Issue 48

A.C. de Oliveria Miranda et alii, Frattura ed Integrità Strutturale, 48 (2019) 611-629; DOI: 10.3221/IGF-ESIS.48.59 611 Focussed on “Crack Paths” Stress Intensity Factor Equations for the Evolution of Surface and Corner Cracks to Through Cracks Antonio Carlos de Oliveira Miranda, Rafael Marques University of Brasília, Brasil acmiranda@unb.br, marquessilvarafael15@gmail.com Marco Antonio Meggiolaro, Jaime Tupiassú Pinho de Castro Pontifical Catholic University of Rio de Janeiro, Brasil meggi@puc-rio.br, jtcastro@puc-rio.br A BSTRACT . Part-through surface or corner 2D cracks are commonly found in structural components, even because practically all fatigue cracks tend to start this way. It is a reasonable hypothesis to model them assuming the shape of their 2D fronts can be approximated by an elliptical arc, as supported by many fractographic observations. However, their transition to a 1D through- crack, an important issue in many practical applications, is normally not properly addressed in fatigue life predictions. Although experimental results reveal that the frontiers of surface cracks essentially retain their elliptical shape as they gradually grow into an 1D through-crack, it is usual to assume they are immediately transformed into an 1D through-crack when their depth reaches the cracked component thickness. This crude approximation may create a large jump in stress intensity values, leading to excessively conservative fatigue crack growth predictions; or else, the crude shape jump hypothesis might induce false overload events that can much affect fatigue crack growth retardation models, leading to inadmissible non-conservative life predictions. To minimize such problems, an improved model to describe the transition of 2D surface cracks to 1D through-cracks is proposed and verified by crack propagation tests in two different materials, 4340 steel and polycarbonate (PC). Moreover, fatigue life predictions based on this improved model are compared with experimental results obtained with these two materials. K EYWORDS . 2D-1D crack transition; SIF equations; fatigue analysis. Citation: de Oliveira Miranda, A.C., Marques, R., Meggiolaro, M.A., Pinho de Castro, J.T., Stress Intensity Factor Equations for the Evolution of Surface and Corner Cracks to Through Cracks, Frattura ed Integrità Strutturale, 48 (2019) 611-629. Received: 29.11.2018 Accepted: 30.01.2019 Published: 01.04.2019 Copyright: © 2019 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 atigue cracks tend to nucleate on surfaces, usually at a notch tip whose stress concentration effects cannot be neglected in fatigue analyses. Most fatigue cracks initiate and initially grow in 2D with a crack front that can be approximated by an elliptical arc, see e.g. the ASM Fractography Handbook for plenty of evidence to support this F