PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2396–2403 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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This APL takes the form:

d a* /d N = C* ∆ K m*

(2)

where C* and m* are the actual Paris coefficients of the material given in Table 3. Fig. 8 shows how the cold drawing manufacture process is beneficial from the fracture mechanics viewpoint, so the improvement of fatigue performance can be attributed to the increase of the actual, physical or real fatigue propagation length in the cold drawn steel (associated with the corresponding increase of micro-roughness after cold drawing, with shorter and more angled micro-deflections). It is seen that the APLs in both materials become closer when considering the real fatigue crack advance, and are plotted more separated when CPLs are represented. 5. Conclusions On the basis of the micro- and macro-approach to the phenomenon of propagation of fatigue cracks in pearlite, both randomly-oriented or non-oriented (hot rolled pearlitic steel bar) and oriented (cold drawn pearlitic steel wire) , the following conclusions can be drawn: (i) Fatigue crack propagation in pearlitic steel takes place as a consequence of micro-plastic tearing. The cold drawn wire exhibits a pattern resembling micro-tearing, these events being of lower size and more curved aspect than those associated with the hot rolled bar. (ii) Fatigue cracks are trans-colonial and trans-lamellar in both steels. As a matter of fact, fatigue crack propagation can be classified as tortuous, with certain quantity of micro-discontinuities, branchings (frequently bifurcations also appear) as well as local deflections. (iii) Fatigue fracture in the cold drawn pearlitic wire exhibits an appearance consisting of micro-roughness. The total fractured surface is greater than in the hot rolled bar (base material). The increase of the stress intensity factor (SIF) range, ∆ K , also produces higher micro-roughness in the fracture surface. (iv) Two laws of fatigue crack growth can be evaluated in the materials: the Conventional Paris Law (CPL) for transverse crack advance in global mode I and the Actual Paris Law (APL) for inclined crack advance in local mixed-mode, with micro-crack deflections in the fatigue crack path ( locally multiaxial fatigue crack growth ). Acknowledgements The authors wish to kindly acknowledge the financial support provided by the following Spanish Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002-01831), Ministry for Education and Science (MEC; Grant BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810), Ministry for Economy and Competitiveness (MINECO; Grant BIA2011-27870), Junta de Castilla y León (JCyL; Grants SA067A05, SA111A07 and SA039A08) and Fundación Samuel Solórzano Barruso. References Costa, J.E., Thompson, A.W., 1982. Hydrogen Cracking in Nominally Pearlitic 1045 Steel. Metallurgical Transactions 13A, 1315-1318. Kitagawa, H., Yuuki, R., Ohira, T., 1975. Crack-Morphological Aspects in Fracture Mechanics, Engineering Fracture Mechanics 7, 515–529. Korda, AA., Mutoh, Y., Miyashita, Y., Sadasue, T., 2006a. Effects of Pearlite Morphology and Specimen Thickness on Fatigue Crack Growth Resistance in Ferritic-Pearlitic Steels. Materials Science and Engineering A 428, 262–269. Korda, AA., Mutoh, Y., Miyashita, Y., Sadasue, T., Mannan, SL., 2006b. In situ Observation of Fatigue Crack Retardation in Banded Ferrite Pearlite Microstructure due to Crack Branching, Scripta Materialia 54, 1835–1840. Mutoh, Y., Korda, AA., Miyashita, Y., Sadasue, T., 2007. Stress Shielding and Fatigue Crack Growth Resistance in Ferritic-Pearlitic Steel, Materials Science and Engineering A 468–470, 114–119. Noroozi, AH., Glinka, G., Lambert, S., 2008. Prediction of Fatigue Crack Growth Under Constant Amplitude Loading and a Single Overload Based on Elasto-Plastic Crack Tip Stresses and Strains, Engineering Fracture Mechanics 75, 188–206. Sadanada, K. Vasudevan, AK., 2004. Crack Tip Driving Forces and Crack Growth Representation Under Fatigue, International Journal of Fatigue 16, 39-47. Stoychev, S., Kujawski, D., 2005. Analysis of Crack Propagation Using ∆ K and K max , Engineering Fracture Mechanics 27, 1425-1431. Suresh, S., 1983. Crack Deflection: Implications for the Growth of Long and Short Fatigue Cracks, Metallurgical and Materials Transactions 14A, 2375–2385.