Separating Fact from Fiction on the "Pit-To-Crack Transition"

Concerning any (military or commercial) application, statements like “more than 80% of military aircraft structural failures are initiated from corrosion pits” lack factual evidence. Such statements contradict real-world field experience and modern materials science. In recent decades, beginning probably since the late 1970s, there have been numerous unjustified speculations regarding the possibility/role of “pit-to-crack transition” (PTCT) in the service failures of structural components, including military aircraft and fatal crashes. Contradicting the fracture-mechanics theory, it is too often postulated by researchers that “crack initiation is assumed to occur at pits that are sufficiently deep that the stress intensity factor, K_I, for the prevailing stress level exceeds K_ISCC [at the pit base], the critical stress intensity factor for the initiation of slow crack growth rate for the material in contact with the particular environment of interest.” Moreover, when examining actual service fractures and seeing pitting on the component’s surface, many investigators quickly concluded that crack growth started from a pit. It appears that these individuals do not require factual data to support their conclusions. They rely on information they have heard or read about PTCT and firmly believe in this idea, which has not been validated in engineering practice. The first part of this paper, titled “Fracture Mechanics,” explains actual fracture mechanics is not applicable to laboratory studies of PTCT. The second part, “Field Experience,” examines actual cracks found in aircraft components (rather than laboratory samples), where the origins of cracks were either unrelated to any pits or mistakenly attributed to surrounding pits without any factual evidence to support such claims. This confusion typically arises when the misleading concept of PTCT is applied.