Issue34

G. M. Domínguez Almaraz et alii, Frattura ed Integrità Strutturale, 34 (2015) 498-506; DOI: 10.3221/IGF-ESIS.34.55 501 furthermore, an increase on applying load (from 2.25 to 6.75 MPa), is accompanied by a low increase on the yield stress. These results are in accordance with the fact that, under the describing testing conditions, the thermal domain is negligible and mechanical domain becomes predominant. a) b) Figure 4 : Ultrasonic fatigue testing of polymer ABS immeresed in water a) and in oil b) . No cavitation effect was observed during ultrasonic fatigue tensting with specimens immersed in water or in oil. a) b) Figure 5 : Yield stress measured by tensile tests after ultrasonic fatigue testing in water and two applying loads a) , yield stress measured by tensile test after ultrasonic fatigue testing in oil and two applying loads b) . No appreciable difference is observed for the yield stress evolution under ultrasonic fatigue testing when testing specimen is immersed in water or oil; however, in both cases a low increase on yield stress is registered in increasing the applying load. Another aspect of these results is the fact that the experimental tendencies grow up rapidly in the first 100 seconds of ultrasonic fatigue testing, and they tend to an asymptotical value afterwards. This behavior should be attributed to very high fatigue testing frequency, to very low applying loading and the control of temperature by the cooling environments (water and oil). The measured temperature at the specimen neck section during fatigue testing was close to 33  C. Assuming mechanical domain under the described conditions, it is expected that the low increase on yield stress of testing material during ultrasonic fatigue testing is a combination of aging and rejuvenation processes  11, 12  ; even though, the last process is not fully recognized  13  .