PSI - Issue 13

5

Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 554–559 M. Iordachescu, A. Valiente, E. Scutelnicu/ Structural Integrity Procedia 00 (2018) 000–000

558





 F  1

 3  2 3  4    a 2  2  a  3

(2)

3 WB  Y 2 3

Fig. 3. a) Sketch of the plane stress plastic collapse model for precracked CT specimens; b) Ideal yield strength, σ Y vs. plastic component of COD, δ p /W in the fracture tests of CTs containing NW, RW and BM; (300 MPa and 560 MPa are the BM yield and tensile strengths, respectively); c) Moment-rotation diagrams of BM, NW and RW The numerical values that result from Eq. (2) substantially coincide with those given by Kumar et al. (1991) for the limit load of the CT specimen under plane stress. The plastic component of COD (relative to W) provides the rotation angle Θ from the mechanism of plastic collapse:     p /(  x   a ) (3) Eq. (1) and Eq. (3) permit to obtain the rotation angle Θ of the tested CTs to be obtained from the crack sizes given by the elastic unloadings made in the fracture test. The moment of the load, F, at the rotation center of CT is:  M  FW(  a   x ) (4) The pairs of values M and Θ resulting by particularizing the Eq. (1), Eq. (3) and Eq. (4) with the experimentally obtained values F, and   a provide the empirical moment – rotation diagram that characterizes the plastic hinge behavior of the tested CT specimens. The strength and the ductility of the resistant ligaments corresponding to BM or HAZs, of NW or RW are the determinant factor of the diagrams. The adequacy of the model can be assessed by checking the consistency between the yield and tensile strength of each tested material given in Fig. 2c and the values of the ideal yield strength σ Y , of Eq. (2). These were derived from Eq. (2) for each pair of values F and   a measured in the CT tests, once the plastic yielding of the ligament initiates. Fig. 3b shows the obtained values of ideal σ Y plotted against the plastic component of COD, also measured in the CT tests. For the BM, whose stress-strain curve is the only one fully known, the deviation between the two performed tests is significant; in both cases σ Y increases with the deformation, although all the recorded values remain between 300 MPa (yield strength) and 560 MPa (tensile strength, in true value). Therefore, in BM case, the variation of σ Y is in agreement with the strain hardening capacity shown by this material. In the NW, the σ Y values significantly differ in the two fracture tests, but hardly vary throughout the same test, once crack growth initiates (shown by the filled circles in Fig. 3b). A unique value of σ Y higher than 560 MPa is fully consistent with the plastic collapse model and does not contradict the strain hardening capacity of the NW HAZ. This is due to the material heterogeneity in the cylindrical specimen containing NW, which only provides the lower bounds of HAZ characteristics when tension tested.    p    p    p