Andrea Spagnoli

Multiaxial high-cycle fatigue criterion for hard metals

Abstract Some multiaxial high-cycle fatigue criteria based on the so-called plane approach are reviewed. According to such an approach, the critical plane where the fatigue life assessment should be performed can be determined by maximising the amplitudes and/or values of some stress components. In the present paper, the critical plane orientation is correlated with the averaged principal stress directions deduced through the weight function method, and new fatigue failure criterion is proposed. The results derived by applying the present criterion and the other critical plane criteria analysed are compared with experimental data related to different brittle (hard) metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states.

A fractal analysis of size effect on fatigue crack growth

As is well-known, strength of materials is influenced by the specimen or structure size. In particular, several experimental campaigns have shown a decrease of the material strength under static or fatigue loading with increasing structural size, and some theoretical arguments have been proposed to interpret such a phenomenon. As far as fatigue crack growth is concerned, limited information on size effect is available in the literature. In the present paper, by exploiting some concepts of fractal geometry, new definitions of fracture energy and stress intensity factor based on physical dimensions different from the classical ones are discussed. Then, a size-dependent crack growth law (expressing crack growth rate against stress intensity factor range) is proposed. Finally, such a law is herein used to interpret relevant experimental data related to concrete.

Expected principal stress directions under multiaxial random loading. Part I: theoretical aspects of the weight function method

As has been observed experimentally by many authors, the position of the fatigue fracture plane appears to strongly depend on the directions of the principal stresses or strains. In Part I of the present work the expected principal stress directions under multiaxial random loading are theoretically obtained by averaging the instantaneous values of the three Euler angles through some suitable weight functions which are assumed to take into account the main factors influencing fatigue behaviour. Then, in Part II, it is examined how such theoretical principal directions determined by applying the proposed procedure are correlated to the position of the experimental fracture plane for some fatigue tests reported in the literature.

Expected principal stress directions under multiaxial random loading. Part II: numerical simulation and experimental assessment through the weight function method

In Part I of the present work, the theoretical aspects of a proposed procedure to determine the expected principal stress directions under multiaxial random loading have been discussed. This procedure consists of averaging the instantaneous values of the three Euler angles through weight functions. In Part II here, a numerical simulation is presented to illustrate the above theoretical method. As an example, the algorithm proposed is applied to some experimental biaxial in- and out-of-phase stress states to assess the correlation between the expected principal stress directions and the position of the experimental fatigue fracture plane for such tests.

A new non-contacting non-destructive testing method for defect detection in concrete

A new non-destructive testing (NDT) method for defect detection in concrete structures is presented. The method is based on the dynamic response of flawed concrete structures subjected to impact loading. Conversely to similar NDT techniques, such as the impact-echo method, the present method uses non-contacting devices for both impact generation (a shock tube producing shock waves) and response monitoring (laser vibrometers measuring concrete surface velocity). Experimental and numerical (finite element) studies have been carried out for concrete specimens containing artificial defects (penny-shaped cracks parallel to the free surface) with varying length and depth. According to the experimental and numerical results, it appears that the present method enables an effective detection of defects, particularly in the range of shallow defects.

Elastic buckling and postbuckling behaviour of widely-stiffened conical shells under axial compression

In thin-walled shells of revolution with widely-spaced meridional stiffeners loaded in compression, a local panel buckling may develop. For the particular case of cylindrical panels, the buckling and postbuckling behaviour has been investigated in detail, whereas limited research is available for the more general case of conical panels. In this paper, the linear and non-linear elastic buckling response of the conical panel is studied for a wide range of shell and stiffening parameters by means of an appropriate finite element model. The classical buckling load is determined on the basis of linear analysis. The imperfection sensitivity is studied through non-linear analysis of imperfect conical panels with imperfections affine to the critical mode. Different aspects of the behaviour are quantified through suitably defined curvature parameters.

Part-through cracks in pipes under cyclic bending

Abstract The propagation of a circumferential external surface crack in a metallic round pipe under cyclic bending loading is examined through a two-parameter theoretical model. A finite element analysis is carried out to determine the stress-intensity factor distribution along the front of the flaw, which is assumed to present an elliptical-arc shape with aspect ratio α = a el / b el ( a el , b el =ellipse semi-axes). The relative depth ξ of the deepest point on the front is equal to the ratio between the maximum crack depth, a , and the pipe wall thickness, t . The parameter R / t , with R =internal radius of the pipe, is made to vary from 1 to 10. The fatigue growth of the surface flaw occurs according to preferred paths in the diagram of α against ξ .

A new high-cycle fatigue criterion applied to out-of-phase biaxial stress state

A new multiaxial high-cycle fatigue criterion based on the so-called critical plane approach is presented. According to such a criterion, the critical plane orientation is proposed to be correlated with the averaged principal stress directions deduced through the weight function method. Then the fatigue failure assessment is performed by considering a nonlinear combination of the maximum normal stress and the shear stress amplitude acting on the critical plane. The proposed criterion is applied to a general sinusoidal biaxial stress state, for which analytical formulae can be derived. The theoretical results calculated according to the present criterion, together with those of other common critical plane criteria, are compared with experimental data related to different brittle (hard) metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states.

Fatigue growth simulation of part-through flaws in thick-walled pipes under rotary bending

Abstract The fatigue growth of a circumferential external surface flaw in a thick-walled round pipe subjected to rotary bending is simulated numerically. A three-dimensional finite element analysis is carried out to obtain the stress–strain field of the cracked structural component for any position of the flaw. The crack front is assumed to have an elliptical-arc shape during the whole propagation, as has been deduced from several experimental investigations. The results for rotary bending are compared to those for cyclic bending.

Buckling design of conical shells based on validated numerical models

In most shell buckling codes, guidance on the design of conical shells is restricted to unstiffened cones and even in this case the clauses are based on the procedures for cylindrical shells. Virtually no guidance is offered on stiffened cones and the particular characteristics of conical shells are not treated in detail. In this paper, use is made of finite element analysis to quantify critical elastic response and imperfection sensitivity through numerical models, whose adequacy has been quantified through comparisons with test data. The finite element results obtained were aimed at validating existing design recommendations for unstiffened cones and at developing a design approach for stringer-stiffened cones under compression, with a philosophy and format compatible with the European Shell Buckling Recommendations (ECCS).