Issue34

G. M. Domínguez Almaraz et alii, Frattura ed Integrità Strutturale, 34 (2015) 498-506; DOI: 10.3221/IGF-ESIS.34.55 500 frequency of testing specimen; the corresponding value was 20039 Hz. Additionally, in Figure 3 is plotted the evolution along the specimen of stress and displacement under resonance conditions (generation of a stationary elastic wave). Figure 2 : Modal analysis to obtain the logitudinal frequency of vibration for testing ABS specimen. a) b) Figure 3 : Evolution of stress (Pa) along testing specimen under a stationary elastic wave a) , and evolution of displacement (m) along testing specimen under same conditions b) . The dimensions of testing specimen were as little as possible (fitting the resonance conditions), in order to increase the surface to volume ratio (reduction of thermal domain). Furthermore, testing specimens were immersed in water or oil for cooling purpose, Fig. 4, and the applying load was considerably low (5 to 15% the yield stress of this material), in order to approach the mechanical domain and reduce the thermal influence. A principal aspect to achieve ultrasonic fatigue testing on this polymer was the attachment of testing specimen to the ultrasonic fatigue machine; however, no details are provided in this paper. Measures of yield stress (tensile tests at strain rate 10 -3 s -1 ), after ultrasonic fatigue testing were carried out in order to investigate the evolution of this property under the following conditions: a) previous ultrasonic fatigue testing, b) low loading (5 – 15% of yield stress), and c) cooling environment (water and oil). Figure 5 a) presents the yield stress evolution for testing specimens immersed in water and for two applying loads: 2.25 and 6.75 MPa, under fatigue stress ratio R= -1. In Fig. 5 b) are plotted the experimental points for the yield stress of this polymer immersed in oil and undergoing the same applying loads and fatigue stress ratio. The increase on yield stress for both cooling liquids is quite small;