PSI - Issue 42

Pedro R. da Costa et al. / Procedia Structural Integrity 42 (2022) 1560–1566 Pedro R. da Costa/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction After more than a century since its acknowledgement, material fatigue damage and consequent failure is still a significant research field with an imperative impact on machinery, maintenance, structural design, and construction. Its vast complexities and influential factors together with an ever-increasing range of materials and innovative manufacturing processes have fueled a large scientific community around its failure study, how to characterize it and how to prevent it. The present study focused on the fatigue study of an AISI P20 steel in the Very High Cycle Fatigue regime (VHCF) subjected to three different stress states. The VHCF range was established to be the fatigue failure beyond the one million cycle mark, more explicitly between 10 6 to 10 9 cycles (2011). Conventionally fatigue tests conducted to standardized specimen geometries are applied in servo-hydraulic or electromechanical motorized machines where a dynamic load cycle is applied. This type of conventional fatigue method/machinery has a limited cycle application frequency that makes the experimental testing in the VHCF range unreliable. One made comparison was conducted by Zhang et al. (2013) when testing Inconel 718 alloy at different frequencies. They compared the VHCF behaviour and fracture results between ultrasonic and rotary-bending fatigue testing methods. One single specimen in rotary-bending at 52.5 Hz took 45 days to reach 2.04*10 8 cycles, while in ultrasonic fatigue testing at 20 kHz, a specimen took close to four days to achieve failure at 5.37*10 9 cycles. The ultrasonic machines do not follow the same stress-inducing methods as conventional fatigue machinery to achieve such high testing frequencies. Mason designed the first Tension-compression ultrasonic machine in 1950 (Bathias and Paris 2005). Several published research studies have followed Mason’s high frequency inducing machine (Costa (2021); Karr et al. (2016)). Also, other different machines and methods since then were developed based on the same concepts that allow for different stress state fatigue test as: pure torsion by Marines Garcia et al. (2007), bending by Xue et al.(2007) and cruciform by Costa et al. (2021). Their development expanded the ultrasonic fatigue testing possibilities, just as conventional machinery development has followed. As aforementioned, the present study applied three different fatigue stress states through the use of three different ultrasonic fatigue machines. The tension-compression machine is similar to the Mason and the referenced research. The pure torsion machine follows a constructed design by Garcia et al. (2007) which transforms the axial cyclic displacement of an axial piezoelectric transducer into cyclic rotation. A third tension-torsion ultrasonic machine was developed by Costa et al. (2019). This unique ultrasonic method utilizes a special component to transform only partly the transducer cyclic axial displacement into rotation, inducing a rotation/axial displacement combination into a particular cylindrical specimen design. The follow subchapter briefly details the three ultrasonic methods main concepts to introduce the three methods better, how they work and the followed experimental methodology.

Nomenclature EDS

Energy Dispersive Spectroscopy

FEA

Finite Element Analysis Longitudinal Horn

LH

SEM

Scanning Electron Microscope

TH

Torsional Horn

1.1. Ultrasonic fatigue machines and methodologies All ultrasonic fatigue machines use vibration and resonance base concepts as the means to achieve such high cycle frequency with a relative low power input. The resonance allows for a high displacement, low force dynamic deformation of a specially designed setup, focusing on achieving the highest stress state in one single region in a material sample (the fatigue testing region), with the desired stress cyclic state.