Issue 42

P. J. Huffman et alii, Frattura ed Integrità Strutturale, 42 (2017) 74-84; DOI: 10.3221/IGF-ESIS.42.09 75 dependent on the cyclic stress-strain behaviour of the material. This new fatigue crack propagation model was proposed by Huffman based on Walker- like strain-life relation. This model is applied to FCG data available for the P355NL1 pressure vessel steel. A comparison of the experimental results and the Huffman crack propagation model is made. K EYWORDS . F atigue Crack Growth; Strain Energy; Unigrow Model; Pressure Vessel Steel. Accepted: 21.06.2017 Published: 01.10.2017 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 atigue crack initiation, usually modelled by strain-life or stress-life, has traditionally been considered to be a separate physical phenomenon from fatigue crack propagation. However, some more recent models of fatigue crack growth have been based on the assumption that each crack growth increment is physically similar to the initiation process. That is, the individual crack growth increments are successive initiations of the crack locally at the crack tip [1-4]. Such models consider fatigue crack initiation and propagation to be physically similar, and they can be used in a unified approach to calculate total fatigue life as the sum of initiation and propagation [5-7]. Some authors, as Glinka [2], Peeker and Niemi [3], Noroozi et al. [4,6,7], Hurley and Evans [5] developed approaches to represent fatigue crack propagation using local fatigue models based on strain parameters. Glinka was one of the precursors to describe the fatigue crack propagation modelling using a strain-based fatigue relation [2]. Similarly, the model proposed by Peeker and Niemi [3] allowed the near threshold fatigue crack propagation data and the stable crack growth to be described. For the near threshold fatigue crack propagation, the authors derived analytical relations which are functions of the strain-life relation constants. Hurley and Evans [5] proposed the use of an elastoplastic finite element analysis to compute the process zone and using the Walker-like strain correlated directly with the fatigue life from experimental data thru a power relation to correlate with the fatigue crack increment. Other authors, such as, Correia et al. [8-14] and Hafezi et al. [15] used the strain and SWT fatigue local relations [16], based on UniGrow model [4,6,7] to predict the fatigue crack propagation using the numerical analysis to obtain residual stresses distribution. Correia et al. [9,17,18] proposed a procedure to derive probabilistic S–N–R fields for notched structural details or mechanical components, which is based on the UniGrow model and numerical analysis aiming at computing the elastoplastic stresses and strains at process zone ahead the crack tip. Alternatively, analytical methods such as the ones proposed by Neuber [19] and Moftakhar et al. [20] may be applied to perform the elastoplastic analysis taking into account the elastic stress/strain fields computed around the crack tip, using available Linear Elastic Fracture Mechanics solutions [4,20,21]. Recently, Huffman [1] suggested new developments related with the fatigue crack propagation modelling using strain energy density-based model. Fatigue evaluation of notched details based on unified local probabilistic approaches was also proposed by Huffman [22] considering the Walker-like strain-life relation in conjunction with the probabilistic model proposed by Castillo and Fernández-Canteli [23]. In present research, the Huffman crack propagation model is applied to the P355NL1 pressure vessel steel and a comparison with experimental results is made. O VERVIEW OF LOCAL STRESS / STRAIN APPROACHES atigue crack growth modelling like the Paris law relation, uses Linear-Elastic Fracture Mechanics (LEFM) to describe cracking driving forces. Local approaches, however, explicitly consider stresses and strains near the crack tip, and associate those with the stress intensity factors. Using the assumption that the stresses or strains near the crack tip are related to fatigue crack growth in the same way that global stresses and strains are related to stress-life and strain-life, an association can be made between damage parameters, such as the SWT damage parameter [16] or the damage parameter introduced by Huffman [1] and the fatigue crack growth. F F