Abstract
Abstract Cyclic softening and hardening processes are expressed by the change in the stress amplitude necessary to cause a given strain amplitude. Understanding the cyclic stress-strain behavior of materials is an important step in the complex study of their fatigue behavior. The potential differences between the defect structures of cold-formed materials may also be related to internal stress changes. Considering that wiredrawing and rotary swaging apply distinct forces on the material in order to obtain the product, differences may arise in the defect structures that can, consequently, affect its mechanical behavior. This study aims at evaluating and comparing, by means of strain-controlled fatigue tests, the cyclic behavior of polycrystalline electrolytic copper cold-formed by wiredrawing (WD) and rotary swaging (RS) with 87% area reduction. The fatigue test results evidenced the highest resistance to cyclic deformation presented by WD material in the low cycle regime. It was observed that the strain hardening for both cold forming conditions is related to a great increase of long-range stresses in the defect structure and the cyclic softening is related, mainly, to the subsequent drop of that stresses. The WD internal stresses resulted slightly bigger than those of the RS condition.
Highlights
Understanding the cyclic stressstrain behavior of materials is an important step in the complex study of their fatigue behavior
It is known that the fatigue softening caused by axial tension and compression occurs mainly in FCC metals, like copper, whose stacking fault energy is high enough to allow cross-slip
The dislocations rearrange by cross-slipping during cyclic loading and form a subgrain structure so plastic flow can occur at lower stresses within the microstructure than in the prior work hardened condition
Summary
Understanding the cyclic stressstrain behavior of materials is an important step in the complex study of their fatigue behavior. The study of cyclic plasticity includes investigations on the dislocation structures and the stressstrain relationships. Depending on the component’s initial state, the material might suffer cyclic hardening or softening, or even keep a steady state regime. It is not uncommon to observe distinct behavior characteristics in the same material, depending on the initial conditions and the cyclic loading parameters (Carstensen, 1998; Howard et al, 2017). Fatigue softening is the reduction of tensile strength in cold worked metals caused by cyclic loading and understood in terms of the internal structure of the metal
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