Variation of Mechanical Characteristics for Thin Wall Tubes Processed in Ultrasonic Field - SUSAN, Mihai; ACHIŢEI, Dragoş Cristian; ŞTEFANICĂ, Roxana Gabriela; GAVRILĂ, Bogdan Lucian (The Annals of “Dunarea de Jos” University of Galati. Fascicle IX, Metallurgy and Materials Science, 2009)