Issue 43

L.C.H. Ricardo, Frattura ed Integrità Strutturale, 43 (2018) 57-78; DOI: 10.3221/IGF-ESIS.43.04 70 crack closure analysis or propagation. Matos & Nowell [66] present a literature review of the phenomenon of plasticity- induced fatigue crack closure under plane strain conditions and mention that there are controversial topics concerning the mechanics of crack propagation. In general there is no consensus in the scientific community. Fleck [67] used finite elements to simulate plasticity induced crack closure under plane strain conditions and predicted that the nature of the closure process changes from continuous to discontinuous after a sufficient increment of crack growth. He suggested that closure involves only a few elements relatively distant from the current crack tip and the closure levels decay steadily as the crack grows beyond its initial length. In the limit, the closure would not occur at all. In Singh et al. [67] the authors provide a review of some crack propagation issues. The paper covers the transients and single overload effects as well as the plasticity induced crack closure. In this topic Singh et al [68] presented a discussion regarding how the researchers normally work in crack propagation simulation considering overload-induced crack closure. Lei [69] determine the crack closure by finite element method in a compact specimen. In the work Lei [69] use ABAQUS [70] to perform the crack propagation simulation using the crack face method was good agreement with experimental data. D ESCRIPTION OF MODEL compact tension specimen was modeled using a finite element code, MSC/Patran, r1 [71] and ABAQUS Version 68 [70] used as solver. Half of the specimen was modeled and symmetry conditions applied. A plane stress constraint is modeled by the finite element method covering the effects in two dimensional (2D) small scale yielding models of fatigue crack growth under variable spectrum loading,Fig.10, and the boundary conditions are presented in Fig. 11. The finite element models has triangle and quadrilateral elements with quadratic formulation and spring elements, SPRING1, used to node release in crack surface (this element works only in the y direction). Fatigue Design & Evaluation (FD&E) committee from SAE (Society of Automotive Engineers) has standard fatigue files. The present work used a standard suspension load history. Fig. 10 presents a modified load history, adapted from the FD&E/SAE histogram considering only tractive loads. The maximum load used was scaled to produce a K max  0.6 K IC , using eq. (4.1), where K IC is the critical stress intensity factor of adopted material in the present study. With the value of K max from K IC computed as mentioned above is computed the maximum load using Eq. 3.1 to be applied in the specimens as explained in next.In the analysis, fatigue crack growth is simulated by releasing the crack tip node at P min , followed by a single loading cycle P min  P max  P min, Fig. 10. The force is divided in nine steps between loads P min - P max and nine steps between the P max -P min ,in each cycle. The smallest element size, 0.025 mm, was estimated based on the plastic zone size ( r p ) ahead of the crack tip and computed by eq. (4.2). Only the first 20 reverses from load history shown in Fig. 7 were used to identify crack opening/closing and retardation effects. Figure 10: FD&E SAE Suspension Modified Load History. A