Issue34

G. M. Domínguez Almaraz et alii, Frattura ed Integrità Strutturale, 34 (2015) 498-506; DOI: 10.3221/IGF-ESIS.34.55 499 Concerning crack initiation and propagation on polymeric materials, the first stage may imply close to 95% of total fatigue life  7  ; however, no clear division is defined for these two stages in most of polymeric materials. For PC (polycarbonate), PMMA (poly-methyl-methacrylate) and PP (polypropylene), fatigue tests under uniaxial extension have shown that first stage covers nearly 99% of total fatigue life and that crack initiation is anticipated by an important plastic deformation  8  . From thermo-mechanical point of view, two fatigue testing modalities are available on polymeric materials: isothermal fatigue testing and non-isothermal fatigue testing. The first modality is characterized by the true mechanical fatigue response, providing that heat dissipation is very low or eliminated. Under isothermal testing and cyclic stress loading or constant stress loading, an important increase on yield stress is observed as a consequence of physical aging  9, 10  ; this behaviour is enhanced with the stress loading. For quenched polycarbonate at room temperature (23  C), under isothermal conditions and cyclic stress in tension, an appreciable increase on yield stress is observed after 100 sec of testing, when the applying load is 55 MPa  4  , (yield stress of polycarbonate at 23  C and constant low strain rate is close to 73 MPa). T ESTING MATERIAL hermoplastic material ABS was used to carry out ultrasonic fatigue testing at low loading (5 to 15% in regard the yield stress of this material: 45 MPa), and at room temperature (23  C). Table I shows the principal mechanical and physical properties of this polymer at room temperature. ABS sheets of 210 x 270 mm and 8 mm of thickness were machined to obtain the specimen profile as shown in Fig. 1. Property Units Value Elastic modulus Poisson modulus Density GPa - Kg/m 3 2.3 0.4 1040 Yield stress Glass transition temperature MPa  C 45 105 Table I : Principal mechanical and physical properties of polymeric material ABS. Figure 1 : Ultrasonic fatigue specimen dimensions (mm) for polymer ABS. Experimental testing Ultrasonic fatigue testing on solid materials requires the resonance condition of vibrating system. This is attained when the natural frequency in longitudinal direction of testing specimen is close to vibration source frequency (20 KHz); under this condition, a stationary elastic wave along the specimen is obtained. If testing material is selected (polymer ABS), the mechanical and physical properties are fixed and the longitudinal natural frequency close to 20 KHz is accomplished with the modification of the specimen dimensions. Figure 1 shows the specimen dimensions fulfilling the resonance condition for this polymer. In Figure 2 is presented the modal analysis by Finite Element method in order to obtain the natural T G = 20 mm H = 5 mm (variable) I = 5 mm W = 10 mm T = 8 mm

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