Proceedings of the Institution of Mechanical Engineers,Part C: Journal of Mechanical Engineering Science

The science of biomimetics seeks to gain inspiration from nature for the development of new engineering solutions. This article reviews some of the work done to understand fatigue-resistant structures in nature. Evolution has created materials and components which are highly resistant to cyclic loading, using a number of interesting strategies: (i) careful control of geometry to minimise stress concentration; (ii) sophisticated use of fibre composites taking advantage of anisotropy, fibre insertions at joints and functional grading, to create structures with no weak interfaces; (iii) hollow, tubular components with optimised fatigue strength taking account of all possible failure modes, and; (iv) continuous monitoring and repair of fatigue cracks in service. In each case an attempt has been made to quantify the potential improvement and to outline the possibilities for future transport applications.

The relationships between the stress concentration factors K_ts to the depths h and diameters Φ of blind holes were investigated based on the finite element method. The E-Mode, built-in the lock-in thermographic technique, was used to determine the stress distribution around blind holes and to predict the variation of K_ts. Good predictions were achieved between the thermography and finite element method. The Altair Li software and Altair software were, separately, used to study the variations of the intrinsic dissipation and temperature increment during fatigue process for the purpose of damage status analysis. These two signals were considered as fatigue damage markers to rapidly predict the fatigue limits of components with different blind holes and then to determine the fatigue notch coefficient K_f. Theoretical results validate the capability of the lock-in thermography for fatigue evaluation.

A series of fatigue tests is carried out to investigate the behavior of microcrack initiation and propagation in Al 7075-T6. Plate specimens with semi-circular single edge notch are tested under uniaxial loading at different load amplitudes. Optical microscopy is used to detect the initiation and growth of short cracks. Continuum damage mechanics is used to model the crack initiation. After the initiation, crack growth is studied using fracture mechanics models. For both regions of crack initiation and growth, Bayesian estimation method is used to account for uncertainties in the parameters of the model. Results of the entire fatigue life, including initiation and growth, are compared with experiments. Good agreement is observed.

In this work, the mechanical strength of automotive exhaust flexible couplings subjected to thermo-mechanical fatigue and corrosion has been investigated. Five different types of flexible coupling have been considered, which is realized by four different metallic materials: three stainless steels (AISI 309, AISI 321, and AISI 321 Ti) and a nickel alloy (Incoloy 825). These components have been tested by a specific procedure developed to reproduce the real working conditions of the flexible joints (mechanical stresses, thermal stresses, and chemical attacks). The aging procedure consisting of different cycles of fatigue, heating and corrosion was performed. The performance of components was compared in terms of variation of both mechanical strength and the effect of corrosion, by means of the critical to quality approaches. The fatigue aging process presented in this work reproduced properly the real damage conditions on automotive flexible coupling, and the analysis of the critical to quality results shows that the best components for this application are those made up of Incoloy 825.

Fatigue is an important design criterion for welded structures subjected to cyclic loading. Several approaches for fatigue strength assessment have been developed which are either based on Woehler S–N curves and damage accumulation rule or on crack propagation law. The paper briefly reviews the different approaches, highlighting their advantages and limitations. In this connection, the problematic distinction between crack initiation and propagation phases is discussed, followed by considerations about some parameters which have large influence on the fatigue behaviour of welded joints but are considered differently in the approaches, such as plate thickness and stress gradient effects, multiaxial stress states, welding-induced distortions and residual stresses. Finally, ways of improving the fatigue behaviour of welded structures, either during design by reducing the stress concentration or during fabrication by improved quality or post-weld treatment or else by special material characteristics, are addressed. Emphasis will be placed on recent developments and challenges for the future from a personal perspective of the author.

Many mechanisms and processes can cause deterioration and ultimately failure of water distribution pipes during in-service operation, amongst these is damage caused by metal fatigue. This paper summarises an attempt at formalising a novel methodology suitable for estimating the number of years taken for a through thickness fatigue crack to form in this complex scenario. The devised method is based on the so-called modified Wöhler curve method and can be applied to estimate fatigue damage of water pipes independently from the degree of multiaxiality and non-proportionality of the load history. The computational approach of the proposed fatigue life estimation technique makes full use of an incremental procedure: fatigue damage is evaluated year by year by assuming that all variable involved in the process can change over time. The detrimental effect of corrosion pits is directly accounted for by treating them as conventional notches whose size increases with time. Finally, by taking as reference information the number of years for a blowout hole to form, the proposed approach is used to show how the lifetime of grey cast iron pipes can be remarkably shortened by fatigue.

With the increasing progress of the technological development in the transport industry, the required fatigue life has increased, so it is very important to determine a safe fatigue strength for 10⁹ cycles. Nowadays, the very high cycle fatigue constitutes one of the main fatigue design criteria for applications in transport industry. In this paper, the infrared thermography and an energetic approach were applied to investigate a tool steel in very high cycle fatigue regime. The traditional energetic approach was developed in order to extend it in very high cycle fatigue regime and to predict the S-N curves. The failure mechanism of the investigated steel was evaluated by means of scanning electron and optical microscopies in order to assess if the nature of microstructure and the metallurgical defects, in terms of inclusions and pores, can influence the crack initiation.

This paper presents an experimental study of the mechanical and thermal behavior of titanium samples (Grade 2 and Grade 4) with different grain sizes under cyclic loading. The self-heating test demonstrates that the structure of the material has a strong effect on the dissipation ability of titanium. The threshold of energy dissipation corresponding to the transition through the fatigue limit is shown for coarse-grained titanium. On contrary, submicrocrystalline samples exhibit the dependence of continuous energy dissipation on the applied stress amplitude. Analysis of the fatigue properties of titanium in a gigacyclic regime provides evidence that grain grinding improves substantially the fatigue properties of the material.

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.

The joining techniques of lightweight and strong materials in the transport industry (e.g. automotive, aerospace, shipbuilding industries) are very important for the safety of the entire structure. In these industries, when compared with other joining methods, the use of adhesively bonded joints presents unique properties such as greater strength, design flexibility, and reduction in fuel consumption, all thanks to low weight. The aim of this study was the analysis of the tensile fatigue behavior of adhesively bonded glass fiber/epoxy laminated composite single-lap joints with three different specimen types including 30, 40 and 50 mm overlap lengths. In this study, composite adherents were manufactured via vacuum-assisted resin transfer molding and were bonded using Loctite 9461 A&B toughened epoxy adhesive. The effect of a surface treatment method on the bonding strength was considered and it led to an increment of about 40%. A numerical analysis based on a finite element model was performed to predict fatigue life curve, and the predicted results showed good agreement with the experimental investigation.

One of the most important issues associated with liquefied natural gas (LNG) storage tanks, such as LNG carrier cargo tanks and land LNG tanks, is their structural integrity. In order to ensure the operating life and safety of LNG storage tanks used under operating conditions such as thermal and cyclic loadings, the securing of safety evaluations for fatigue performance is considered to be of particular importance. There have been various efforts to reduce the production costs of LNG storage tanks, such as the optimum selection of materials and the development of new low temperature materials. This, the motivation of this study is to evaluate new material candidates for LNG storage tanks. This study begins with a comprehensive review of the characteristics of low temperature alloys such as SUS 304L, Invar, A5083 and 9% Ni steel that are widely used for LNG storage tanks. Then, the fatigue characteristics of a newly developed low temperature material, 7% nickel steel are investigated. Finally, the fatigue performance of 7% nickel steel is compared with that of 9% nickel steel.

A historical review was carried out, in order to point out the phases leading to investigate the fatigue resistance of materials and mechanical components, using quick and direct methodologies. Starting from researches performed in the early 1980s, the progresses in the research conducted by the Catania group and other investigators, significantly contributing to the increasing interest in the thermographic analysis of the fatigue phenomenon and, more specifically, to the experimental evaluation of the energy release under fatigue loading, were described.

The welded structures used in the naval field are generally subjected to fluctuating stress over time. In some structural welded details, due to changing loading conditions, significant elastic-plastic deformation can arise, which may lead to the failure of the structure after a relatively low number of cycles. The aim of this scientific work was to investigate the behavior of welded T-joints under low-cycle fatigue using full-field techniques: digital image correlation and infrared thermography. Low-cycle fatigue tests were carried out on welded “small-scale” specimens with the aim of obtaining loading and boundary conditions similar to those that occur in “large-scale” components in their real operating conditions. A nonlinear finite element analysis was also performed. The material curves, relative to different zones (base material, heat-affected zone, weld), were obtained by hardness measurements, which were done by means of a fully automated hardness scanner with high resolution. This innovative technique, based on the ultrasonic contact impedance method, allowed to identify the different zones (base material, heat-affected zone, weld metal) and to assess their cyclic curves, which were considered in the finite element model. Finally, the finite element model was validated experimentally comparing the results with the measurements obtained using the digital image correlation technique. The applied procedure allows providing useful information to the development of models for the prediction of fracture and fatigue behavior of the welded joints under the low-cycle fatigue loading.

Luca Susmel

Nominal stresses and Modified Wöhler Curve Method to perform the fatigue assessment of uniaxially loaded inclined welds. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 2014, Vol 228 No 16, pp 2871–2880.

This article was inadvertently published early. It was intended for publication in the Special Issue ‘Fatigue Design and Analysis in Transportation Engineering’ edited by V Crupi, W Fricke and E Guglielmino, Vol 229 No 7, published May 2015, where it is reprinted for the convenience of print readers only. Online readers please access this article as above at DOI 10.1177/0954406214522991.