A B S T R A C T In the present paper, the fatigue strength estimation capabilities of the modified C-S (Carpinteri-Spagnoli) criterion are improved by employing the Maximum Rectangular Hull (MRH) method proposed by the first author. The C-S criterion is a multiaxial high-cycle fatigue criterion based on the critical plane approach and takes into account both shear stress (Mode II) and normal stress (Mode I) mechanisms to evaluate the orientation of the critical plane. The fatigue damage parameter used is given by a nonlinear combination of the equivalent normal stress amplitude, N a,eq , and the shear stress amplitude, C a , acting on the critical plane. In the present paper, the shear stress amplitude is evaluated through the MRH method. Some experimental data available in the literature are compared with the theoretical estimations, concluding that the multiaxial fatigue strength evaluations provided by the C-S criterion are improved when C a is computed applying the MRH method instead of the Minimum Bounding Circle (MBC) method.Keywords critical plane approach; modified C-S criterion; multiaxial high-cycle fatigue; Maximum Rectangular Hull method; stress-based criterion.
N O M E N C L A T U R Ea = amplitude C = shear stress vector acting on the critical plane C a = shear stress amplitude C 1,a (ξ), C 2,a (ξ) = half sides of the rectangular hull f y = material yield stress I = error index m = mean value max = maximum value N = normal stress vector perpendicular to the critical plane N a,eq = equivalent normal stress amplitude PXYZ = fixed frame P123 = frame of the principal stress axes P123 = frame of the weighted mean principal stress axes S w = stress vector at material point P, related to the critical plane t = time T = observation time interval w = vector normal to the critical plane α = phase angle between longitudinal (axial) normal stress σ x and tangential (hoop/circumferential) normal stress σ y β = phase angle between longitudinal (axial) normal stress σ x and shear stress τ xy
Low fracture toughness of concrete represents a serious shortcoming. An effective way to improve the concrete toughness is represented by the dispersion (during mixing) of discontinuous fibres into the concrete mix. The principal beneficial effect of fibres is the crack bridging in the cementitious matrix, providing resistance to crack propagation before fibre debonding and/or pulling out or failure. In the present paper, the fracture behaviour of FRC (fibre reinforced concrete) specimens is examined, with micro-synthetic polypropylene fibrillated fibres being randomly distributed in concrete. The modified two-parameter model, proposed by the authors to calculate Mode I plain-stain fracture toughness for quasi-brittle material, is able to take into account the possible crack deflection (kinked crack) during stable crack propagation.
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