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The 21st century brought new and complex technological challenges, which need to be solved. Of primary importance is the global energy transition that pushes forward the research and innovation in order to achieve the goal of replacing the existing non-renewable energy sources with new renewable and efficient ones, with positive effects on the world climate. In many countries worldwide mid- and long-term goals have been set out in order to reduce the greenhouse gas emissions. Germany, among others, intends to reduce the emissions by 80 to 95% within 2050, compared with 1990 levels. The achievement of this goal is aimed to be realized by the development of new and more efficient energy sources, but also by substantial investments in electromobility (the goal is to bring one million electrically driven vehicles onto German streets by 2020).
Another important challenge came into play following the global financial crisis, which pushed many industries to reduce their operational and maintenance costs. In particular, the life-cycle management of a component has become of primary importance. In some cases it has been shown that the underestimation or the lack of awareness in ageing of plants led to incidents due to the loss of technical integrity. Other studies demonstrated that the component life of many of the long-life components could be extended up to 50% without compromising safety.
The third, not less important, challenge is driven by the development of new technologies and materials. The trends show a large investment in additive manufactured metal components and new metallic materials which have to assure longer in-service life, lightweighting and efficient recycling.
This works aims at addressing part of these problems by proposing a fracture mechanics-based analytical procedure for the determination of the fatigue lives of engineering components and particularly of welded joints. In fact, an analytical tool, which is able to simulate the main failure mechanisms of weldments under fatigue loading, brings many advantages: i) drastic reduction of long and expensive experimental tests; ii) optimization of the geometry and production processes (possible weight reduction); iii) better estimate of the safety margins used in the design of the components (life extension without safety issues).
The results show that the model can fairly well predict the lives of the welded joints investigated experimentally, irrespective of the geometry and loading conditions.
Approximation of the crack driving force for cracks at notches under static and cyclic loading
(2017)
The work deals with the efficient calculation of the elastic-plastic crack driving force (J-integral for monotonic loading andΔJ-integral under cyclic loading) for short cracks at notches as essential parameter for the reliable static and fatigue assessment of notched structures. The J- or ΔJ-integral is calculated based on analytical solutions for stress intensity factors, estimated by means of well-known weight function solutions in the case of cracks under power-law stress distributions. A plasticity-correction function is applied to the stress intensity factors to obtain the final expression of the crack driving force. The comparison between analytical solutions and finite element calculations in case of cracks at the weld toe in welded joints shows good agreement.
Approximation of the crack driving force for cracks at notches under static and cyclic loading
(2017)
The work deals with the efficient calculation of the elastic-plastic crack driving force (J-integral for monotonic loading andΔJ-integral under cyclic loading) for short cracks at notches as essential parameter for the reliable static and fatigue assessment of notched structures. The J- or ΔJ-integral is calculated based on analytical solutions for stress intensity factors, estimated by means of well-known weight function solutions in the case of cracks under power-law stress distributions. A plasticity-correction function is applied to the stress intensity factors to obtain the final expression of the crack driving force. The comparison between analytical solutions and finite element calculations in case of cracks at the weld toe in welded joints shows good agreement.
Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The actual part qualification and certification procedures make use of spin-tests conducted on production-similar disks. The aim of this work is to present part of a fracture mechanics-based procedure under development which aims at replacing the tests on production-similar disks with lab tests on fracture mechanics specimens. The finite element simulation of the cracked disk considers the real thermal and mechanical loading conditions. In order to design a lab representative specimen, beside the crack driving force, expressed in terms of 𝐽-integral, also the constraint to plastic deformation e.g., stress triaxiality, at the crack-tip must be similar for the same crack in the specimen and in the disk. This has been achieved and as expected, both the highest 𝐽 -integral and constraint factor are calculated at the same location along the crack front for both disk and specimen. The results of the structural integrity assessment in the form of a Failure Assessment Diagram (FAD) show good agreement between designed specimen and disk both in terms of expected failure mode and value of the critical speed. In addition, probabilistic aspects are also considered in the calculations.