Analysis of Thermomechanical Fatigue Behaviour of Diesel Engine Cylinder Head Materials in Nigerian Operational Conditions

Premature failure of diesel engine cylinder heads through thermomechanical fatigue cracking is a significant maintenance cost driver for fleet operators and public transport companies across Nigeria, where severe engine loading cycles, poor fuel quality, and inadequate cooling system maintenance accelerate the thermal and mechanical damage accumulation in cylinder head structures. This study analyses the thermomechanical fatigue behaviour of grey cast iron and compacted graphite iron cylinder head materials under loading spectra representative of Nigerian heavy commercial vehicle duty cycles. In-service cylinder head temperature profiles were measured on a Volvo FH12 truck engine during a standardised test drive covering Lagos to Ibadan highway conditions, using thermocouple arrays embedded at critical locations in the fire deck, valve bridge, and port regions. Experimental thermomechanical fatigue specimens of both materials were tested on a servo-hydraulic testing machine under strain-controlled in-phase and out-of-phase cycling conditions at temperature ranges of 50 to 350 degrees Celsius, selected to match recorded in-service profiles. Fatigue life characterisation using the Ostergren energy-based damage parameter produced satisfactory correlations for both materials and cycling conditions. Results showed that compacted graphite iron exhibited a 2.3-fold thermomechanical fatigue life advantage over grey cast iron under the most damaging out-of-phase cycling condition, due to its superior thermal conductivity and reduced temperature gradient-induced stress. The study provides materials selection guidance for cylinder head upgrades in the Nigerian heavy vehicle fleet. Keywords: thermomechanical fatigue, cylinder head, diesel engine, compacted graphite iron, Nigerian road conditions.