Automation in strain and temperature control on VHCF with an ultrasonic testing facility.

Abstract

Increased safety and reliability in mechanical components has become a subject of prime importance in 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, the use of conventional machines for fatigue testing is very time consuming and costly for VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to IE8 to IE10 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 ongoing development. Because of 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 a working frequency of 20 kHz. 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 the temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data are acquired, managed, and processed with a data acquisition device working at a 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 strain and temperature-control issues. Comparisons are made of tests carried out with and without both displacement and temperature control on two metallic alloys, copper 99 % and carbon steel, with the determination of strength-life (S-N) curves.