In the Strain Energy Density (SED) approach for fatigue strength assessments of welded joints a well-defined control volume is considered. This volume surrounds the weld root or weld toe, both modelled like sharp (zero radius) V-notches with different opening angles. The volume becomes a circular sector under plane strain conditions, with the radius being about 0.3 mm for welded joints made of structural steel. The mean value of the SED mainly depends on the singular stress fields when the main plate thickness is large enough, whereas the influence of the T-stress component cannot be neglected in the case of thin-walled welded joints. Both contributions are directly accounted for by using finite element models, also when the relevant meshes are quite coarse. This fact makes the application of the SED approach easier than any stress-based approach in the case of complex structures. Due to three-dimensional effects, a non conventional out-of-plane singular mode can be present, in addition with respect to modes I and II of the Williams’ solution. This out-of-plane mode, analogous to the Mode III, is discussed here with reference to welded (seam) lap joints under tensile-shear loads

The paper deals with the application of an efficient finite element (FE) model for the failure analysis of bonded structures. Aim of the work is to assess the accuracy and applicability of the computational model in the prediction of the post-elastic response of large and complex bonded structures. In order to overcome the limitations encountered in the technical literature, such as the use of special elements, the present work assesses the applicability of a reduced computational method, previously presented by the authors. The method is based on standard modeling tools, which are available in most of commercial FE packages. The method describes the adherends by semi-structural elements (plates or shells), and the adhesive by means of a single layer of cohesive elements. This work applies the proposed reduced method to a complex, industrial-like, structure. A square thin-walled beam is considered, made of two different portions joined head to head by overlapping thin plates on each side. The beam is loaded by a three point bending fixture up to failure which originates an indirect, complex stress field on the bonded region. The benchmark for the computational analyses are the force-displacement curves obtained by experimental tests on two different geometries. The comparison with the experimental data shows a good accuracy of the proposed method in terms of structure stiffness, maximum load and post-elastic behaviour up to the collapse of the structure. The numerical precision and the computational speed make the proposed method very useful for the efficient analysis of complex bonded structure, both for research and industrial purposes.

The choice of the proper weld size for cruciform joints can be a critical topic especially in case of different thickness of the welded plates. According to technological recommendations the size of the weld bead should not exceed the thickness of the smallest plate. On the other hand, design standards do not suppose the joint static and fatigue resistance to be dependent on the weld size provided that it is thick enough to avoid the failure of the weld itself instead of the failure of the welded plate. The aim of this work is to study, both from a theoretical and an experimental point of view, the effect of different weld sizes on the fatigue resistance of cruciform joints.

Welded joints are frequently locations for cracks initiation and propagation that may cause fatigue failure of engineering structures. Biaxial or triaxial stress-strain states are present in the vicinity of welded joints, due to local geometrical constraints, welding processes and/or multiaxial external loadings. Fatigue life evaluation of welded joints under multiaxial proportional (in-phase) cyclic loading can be performed by using conventional hypotheses (e.g. see the von Mises criterion or the Tresca criterion) on the basis of local approaches. On the contrary, the fatigue life predictions of welded joints under non-proportional (out-of-phase) cyclic loading are generally unsafe if these conventional hypotheses are used. A criterion initially proposed by the authors for smooth and notched structural components has been extended to the fatigue assessment of welded joints. In more detail, fatigue life of welded joints under multiaxial stress states can be evaluated by considering a nonlinear combination of the shear stress amplitude (acting on the critical plane) and the amplitude and the mean value of the normal stress (acting on the critical plane). In the present paper, fatigue lifetimes predicted through the proposed criterion are compared with experimental fatigue life data available in the literature, related to fatigue biaxial tests.

Fatigue design of welded structures is primarily based on a nominal stress; hot spot stress methods or local approaches each having several limitations when coupled with finite element modeling. An alternative recent structural stress definition is discussed and implemented in a post-processor. It provides an effective means for the direct coupling of finite element results to the fatigue assessment of welded joints in complex structures. The applications presented in this work confirm the main features of the method: mesh-insensitivity, accurate crack location and life to failure predictions.

Following their research, the authors remarked on the importance of the knowledge of the temperature for stressed material (mechanical component). They show as a temperature (third parameter) is an important parameter to characterise dynamically the material. Being that the thermal release is a function of the applied energy that leads to the failure, the knowledge of parameters linked to energy leads to new hypothesis and definitions of fatigue limit and lifespan. A thermal analysis permits one to evaluate parameters related to the amount of energy to failure of tested material. In [4] A. Risitano and Others, had remarked that during a static traction test, the beginning of plastic behaviour (linked to applied stress) was definable by analysing of the correspondent temperature curve. In the same work, they remarked about the low influence of the test velocity. Still working with high level temperature sensors, the authors observed that during static tests, the temperature variation of the surface specimen, permits to associated the lower dynamic fatigue limit s0 to a “temperature limit” T0 coincident to the end of thermo-elastic phase. In this case a qualitative physical model, able to give and justify the possibility to evaluate the classic fatigue limit by experimental knowledge of thermo-elastic behaviour is discussed. As an example, the results of traction tests performed on two rectangular section specimens notched with one hole each, where the change of linearity was evident, are reported. The corresponding value of stress was coincident with the fatigue limit s0 for R= - 1, found by traditional method.

This paper summarises an attempt of devising a new engineering approach, based on the so-called Modified Wöhler Curve Method, suitable for estimating fatigue lifetime of both steel and aluminium welded connections subjected to variable amplitude multiaxial fatigue loading. The Maximum Variance Method is used to determine the orientation of the critical plane and the cycle counting is directly performed in terms of shear stress resolved along the maximum variance direction. The accuracy of the proposed methodology was checked by using two different datasets taken from the literature and generated by testing both steel and aluminium tube-to-plate welded connections under in-phase and 90° out-of-phase variable amplitude bending and torsion. The proposed fatigue life assessment technique was seen to be highly accurate, resulting in estimates falling within the scatter bands of the curves used to calibrate the Modified Wöhler Curve Method itself. This seems to strongly support the idea that the proposed approach can be considered as an effective engineering tool, capable of performing multiaxial fatigue assessment under variable amplitude loading by fully complying with recommendations of the available standard codes.

The paper presents the so-called Peak Stress Method applied to the analysis of the fatigue strength of fillet-welded joints failing from the weld toe. Such method is an engineering application of the local approach based on the mode I Notch Stress Intensity Factor (N-SIF), which assumes the weld toe profile as a sharp V-shaped notch having a toe radius equal to zero. According to the Peak Stress Method, the design stress is the elastic peak stress calculated at the point of singularity by means of a finite element analysis, where the element size has a fixed value, for example equal to 1 mm in the present work. Due to its simplicity, the Peak Stress Method proved to be convenient in practical applications.

Fatigue resistance of welded joints represents an interesting and challenging studying field from a scientific point of view, with valuable and important practical consequences. In the specific, the main target is to establish in general the proper quantitative verification and design procedures, based on theoretical achievements. The actual procedures and standard codes are extremely useful for most engineering applications, in which elevated safety factors have to be used. On the other hand the mechanical behavior of welded material is different from the base metal; the weld bead material has different mechanical characteristics because of microstructural changes and other local effects, due to extremely variable weld toe geometry, resulting in notch effects and coupled with relevant residual stresses not to be ignored. In this work, a brief resume of the principal design methods is primarily reported for the estimation of residual life expectancies of welded joints, leading to different engineering approaches; among them, the method based on local deformations measurements is presented as reliable verification tool, already analyzed and modified by several authors in the past. The local deformation approach has been deeply applied and verified by the authors and the main advantage and characteristics are explained, as well as the present limitations to be observed in the frame of many experimental fatigue testing activities.

In this paper, a non-local equivalent stress is calculated by solving a second order differential equation of implicit type. The solution is obtained by assuming a linear elastic constitutive behaviour and the maximum principal stress as equivalent stress. Fatigue behaviour of steel welded joints is taken into account and a general fatigue scatter band is proposed. The non local stress is computed, namely the effective stress, by means of a completely numerical solution of local elastic stress field. In complex 3D welded details the critical point turns out from the analysis and it is not assumed a priori.

Results of fatigue tests on adhesive lap joints of thick (10 mm) composite laminates are presented and discussed. Specimens of different overlap length (from 25 to 110 mm), different shape (with and without taper) and different materials (composite on composite, composite on steel) were fatigue tested. In order to investigate on the relationship between peak elastic stresses in the adhesive layer and fatigue life, a 2D structural analysis of the joints by the finite element method was performed. This analysis suggested that peak elastic stresses in the adhesive layer could be adopted as a design criterion, at least as an engineering tool for industrial applications. The role of crack propagation is also discussed, on the basis of some observations during fatigue tests.

The employment of standard codes in fatigue design of aluminium alloy structures has been the subject of many recent papers where both the comparison among different design standards and the lack of application, even in the latest European codes, of reliable theoretical results were discussed. The aim of this work is to deepen the comparison between the Italian code UNI 8634, recently withdrawn, and the European standard Eurocode 9, published in 2007, through the comparison of experimental data taken from literature with the corresponding design curves proposed by both of codes. In this way, the differences between the fatigue resistance data will be pointed out together with the effects of neglecting well known and reliable theoretical results.

The structural durability design of complex welded structures should not rely only on one single design method but should apply different methods for assuring the reliability of the assessment. In this context the application of the structural stress concept, notch stress concept and crack propagation concept are discussed through the example of K-nodes used in energetic offshore constructions like oil platforms or wind power plants, presenting the state of the art.

This work deals with the prediction of the fatigue crack growth of bonded joints. Traditionally cohesive zone model is used for the simulation of failure of bonded joint under quasi-static and impact conditions. On the other hand fracture mechanics concepts are used for the prediction of fatigue crack growth in bonded joint. In this work a modified cohesive damage model, accounting for the fatigue crack growth, was implemented in the commercial software ABAQUS. The fatigue damage evolution in the cohesive element is related to the number of cycles through the experimental fatigue crack growth behavior. An automated procedure, valid for any from the geometry and load distribution, was also defined to compute the Strain Energy Release Rate, the necessary input to define the instantaneous fatigue crack growth rate. The model is validated by comparing the simulation to fatigue crack growth tests.

Orthotropic decks were applied to the long span bridges after World War II due to several advantages, such as light weight, high strength, few deck joints, durability, rapid construction, life-cycle economy. The fatigue problem of orthotropic decks was realized twenty years ago since fatigue failure was found. In the past two decades large amount of studies and investigations were carried out and fruitful achievements were obtained. It was found that most of the fatigue cracks were occurred at the welded connection details, such as rib-to-deck plate, rib-to-diaphragm, and rib-to-diaphragm-to-deck plate (RDDP). These connections are sensitive to fatigue cracking due to high concentrated stress and residual stress at welded connections. In this paper practical fatigue failure cases at the welded connections, ease to occur fatigue cracking, are presented, and analyzed through a numerical modeling of orthotropic deck via FE (finite element) software. Furthermore, the improvement technologies of fatigue are also discussed. The results of the analysis can be contributed to the evaluation of the fatigue design for the orthotropic deck.