PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 718–723 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000 – 000

722

5

10 -6

10 -6

Hot rolled bar Cold drawn wire

Hot rolled bar Cold drawn wire

10 -7

10 -7

da/dN (m/cycle)

da/dN (m/cycle)

10 -8

10 -8

10

20

30

40 50

10

20

30

40 50

 K (MPam 1/2 )  K (MPam 1/2 ) Fig. 5. Conventional Paris Laws (CPLs; left) and Actual Paris Laws (APLs, right) for both materials. Table 2 shows the Paris coefficients C and m (CPL), together with C* and m * (APL) for both materials. Table 2. Conventional and actual Paris coefficients of the materials (units for d a /d N in m/cycle and Δ K in MPam 1/2 ). ͵Ǥʹ Fig. 5 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. An additional reason for the improvement of fatigue performance with cold drawing is the blocking of dislocational movement (a basis for plastic crack advance) produced by the cementite lamellae, they being oriented with a higher angle in the cold drawn material and thus increasing the blocking effect and retarding the crack growth rate, because the cementite lamellae act as barriers or obstacles for dislocation movements, thereby retarding plastic crack advance by fatigue in the steel. 8. 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 path 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. (ii) 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 ). Steel C 5.3·10 Ǧͳʹ 4.1·10 Ǧͳʹ m ͵ǤͲ ͵ǤͲ C 3.3·10 Ǧͳʹ 3.2·10 Ǧͳʹ m ͵Ǥʹ Hot rolled bar Cold drawn wire