PSI - Issue 2_B

V Shlyannikov / Procedia Structural Integrity 2 (2016) 744–752

745

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Author name / Structural Integrity Procedia 00 (2016) 000–000

the stress-strain state changes between plane stress and plane strain. From a practical point of view, the most useful approach for assessing the fracture resistance of materials, components and structures would involve one common parameter, which, unlike the two parameter models and the higher order term solutions, would preserve the one term representation. Nevertheless, the basic parameters of the model must be modified such that they are able to take into account both the in-plane and out-of-plane constraint effects. In this paper, the new fracture resistance parameters of materials and structures in the form of the plastic and creep stress intensity factors is employed to generalization and solution actual fracture mechanics problems. It is further demonstrated that the nonlinear stress intensity factors accounting for the in-plane and out-of-plane constraint effects can be used to characterize the static and cyclic fracture resistance as well as structural integrity as a unified single parameter for a variety of cracked body configurations and loading conditions. 2. Plastic stress intensity factor The plastic stress intensity factor Kp in pure Mode I can be expressed directly in terms of the corresponding elastic stress intensity factor as follows: where K K w / 1 1  is elastic SIF normalized by a characteristic size of cracked body,  and n are the hardening parameters, λ= a/w is the dimensionless crack length, w is specimen width, σ is the nominal stress, and σ 0 is the yield stress, I n is governing parameter for 3D-fields of the stresses and strains at the crack tip. Shlyannikov and Tumanov (2014,a) suggested the procedure for calculating the governing parameter of the elastic–plastic stress–strain fields in the form of I n for the different specimen geometries by means of the elastic–plastic FE-analysis of the near crack-tip stress-strain fields. In this study, the numerical integral of the crack tip field I n changes not only with the strain hardening exponent n but also with the relative crack length c/w and the relative crack depth a/t .                                                      d u u d u du d u du n n I n c w a t FEM FEM r FEM r FEM rr FEM FEM r FEM r FEM r FEM FEM rr n FEM e FEM n cos . ~ ~ ~ ~ 1 sin ~ ~ ~ ~ ~ ~ cos ~ 1 ( , ,( / ),( / )) 1 (2) The subject for the experimental study in this part of the work is carbon steel 34ХН3МА, whose main mechanical properties are listed in Table 1. The single-edge-notched bend (SENB) and compact (CT) specimens were used in both experimental studies and numerical analyses. In addition to the standard ASTM thickness-to width ratio B/W = 0.5, SENB specimens with B/W = 1.0 were prepared. The relative crack length a/W after inserting a fatigue precrack varied in the range of 0.3-0.62. The three type of CT specimens with the ratio B/W = 0.1, 0.2, and 0.4 were chosen with the relative crack length a/W change of 0.35-0.645 after precracking. Several sets of specimens of both configurations, which include the change of the in-plane constraint, the out-of-plane constraint and both of them, were investigated. It is found that elastic stress intensity factor K max varied in the range of 52.15 68.12 MPa  m and 59.49-77.46 MPa  m for SENB and CS, respectively. Table 1. Main mechanical properties of Steel 34ХН3МА Properties Static Cyclic  0 MPa  b MPa  %  u MPa E GPa n α  ' f MPa  ' f n' 34ХН3МА 790 992 56 1455 196.4 7.49 2.39 1488 0.74 6.67     1/ 1 n      1/ 1 n  ( / ) Y a w ; , ,( / ) ( / ) n a w  ( / ) a w Y a w  , ,( / ) n a w  1 1 2 1 2    0  0  2 1 K I I K K FEM n FEM n p                            (1)                  n 1