PSI - Issue 7

Kentaro Wada et al. / Procedia Structural Integrity 7 (2017) 391–398 K. Wada et al./ Structural Integrity Procedia 00 (2017) 000–000

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2. Material and methods 2.1. Material and specimen

The material used in this research was the bearing steel, SAE52100, with a chemical composition of 1.00% C, − 0.26% Si, − 0.36% Mn , − 1.44% Cr , − 0.0006% O and − 0.002% Ti. The material was first heat-treated at 840 °C for one hour in a deoxidizing gas, then subsequently oil-quenched and tempered at 240 °C for two hours. The 0.2% proof stress, σ 0.2 , tensile strength, σ B , and Vickers hardness, HV , measured with a load of 9.8 N were 2161 MPa, 2432 MPa and 703, respectively. Fig. 1 (a) presents the shape and dimensions of the specimen used for the fatigue tests. The specimen surface was electro-polished after being buffed with diamond paste. An artificial, sharp-notch defect (shown in Fig. 1 (b)) was introduced in the center of the specimen, achieved via electrical discharge machining. 2.2. Fatigue tests Tension-compression fatigue tests were conducted at stress ratios of − 10, − 5, − 3 and − 1, in ambient air at room temperature, to determine the fatigue limits and FCG thresholds, Δ K th and K max th . The fatigue limit at each stress ratio was established from the maximum stress amplitude at which the specimen did not fail after 10 7 loading cycles. Δ K th and K max th were calculated by the following equations (Murakami, 1985):

0.65 2 σ ∆ = ⋅ ⋅ K

π

(1)

area

th

a

max th (2) where, √ area is the square root of the projected area of crack or defect, σ a is the stress amplitude and σ max is the maximum stress. The fatigue test was halted periodically so as to observe fatigue cracks by the plastic replica method. max 0.65 σ = ⋅ π K area

(b)

(a)

Fig. 1: Shape and dimensions of the specimen in mm (a) and the artificial defect in μ m (b).