Giga-Cycle Fatigue Behaviour of Engineering Metallic Alloys 

Today, the fatigue behaviour of engineering metallic alloys for a number of loading cycles surpassing the conventional fatigue limit (10^7) has become a question of major interest. More and more components (ex: modern transportation systems, energy harvesting structures) are subjected to load scenarios of small stress amplitudes that must endure a number of loading cycles up to 10^8 or even beyond, in a range which is either called Giga-Cycle Fatigue (GCF), Ultra-High-Cycle Fatigue (UHCF) or even Very-High-Cycle Fatigue (VHCF). It has been demonstrated for that loading regimes, fatigue cracks may initiate from inclusions inside of the material, instead of the typical surface fatigue cracks. Therefore, the fatigue behaviour in the UHCF regime exhibits extra dependency on material microstructure and extrapolations from high-cycle fatigue to this fatigue regime are questionable. Fatigue limits are often conventionally defined due to limitations in fatigue testing for very long durations, which can lead to unsafe designs.  

Fatigue testing in the UHCF regime is then mandatory, but conventional testing equipment cannot provide the answers in an affordable cost and time. The use of high-speed ultrasonic (20kHz) fatigue testing machines has being proposed as a method of evaluating these fatigue characteristics. Nevertheless, this testing procedure also raises many challenges, namely: i) possible frequency effects on both fatigue crack initiation and crack growth; ii) thermal effects due to heating during cycling; iii) the difficulty of defining new testing specimen geometries to comply with constrained real component dimensions; iv) definition of testing procedures to generate multiaxial fatigue test procedures. 

The proposed research project aims at performing the UHCF characterization of three metallic alloys resulting from distinct manufacturing processes and, therefore with different typical microstructures, including internal defects. A hot-rolled structural steel (tentatively S355 or S690) used for large structural applications such as bridges and wind towers will be investigated. For this material, there is no limitations as regards the specimen’s geometries, allowing testing using recommended specimens. It is expected to test this material under both constant amplitude and block loading. Also, fatigue notch sensitivity will be investigated using notched specimens. Multiaxial fatigue tests will be also performed. Also, for this material, fatigue tests will performed in conventional machines (low frequency) till 10^6 cycles is order to perceive the equivalence between the two testing procedures. 

A light cast metallic alloy will be investigated (tentatively Al-Si cast alloy) which is used for energy efficient transports. In this case the possibly of extracting specimens from real components will be the focus of research. Cast components are typically designed to exhibit thin sections, which hampers the extraction of traditional round specimens. Therefore, for this material, alternative specimens design should be sought after, namely of planar geometry. A new testing procedure will be developed to allow proper ultrasonic fatigue testing using existing equipment with required adaptations. The cast alloy is generally characterized by higher internal defects than hot-rolled materials, which will allow to investigate the effect of different defects density UHCF damage. Finally, a third material (to be selected between Al or steel alloys according availability) produced by additive manufacturing (L-PBF) will be investigated under UHCF regime. It is expected that this material will be the one showing higher internal defects density. The possibility to compare the additive with the conventional counterpart will be investigated as well. For these latter two materials, only uniaxial and constant amplitude loading will be investigated.  

 Since the UHCF damage is dominated by internal defects, statistical distribution of the internal defects will be evaluated and their relation with fatigue strength will be established. Also, fracture mechanics will be applied to correlate the microstructural defects with the fatigue resistance of the materials attempting to propose models for the UHCF damage.

1. Laboratório Associado de Energia, Transportes e Aeronáutica (LAETA)

Associated Laboratory in the fields of Energy, Transportation and Aeronautics, which involves a total of 113 Integrated Post-Doc researchers, 2.8 ISI papers/Researcher, 5 patents awards, 41 FCT projects, and 27 concluded PhD thesis. In the last evaluation performed by the FCT, LAETA was graded as ‘Excellent’.

2. INEGI - Instituto de Ciência e Inovação em Engenharia Mecânica e Engenharia Industrial

The INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineering from the University of Porto is an industry-oriented RTO with research activity in the areas of new technologies for advanced production processes, experimental mechanics, applied mechanics, energy, new materials, biomechanics, and the development of new products and systems. INEGI has more than 100 associate members, and a workforce of about 250 people, including more than 120 PhD researchers, being the largest research group in the Mechanical Engineering in Portugal. INEGI’s research infrastructure includes a broad set of well-equipped laboratories and an extensive engineering tools base for supporting its R&D activity as well as the production of industrial or commercial prototypes. INEGI is actively participating in several national and international expert panels, consulting committees, groups and societies, sectorial industry clusters (automotive, aeronautics, tooling, additive manufacturing, sea economy, health and energy). The figures related to INEGI’s scientific production in 2019 resume the performance of the FCT R&D unit LAETA.

3. Instituto de Engenharia Mecânica (IDMEC)  

IDMEC (https://www.idmec.tecnico.ulisboa.pt/) is a private non-profit association of science, technology and training of Instituto Superior Técnico (IST), of the University of Lisbon. The activity of IDMEC is oriented towards five key strategic objectives; (i) to promote R&D actions, (ii) to promote technology transfer, (iii) to develop advanced training programmes, (iv) to evaluate and audit new entrepreneurial processes and (v) to develop integrated management programmes in energetic, logistic and technical areas. The activity of IDMEC is mainly focused on mechanical engineering and its structure is organized in five Centers: Mechanical Design; Intelligent Systems; Manufacturing and Industrial Management; Energy and Fluid Mechanics; Aerospace Science and Technology, supported by well-equipped laboratories that play a significant role in national and international R&D in different areas of mechanical engineering. IDMEC is one of the units of the research institution LAETA - Associated Laboratory for Energy, Transports and Aeronautics. 

Why test in ultrasonic frequencies? 

With the ultrasonic fatigue testing system,with the frequency 20 kHz, fatigue tests in the Very-High-Cycle-Fatigue (VHCF) range can be performed time- and cost-efficiently.

Results can be obtained after 10 minutes, for fatigue behaviour and the fatigue strength at 107 cycles and after 6 days will be reach at 1010 cycles, this number of cycles, in a conventional system, would take one year to complete.

Filipe G.A. Silva (Post-Doctoral Contract) 

Felipe Fiorentin (PhD Scholarship)

Orlando Oliveira (BII);

David Peliteiro (BII)

Tomás Brito (BII)

José Henrique Costa Lopes (BII)

Michael Ascenção Gouveia 

Master's thesis title: Giga-cycle Fatigue Behaviour of the Structural Steel S690     URL: https://hdl.handle.net/10216/143741

Ricardo Vieira Lobo

Master's thesis title:   Development of an Ultrasonic Frequency Equipment for Fatigue Testing      URL: https://hdl.handle.net/10216/144559 

Orlando Miguel Gonçalves de Oliveira

Master's thesis title: Development of human-machine interface for ultrasonic fatigue testing machine      URL: https://hdl.handle.net/10216/145566 

David Peliteiro

Master's thesis title: Giga-Cycle Fatigue Behavior of high pressure die-casting aluminum alloys        URL: https://hdl.handle.net/10216/153495

Tomás Moutinho de Azevedo Brito

Master's thesis title: Comportamento à fadiga de ligas de alumínio no regime supercíclico de extrema longa duração URL: 

José Henrique Costa Lopes 

Master's thesis title:

VESTAS Prize awarded to student Michael Ascenção Gouveia in the academic year 2021/22 for the dissertation presented in the Master in Mechanical Engineering, entitled “Giga-cycle Fatigue Behavior of the Structural Steel S690” 

Felipe Klein Fiorentin  

PhD's thesis title: High-Cycle and Ultra-High Cycle Fatigue Behaviour of Directed Energy Deposited Inconel 625     URL: https://hdl.handle.net/10216/149204 

Vitor Gomes

PhD's thesis title:     A Methodology for Fatigue Performance Analysis of Leaf Springs Suspensions in Freight Wagons  URL: https://hdl.handle.net/10216/155893

Rita Dantas

PhD's thesis title: Fatigue behaviour of S690 structural steel for ocean systems applications      URL: