ORAL/POSTER REFERENCE: ICF100473OR A METHOD FOR ACCELERATION OF DECREASING ∆K FCGR TESTS S.Tarafder1, S.Sivaprasad1, M.Tarafder1 and K.K.Ray2 1 National Metallurgical Laboratory, Jamshedpur 831 007, INDIA 2 Dept. of Met. & Mats. Engg., IIT, Kharagpur 721 302, INDIA ABSTRACT An alternative method of accelerating decreasing ∆K FCGR tests has been proposed in this paper in order to address experimental difficulties that are often faced when conducting such tests using the conventional technique. The equation for the envelope that is to be followed for reducing the ∆K level as the crack grows has been derived from considerations of decreasing the monotonic plastic zone size at a constant rate. Through experimental assessment of the alternative method and examination of crack closure effects, it has been shown that it does not lead to the accumulation of crack growth retardation effects. The new method has been shown to be particularly suitable for high strength materials. The employment of the method for obtaining threshold regime corrosion fatigue crack growth data has been demonstrated. KEYWORDS Fatigue crack growth, FCGR test method, Decreasing ∆K test, Crack closure, HSLA steel INTRODUCTION Decreasing ∆K fatigue crack growth rate (FCGR) tests are indispensable for obtaining fatigue crack growth resistance of materials at low levels of crack driving force. Conventionally such tests are conducted by a load-shedding procedure laid down in the ASTM standard E647 [1]. In this procedure, suggested by Saxena et al. [2], loads are progressively reduced as the crack length a increases such that the ∆K envelope of the test is forced to follow the relation ∆K = ∆Ko e C(a-a o ) (1) In the above equation, ∆Ko and ao are the stress intensity factor (SIF) range and crack length respectively with which the test is started, and C is a negative constant (standardised at –0.08 mm-1). Eqn. 1 was obtained based on the requirement that the fractional change in the monotonic plastic zone size associated with the fatigue crack remains constant with increase in a so as to preclude the accumulation of overload retardation effects. Figure 1 gives a schematic of the form of the ∆K vs a curve described by Eqn. 1. From the nature of the conventional ∆K envelope shown in Figure 1, it can be envisaged that as lower ∆K values are achieved, cracks have to be grown through larger and larger increments in order to produce a given reduction in ∆K. Due to this, and because crack growth rates decrease as the ∆K is lowered, longer time intervals are required to produce progressive reductions in ∆K. For growing down to low values of ∆K
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