Issue 35

G. Gobbi et alii, Frattura ed Integrità Strutturale, 35 (2016) 260-270; DOI: 10.3221/IGF-ESIS.35.30 262 implementation of 2D plane stress and plane strain models are proposed. Hydrogen concentration estimated at the crack tip is discussed. C OHESIVE ZONE MODEL SETUP tarting from experimental results obtained by toughness tests on AISI 4130 steel in hydrogen uncharged and charged conditions [17], a finite element cohesive zone model is built to reproduce the mechanical response of the material. The model, developed using Abaqus software, simulates a 2D C(T) specimen. Exploiting the symmetry of the geometry, only half part of the specimen is considered and a line of cohesive elements is implemented all over the crack propagation path. These zero thickness elements are connected to the continuum elements on the symmetry plane. In this region, the size of the mesh reaches the smallest dimension, around 10 µm, to ensure proper resolution of results. This mesh size is the result of a convergence study. Mechanical properties of continuum elements are assigned according to the data obtained by experimental tensile tests carried out on AISI 4130 steel. Fig. 1 depicts the stress-displacement curve of the base material together with the specific values of Young’s modulus, Poisson’s ratio and yielding stress. Instead, peculiarity of cohesive zone model is the definition of a phenomenological law, identified as traction separation law (TSL) describing the constitutive behavior of the material inside the cohesive elements. The cohesive behavior is expressed by a stress-displacement function that can present different shapes according to the crack propagation response of the material. Usually, for steels or metals the most noted are Gaussian or trapezoidal shapes in which it is possible to split elastic, plastic and failure parts. For the current case, the TSL presents a trapezoidal shape defined by the four parameters shown in Fig. 1: σ 0, δ 0 , δ N , δ F . Figure 1 : Experimental tensile curve and values of mechanical parameters used for continuum elements. TSL shape as stress- displacement trend. Contrary to other authors that customized the user defined elements (UEL) subroutine to define the traction separation law of cohesive elements [18, 19], this work presents an easier approach. In fact, the TSL is designed exploiting the traction-separation model available in Abaqus, which assumes an initial linear elastic behavior followed by an initiation and evolution of damage. S