Issue34

G. M. Domínguez Almaraz et alii, Frattura ed Integrità Strutturale, 34 (2015) 498-506; DOI: 10.3221/IGF-ESIS.34.55 502 U LTRASONIC FATIGUE RESULTS ON POLYMER ABS xperimental ultrasonic fatigue results for this polymer are shown in Fig. 6 a) and b), immersed in water and oil respectively. The range of applying load in both cases was low: between the 5 and 15% the yield stress of this material (45 MPa at 23  C and at strain rate reference of 10 -3 s -1 ). a) b) Figure 6 : Ultrasonic fatigue endurance of polymeric material ABC immersed in water a), and in oil b). Ultrasonic fatigue endurance on thermoplastic ABS is close to one millions of cycles when the applying load is about 15%regarding the yield stress of this polymer, independently of cooling liquid used. Furthermore, dispersion of experimental points is higher for high applying load (from 4 to 6.75 MPa); it decreases for low loading or long fatigue life. Concerning crack initiation and propagation under ultrasonic fatigue testing associated with the described testing conditions, Fig. 7 a) and 7 b) show the corresponding images for the specimens immersed in water and in oil, respectively. a) b) Figure 7 : Crack initiation and propagation under ultrasonic fatigue testing of polymer ABS immersed in water a) , and in oil b) . Physical-chemical reactions are expected to take place on the interface of polymer surface and the cooling liquid during ultrasonic fatigue testing, such as the swelling phenomenon  14-16  . Swelling is the absorption process of the solvent (water or oil) penetrating the polymer and this process should be enhanced by the very high frequency loading. The tail phenomenon  17, 18  , or the transition process of damage accumulation and the corresponding plastic deformation is observed at the first stage of crack, particularly for specimens immersed in water, Fig. 7 a). In most of testing specimens it was observed a bubble or protuberance at the neck section that is caused by the expansion of micro-porosities inside the specimen when loading at very high frequency, Fig. 8. The swelling contribution for this protuberance should be negligible. E Crack initiation Applying load