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.
A B S T R A C T The myriad applicability of the frequency-domain critical plane criterion is outlined in order to evaluate and track the progression of fatigue damage in metallic structures subjected to high-cycle multiaxial random vibrations. The fatigue assessment using the given criterion is performed according to the following stages: (i) critical plane definition, (ii) power spectral density evaluation of an equivalent normal stress and (iii) computation of the damage precursor and fatigue life. The frequency-domain critical plane criterion is validated using experimental results related to (a) AISI 1095 steel cantilever beams under nonlinear base vibration, (b) 18G2A steel and (c) 10HNAP steel round specimens under random non-proportional combined flexural and torsional loads.Keywords damage precursor; frequency domain; high-cycle fatigue; multiaxial loading; random loading; vibration fatigue. Z ' = rotated frame Puvw = reference system related to the critical plane S eq (ω) = equivalent PSD function s xyz (t) = stress vector in PXYZT = time interval of observation T cal = fatigue life determined through calculations T exp = fatigue life through experiments α n = nth bandwidth parameter, with n = 1,2,3, … γ = rotation about the w axiŝ 1;2 and3 = rotation about the X ' axis λ n = nth spectral moment, with n = 1,2,3, … σ af = fatigue limit for fully reversed normal stress (R = À 1) σ 2 6';6' = variance of s 6 ' t ð Þ σ 2 6};6} = variance of s 6 } t ð Þ τ af = fatigue limit for fully reversed shear stress (R = À 1) ω = pulsation Correspondence: S. Vantadori.
The goal of the present paper is to discuss the reliability of a strain-based multiaxial Low-Cycle Fatigue (LCF) criterion, recently proposed by some of the present authors, in estimating the fatigue lifetime of metallic structural components weakened by sharp notches. Such a criterion, based on the critical plane approach, is formulated according to the control volume concept related to the Strain Energy Density (SED) criterion: a material point located at a certain distance from the notch tip is assumed to be the verification point where to perform the fatigue assessment. The above distance is assumed to be a function of both the biaxiality ratio (applied shear stress amplitude over normal stress amplitude) phase angle between transversal normal strain t and axial normal strain z ur , ut , uz direction cosines of u -axis vr , vt , vz direction cosines of v -axis wr , wt , wz direction cosines of w -axis phase angle between shear strain zt and axial normal strain z a Manson-Coffin shear strain amplitude zt shear strain angle between the averaged principal strain direction 1 and the normal w to the critical plane
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