Fatigue behaviour of graphite and interpenetrating graphite–aluminium composite up to 109 load cycles

Fatigue behaviour of polycrystalline, isotropic graphite FU2590 (porosity 13 vol%) and of FU2590 infiltrated with the aluminium alloy AlSi7Mg (FU2590/AlSi7Mg) is investigated. The interpenetrating graphite–aluminium composite is produced by squeeze casting infiltration, where the open porosity of FU2590 (10–11 vol%) is infiltrated with AlSi7Mg. Fully reversed bending fatigue tests at cycling frequency 25 Hz and fully reversed tension–compression experiments at 20 kHz are performed. Fatigue data of FU2590 measured with both methods are comparable when presented vs. the normalized maximum stress, i.e. maximum tension stress of a cycle divided by the bending or tensile strength, respectively. The mean endurance limit of FU2590 at 109 cycles under tension–compression loading is at 23.0 ± 0.9 MPa (normalized maximum stress 0.65 ± 0.03), and 26.1 ± 1.5 MPa (normalized maximum stress 0.55 ± 0.03) is found for FU2590/AlSi7Mg. The increased endurance limit of FU2590/AlSi7Mg is mainly caused by the 15% higher stiffness compared with FU2590. In reversed bending experiments, at cycles to failure below 107, the composite shows about 30% higher cyclic strength. The beneficial influence of infiltration on static properties is more pronounced, and 50% higher bending strength and 35% higher tensile strength are determined. In the graphite, fatigue cracks predominantly initiate at porosity. In the composite, cracks initiate at pores, which are filled with AlSi7Mg. Interface failures and failures of the aluminium alloy limit the beneficial influence of infiltration on the cyclic properties.