Fatigue life assessment of thin walled welded joints

Lightweight design is a crucial aspect to consider in order to achieve the reduction of green house gasses emission of vehicles, which will be enforced in the next years. For this reason the use of high strength steel is rapidly increasing for automotive applications. In this prospect fatigue life assessment of thin walled welded structures is one of the field where research is needed. Since fatigue is one of the main causes of failure, and a welded joint is generally a critical spot for fatigue crack nucleation. In the present work the effective notch stress theory will be applied to a database of experimental tests of thin walled specimens. It will be shown that, by means of a properly defined effective stress, results from very different test series can be brought together in an S-N curve with a tight scatter band. Both the stress averaging approach according to Neuber and the critical distance approach according to Taylor will be applied in order to obtain the desired effective stress. Their application will be optimized for thin walled structures using the scatter of the S-N curve as main decisional parameter. A reference S-N curve with a survival probability of 97% for both methods will be proposed, which could be utilized in design procedures. The possibility of new more complex kernels for the definition of the effective stresses will be discussed

Fatigue endurance of welded joints subjected to different blocks of bending and torsion loading

The fatigue strength of pipe-to-plate welded joints under bending, torsion and combined (in-phase and out-of-phase) bending and torsion has been already investigated in previous works by the authors. The specimen consisted of a pipe joined by seam welding to a plate. Both the pipe and the plate were made of S355JR steel. The test apparatus allows to apply any combination of proportional and non-proportional bending and torsion loads to the specimen. For the analysed specimens failure originated mainly from the weld root, where a severe notch is present, even if some failure from the weld toe was observed in case of bending loading. However, the crack propagation and fracture surface under bending and under torsion were significantly different. For this reason, in order to investigate any possible influence of the loading order on the fatigue endurance, the effect of different loading blocks was analysed in this work. This subject has not been widely investigated in the technical literature about welded joints. In a first series of tests, specimens were loaded in bending for a given fraction of the estimated endurance and then were loaded in torsion till failure. A similar series of tests was then conducted by varying the loading order: specimens were loaded in torsion for a given fraction of the estimated endurance, followed by a block of bending loading till failure. The whole test campaign was repeated for two different fractions of the estimated life, i.e. 0.3 and 0.45, respectively. The failure was intended as the presence of a through the thickness crack, whose presence was monitored by a drop in the internal pressure of the pipe. Results are discussed in terms of the Miner’s rule based on nominal stresses and to the cumulative damage suggested by Eurocode and IIW

Effects of plate stiffness on the fatigue resistance and failure location of pipe-to-plate welded joints under bending

A series of tests have been carried out using specimens made of a tube, having a thickness of t=10 mm, joined to a plate by fillet welding. Two different kinds of specimen were employed, differing in the plate geometry (stiffness). Both kinds of specimen were tested under bending (prevalent load) and shear loading in as welded conditions. Different initiation regions for the fatigue cracks were found and significantly different fatigue resistances were obtained for the two geometries in terms of the nominal stress approach (or in terms of applied load vs cycles to failure). Two local methods for the fatigue life assessment were then applied to independently analyse the experimental results: the fictitious notch rounding approach proposed by Radaj, which is also recommended by some international standards and the more recently proposed peak stress method, which is based on the NSIF concept. It is shown that the nominal stress method, which is by far the simplest method among those recommended in standards for analysing the joint under study, fails to explain the observed different endurances. On the other side, the methods based on local stresses account for the different joint stiffness and provide a reduced scatter in the results. However, even if local approaches, accounts for differences in the structural behaviour of the joint, the knowledge of the actual geometry of the weld need to be accounted for, in order to be able to identify the fatigue crack initiation region. For a design purpose, a safe prediction of the fatigue endurance of the joint can be obtained by all the analysed methods, if the corresponding recommended design curve is used

On the application of a critical plane approach to the life assessment of welded joints

Abstract In the present work, the Fatemi-Socie approach is adopted in order to analyze the fatigue endurance of welded joints under multiaxial loads. This critical plane criterion has already been successfully applied to plain or notched components, however, it is not spread in the assessment of welded joints, yet. This work is focused on the practical implementation issues related to this particular application, which has not been discussed in the literature. The described procedure is adopted for the assessment of one hundred experimental tests and some preliminary results are shown. The specimen under investigation is a pipe-to-plate fillet joint made out of structural steel (S355JR). The tests were performed under both uniaxial and multiaxial, i.e. combined in-phase and out-of-phase bending and torsion, load conditions with a constant amplitude at the laboratories of the University of Pisa, Italy

Parachute emergency landing simulation and enhanced composite material characterization for General Aviation aircraft

General Aviation (GA) aircraft crashworthiness of the vehicle when it hits the ground after the parachute deployment is an important issue. The current dynamic emergency landing regulation (CS 23.562) defines the maximum human tolerant accelerations under both vertical and horizontal directions. This article aims to compare two different aircraft configurations: metal low-wing and composite high-wing ones. Both are two-seats and single-engine GA aircraft. The purpose of the analysis is to check whether the seats and restraint systems met human injury tolerance standards and to determine the possible impact on passengers in the cabin space due to shock loads. Finite element analysis of the fuselage sections for both configurations is performed using the commercial LS-Dyna solver. An extensive campaign of experimental tests has been performed on the composite samples for tuning and validating the model and to find the transition from an undamaged up to totally collapsed sample. The material of the composite fuselage has been characterized through experimental tests. The adopted material model has been refined to match with the performed experimental analysis, allowing high-fidelity modeling. A parametric analysis has been performed to determine the optimal impact angle in terms of lumbar injuries and loads transmitted by the seat belt due to aircraft contact with the ground, thereby increasing the level of safety. The investigations carried out may be an important indicator of the design of the parachute system

Fatigue assessment of a FSAE car rear upright by a closed form solution of the critical plane method

Material fatigue is extensively discussed and researched within scientific and industrial communities. Fatigue damage poses a significant challenge for both metallic and non-metallic components, often resulting in unexpected failures of in-service parts. Within multiaxial fatigue assessment, critical plane methods have gained importance due to their ability to characterize a component's critical location and detect early crack propagation. However, the conventional approach to calculate critical plane factors is time-consuming, making it primarily suitable for research purposes or when critical regions are already known. In many real-world scenarios, identifying the critical area of a component is difficult due to complex geometries, varying loads, or time limitations. This challenge becomes particularly crucial after topological optimization of components and in the context of lightweight design. Recently, the authors proposed an efficient method for evaluating critical plane factors in closed form, applicable to all cases that necessitate the maximization of specific parameters based on stress and strain components or their combination. This paper presents and validates the proposed methodology, with reference to a rear upright of a FSAE car, which is characterized by a complex geometry and is subjected to non-proportional loading conditions. The efficient algorithm demonstrated a substantial reduction in computation time compared to the standard plane scanning method, while maintaining solution accuracy

Light Electrospun Polyvinylpyrrolidone Blanket for Low Frequencies Sound Absorption

Light polymeric soundproofing materials (density = 63 kg/m3) of interest for the transportation industry were fabricated through electrospinning. Blankets of electrospun polyvinylpyrrolidone (average fiber diameter = (1.6 ± 0.5) or (2.8 ± 0.5) μm) were obtained by stacking disks of electrospun mats. The sound absorption coefficients were measured using the impedance tube instrument based on ASTM E1050 and ISO 10534–2. For a given set of disks (from a minimum of 6) the sound absorption coefficient changed with the frequency (in the range 200–1600 Hz) following a bell shape curve with a maximum (where the coefficient is greater than 0.9) that shifts to lower frequencies at higher piled disks number and greater fiber diameter. This work showed that electrospinning produced sound absorbers with reduced thickness (2–3 cm) and excellent sound-absorption properties in the low and medium frequency range

Non Monotonous Effects of Noncovalently Functionalized Graphene Addition on the Structure and Sound Absorption Properties of Polyvinylpyrrolidone (1300 kDa) Electrospun Mats

Graphene is an attractive component for high-performance stimuli-responsive or 'smart' materials, shape memory materials, photomechanical actuators, piezoelectric materials and flexible strain sensors. Nanocomposite fibres were produced by electrospinning high molecular weight Polyvinylpyrrolidone (PVP-1300 kDa) in the presence of noncovalently functionalised graphene obtained through tip sonication of graphite alcoholic suspensions in the presence of PVP (10 kDa). Bending instability of electrospun jet appears to progressively increase at low graphene concentrations with the result of greater fibre stretching that leads to lower fibre diameter and possibly conformational changes of PVP. Further increase of graphene content seams having the opposite effect leading to greater fibre diameter and Raman spectra similar to the pure PVP electrospun mats. All this has been interpreted on the basis of currently accepted model for bending instability of electrospun jets. The graphene addition does not lower the very high sound absorption coefficient, α, close to unity, of the electrospun PVP mats in the frequency range 200⁻800 Hz. The graphene addition affects, in a non-monotonous manner, the bell shaped curves of α versus frequency curves becoming sharper and moving to higher frequency at the lower graphene addition. The opposite is observed when the graphene content is further increased

Fatigue life assessment of welded joints

Fatigue is one of the most common failure mechanisms of structural components. Many fatigue failures have become popular throughout engineering history since the tragic railroad accidents that motivated Wöhler to begin the first investigations of the fatigue damage process. Since then a huge deal of work has been carried out ot the subject by several generations of engineers leading to a deeper comprehension of the complex phenomena related to the damages occurring in structural components due to time varying loads. However, many aspects of the subject remains unclear and several questions are still open for discussion. The present work tries to give an answer to some of those questions. In particular, it is focused on the fatigue life assessment under multiaxial and variable amplitude loads with regard to the application to welded joint connections. Fatigue is a classical topic in engineering design and a subject of wide practical interest since it is one of the most common causes of failure for structural components

Fatigue life assessment of welded joints by two local stress approaches: The notch stress approach and the peak stress method

This paper presents a comparison between two local methods that can be used for the fatigue life assessment of welded joints: the notch stress approach and the peak stress method. Even if they have been derived starting from entirely different theoretical backgrounds, both methods are based on the evaluation of a local stress acting at the notch tip of the seam weld, which is usually obtained through a finite element simulation. Said methods are applied to a collection of experimental data from three kinds of welded joints, which are characterized by different geometric parameters and, in particular, by a wide range of the element thickness. A total of 177 experimental tests are evaluated in this investigation, 31 of which are taken from literature while the other 152 are published here for the first time. The performance of the two methods is discussed in terms of the statistical dispersion in the analysed data. This is accomplished comparing the scatter bands obtained from a regression procedure in the S-N plot. In addition, some practical aspects related to the implementation of both methods are discussed, with particular attention to the ease of use offered, considering that this may represent a relevant aspect for the method's diffusion in practical applications