Experimental evaluation of CTOD in constant amplitude fatigue crack growth from crack tip displacement fields

J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 41 (2017) 157-165; DOI: 10.3221/IGF-ESIS.41.22 Focused on Crack Tip Fields Experimental evaluation of CTOD in constant amplitude fatigue crack growth from crack tip displacement fields J.M. Vasco-Olmo, F.A. Díaz University of Jaén, Spain F.V. Antunes University of Coimbra, Portugal M.N. James University of Plymouth, UK ABSTRACT. In the current work an experimental study of the crack tip opening displacement (CTOD) is performed to evaluate the ability of this parameter to characterise fatigue crack growth. A methodology is developed to measure and to analyse the CTOD from experimental data. The vertical displacements measured by implementing Digital Image Correlation on growing fatigue cracks are used to measure the CTOD. Fatigue tests at R ratios of 0.1 and 0.6 were conducted on compact-tension specimens manufactured from commercially pure titanium. A sensitivity analysis was performed to explore the effect of the position selected behind the crack tip for the CTOD measurement. The analysis of a full loading cycle allowed identifying the elastic and plastic components of the CTOD. The plastic CTOD was found to be directly related to the plastic deformation at the crack tip. Moreover, a linear relationship between da/dN and the plastic CTOD for both tests was observed. Results show that the CTOD can be used as a viable alternative to ΔK in characterising fatigue crack propagation because the parameter considers fatigue threshold and crack shielding in an intrinsic way. This work is intended to contribute to a better understanding of the different mechanisms driving fatigue crack growth and the address the outstanding controversy associated with plasticity-induced fatigue crack closure. Citation: Vasco-Olmo, J.M., Díaz, F.A., Antunes, F.V., James, M.N., Experimental evaluation of CTOD in constant amplitude fatigue crack growth from crack tip displacement fields, Frattura ed Integrità Strutturale, 41 (2017) 157-165. Received: 28.02.2017 Accepted: 15.04.2017 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. KEYWORDS. Crack tip opening displacement; Fatigue crack growth; Plastic deformation; DIC. 157 J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 41 (2017) 157-165; DOI: 10.3221/IGF-ESIS.41.22 INTRODUCTION F atigue crack growth has been traditionally characterised by the Paris law [1], that relates the crack growth per cycle, da/dN to the stress intensity factor range, ΔK. However, there are several controversial issues and unanswered questions in this field. The procedures for analysing constant amplitude fatigue under small-scale yielding conditions are well established, although a number of uncertainties remain. Variable amplitude loading, large-scale plasticity, and short cracks introduce additional complications that are not fully understood. In addition, in many materials, it is virtually impossible to characterise the fracture behaviour with lineal elastic fracture mechanics (LEFM), and an alternative fracture mechanics model is required. Elastic-plastic fracture mechanics (EPFM) is applied to materials that exhibit nonlinear behaviour (i.e., plastic deformation). Hence in the authors’ view, the linear elastic ΔK parameter should be replaced by nonlinear crack tip parameter since fatigue crack growth is governed by nonlinear processes at the crack tip. Two elastic-plastic parameters have been proposed to be related with crack tip plastic deformation, the crack tip opening displacement (CTOD) and the J contour integral. Both parameters describe crack tip conditions in elastic-plastic materials, and they can be used as a fracture criterion. CTOD is a local parameter, while the J integral is used as a global criterion based on the quasi-strain energy release rate. Critical values of CTOD or J give nearly size-independent measurements of fracture toughness, even for relatively large amounts of crack tip plasticity. There are limits to the applicability of these parameters but they are much less restrictive than the validity requirements of LEFM. In this work, CTOD is the parameter used to characterise fatigue crack growth. CTOD was first observed by Wells [2] when he was attempting to measure KIC values in a number of structural steel. Wells found that these materials were too tough to be characterised by LEFM. While examining fractured test specimens, Wells noticed that the crack faces had moved apart prior to fracture; plastic deformation had blunted an initially sharp crack, resulting in a finite displacement at the crack tip. The degree of crack blunting increased in proportion to the toughness of the material. This observation led Wells to propose the opening at the crack tip as a measurement of fracture toughness. Nowadays, CTOD is a classical parameter in elastic-plastic fracture mechanics and it has a high importance for fatigue analysis. Crack tip blunting at maximum load and the crack tip re-sharpening at minimum load were used to explain fatigue crack growth [3]. CTOD has been experimentally measured using extensometers located remotely to the crack tip. Thus, in compact tension (CT) specimens an extensometer with blades is located at the mouth of the specimen notch to measure the opening of the specimen [4]. In the case of middle tension (MT) specimens a pin extensometer is placed at the centre of the specimen by fixing it into two small holes [5]. Recently, full field optical techniques have become very popular for the analysis of structural integrity problems. Among them Digital Image Correlation (DIC) technique can be considered because the displacement fields at the vicinity of the crack tip can be measured with high level of accuracy [6]. Thus, in this work DIC is implemented to measure the CTOD from the relative displacement between both crack flanks. Moreover, a finite element analysis has also been employed to measure the CTOD numerically. The displacement at the first node behind the crack tip is generally used as an operational CTOD [7]. (a) (b) Figure 1: (a) Dimensions (mm) of the CT specimen tested. (b) Experimental set-up used to measure DIC data during fatigue testing. In previous work, Antunes et al. [8] developed a numerical study to quantify the CTOD in a MT specimen for two aluminium alloys in order to analyse the applicability of this parameter to characterise fatigue crack growth. A relationship was found between da/dN and the plastic CTOD range for the 6082-T6 aluminium alloy independent of stress ratio, showing that the CTOD can be a viable alternative to ΔK in the analysis of fatigue crack propagation. Thus, in the current 158 J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 41 (2017) 157-165; DOI: 10.3221/IGF-ESIS.41.22 work an experimental stu (...truncated)