Issue 41

J.A.O. González et alii, Frattura ed Integrità Strutturale, 41 (2017) 227-235; DOI: 10.3221/IGF-ESIS.41.31 227 Focused on Crack Tip Fields On DIC measurements of  K eff to verify if it is the FCG driving force Julián Andrés Ortiz González, Jaime Tupiassú Pinho de Castro, Giancarlo Luis Gomez Gonzáles, Marco Antonio Meggiolaro, José Luiz de França Freire Pontifical Catholic University of Rio de Janeiro, PUC-Rio, R. Marquês de São Vicente 225, Rio de Janeiro, 22451-900, Brazil julian@aluno.puc-rio.br, jtcastro@puc-rio.br , gonzalesglg@aaa.puc-rio.br, meggi@puc-rio.br, jlfreire@puc-rio.br A BSTRACT . Redundant data obtained under quasi-constant {  K , K max } loading conditions is used to verify if the effective stress intensity factor (SIF) range  K eff  K max  K op is indeed the fatigue crack driving force. The crack opening SIF K op is measured along the entire crack path on DC(T) low carbon steel specimens by a series of strain gages bonded along the crack paths, by a strain gage bonded on their back faces, and by a digital image correlation technique. All such measurements showed a significant K op decrease as the crack sizes increased, while the fatigue crack growth rates remained essentially constant both in the thin and thick specimens, a behavior that cannot be explained by  K eff arguments. K EYWORDS . Fatigue crack driving forces; opening load measurements;  K eff limitations. Citation: González, J.A.O., Castro, J.T.P., Gonzáles, G.L.G., Meggiolaro, M.A., Freire, J.L.F. On short cracks that depart from elastoplastic notch tips, Frattura ed Integrità Strutturale, 41 (2017) 227-235. Received: 28.02.2017 Accepted: 03.05.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION t is well known that the order of load events can much affect fatigue crack growth (FCG) lives, inducing sequence effects by several mechanisms. Such mechanisms can act along the crack faces, so before the crack tip (like fatigue crack closure induced by plasticity, roughness, phase transformation, and/or oxidation); or at the crack tip (such as blunting, kinking, or bifurcation of the crack tip); or else ahead of the crack tip (like residual stresses and/or strains in the uncracked residual ligament) [1-2]. Moreover, these mechanisms are not exclusive, and their relative importance may depend on many factors, among them at least: load and overload (OL) ranges and maxima; number of OL cycles; crack and uncracked residual ligament ( rl ) sizes; transversal constraints along the crack front; residual stresses around the crack tip; material microstructure; and environment. In many cases, a load order mechanism can be so dominant that the others may become negligible. However, in other cases such mechanisms can compete (e.g. OL-induced crack tip bifurcation can reduce the subsequent opening load and decrease crack closure effects), or else act symbiotically (e.g. plasticity-induced martensitic transformation tends to increase the material volume inside the plastic zones and thus the residual stresses ahead, as well as the crack opening loads behind the crack tip). Since so many variables can affect the FCG behavior under variable amplitude loads (VAL), there is yet no consensus on how to model properly this most important practical problem. Many fatigue experts defend I