens and the replication of tests is frequently necessary. 4.6 The possiblity of relaxation during the exposure period should be considered especially when specimens are exposed at elevated temperatures. Relaxation can be estimated if
14、creep data are available for a simultaneous effect of the test environ- ment. The difference in thermal expansion should also be con- sidered. 5 Specimens 5.1 General 5.1.1 Identification marks or numbers should be permanently inscribed at each end of the specimen. This is the region of lowest stres
15、s and the identification marks will therefore not initiate cracking. 5.1.2 Specimens for determination of mechanical properties shall be taken from the same heat treatment batch, and preferably from the same piece of material, as the stress cor- rosion specimens. 5.2 Types of Specimens 5.2.1 Bent-be
16、am stress corrosion specimens are usually flat strips of metal of uniform rectangular cross-section and uniform thickness. They may alternatively be lengths of wire or rod of uniform circular cross-section. 5.2.2 Bent-beam stress corrosion tests may also be carrried out on specimens having a gauge l
17、ength of uniform rectangular or circular cross-section with threaded ends of larger cross- section as described in IS0 7539-4. 5.3 Surface finish 5.3.1 Wire or rod specimens and flat specimens cut from sheet, plate and extruded sections may be tested with the original surface retained. This is often
18、 desirable as the structure of the original surface may be different from that of the layers of metal beneath. 5.3.2 If it is desired to exclude the effects of variations in the original surface conditions for a comparison of different alloys, the specimens should be finished by grinding or machinin
19、g to a depth of at least 0.25 mm. This is usually sufficient to eliminate original surface imperfections without completely removing any outer recrystallized layer. The maximum depth of machin- ing or grinding of the surface should be decided after studying the structure of the material as shown in
20、an etched metallographic section. It is desirable to remove the required amount of metal in several steps by alternatively machining or grinding opposite surfaces. This practice minimizes warping due to unequal residual stresses introduced by machining. All edges should be similarly ground or machin
21、ed to remove any cold worked material remaining from shearing. 5.3.3 Chemical or electrochemical treatments are generally inappropriate for flat rectangular section specimens as attack at the edges tends to be greater and less easy to control than on the faces. 5.3.4 If chemical or electrochemical t
22、reatments are employed, care must be taken t ensure that the conditions used do not result in selective phase attack on the metal or leave a deposit of undesirable residues on the surface. 5.3.5 Chemical or electrochemical treatments that generate hydrogen on the specimen surface must not be used on
23、 materials that are susceptible to hydrogen-induced damage. 5.3.6 Before testing, the specimens should be degreased to remove surface contamination; they should then be tested irn- mediately, or stored in such a way as to avoid contamination or deterioration until they can be tested. 5.4 Methods of
24、stressing 5.4.1 Constant strain methods 5.4.1.1 Modes of loading Figure 1 showssix methods of stressing specimens under nominally constant strain conditions. The two-point loaded, three-point loaded and four-point loaded specimens represent the three basic modes of loading used for bent-beam speci-
25、mens. The double-beam specimen, fully supported specimen and lever-loaded specimen may be regarded as special cases of four-point loading. 5.4.1.2 Two-point loading 5.4.1.2.1 The maximum stress in a two-point loaded specimen occurs at the mid-point of its convex surface and decreases to zero at the
26、specimen ends. 5.4.1.2.2 Flat two-point loaded specimens should be approxi- mately 15 mm to 25 mm wide by 110 mm to 255 mm long as shown in figure 1 a). The specimen thickness, t, exact length, L, and holder span, H, are selected to give the required stress calculated according to 5.4.1.2.4 and to g
27、ive a value for (L - H)/H between 0,Ol and 0.50 to keep the error in calculating stress within acceptable limits. A specimen of thickness 0.8 mm to 1,8 mm with a holder span of 175 mm to 215 mm has proved convenient when working with very high strength steels and with aluminium alloys, with test str
28、esses ranging from about 200 MN/m2 for aluminium to 1 500 MN/m2 for steel. 5.4.1.2.3 Care should be taken when fitting specimens into their holders to avoid overstresing, distortion or misalignment. 2 L H 4 a) Two-point loaded specimen h H b) Three-point loaded specimen - c) Four-point loaded specim
29、en Weid Weid d) Double-beam specimen h ni w- e) Fully supported specimen Dimensions in millimetres P Specimen lit Assembly 1 (ii Specimen is the maximum deflection in metres; L = (ktE/a) sin- (HalktE) H is the distance between outer supports in metres. where L is the specimen length in metres; 5.4.1
30、.4 Four-point loading U E metre; between the inner supports. is the maximum stress in newtons per square metre; is the modulus of elasticity in newtons per square 5.4.1.4.1 Four-point loading gives a uniform longitudinal tensile stress in the convex surface of the part of the specimen H is the holde
31、r span in metres; f is the thickness of specimen in metres; k = 1,280, an empirical constant. The equation should be used only with HalktE = 1,0 This equation can be solved by computer, by trial and error, or by using a series expansion of the sine function. 5.4.1.2.5 A more rigorous calculation of
32、stress may be based on a theoretically exact hrge deflection analysis. Calculation of stresses above the limit of elasticity may be carried out on the basis of an elastic-plastic analysis. 5.4.1.3 Three-point loading 5.4.1.3.1 The maximum tensile stress in a three-point loaded specimen occurs at the
33、 mid-point of its convex surface and decreases linearly to zero at the outer supports. A disadvantage of three-point loaded specimens is the possiblity of crevice cor- rosion occurring at the central support close to the region of maximum tensile stress. The pressure of the central support also intr
34、oduces unknown bi-axial stresses in the region of maxi- mum calculated longitudinal tensile stress. The stress decreases linearly to zero from the inner supports to the outer supports. The relatively large area of uniformly stressed material makes four-point loaded specimens generally preferable to
35、two or three-point loaded specimens and particularly suitable for tests of welded material and for studies of protection by sprayed metal or organic coatings. 5.4.1.4.2 Four-point loaded specimens are commonly flat strips 15 mm to 50 mm wide and 110 mm to 250 mm long. The thickness of the specimen i
36、s usually dictated by the mechanical properties of the material and the product form available. Specimen dimensions can be modified to suit specific needs but the approximate dimensional proportions should be pre- served. 5.4.1.4.3 The specimen is supported near the ends and bent by forcing the two
37、inner supports against it in the manner shown in figure 1 ci. The two inner supports must be located symmetrically about the line midway between the outer sup- ports. 5.4.1.4.4 The elastic stress in the convex surface of the part of the specimen between the inner supports is calculated from the rela
38、tionship U = 12Ety/(3H2 - 4A*) where a metre; E is the modulus of elasticity in newtons per square metre; t y metres; 5.4.1.3.2 Three-point loaded specimens are usually flat strips 15 mm to 50 mm wide and 110 mm to 250 mm long. The thick- ness of the specimen is usually dictated by the mechanical pr
39、o- perties of the material and the product form available. Speci- men dimensions can be modified to suit specific needs but the approximate dimensional proportions should be preserved. 5.4.1.3.3 The specimen is supported near the ends and bent by forcing a ball-ended screw against it at its mid-poin
40、t as shown in figure 1 b). is the maximum tensile stress in newtons per square is the thickness of specimen in metres; is the maximum deflection between outer supports in H A is the distance between outer supports in metres; is the distance between inner and outer supports in 5.4.1.3.4 The elastic s
41、tress at the mid-point of the convex sur- face is calculated from the relationship O = 6EtyIH2 metres. where The dimensions are often chosen so that A = Hl4. u metre; is the maximum tensile stress in newtons per square 5.4.1.4.5 An alternative method of calculating the elastic stress between the inn
42、er supports is to use the relationship 4 E metre; U = 4EtylhZ is the modulus of elasticity in newtons per square CE“ ENxISOx7539- 2 95 3404589 0107096 T99 IS0 7539-2 1989 (E) where h y is the distance between inner supports in metres; is the deflection between inner supports in metres. NOTE -. This
43、equation is a special case of the equation in 5.4.1.4.4 when A = O. 5.4.1.4.6 The above relationships are based on small deflec- tions (ylH or ylh *LN -,-. “ _.% i “ . CEN EN*ISO*7539- 2 95 3404589 O307098 861 BS 6980 : Part 2 : 1990 Publications referred to See national foreword. CEN EN*ISO*?537- 2
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