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ASTM E647-13a
Standard Test Method for Measurement of Fatigue Crack Growth Rates (Includes all amendments And changes 7/2/2014).
Automaticky přeložený název:
Standardní zkušební metoda pro měření únavových trhlin rychlost růstu
NORMA vydána dne 15.10.2013
Informace o normě:
Označení normy: ASTM E647-13a
Poznámka: NEPLATNÁ
Datum vydání normy: 15.10.2013
Kód zboží: NS-47505
Počet stran: 50
Přibližná hmotnost: 150 g (0.33 liber)
Země: Americká technická norma
Kategorie: Technické normy ASTM
Anotace textu normy ASTM E647-13a :
Keywords:
constant amplitude, crack size, fatigue crack growth rate, stress intensity factor range, ICS Number Code 77.040.10 (Mechanical testing of metals)
Doplňující informace
| Significance and Use | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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5.1 Fatigue crack growth rate expressed as a function of crack-tip stress-intensity factor range, d a/dN versus Δ5.1.1 In innocuous (inert) environments fatigue crack growth rates are primarily a function of Δ5.1.2 Expressing da/dN as a function of Δ5.1.3 Fatigue crack growth rate data are not always geometry-independent in the strict sense since thickness effects sometimes occur. However, data on the influence of thickness on fatigue crack growth rate are mixed. Fatigue crack growth rates over a wide range of ΔK have been reported to either increase, decrease, or remain unaffected as specimen thickness is increased. Thickness effects can also interact with other variables such as environment and heat treatment. For example, materials may exhibit thickness effects over the terminal range of da/ dN versus Δ5.1.4 Residual stresses can influence fatigue crack growth rates, the measurement of such growth rates and the predictability of fatigue crack growth performance. The effect can be significant when test specimens are removed from materials that embody residual stress fields; for example weldments or complex shape forged, extruded, cast or machined thick sections, where full stress relief is not possible, or worked parts having complex shape forged, extruded, cast or machined thick sections where full stress relief is not possible or worked parts having intentionally-induced residual stresses. Specimens taken from such products that contain residual stresses will likewise themselves contain residual stress. While extraction of the specimen and introduction of the crack starting slot in itself partially relieves and redistributes the pattern of residual stress, the remaining magnitude can still cause significant error in the ensuing test result. Residual stress is superimposed on the applied cyclic stress and results in actual crack-tip maximum and minimum stress-intensities that are different from those based solely on externally applied cyclic forces or displacements. For example, crack-clamping resulting from far-field 3D residual stresses may lead to partly compressive stress cycles, and exacerbate the crack closure effect, even when the specimen nominal applied stress range is wholly tensile. Machining distortion during specimen preparation, specimen location and configuration dependence, irregular crack growth during fatigue precracking (for example, unexpected slow or fast crack growth rate, excessive crack-front curvature or crack path deviation), and dramatic relaxation in crack closing forces (associated with specimen stress relief as the crack extends) will often indicate influential residual stress impact on the measured da/dN versus ΔK result. 5.1.5 The growth rate of small fatigue cracks can differ noticeably from that of long cracks at given Δ5.1.6 Crack closure can have a dominant influence on fatigue crack growth rate behavior, particularly in the near-threshold regime at low stress ratios. This implies that the conditions in the wake of the crack and prior loading history can have a bearing on the current propagation rates. The understanding of the role of the closure process is essential to such phenomena as the behavior of small cracks and the transient crack growth rate behavior during variable amplitude loading. Closure provides a mechanism whereby the cyclic stress intensity near the crack tip, ΔK eff, differs from the nominally applied values, ΔK. This concept is of importance to the fracture mechanics interpretation of fatigue crack growth rate data since it implies a non-unique growth rate dependence in terms of ΔK, and R (8).6 5.2 This test method can serve the
following purposes:
5.2.1 To establish the influence of fatigue crack growth on the life of components subjected to cyclic loading, provided data are generated under representative conditions and combined with appropriate fracture toughness data (for example, see Test Method E399), defect characterization data, and stress analysis information 5.2.2 To establish material selection criteria and inspection requirements for damage tolerant applications. 5.2.3 To establish, in quantitative terms, the individual and combined effects of metallurgical, fabrication, environmental, and loading variables on fatigue crack growth. |
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| 1. Scope | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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1.1 This test method2 covers the determination of fatigue crack growth rates from near-threshold to Kmax controlled instability. Results are expressed in terms of the crack-tip stress-intensity factor range (ΔK), defined by the theory of linear elasticity. 1.2 Several different test procedures are provided, the optimum test procedure being primarily dependent on the magnitude of the fatigue crack growth rate to be measured. 1.3 Materials that can be tested by this test method are not limited by thickness or by strength so long as specimens are of sufficient thickness to preclude buckling and of sufficient planar size to remain predominantly elastic during testing. 1.4 A range of specimen sizes with proportional planar dimensions is provided, but size is variable to be adjusted for yield strength and applied force. Specimen thickness may be varied independent of planar size. 1.5 The details of the various specimens and test configurations are shown in Annex A1 – Annex A3. Specimen configurations other than those contained in this method may be used provided that well-established stress-intensity factor calibrations are available and that specimens are of sufficient planar size to remain predominantly elastic during testing. 1.6 Residual stress/crack closure may significantly influence the fatigue crack growth rate data, particularly at low stress-intensity factors and low stress ratios, although such variables are not incorporated into the computation of ΔK. 1.7 Values stated in SI units are to be regarded as the standard. Values given in parentheses are for information only. 1.8 This test method is divided into two main parts. The first part gives general information concerning the recommendations and requirements for fatigue crack growth rate testing. The second part is composed of annexes that describe the special requirements for various specimen configurations, special requirements for testing in aqueous environments, and procedures for non-visual crack size determination. In addition, there are appendices that cover techniques for calculating da/dN, determining fatigue crack opening force, and guidelines for measuring the growth of small fatigue cracks. General information and requirements common to all specimen types are listed as follows:
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