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ISO 7539-6-2011 pdf free download
ISO 7539-6-2011 pdf free download.Corrosion of metals and alloys — Stress corrosion testing — Part 6:
Preparation and use of precracked specimens for tests under constant load or constant displacement
Corrosion des métaux et alliages — Essais de corrosion sous contrainte — Partie 6: Preparation et utilisation des éprouvettes prefissurées pour essais sous charge constante ou sous déplacement constant.
7.6.1 Either constant load or constant displacement specimens can be used for the determination of scc by crack initiation.
7.6.2 A series of specimens is required to allow the value of Kc to be determined by crack initiation. Two approaches may be adopted, as described in a) and b).
a) In cases where time is at a premium but the availability of specimens and testing apparatus is plentiful, it is most appropriate to simultaneously expose a series of specimens stressed to different K11 levels encompassing the range within which it is anticipated that Klscc will he.
b) In circumstances where time permits, the value of Kiscc can be determined with greater certainty and economy of specimens and testing apparatus by means of the binary search procedure. This requires that an initial specimen be used to determine the fracture toughness of the material, Ki (or K0 if invalid), using recommended procedures. This value establishes the upper bound to KISCC. The first stress corrosion test shall then be conducted at an initial stress intensity of half Ki with subsequent tests at other fractions of Kp in accordance, for example, with the schedule given In ISO 7539-1, depending on whether or not failure (or crack extension) occurred in the preceding tests.
7.6.3 Where constant displacement specimens are used, the deflection shall be applied in accordance with the recommendations made in 7.5.2 and 7.5.3. For constant load specimens, the load required to produce the desired stress intensity can be calculated by means of relationships such as those given in Figures 12 and 13. The testing machine used to apply the load shall permit the applied load to be measured to an accuracy of ±1 % and loading fixtures shall allow the load to be applied smoothly following exposure to the test environment.
7.6.4 The test period commences as soon as the required load or displacement is applied. An arbitrary test duration shall be chosen for the determination of a preliminary KISCC value above which stress corrosion cracking will initiate. This duration will depend upon the material and environment in question and shall be agreed upon between the parties concerned. However, for preliminary testing, times of 10 h for titanium alloys, 100 h for ultra-high-strength low alloy steels and 1 000 h for lower strength steels, high alloy steels of the maraging type and aluminium alloys may be appropriate minima.
7.6.5 Duflng testing, crack length may be monitored optically at intervals, or continuously by means of electrical resistance, back-face strain, displacement gauge or alternative techniques, depending on the experimental circumstances. These measurements may facilitate the detection of crack initiation and enable crack growth rates to be determined as a function of stress intensity.
7.6.6 On completion of the test period, the specimen shall be inspected for signs of failure. If intact, it shall be broken open and the minimum and maximum fatigue precrack lengths measured, if possible to the nearest 0,5 % H. The length of the fatigue precrack shall also be measured at both edges and at the following three positions from one edge: 0,258, 0,508 and 0,75B.
The average of these last five measurements shall be used as the effective initial crack length, a0, in the calculation of Kosoc.ISO 7539-6-2011 pdf free download.
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