Strength and failure mechanism of gray cast iron under various loading conditions. Part 1. Static crack resistance, tensile and torsional strength of the samples

The authors studied the samples of as-cast ferritic-pearlitic GG15 gray cast iron with lamellar graphite (σ_u = 121 ± 7 MPa). Tension of 20 mm diameter cylindrical samples was performed on N50KT testing machine. Static crack resistance tests of cast iron were carried out at temperatures of 20, –70 and –196 °C on 10 and 20 mm thick prismatic samples using a three-point bending scheme on Instron 8802 testing machine. The authors estimated the implementation of plane strain (PS) conditions by the t/(K_Q/σ_0.2)² ≥ 0.6 criterion specified in GOST 25.506 – 85, as well as by the h_max/t < 10^–2 criterion, where hmax is the maximum depth of the plastic zone under the fracture surface, t is the sample thickness. Depth of the plastic zones beneath the fractures surface was determined using X-ray diffraction. Tensile tests on cylindrical samples with a 10 mm diame­ter and 50 mm length were conducted at 20 °C using MK-50 tester. The authors examined the fractures surface microrelief using SIGMA scanning electron microscope (ZEISS). Static crack resistance tests of cast iron showed that at 20 °C, the PS condition according to GOST 25.506 – 85 is met only for 20 mm thick samples at –70 °C. However, according to the h_max/t criterion, the PS condition is met for all the samples, regardless of thickness and testing temperature. Therefore, the obtained static crack resistance values ​​(20.6 ± 1.5 MPa·m^1/2) correspond to K_1C. To evaluate the PS conditions of GG15 cast iron, it is proposed to use the criterion t/(K_Q/σ_0.2)² ≥ 0.38. In the static failure area, regardless of the sample thickness and test temperature, cast iron destruction occurred via intergranular brittle failure and cleavage mechanisms. Results from tensile tests of the samples showed that crack initiation occurred at graphite inclusions near the sample surface. Subsequent sample failure occurred at an angle to the sample cross-section by a cleavage mechanism.

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