A Study of Naturally Initiating Notch Root Fatigue Cracks Under Spectrum Loading

Abstract

The kinetics of short cracks at notches were studied in 2014-T6511 Al-Cu alloy extruded bar stock under modified versions of FALSTAFF and TWIST load spectra. Experiments were conducted on test coupons of different thickness and at different stress levels. The effects of these and other parameters on the density and growth rate patterns of multiple cracks were investigated. The quality of fractographic crack growth rate measurements was found to be superior to surface replica technique. It was also observed that artificially initiated fatigue cracks grow slower than naturally initiated ones. Interaction of naturally initiating multiple cracks was quantified.

Fatigue crack growth at stress concentrators under spectrum loading

“…Crack growth rate data obtained using optical fractography exhibit good consistency as well as continuity in marker patterns on the fracture surface comparable with long-crack growth rates [5]. This is in contrast with observations in the literature that tend to suggest that the crack growth behaviour of short cracks is characterized by high levels of scatter.…”

“…Crack growth rate versus crack length information was extracted after specimen failure by optical fractography. In a previous study, it was noted that optical fractography provided a consistent estimate of crack growth rate and crack interaction compared with the plastic replication technique that has been widely used by other researchers in short-crack studies [5].…”

“…Further, this acceleration effect settles down quickly as the new crack front and shape are formed after merger of two cracks. Thus the earlier assumption of continuous crack rate versus crack depth data before and after the merger of two initiators [5] growth for the backtracking crack front appears to be justified. The fracture surface of the lug specimen tested under constant-amplitude loading was examined under a JEOL scanning electron microscope.…”

Studies on fatigue crack growth for airframe structural integrity applications

“…With increase in stress level, a larger fraction of total life is spent on FCP {when viewed from a given initial crack length to failure). In fact, recent studies on short crack growth at notches revealed that crack growth at high stress level can commence without any incubation period (Newman et al 1992;Prakash et al 1994).…”

“…The study provided many significant observations (Prakash et al 1994; i Surface replica based measurements provide an exaggerated measure of scatter in growth rate. Short crack growth rates do not exhibit more scatter than long cracks at comparable growth rates.…”