PSI - Issue 68

Mihaela Iordachescu et al. / Procedia Structural Integrity 68 (2025) 1147–1152 Iordachescu M. et al. / Structural Integrity Procedia 00 (2025) 000–000

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The measured crack growth rate da/dN is the mean value resulting from dividing the crack depth increment during each loading step by the corresponding number of fatigue cycles, whereas the related stress intensity range ∆K is obtained by particularising Eq (2) for the mean values of the elliptic crack semi-axes measured at the initial and final crack fronts during each loading step. 3.2. Damage tolerance The results of the fracture tests performed by tensile loading the cracked cylindrical specimens after being fatigue tested were analysed by means of the engineering diagram given in Fig. 5, where failure load was plotted vs. the cracked area, both as respective fractions of the bearing load capacity and the cross section of the uncracked specimen. The diagram includes the theoretical limits of the failure processes respectively controlled by the yielding capacity and toughness of the steel. These indicate the areas respectively enclosed by the solid lines 1, 2 and the area 3 marked in light red, that correspond to: 1 - plastic collapse under combined tension and bending loading; 2 – plastic collapse in simple tension and 3 – toughness controlled collapse. The third area predicts the collapse of specimens for the envelope of the crack shapes determined in the fatigue tests for the toughness value of 82 MPa√m that best fits the experimental data, whereas the first two respectively predict the plastic collapse of the un-cracked ligament in pure tension and the plastic collapse in tensile-bending. The positions occupied in the diagram by the experimentally obtained failure data with respect to these areas indicate that toughness is not the cause of an unexpected failure before plastic collapse. Therefore, the experimental results show that the toughness and ductility with which the bar steel is endowed are high enough to prevent a premature failure even in the presence of severe cracks.

Fig. 5. Damage tolerance diagram with the experimental data and the theoretical failure limits.

4. Conclusions According to this research, the studied high-strength lath martensite steel bars manufactured for structural uses combine toughness and ductility in the due proportion to prevent premature failures even in the presence of severe stress concentrators. The fatigue behaviour of the bar steel follows the Paris law with constants that denote a fatigue resistance analogous to ferrite-perlite structural steel and significantly higher than the martensitic ones.