International Journal of Fatigue 22 (2000) 593–600 www.elsevier.com/locate/ijfatigue An alternative method of decreasing K FCGR testing S. Sivaprasad a , S. Tarafder a,* , M. Tarafder a , K.K. Ray b a Fatigue and Fracture Group, National Metallurgical Laboratory, Jamshedpur-831 007, India b Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721 302, India Received 28 July 1999; received in revised form 1 March 2000; accepted 1 March 2000 Abstract The conventional technique of decreasing K FCGR testing often presents experimental difficulties in terms of the length of crack that must be accommodated and the time required to reach low values of K. Addressing these issues, an alternative method of conducting such tests has been proposed. A relation according to which K must be decreased has been derived assuming that the monotonic plastic zone size can be decreased at a constant rate with increase in crack length without adversely affecting the integrity of the FCGR data. Through experimental assessment of the alternative method and examination of crack closure effects, such an asumption has been vindicated. It has been shown that this method is particularly suitable for high strength materials and that it provides great advantages when conducting FCGR tests at low frequencies. The employment of the method for obtaining threshold regime corrosion fatigue crack growth data has been demonstrated for HSLA steels.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Fatigue test methods; Corrosion fatigue; HSLA steels 1. Introduction Decreasing K fatigue crack growth rate (FCGR) tests are conventionally conducted by a load-shedding pro- cedure as documented in the ASTM standard E 647 [1, pp. 569–596]. In this procedure, loads are progressively decreased as the crack length increases during a FCGR test such that the K envelope of the test is forced to follow the relation K=K o e C(a-a o ) (1) In the above equation, K o and a o are the stress intensity factor range and crack length respectively with which the test is started. C is a negative constant, the value of which has been standardised at -0.08 mm -1 so as to ensure that there are no delay effects originating from the progressive unloading. The procedure for conducting decreasing K FCGR tests, as described above, was proposed by Saxena et al. [2]. It is based on the requirement that the monotonic plastic zone associated with K max of the fatigue cycle * Corresponding author. Fax: + 91-0657-426527. E-mail address: star@csnml.ren.nic.in (S. Tarafder). 0142-1123/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII:S0142-1123(00)00029-3 should be decreased ‘at a rate such that the fractional change in the (plastic) zone size remains constant with increase in a’. Mathematically, this requirement can be expressed as 1 r dr da =C' (2) where C' is a constant. The monotonic plastic zone size r is given by [3] r= 1 2p  K max s y  2 (3) where s y is the yield stress of the material under test. Substitution of Eq. (3) into Eq. (2), and integration after rearrangement yields Eq. (1) for FCGR tests under con- stant load ratio, R. It may be noted that the constants in Eqs. (1) and (2) are related as C=C'/2. A schematic of the reduction of K with increase in crack length as per Eq. (1) during a decreasing K FCGR test is shown in Fig. 1. Tests are normally started with a sufficiently high K o that will ensure easy initiation of fatigue cracks. It can be visualised from the figure that in order to grow down to low values of K, fatigue cracks of substantial lengths have to be accom-