Failure criteria of heat-stressed parts of piston engines and the review of methods for assessment of pistons durability

During designing, calculation and further development of an engine it is necessary to take into account all the values that define its lifetime. In variable modes of tractor diesel engine, the stress level of piston changes over time, which leads to the appearance of cracks on the edge of combustion chamber. It especially applies to the semi-open combustion chambers. The appearance and growth of crack to a critical length often leads to failure of the combustion chamber edge. In this regard, the selection of failure criteria of pistons in operation is an urgent task. The durability assessment should be made on the criteria of the thermal fatigue strength that could be divided into three groups: the deformation criteria, the deformation and kinetic criteria, the energy criteria. Deformation criteria is the most widely used. Taking into account the active loading in conditions of uniaxial tension, the damage accumulation related to the material plastic deformation, or the peak deformation criterion is determined. Along with experimental methods consuming time and money, the other reliability assessment methods based on the numerical calculations, such as finite element method become common use. The fatigue material properties when loading a small number of cycles are determined by the Coffin-Manson equations in the form introduced by J. Morrow for the amplitudes of elastic and plastic deformations, depending on the number of cycles to failure. The analysis of the works of various authors shows that the crack growth process is not fully considered. The lack of theoretical approach to determining the remaining lifetime demands the improvement of calculation method which should take into account the temperature difference on the surface of piston, gas pressure and piston geometric dimensions affecting the stress concentration.

thermal strength, damages, durability, strength criteria, deformations, low-cycle fatigue