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

Abstract

The necessity of increased safety and reliability in mechanical components or structures became a subject of prime importance over the years. Therefore, to have a proper understanding of the damage and rupture mechanisms in materials subjected to fatigue at Very High Cycle Fatigue (VHCF) regimens is extremely important nowadays. However, using conventional fatigue testing machines to carry out VHCF tests can be very time-consuming and expensive; for instance, making a fatigue test at 30 Hz as working frequency would take more than one year to reach 109 cycles. Ultrasonic fatigue testing machines are being used to perform materials testing in the range of 108 to 1010 fatigue cycles. The so-called VHCF regimen is now under intense studies in which concerns the performance of ultrasonic fatigue testing machines themselves. 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 continuous development. Due to the high frequencies involved in VHCF testing, the heat generated on the specimens greatly affects the parameters that influence fatigue behavior. The objective of this paper is to describe the design, construction, instrumentation and operation of an ultrasonic fatigue testing machine of 20 kHz working frequency, with automatic strain and temperature control. In order to achieve fully automated tests, a closed loop control system was developed to use monitored temperature and displacement amplitude of the specimen to set the length of the powered and the cooling periods of the machine. The monitoring of the displacement, measured here at the bottom face of the specimen, is carried out using a high resolution laser. The specimen´s temperature is monitored online through a pyrometer and an infrared thermography camera. Data is acquired, managed and processed with a National Instruments DAQ device working at 400 kHz sampling frequency. The software was developed in-house using the LabView® package. The present paper describes the advantages and drawbacks of metal fatigue testing at very high frequencies 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. Fatigue tests were carried out on two metallic materials, copper 99% and carbon steel, with the determination of S-N curves.