Issue 36

L. C. H. Ricardo et alii, Frattura ed Integrità Strutturale, 36 (2016) 201-215; DOI: 10.3221/IGF-ESIS.36.20 207   K eff and others conditions being nominally similar, it is possible that other crack tip aspects also affect crack growth, such as crack tip blunting and strain hardening, Schijve 45 . Figure 5 : Definitions of K Values, Schijve [41]. Newman and Armen [42-44] and Ohji et al. [45] were the first to conduct the two dimensional analysis of the crack growth process. Their results under plane stress conditions were in quantitative agreement with experimental results by Elber [21], and showed that crack opening stresses were a function of R ratio (S min /S max ) and the stress level (S max /  0 ), where  0 is the flow stress i.e: the average between  ys and  u (ultimate stress). Blom and Holm [46] and Fleck and Newman [47-48] studied crack growth and closure under plane-strain conditions and found that cracks did close but the cracks opening levels were much lower than those under plane stress conditions considering same loading condition. Sehitoglu et al. [49] found later that the residual plastic deformations cause the crack closing. McClung [50-52] performed extensive finite element crack closure calculations on small cracks at holes, and various fatigue crack growth models. Solanski et.al [53] found that S max /  0 could correlate the crack opening stresses for different flow stresses (  0 ). This average value was used as stress level in the plastic zone for the middle crack tension specimen McClung [52] found that K analogy, using K max /K 0 could correlate the crack opening stresses for different crack configurations for small scale yielding conditions where K 0 =  o  (  a) . ( K -analogy assumes that the stress-intensity factor controls the development of closure and crack-opening stresses, and that by matching the K solution among different cracked specimens, an estimate can be made for the crack opening stresses.) D ESCRIPTION OF THE MODEL compact tension specimen was modeled using a finite element code, MSC/Patran, r1 [54] and ABAQUS Version 68 [55] 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. 7, and the boundary conditions are presented in Fig. 6. The finite element model 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. 7 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. A