Specific Features of Crack Propagation Under the Conditions of Static and Low-Cycle Loading
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SPECIFIC FEATURES OF CRACK PROPAGATION UNDER THE CONDITIONS OF STATIC AND LOW-CYCLE LOADING V. А. Skachkov1, 2 and O. R. Berezhnaya1
UDC 629.3:667.64
We study the influence of elliptic fatigue cracks of the fracture toughness of high-strength steel and the influence of static and low-cycle loading on the process of crack growth. Keywords: fatigue crack, fracture toughness, stress intensity factor.
The premature failures of unique structures made of high-strength steels caused by surface defects, such as undercuts, laps, and cracks, may lead to catastrophic consequences. Thus, it is an urgent problem to study their influence on the brittle fracture of steels of this kind.
In analyzing the brittle fracture of high-strength steels [1–3], it was established that its main cause is the presence of surface and undersurface defects in the form of laps formed in the process of rolling and cracks formed in the region of the welds. To study this phenomenon, we use the stress intensity factors (SIF; fracture toughness) under the conditions of plane strained state K IC and in the plane stressed state K C [4, 5].
The aim of the present work is to determine the SIF K IC of SP-53 high-strength steel and propose a pro-
cedure for the evaluation of brittle fracture under the conditions of long-term static and low-cycle loading. For this purpose, we used specimens with fatigue elliptic cracks obtained in the mode of resonance vibrations with an amplitude of 10 mm (Fig. 1). As the initial crack appears, the frequency of resonance vibrations becomes lower and their amplitude decreases to 5 mm. Adjusting the frequency of the generator to the shifted resonance, we increased the amplitude of vibrations to 10 mm. In this case, it becomes possible to obtain cracks with given sizes (Fig. 2а), namely, with length 2C and depth l . The fracture toughness is given by the Irwin formula
K IC = 1.1 πσ p
Here,
l
is the depth of the crack, mm,
l . Q
(1)
σ p is the fracture stress of the specimen containing the crack,
kN/mm 2 , and Q is the aspect ratio for the elliptic crack:
2
⎛ σp ⎞ Q = Φ − 0.212 ⎜ , ⎝ σ 0.2 ⎟⎠ 2
1 2
(2)
Zaporizhzhya State Engineering Academy, Zaporizhzhya, Ukraine. Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 51, No. 1, pp. 108–110, January–February, 2015. Original article submitted July 16, 2013. 1068-820X/15/5101–0121
© 2015
Springer Science+Business Media New York
121
V. А. SKACHKOV
122
AND
O. R. BEREZHNAYA
Fig. 1. Schematic diagram of an installation for the formation of surface elliptic fatigue cracks in the specimens: (1) specimen;
(2) magnetic wire; (3) excitation coil; (4) magnetizing coil; (5) power amplifier; (6) voltage stabilizer; (7) generator of variable signals.
Fig. 2. Elliptic surface fatigue cracks: (а) initial crack; (b) initial and grown crack.
Fig. 3. Dependences of the aspect ratio of the crack on the ratio of its depth to its length for the following values of σ p /σ 0.2 : (1) 1.0; (2) 0.8; (3) 0.6; (4) 0.4.
wher
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