Fractography and Crack Initiation of Very-High-Cycle Fatigue for a High Carbon Low Alloy Steel

Abstract

Key words: fractography, very-high-cycle fatigue, crack initiation Abstract: Very-High-Cycle Fatigue (VHCF) is the phenomenon of fatigue damage and failure of metallic materials or structures subjected to 10 8 cycles of fatigue loading and beyond. This paper attempts to investigate the VHCF behavior and mechanism of a high strength low alloy steel (main composition: C-1% and Cr-1.5%; quenched at 1108K and tempered at 453K). The fractography of fatigue failure was observed by optical microscopy and scanning electron microscopy. The observations reveal that, for the number of cycles to fatigue failure between 10 6 and 4×10 8 cycles, fatigue cracks almost initiated in the interior of specimen and originated at non-metallic inclusions. An "optical dark area" (ODA) around initiation site is observed when fatigue initiation from interior. ODA size increases with the decrease of fatigue stress, and becomes more roundness. Fracture mechanics analysis gives the stress intensity factor of ODA, which is nearly equivalent to the corresponding fatigue threshold of the test material. The results indicate that the fatigue life of specimens with crack origin at the interior of specimen is longer than that with crack origin at specimen surface. The experimental results and the fatigue mechanism were further analyzed in terms of fracture mechanics and fracture physics, suggesting that the primary propagation of fatigue crack within the fish-eye local region is the main characteristics of VHCF.

Very-high-cycle fatigue behavior of a structural steel with and without induced surface defects

“…For high-strength steels, cracks are prone to initiate at specimen subsurface or interior [9][10][11][12][13] in VHCF regime, and a distinct crack initiation feature of ''fish-eye" morphology embracing a region of ''fine-granular-area (FGA)" usually presents at fracture surface. Another typical feature of VHCF behavior for highstrength steels is that the S-N curve is a stepwise or duplex shape [14][15][16][17][18][19][20][21] with regard to surface-induced and interior-induced crack initiation, respectively. Bathias and Paris [22] investigated several high-strength steels via ultrasonic fatigue test technique and revealed that subsurface crack initiated from either nonmetallic inclusions or other microstructural inhomogeneities.…”

Experimental and theoretical investigation of environmental media on very-high-cycle fatigue behavior for a structural steel

“…In recent years, the investigation of VHCF on metallic materials has attracted an increasing number of investigators in the fatigue research field (e.g. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]) due to the growing requirements of engineering applications, including aircraft, automobile, ship, railway, bridge, etc., for which the metallic components and structures need to endure a fatigue life larger than 10 7 or 10 8 loading cycles, and even requiring 10 10 -10 11 life cycles of endurance in some vital cases.…”

“…One typical characteristic of VHCF for high-strength steels is that the S-N curve includes two parts corresponding to subsurface and surface crack initiation, resulting in a stepwise or duplex shape [7][8][9]. Another distinct feature of crack initiation for VHCF is identified as a fisheye pattern on the fracture surface, which is mostly located at the specimen subsurface region and originates from a nonmetallic inclusion [3,[8][9][10].…”

“…Another distinct feature of crack initiation for VHCF is identified as a fisheye pattern on the fracture surface, which is mostly located at the specimen subsurface region and originates from a nonmetallic inclusion [3,[8][9][10]. Murakami et al [3] reported that the mechanism of subsurface crack initiation and the formation of the fisheye pattern are associated with the interaction of hydrogen embrittlement and cyclic damage.…”

Crack propagation mechanism and life prediction for very-highcycle fatigue of a structural steel in different environmental medias

“…INTRODUCTION ery-high-cycle fatigue (VHCF) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] of metallic materials is regarded as fatigue failure at stress levels below the conventional fatigue limit and the corresponding fatigue life beyond 10 7 loading cycles. Lots of modern engineering structures and components, such as airplanes, turbines, automobiles and high speed trains are expected to endure the safe performance in the range of 10 7 -10 10 load cycles.…”