SYMPOSIA PAPER Published: 01 January 1976
STP28657S

Brittle Fracture ( ) Behavior of Cracks Emanating from Notches

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Experimental and analytical studies were conducted under AISI sponsorship to determine the influence of short cracks emanating from notches on fracture behavior. All studies were conducted by using concepts of linear-elastic fracture mechanics (LEFM). The experimental studies were conducted on AISI 4340 steel at a high strength level (σys = 215 ksi = 1480 MN/m2) under plane-strain (KIc) conditions by using various types of fatigue-cracked as well as notched and fatigue-cracked specimens. The analytical studies were conducted by using recently developed theoretical KI analyses for notched and fatigue-cracked specimens.

The experimental results obtained by using various types of standard fatigue-cracked specimens demonstrated that the basic fracture-toughness behavior of 4340 steel is repeatable and reproducible within a relatively narrow range (KIc = 70±5 ksi √in. = 77±5.5 MNm−3|2). Limited experimental results on notched and fatigue-cracked double-edge-notched tension (DENT) specimens verified the accuracy of a recent theoretical KI analysis by Tada. Results for these notched and cracked DENT specimens exhibited good agreement with the baseline KIc behavior, but two to three times the amount of scatter usually observed in standard KIc tests (±5 to 10 percent). Tests demonstrated no significant differences in the basic fracture behavior (apparent KIc) of either pure-tension (DENT) specimens or essentially pure-bending (three-point-bend) specimens when tested with cracks emanating from notches. Apparent KIc results calculated by using traditional equations, in which the presence of the notch is ignored (treated as a crack of equivalent length), were elevated above the intrisic KIc level by less than 30 percent for all investigated combinations of specimen type, notch type, and the length of the fatigue crack beyond the notch tip (ΔaF).

The theoretical studies were conducted to develop an analytical criterion [(ΔaFaNρ)=¼] that predicts the depth beyond which a notch with an emanating crack can be treated as a crack of equivalent length. The engineering significance of this criterion is discussed. For values of the parameter that are greater than the criterion, traditional KI equations are accurate within ±5 to ±10 percent. For values less than the criterion, the stress-intensity (KI) at the tip of a short fatigue crack (of length ΔaF) emanating from a dull notch (of length, aN, and radius ρ) is always less severe than a crack of equivalent length, and the use of traditional KI equations can grossly overestimate the actual KI at the crack tip. For this short crack-length range, KI values must be calculated by using appropriate theoretical analyses that account for notch effects in order to minimize the conservative results of traditional analyses.

Included in the present work is a state-of-the-art summary of currently available theoretical KI analyses for cracks in the near vicinity of notches.

Author Information

Novak, SR
Research Laboratory, United States Steel Corporation, Monroeville, Pa.
Barsom, JM
Research Laboratory, United States Steel Corporation, Monroeville, Pa.
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Developed by Committee: E08
Pages: 409–447
DOI: 10.1520/STP28657S
ISBN-EB: 978-0-8031-4676-1
ISBN-13: 978-0-8031-0318-4