Automation in Strain and Temperature Control on VHCF with an Ultrasonic Testing Facility

Abstract

Increased safety and reliability in mechanical components became a subject of prime importance over the past recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand Very High Cycle Fatigue (VHCF) loadings is extremely important nowadays. However, conventional machines used for fatigue testing are very time-consuming and costly in order to perform VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to 1E 8 to 1E 10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain-rate, temperature and frequency) is still a matter of concern and on-going development. Due to the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at 20 kHz working frequency. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera and displacement is monitored at the free end of the specimen with a high resolution laser. The machine's power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data is acquired, managed and processed with a data acquisition device working at 400 kHz sampling frequency.The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on the strain and temperature control issues. Comparison of tests carried out with and without both displacement and temperature control are made on two metallic alloys, copper 99% and carbon steel, with the determination of S-N curves.

Development of a Very High Cycle Fatigue (VHCF) multiaxial testing device

“…Because of the high frequencies used on this type of specimen testing, material temperature control represents a challenge on the completion of such tests [1,5]. Still, because of the fact that the specimen heats up faster on regions where stresses are higher, thermographic imaging may be used to evaluate an approximation of the stress profile on the specimen.…”

Cruciform specimens' experimental analysis in ultrasonic fatigue testing

“…The specimens are attached to the machine, composed by horn, booster, and transducer, at only one of the specimen's extremities. The used machine was built at Instituto Superior Técnico, laboratories in Lisbon as described in Freitas et al and Lage et al The system's component's set is represented in Figure .…”

“…The used machine was built at Instituto Superior Técnico, laboratories in Lisbon as described in Freitas et al 23 and Lage et al 25 The system's component's set is represented in Figure 3.…”

Mechatronics

“…Other applications include micro-positioning, semiconductor, silicon wafers, lenses or circuitry production, vibration measurement and differential thickness measurement. Figure 69 shows an application in which a FLW laser nano sensor is being used to determine the displacement at the tip of a specimen being tested in a Very High Cycle Fatigue (VHCF) machine [32]. In a VHCF test the specimen is designed to have the same fundamental axial frequency as the excitation source.…”

“…Also, their life is shorter than the specimen being tested, and will eventually stop functioning before the end of the test. As an alternative, a laser nano sensor is used to measure the displacement at the tip of the specimen, which can be related to the strain in the middle of the specimen, and thus stress, using analytical equations [32].…”