Stress criterion of delamination cracking in 2091 aluminum alloys

The measurement of ultrasonic wave velocity has been used to detect the delamination cracking during tensile tests in an 2091 T-8 Al-Li alloy at both liquid helium and room temperature. The measured stresses for initiation of delamination cracking are 335 and 357 MPa, for the cryogenic and room temperatures, respectively. The density of delamination cracks in the alloy is estimated from the observed reduction in ultrasonic wave velocity and resultant decrease in modulus of elasticity. It is estimated that the interfacial debonding strengths normal to a grain boundary are 116 and 113 MPa, for the cryogenic and room temperature, respectively. Fracture mechanics analysis has been used to estimate the onset of delamination cracking in the case of a cracked tensile specimen. The predicted levels of far field applied stresses are much lower than that for macroscopic crack initiation at both cryogenic and room temperatures, respectively.