Effect of numerical parameters on plastic CTOD

F.V. Antunes et alii, Frattura ed Integrità Strutturale, 41 (2017) 149-156; DOI: 10.3221/IGF-ESIS.41.21 Focused on Crack Tip Fields Effect of numerical parameters on plastic CTOD F.V. Antunes, R. Simões, R. Branco, P. Prates University of Coimbra, Portugal , http://orcid.org/0000-0002-0336-4729 , http://orcid.org/0000-0003-2471-1125 , http:// orcid.org/0000-0001-7650-9362 ABSTRACT. Fatigue crack growth (FCG) is associated with irreversible and non-linear processes happening at the crack tip. This explains different problems observed in the use of da/dN-K curves, namely the inability to explain stress ratio and load history effects. The replacement of K by nonlinear crack tip parameters, namely the crack tip opening displacement (CTOD) is an interesting alternative. However, the determination of CTOD, using the finite element method, depends on different numerical parameters, not sufficiently studied so far. The objective here is to study the effect of these parameters on plastic CTOD, and therefore on da/dN-CTODp curves. A transient behaviour was found at the beginning of numerical crack propagation which is linked to the formation of residual plastic wake. Therefore, a minimum number of crack increments is required to obtain stabilized values. On the other hand, the predicted CTODp decreases with the distance to crack tip. Close to the crack tip, sensitivity to the measured values is much higher, but it also exists at remote positions. In addition, the mesh has a relatively low influence on CTODp. Finally, the effect of the number of load cycles between crack increments greatly depends on material properties. Citation: Antunes, F.V., Simões, R., Branco, R., Prates, P., Effect of numerical parameters on plastic CTOD, Frattura ed Integrità Strutturale, 41 (2017) 149-156. 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 (CTOD); Plastic CTOD; Finite element method; Numerical parameters. INTRODUCTION T he analysis of fatigue crack propagation is usually conducted by relating the crack advance per unit cycle, da/dN, to the stress intensity factor range, K. Nevertheless, da/dN-K relations have several limitations, namely: (i) such curves are completely phenomenological, not derived from physics, and the fitting parameters have units with no physical justification; (ii) such curves are only valid in the small-scale yielding range; (iii) and da/dN depends on 149 F.V. Antunes et alii, Frattura ed Integrità Strutturale, 41 (2017) 149-156; DOI: 10.3221/IGF-ESIS.41.21 other parameters, including the stress ratio and the load history. In order to overcome the difficulties related to the application of K to the analysis of fatigue crack growth, several concepts have been proposed, namely crack closure, partial crack closure, T-stress or the CJP model. In authors’ opinion, the linear elastic K parameter must be replaced by non-linear crack tip parameters, because fatigue crack growth is effectively linked to non-linear processes happening at the crack tip. Different parameters have been proposed to quantify crack tip plastic deformation, namely the plastic strain range, the energy dissipated around the crack tip and the crack opening displacement (COD) 1. The crack opening displacement (COD) is a classical parameter in elastic-plastic fracture mechanics, still widely used nowadays [2]. It has also great importance for fatigue analysis. Crack tip blunting under maximum load and re-sharpening of the crack-tip under minimum load were used to explain fatigue crack growth under cyclic loading [3]. Additionally, it was shown by various authors that there is a relationship between the striation spacing (related to the amplitude of crack tip blunting over a full fatigue cycle) and the crack growth rate [4]. The experimental measurement of COD is usually made remotely to crack tip. In CT specimens an extensometer with blades is used to measure the opening of the specimen at the edge, usually called crack mouth opening displacement (CMOD). In the M(T) specimen a pin extensometer is placed at the center of the specimen, fixed in two small holes to avoid sliding. However, optical techniques have been gaining increased relevance. Nevertheless, the crack tip opening displacement, CTOD, has only been measured numerically or analytically. In the finite element analysis, the displacement of the first node behind the crack tip is generally used as an operational CTOD [5]. The crack profiles also express the crack opening displacements, and are interesting to analyze the effect of load history. In a previous work [6], da/dN was related with the range of plastic CTOD, CTODp, for the 7050-T6 aluminum alloy. It was found to be a viable and interesting alternative to K, since it is a local parameter that quantifies crack tip plastic deformation, which is expected to control fatigue crack growth. Additionally, it includes naturally the effect of crack closure and fatigue threshold. The relation between the numerical CTODp and the experimental da/dN values was used to predict fatigue crack growth rates for other loading conditions. The CTODp was predicted numerically at the first node behind crack tip, at a distance of 8 m from it. However, there are several numerical parameters which may affect by the magnitude of plastic CTOD, and therefore of da/dN versus CTODp relations. The objective here is to study the effect of these parameters on CTODp, namely the measurement node behind crack tip, the crack propagation, the finite element mesh, and the number of load cycles between crack increments. NUMERICAL MODEL T he specimen geometry studied was a Middle-Tension specimen, having W=60 mm, and a small thickness (t=0.2 mm) in order to obtain a plane stress state. A straight crack was modeled, with an initial size, ao, of 5 mm (ao/W=0.083). Since the specimen is symmetric about three orthogonal planes, only 1/8 was simulated considering proper boundary conditions. Pure plane strain state was also modeled constraining out-of-plane deformation. The materials considered in this research were the 6016-T4 (ys=124 MPa) and 6082-T6 (ys=238 MPa) aluminum alloys. The mechanical behaviour was represented using an isotropic hardening model described by a Voce type equation: p Y   ys  Rsat (1  e nv  ) (1) combined with a non-linear kinematic hardening model described by a saturation law:    C x  X sat  σ  X   X   p , X  0   0 X      (2) In the previous equations, Y is the flow stress,  p is the equivalent plastic strain, ys is the initial yield stress, Rsat is the saturation stress, n, Cx and Xsat are material constants, σ  is the deviatoric stress tensor, X is the back stress tensor, and  (...truncated)