Issue 41

D.-Q. Wang et alii, Frattura ed Integrità Strutturale, 41 (2017) 143-148; DOI: 10.3221/IGF-ESIS.41.20 144 crack length, etc., on fatigue crack growth rates. Although several mechanisms for the retardation have been proposed, including crack closure [1-4], crack tip hardening [5], crack branching [6], residual stresses [7] or combined effects [5, 8], the micro-mechanical behavior of the overload is still an open issue. The crack closure concept, though extensively applied with its roles recognized in the overload process, is actually physical only at crack wake; whereas the driving part of crack growth and associated cracking of materials is at crack front. It seems that any interpretation of overload effect is insufficient if crack closure at crack wake is not correlated with cracking process in front of crack tip. Digital image correlation (DIC) technique provides a good solution to strain measurement and has been widely applied in both engineering and laboratory scale researches. As for fatigue tests, the DIC method was found to be very helpful to the determination of plastic zone size [9], crack closure level [3, 4, 10, 11], and microstructural scale strain localization [12]. The DIC would be more powerful to resolve strain distribution if combined with high resolution images from SEM. In this work, the DIC technique combined with in situ SEM was used to study the effect of overload on fatigue crack growth behavior. The strain evolution at crack tip as well as variation of crack opening displacement (COD) were measured and correlated with each other both before and after an overloaded growing fatigue crack. The driving force of fatigue crack growth in case of overload was also discussed. E XPERIMENTAL METHODS Material and specimen he material used in this work is 25Cr2Ni2MoV, a type of rotor steel. The steel has been undergone quenching and tempering heat treatment, and a final tempering process was conducted at 560  C for 40 hours. The chemical composition of the steel in wt.% mainly includes C (0.18  0.27), Si (  0.12), Mn (0.12  0.28), P (  0.015), S (  0.015), Ni (2.05  2.35), Cr (2.15  2.45), Mo (0.63  0.82), V (  0.12), Cu (  0.17) and Fe as the balance. The yield strength and ultimate strength of this steel are 793 MPa and 894 MPa, respectively. The material thus has a low work hardening coefficient. The microstructures are mainly tempered martensites. Standard CT specimens with a  W ratio of 0.5 and a thickness B of 16 mm were firstly prepared and then pre-cracked using high frequency fatigue testing machine. The stress intensity factor range  K during the pre-cracking process was decreased from 15 MPa  m 1/2 to 13.5 MPa  m 1/2 for a first fatigue crack extension of 1.5 mm, followed by another 10% load decrease for another 1.5 mm extension. In total, the pre-crack length was 3 mm. Several dog-bone shaped miniature specimens with a gauge cross-section of 0.5  4 mm 2 and a gauge length of 10 mm were then cut from the CT specimen by Electrical Discharge Machining (EDM) technique. The specimen for in-situ SEM fatigue test was thus prepared with a pre-crack length of 2.286 mm. The shape and dimensions of the standard CT specimen and a small size fatigue specimen is shown in Fig. 1. Figure 1 : Shape and dimensions of standard CT specimen and the machined small size tensile specimen In-situ fatigue test Prior to in-situ fatigue test, the specimen was ground with SiC papers, and polished and chemically etched to reveal the microstructures and then mounted onto a Deben Microtest tensile module with a load capacity of 5 kN equipped into a SEM (Zeiss, EVO series). The loading scheme is shown in Fig. 2. The base line  K was 25 MP  m 1/2 , and the overload ratio T