PSI - Issue 54

Artur Kuchukov et al. / Procedia Structural Integrity 54 (2024) 369–375 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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residual strength; II - stabilization stage, the most extended one, which shows a slow decrease of residual mechanical characteristics, of the order of 5-10% of the original static strength; III - aggravation, characterized by a fall of composite characteristics with fatigue macro-failure. The points determining the stabilization stage limits can be determined as the value of the tangent of the angle at these points [7]. Determination of the stabilization section limit, as the most informative from the point of view of material life prediction, seems to be significant, because knowing the duration of preliminary cyclic loading, at the stabilization area, it is possible to be limited by calculations on underestimated static characteristics of the composite, without carrying out more time-consuming fatigue calculations. Various models are known to describe the process of stiffness degradation of polymer composites as fatigue damage accumulates. One of them was proposed by Mao and Mahadevan [8], the approximation of experimental dependencies in the model is carried out by two power functions. Previously, the authors noted that the appearance of stiffness degradation curves (fatigue sensitivity curves) corresponds to the appearance of some integral probability distribution curves. We developed two models assuming the use of the Weibull distribution law and beta distribution [9] to approximate the variation of strength and deformation characteristics. The verification of these models carried out in [10] demonstrated their high descriptive capability.

Nomenclature E

Young's modulus K E, K B Fatigue sensitivity coefficients N Number of cycles n Relative number of load cycles R Load cycle asymmetry coefficient ν Cyclic frequency σ B , τ B Strength limits σ max Maximum normal stresses τ max Maximum shear stresses

2. Materials Thin-walled FRP tubular specimens, obtained by continuous longitudinal-transverse winding method, based on EC 1200 tek glass fiber and cold-cured epoxy resin KER 828 were prepared for experimental studies. Winding angle: 85°. The length of the samples was 140 mm with the length of the working part 60 mm, inner diameter 25.4 mm, outer diameter 30 mm. Previously, we solved methodological aspects of testing composite tubular specimens, in particular, we gave recommendations on gluing the gripping parts of the tubes into aluminum sleeves with a wall thickness of at least 1.5 mm to prevent slipping out of the grips of the testing system in the process of quasi-static and cyclic

stretching. [11]. 3. Tests results

Experimental study of stiffness and strength changes of FRP tubular specimens under preliminary biaxial cyclic loading of different duration and following quasi-static tensile testing has been performed. The specimens were separated into groups for quasi-static and cyclic tensile, torsional and proportional tensile-torsional tests with different ratios of normal and shear stress tensor components.