A Single Specimen Analytical Method to Evaluate Crack Growth Rate of 7050 Alalloy at Elevated Temperature

It has been recognized that design against service failure in the airframe components, which will be subjected to temperatures more than room temperature, may require accurate prediction of crack propagation rate under creep conditions. In present work, a fracture mechanics approach is considered to study the crack growth rate of high-strength aluminum alloy 7050-T73651, which is of considerable interest for the aircraft industry. Tests were conducted under constant load condition on half inch thick compact tension, CT, specimens in air at 150°C. During the tests, crack lengths were monitored by direct current potential drop, DC-PD, technique. The fracture mechanic parameters such as stress intensity factor KI, path independent energy rate integral c* were employed to correlate with the crack growth rate, da/dt. Using experimentally determined displacement versus elapsed time as well as crack length versus elapsed time and related equations, the stress intensity factor and the c*- integral were calculated. Fracture surfaces were examined using SEM. According to results, log da/dt vs. log KI curves, the crack propagation behavior over discrete ranges of crack growth rates could be assumed linear and then be obeyed as a power law equation in the form of da/dt = A (KI )n . Additionally, the creep equivalent of J-integral termed as c* was more relevant to describe the stress and strain distribution local to a crack tip at elevated temperatures in 7050 Al-alloy. Particles as intermetallic compounds (mostly MgZn2) were observed in some regions of the fracture surfaces. Fractographs showed that cracks were usually cut the particles but in some cases they have passed around them. The extent of yield zones observed on the specimens surfaces, confirmed presence of large scale yielding at the crack tip. Presence of dimples on the fracture surface examined by SEM, was also indicative of highly localized plastic deformation at the crack tip. Observations of 45° shear lips on the side ligaments adjacent to the surface were also indicator of ductile fracture behavior and thus high fracture toughness, as reported by other researchers for L-T and L-S orientations ( L : longitudinal rolling direction, T: long- and S : short transverse direction).

کلیدواژه ها:

crack growth rate ، stress intensity factor ، KI ، path independent energy rate integral c* ، fractograph