Issue 41

A. S. Cruces et alii, Frattura ed Integrità Strutturale, 41 (2017) 54-61; DOI: 10.3221/IGF-ESIS.41.08 56 The cyclic elastic-plastic material properties were obtained from the cyclic uniaxial tests. The properties obtained from the cyclic uniaxial tests have been listed in Tab. 2 & Tab. 3. ASTM standard was followed to conduct these tests. Cyclic strength coefficient, K’ 630.6 MPa Cyclic hardening exponent, n’ 0.10850 Cyclic yield strength, σ’ y 321.3 MPa Fatigue strength coefficient, σ’ f 564.4 MPa Fatigue strength exponent, b -0.0576 Fatigue ductility coefficient, ɛ ’ f 0.1554 Fatigue ductility exponent, c -0.4658 Table 2 : Monotonic properties of St52-3N steel. Cyclic strength coefficient, K’ ɤ 593.8 MPa Cyclic hardening exponent, n’  ɤ 0.1553 Cyclic yield strength, τ’ y 594.2 MPa Fatigue strength coefficient, τ’ f 486.9 MPa Fatigue strength exponent, b  ɤ -0.0668 Fatigue ductility coefficient, ɤ ’ f 0.0662 Fatigue ductility exponent, c ɤ -0.3191 Table 3 : Torsional properties of St52-3N steel. Biaxial tests require both torsional and longitudinal extensometer. It also needs a technician with good skill to place four pins of these devices [24]. As per the ASTM, load was also recommended to be applied at relatively low frequency (normally less than 4 Hz) because the weak connection between extensometer pins and the sample is preserved. A series of in-phase biaxial tests were conducted with the strain levels that were chosen with an intension to fail the samples between 10 4 and 10 6 . The design of these bi-axial fatigue tests was done on the basis of the previous uniaxial test results. Tab. 4 shows the applied axial strain amplitudes ( ɛ a ), and shear strain amplitudes ( ɤ a ) data for these tests. The corresponding axial stress amplitudes (σ a ) and shear stress amplitudes (τ a ) are also shown in Tab. 4 with corresponding fatigue life. Total of 13 samples were tested for this test program. All the load path for these bi-axial tests were kept proportional (Tab. 4). C RITICAL PLANE APPROACHES Fatemi-Socie damage parameter II (FSDP-II) atemi-Socie has recently modified their earlier developed fatigue model [25]. The previous Fatemi-Socie model was based on the concept that the range of shear strain is primary reason to initiate the fatigue crack and the tensile normal stress creates secondary effects of opening the crack face. Fatemi-Socie also introduced the normal stress component to account for the rotation of principle plane axis at the critical plane. Fatemi-Socie have recently modified their damage parameter and introduced a shear stress range term to account for the interaction of normal and shear stress at the critical plane. Fatemi & Socie model resorts on the ratio of maximum normal stress and the range of shear stress to model the interaction. A material dependent parameter k has also been introduced in the parameter. Modified Fatemi- Socie fatigue damage parameter has been shown in Eq. 1 [18].                       , 1 2 ' 2 2 Δ b c f n max max f f f k N N G (1) F