PSI - Issue 25

Pedro R. da Costa et al. / Procedia Structural Integrity 25 (2020) 445–453 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Fatigue damage has special relevance on the life span of mechanical components and structures, as it takes responsibility for a large majority of the registered structural failures in service. The pioneering research work performed by Wohler in the middle of the 19th century, when investigating fatigue failures of railway axles, lead to the concepts of fatigue limit or endurance fatigue and early rules for designing against fatigue. However, it is generally recognized that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states (Manuel de Freitas 2017). Fatigue tests under multiaxial fatigue loading are more recent and was the subject of research and development since mid-XX century, using classical mechanically actuated fatigue testing machines and servo hydraulic fatigue testing machines. Much more recent papers in the literature questions the existence of the so-called fatigue limit introducing the concept of Very High Cycle Fatigue (VHCF) that is gaining notoriety (C. Bathias 1999), largely due to the appearance of ultrasonic fatigue testing machines (Claude Bathias 2006), working at 20 kHz allowing experimental fatigue tests in a reasonable time of testing. Most of such ultrasonic fatigue researches use mainly uniaxial tension-compression machine as in (Huang et al. 2016; Kovacs, Beck, and Singheiser 2013; Sander, Stäcker, and Müller 2018) and more recently a pure torsion ultrasonic machine (Marines-Garcia, Doucet, and Bathias 2007; Mayer et al. 2015; Nikitin, Bathias, and Palin-Luc 2015). Nevertheless, in the region of multiaxial very high cycle fatigue loading, no detailed research has been carried out. One of the main reasons is due to the inexistence of appropriate machinery that can perform multiaxial fatigue tests for very high cycle fatigue loadings in a reasonable time of testing, since mechanical actuated or servo-hydraulic actuated fatigue testing machines have a limited frequency ranges under biaxial loading conditions. Present manuscript is a description of the main equipment and specimens where research is being carried out for performing multiaxial fatigue tests under tension-torsion stress states on very high number of cycles (VHCF). Such method are being developed (P. Costa et al. 2017; Vieira, Reis, et al. 2016) and new ideas are still being implemented for a bigger multiaxial fatigue study range of stress study (P. R. Costa et al. 2019). One ultrasonic machine developed by Palin-Luc et al. is also worth mentioning, where a multiaxial bending stress state is achieved in a disk-shaped specimen (Brugger et al. 2016). Tension/torsion fatigue experiments were also conducted to a railway wheel with stress life and fracture surface results and analysis. Currently most of the experimental fatigue tests are performed in rotating bending or in servo-hydraulic testing machines. Rotating bending machines, originally used by Wholer studies on railway axles, are a low cost solution to carry out fatigue tests, but are limited to the bending applied load. The servo-hydraulic actuation provided over the last decades the best solution for general test systems because of its versatility, fast response and load capacity. Examples of these machines can be found in the catalogue of the most important testing brands, Instron® MTS® etc… The principle of these systems is based on servo-valves that are able to deliver a controlled amount of oil power to an actuator, providing a fast and precise control to that actuator. With an appropriate feedback controller it is possible to control the actuator movement in position, load, velocity, acceleration, sample strain or any other coupled transducer allowing reproducing any reference signal (waveforms, random spectrums etc…) at frequencies that in some cases can exceed 200Hz (generally for most of the fatigue tests the working frequency ranges from 5 – 20Hz). For most of the cases the hydraulic circuit operates as pressure source, keeping a constant oil pressure independently of the flow usage. This is essential for a fast and reliable answer from the machine controller however an enormous power is being lost in the form of heat with large consequences in running and maintenance costs. In the last decade the most important brands of fatigue testing machines made available on the market uniaxial testing machines with a new actuation technology, based on linear actuators, totally electrical. This kind of machines can be found in the catalogue of some brands like Bose® ElectroForce®, Instron® ElectroPuls™ or MTS®. This new technology is alternative to the traditional hydraulic actuators and are suitable for low load capacities dynamic tests, being considered the state-of-the-art of non-hydraulic test systems. The commercial success of these actuators are mainly due to much lower operating costs and many other advantages such as: no consumables needs, much less 2. Multiaxial fatigue testing machines 2.1. Conventional multiaxial fatigue testing