Torsional Fatigue Cracking and Fracture Behaviors of Cold-Drawn Copper: Effects of Microstructure and Axial Stress

doi:10.1007/s40195-019-00965-5

Abstract

Abstract:

The fatigue cracking and fracture behavior of cold-drawn copper subjected to cyclic torsional loading were investigated in this study. It was found that with increasing stress amplitude, the fracture mode of cold-drawn copper gradually changes from a shear fracture on transverse maximum shear stress plane to a mixed shear mode on both transverse and longitudinal shear planes and finally turns to the shear fracture on multiple longitudinal shear planes. Combining the cracking morphology and the relationship between torsional fatigue cracking and the grain boundaries, the fracture mechanism of cold-drawn copper under cyclic torsional loading was analyzed and proposed by considering the effects of the microstructure and axial stress caused by torsion. Because of the promotion of the grain boundary distribution on longitudinal crack propagation and the inhibition of axial stress on transverse crack grown, the tendency of crack propagation along the longitudinal direction increases with increasing stress levels.

Key words: Torsion, Fatigue behavior, Crack propagation, Fracture mechanisms, Axial stress, Copper

Rong-Hua Li, Peng Zhang, Zhe-Feng Zhang. Torsional Fatigue Cracking and Fracture Behaviors of Cold-Drawn Copper: Effects of Microstructure and Axial Stress[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(12): 1521-1529.

Figures/Tables 7

Fig. 1 a Microstructure, b specimen geometry of cold-drawn copper used to fatigue test

Table 1 Tensile and torsional properties of cold-drawn copper

Tension			Torsion
Yield stress, σ_s (MPa)	Uniform elongation, ε_u (%)	Strain to fracture, ε_f (%)	Yield stress, τ_s (MPa)
260?±?1	17.6?±?1.93	41.7?±?2.52	101?±?1

Fig. 2 Surface damage morphologies near the fracture surfaces of cold-drawn copper after torsional fatigue: a, b 75 MPa, c, d 90 MPa, e, f 100 MPa, g, h 150 MPa

Fig. 3 Fracture morphologies of cold-drawn copper after torsional fatigue tests: a-c 75 MPa, d 90 MPa, e 100 MPa, f 150 MPa. b, c show the typical morphology of propagation zone and rapid fracture zone, respectively; the arrow in d denotes propagation direction of longitudinal cracks

Fig. 4 Relationship between torsional fatigue crack and grain boundary of cold-drawn copper at the stress amplitude of 100 MPa: a, b transverse crack, c longitudinal crack, d microstructure on the longitudinal section of specimen

Fig. 5 Relationship between axial stress and time when cold-drawn copper specimen was cycled to half of its total life at different stress amplitudes: a 75 MPa, b 90 MPa, c 100 MPa, d 150 MPa

Fig. 6 Summary of crack propagation and fracture morphologies of cold-drawn copper under cyclic torsional loading: a 75 MPa, b 90 MPa, c 100 MPa, d 150 MPa. (The line segments with arrows denote the width of concentrated damage areas)

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