Abstract
Cast iron used in manufacturing transport facilities and mining equipment may be made layered with various compositions. The results of experimental-analytical investigation into cyclic bending strength of seminatural rectangular cross-section cast iron specimens with a 10–11 mm thick porous layer are presented. The stress ratio r = -0.62, while stress ranges from 70 to 280 MPa. The bending specimens were tested under a great number of loading cycles – 3 × 10⁸ (specimen 1) and 2 × 10⁸ (specimen 2). Up to 1 × 10⁸ cycles cracks propagated only in the surface porous layer. Then the main crack has formed and propagated into depth. The dependences of crack propagation rates on stress intensity factor were established. The presented results show that transitional layer arrests crack propagation, when crack front passes from porous to basic metal. The thresholds of discrete layers were established by testing CT specimens. It has been concluded that crack propagation character and the range of stress intensity factor threshold Δ K_th depend on the structure of the layer.

Key words
cast iron, dross layer, structure, crack, fatigue, threshold

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