Issue34

G. M. Domínguez Almaraz et alii, Frattura ed Integrità Strutturale, 34 (2015) 498-506; DOI: 10.3221/IGF-ESIS.34.55 498 Focussed on Crack Paths Crack initiation and propagation on the polymeric material ABS (Acrylonitrile Butadiene Styrene), under ultrasonic fatigue testing G. M. Domínguez Almaraz, E. Correa Gómez, J.C. Verduzco Juárez, J.L. Avila Ambriz University of Michoacán (UMSNH), Santiago Tapia No. 403, Col. Centro, 58000, Morelia, Michoacán, Mexico dalmaraz@umich.mx, etrasmo@gmail.com, Julio_glenn83@hotmail.com, joavam@hotmail.com A BSTRACT . Crack initiation and propagation have been investigated on the polymeric material ABS (Acrylonitrile Butadiene Styrene), under ultrasonic fatigue testing. Three controlled actions were implemented in order to carry out fatigue tests at very high frequency on this material of low thermal conductivity, they are: a) The applying load was low to limit heat dissipation at the specimen neck section, b) The dimensions of testing specimen were small (but fitting the resonance condition), in order to restraint the temperature gradient at the specimen narrow section, c) Temperature at the specimen neck section was restrained by immersion in water or oil during ultrasonic fatigue testing. Experimental results are discussed on the basis of thermo-mechanical behaviour: the tail phenomenon at the initial stage of fatigue, initial shear yielding deformation, crazed development on the later stage, plastic strain on the fracture surface and the transition from low to high crack growth rate. In addition, a numerical analysis is developed to evaluate the J integral of energy dissipation and the stress intensity factor K, with the crack length K EYWORDS . Crack initiation; Polymeric material; Ultrasonic fatigue; J integral; Stress intensity factor K. I NTRODUCTION olymeric materials used in modern industries present valuable combination of properties, such as: corrosion resistance, high elastic modulus and strength in regard their density, good thermal and electrical insulation, excellent shape design and formability  1  . The physical-chemical and mechanical properties of polymeric materials are modified in use for a wide variety of industrial applications  2, 3  . Often, mechanical loading determines the fatigue endurance properties of these materials; this is the case for thermoplastic polymers such as ABS (Acrylonitrile Butadiene Styrene), for which two fatigue failure modes can be observed: a) the low cycle regime, characterized for failure at low number of cycles and high stress loading, and b) the high cycle regime and low stress loading. The first mode is associated with high heat dissipation and hysteresis process during testing: high stress, high strain or high testing frequency and ductile failure; whereas the second mode presents low energy dissipation inside the hysteresis loops, revealing low influence of mechanical loading or frequency on the specimen temperature  4  . Furthermore, the fatigue testing frequency on glassy polymers is associated with the process of physical aging, since an increase on frequency promotes the heat dissipation and aging process on these materials  5  . In addition, polymers undergoing embrittlement degradation caused by three different physical-mechanical factors: cyclic stress, constant stress and thermo-physical aging; nevertheless, a complete understanding and differentiation of these factors inducing embrittlement degradation is not available  6  . P