Identification of Crack Initiation in Aluminum Alloys using Acoustic Emission

This paper uses multiple specimens, mechanical loading setups and nondestructive evaluation (NDE) techniques to reliably identify crack initiation in aluminum alloy specimens and quantify the associated Acoustic Emission (AE) activity. Compact Tension (CT) and Middle Tension (MT) specimens were tested until crack initiation was verified using optical methods while simultaneously recording AE, Infrared Thermography (IRT) and Digital Image Correlation (DIC). The specimens were loaded under tension and fatigue loading conditions with the prime focus being on identifying, in these controlled experiments, the most sensitive AE features to crack initiation. The changes of such AE features at the time instance of crack initiation was cross-validated by the complementary optical metrology data. In addition to the load drop accompanying the ductile failure process, the synchronous use of the optical NDE techniques provided the opportunity to associate the mechanical behavior of the material to the AE recordings observed during testing. The identified changes in AE features were combined with extensive signal processing which revealed trends that provide strong evidence on the existence of a dominant and quantifiable trend of AE activity which was noted to be directly associated with crack initiation. The onset of cracking in both types of aluminum specimen tested was noted with an increase in the peak frequency and partial powers which was used to define a novel AE damage parameter and shown to robustly identify crack initiation in both tensile and fatigue loading.