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This article assesses the influence of the long stroke piston unit design parameters for the stress-strain state of the cylinder slow-speed stage. The stress-strain state estimation for the piston compressor cylinder slow-speed stage with a cylinder diameter equal to and more than 50 mm, show a significant increase in gaps and leaks in the cylinder-piston pair is observed, as well as an increase in equivalent stresses in the cylinder walls to the value of the yield strength of the wall cylinder material. The analysis allows for optimal engineering of the piston compressor cylinder slow-speed stage for the most promising designs.

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The article proposes a model of fatigue defect growth obtained on the basis of the analysis of the three-dimensional stress state at the apex of the corrosion defect. To study the stress-strain state the actual operational loads of the pipeline were simulated by the finite element method. The scientific novelty of the study is that when developing a model of fatigue defect growth, it is assumed that the growth of the defect, which develops by the opening fracture mode, is controlled only by normal stresses. In one loading cycle, the intensity of the fracture process is determined by both the maximum tensile stresses during the loading period and the maximum compressive stresses during the unloading period. Based on the assumptions made, it has been established that the growth rate of fatigue defects of corrosion origin depends on the magnitude of the coefficient of the average stresses change ahead of the crack front during full loading cycle.

The common growth criterion of fatigue cracks is offered for different degree of the biaxial tension. The specimen tension is investigated. Influence of biaxial loading on growth rate of a fatigue crack and its connection with a triaxial coefficient of a tension which can be calculated in an environ of a crack top is install.