Abstract
Strain-controlled out-of-phase thermo-mechanical fatigue tests at 100–500 °C and various strain ranges were conducted on five cast iron grades, including one lamellar, three compacted and one spheroidal graphite iron. Investigations of graphite morphology and matrix characteristics were performed to associate parameters, such as geometrical features of graphite inclusions and the matrix microhardness, to the thermo-mechanical fatigue performance of each grade. From this, thermo-mechanical fatigue life as a function of maximum stress at half-life, is found to decrease consistently with increasing average graphite inclusion length irrespective of the graphite content. In contrast, no evident correlation between the fatigue life and the matrix microhardness is observed.
Highlights
Heavy-vehicle engine components, such as cylinder heads, are sub jected to rough working conditions during operation, namely high temperatures and mechanical loads
Thermo-mechanical fatigue of cast iron has been experimentally investigated in the past and associated damage processes have been closely related to the microstructure [6,7,8,9,10,11], which consists of graphite inclusions embedded in a steel-like matrix
One pearlitic Cr-Mo-alloyed lamellar graphite iron typically used in cylinder heads, denoted pearlitic LGI290, from a cast staircase with different thickness to obtain different solidification and cooling conditions where the specimens for this paper was manufactured from the same thickness
Summary
Heavy-vehicle engine components, such as cylinder heads, are sub jected to rough working conditions during operation, namely high temperatures and mechanical loads. Due to the compressive inelastic strain accumulated at the high temperature, tensile stresses typically develop as the engine cools down [3]. This repetitive cycle of time-dependent temperature and mechani cal load conditions, typically referred to as out-of-phase thermo-me chanical fatigue (OP-TMF), contributes to crack formation at the valve bridge in the cylinder head [3,4,5]. Accord ingly, previous studies have found that microcracks typically initiate in the matrix starting from fractured or debonded graphite in clusions under cyclic loading at temperatures between room tempera ture and 500 ◦C [6,7,10,15,17]. Based on the observation that graphite inclusions act as pre-existing cracks in lamellar and compacted graphite iron from which fatigue growth in the matrix follows, it is reasonable to suggest that more elongated graphite inclusions will promote higher crack growth rate
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