Giovanni Meneghetti

Fatigue crack initiation and propagation phases near notches in metals with low notch sensitivity

Abstract The paper summarizes the results of experimental work carried out in order to analyse the initiation and propagation of fatigue cracks on plates in a deep drawing steel and in a cast aluminium alloy. The plates were characterized by lateral V and U-shape symmetric notches, with a notch root radius ranging from 0.1 to 10 mm, the notch depth being 10 mm and the plate thickness 2 mm for the steel and 5 mm for the light alloy. In order to estimate the crack initiation life two new parameters are proposed; they are no longer based on the peak values of strain and stress, but on the averaged values of such quantities in the neighbourhood of the notch tip. The dimension of the process-zone is correlated to the intrinsic crack length of the material. The estimates need an elastoplastic approach and a numerical solution. Experimantal data and expected values are compared.

On fatigue limit in the presence of notches: classical vs. recent unified formulations

Abstract Classical formulations for the fatigue strength reduction factor of notched specimen, K f , (such as those by Neuber, Peterson, Heywood) were developed long time ago and have found some success by introducing a material constant (dependent on the tensile strength only) in order to take into account the problem of notch sensitivity. However, being empirical fitting equations, they have serious limitations when their asymptotic behaviour is considered, or when the empirical constants are not directly calibrated with experiments. This is shown in this work by using example data taken from the literature for various steels and alloys, and various notch sizes and shapes. Furthermore, although the material constants can be modified to include fatigue threshold dependence (satisfying the requirements of fracture mechanics), only the Neuber formula has a correct functional form in the entire range of notch sizes and shapes, and indeed appears to be sufficiently conservative in the range of data considered. Improved accuracy is found with a more recent empirical criterion due to Atzori and Lazzarin based on the Smith and Miller classification of notches, and with a new criterion here obtained by making consistent the Atzori and Lazzarin with the Lukas–Klesnil, having a sound interpretation in terms of self-arrested cracks ahead of a rounded notch for which the Creager–Paris stress field is valid. A large number of experimental data are taken from the literature to compare the accuracies of the various criteria.

Fatigue behaviour of AA356-T6 cast aluminium alloy weakened by cracks and notches

Abstract In this paper the static and fatigue behaviour of AA356-T6 cast alloy is analysed in order to provide engineers with some practical rules for a preliminary assessment of this material in an early stage of the design process. The study is divided into two different parts: in the first part a systematic reanalysis of some data taken from literature highlighted the influence of the main metallurgical parameters on the static and fatigue properties of the considered material, whereas in the second part an experimental investigation was performed in order to verify the applicability of the Atzori–Lazzarin diagram to the AA356-T6 cast aluminium alloy.

Structural Analysis of the Interior PM Rotor Considering Both Static and Fatigue Loading

Interior permanent magnet synchronous machines result to be a valid motor topology in case of both high efficiency and high flux-weakening range. These interesting peculiarities can be profitably achieved adopting a salient rotor structure. However, the design of an anisotropic rotor equipped with a multi flux-barrier structure is a very challenging task due to two conflicting requirements: (i) torque performance and (ii) structural integrity. The necessary iron bridges between the multiple rotor cavities, in which the permanent magnets are inset, represent a magnetic short circuit reducing the overall air gap flux. The structural strength assessments under static and fatigue loading of a radially laminated interior permanent magnet rotor are presented. The static resistance is assessed by using a classical approach in mechanical structural design, while the fatigue assessment required careful considerations due to the reduced values of the tip radius of the cavities, which induce severe stress concentration effects. Then the fatigue assessments are carried out using both the classical notch sensitivity theory and a more recent approach based on a non-conventional extension of the Fracture Mechanics discipline. As a result, both static and fatigue strengths are assessed successfully.

The Peak Stress Method for Fatigue Strength Assessment of welded joints with weld toe or weld root failures

The Peak Stress Method (PSM) has been presented in some previous papers, where it was shown that, in plane problems, the singular elastic maximum principal stress, evaluated at the weld toe of fillet-welded joints by means of a finite element analysis, is proportional to the Mode I Notch Stress Intensity Factor (NSIF), In parallel, a model based on the mean value of the strain energy (SED) averaged in a properly defined structural volume surrounding the point of fatigue crack initiation (either the weld toe or the weld root) has been proposed for fatigue strength assessments. The local strain energy values were given as a function of the Mode I and Mode II NSIFs neglecting the influence of the higher order (non-singular) stress terms. In the present work an expression linking the peak stress and the local strain energy value is presented, which can be used in plane problems when Mode II stress fields are non singular (at the weld toe) or of low intensity (at the weld root), By so doing, a single design scatter band valid for either weld root failures or weld toe failures can be calibrated in terms of elastic peak stress evaluated at the critical point.

Estimation of the fatigue strength of light alloy welds by an equivalent notch stress analysis

Abstract In this work the well known local approach to predict the fatigue strength of sharply notched components, based on the analytic expressions of the local stress field as proposed in literature, is applied to welded joints in aluminium alloys in a simplified form oriented to practical applications. A particular value of the general expression of the local stress field parameter is taken into account, which the fatigue strength depends on. Then a simple model is proposed by the authors in order to estimate such a parameter, based on the calculation of a geometric (or structural) contribution to the local stress field, depending on the overall joint geometry, and a local contribution evaluated by considering a specimen with lateral V-notches characterised by the same weld toe profile and a depth proportional to the weld throat thickness. Doing so, the estimation of the fatigue strength of a welded joint can be reduced to the estimation of the fatigue strength of the equivalent V-notch subjected to a remote stress equal to the structural stress (that can be regarded as a ‘hot spot’ stress). Finally a simple fatigue strength diagram, in the form recently proposed by Atzori and Lazzarin and calibrated on experimental fatigue test results, is proposed, so that one can estimate the fatigue strength of a welded joint, in terms of structural stress at a given number of cycles, as a function of the equivalent V-notch depth. By considering this diagram, the scale effect and the effectiveness of the methods to improve the fatigue strength by smoothing the weld toe radius are also taken into account.

Crack propagation analysis in composite bonded joints under mixed-mode (I+II) static and fatigue loading: a damage-based model

In this paper, a first step towards the definition of a unifying model based on the process zone concept has been attempted. The aim of the proposed model is to summarize the fatigue behaviour of composite bonded joints under mixed-mode (I + II) fatigue loading conditions focusing as close as possible on the actual damage mechanisms observed during the fatigue tests, which were found strongly dependent on the mode mixity. The proposed model has been verified on the experimental results obtained from double cantilever beam (DCB), end notch flexure (ENF) and mixed-mode bending (MMB) tests as well as from single-lap bonded joints. Finally, the model is successfully applied to the description of joint behaviour under static mixed-mode loading.

Crack propagation analysis in composite bonded joints under mixed-mode (I+II) static and fatigue loading: experimental investigation and phenomenological modelling

This paper illustrates the results of an extensive experimental investigation on composite bonded joints under mixed-mode (I+II) static and cyclic loading conditions oriented to understand the influence of the mode mixity condition on the crack propagation resistance at the bondline. The double cantilever beam (DCB), end notch flexure (ENF) and mixed-mode bending (MMB) tests were conducted on pre-cracked samples and both fracture toughness and crack propagation resistance were seen to increase, both for static and fatigue loading, respectively, as the mode II contribution increases. The crack propagation and damage evolution were carefully investigated and documented, and a strong dependence of the propagation mode on the mode mixity was found. Fatigue data under the different loading conditions are then described by a phenomenological model based on the strain energy release rate contributions, which represents a useful engineering tool for preliminary design. After that a damage-based model, developed on the basis of the actual damage mechanisms, is presented in a companion paper.

Experimental evaluation of fatigue damage in two-stage loading tests based on the energy dissipation

Heat energy dissipation is a manifestation of damage accumulation in fatigue-loaded components. Once recognized that some mechanical energy has to be expended to fatigue a material, energy partition into heat and stored energy is thought of as a material property in the present testing conditions. However, most of the mechanical input energy is dissipated as heat; therefore, the stored energy is difficult to estimate as difference between the expended and the dissipated energy. In this article heat energy is assumed as an index of fatigue damage. Since it reflects the material response to external loading, heat energy was seen to reduce the scatter of constant amplitude fatigue test results with respect to the use of the stress amplitude. Moreover, two-level fatigue test results could be interpreted with a higher level of accuracy when Miner’s rule was applied in terms of energy rather than stress amplitude.

Practical Application of the N-SIF Approach in Fatigue Strength Assessment of Welded Joints

In the notch stress intensity approach the weld toe is modelled as a sharp V-notch, ρ = 0, and local stress distributions in plane configurations are given on the basis of the relevant Mode I and Mode II Notch Stress Intensity Factors (NSIFs). Then, the local strain energy density over a circular sector surrounding the point of stress singularity can be easily calculated as soon as the NSIFs are known. These parameters include not only the influence of the main geometrical ratios, but also the size effect. Welded joints simply scaled in geometrical proportion have different NSIFs. Whilst the local stress-based evaluation of the NSIFs needs very fine meshes in the vicinity of the points of singularity, which is a drawback of the approach in the presence of complex geometries, the mean value of the elastic strain energy density (SED) on the control volume can be accurately determined by using relatively coarse meshes. Thanks to the SED use, the degree of refinement of FE models is not so different from that usually used to determine the ‘hot-spot stress’ according to the structural stress approach. The present paper deals with this topic, considering both bi-dimensional and three-dimensional welded details and discussing the degree of accuracy and the limits of applicability of the method. An alternative, simplified application of the NSIF approach valid for fillet-welded joints with fatigue failures from the weld toe (the so-called Peak Stress Method -PSM) is also presented.