The influence of the degree of deformation on the discontinuity of the steel-copper wire during stretching and torsion

Introduction.

In world practice, more and more attention is paid to the use of products made of composite materials. A special case is bimetallic steel-copper wire, as well as wire with numerous coatings. Bimetallic wire is widely used in the electrical industry and is subjected to high tensile and torsion stresses, in particular, when connecting its ends by twisting, which is accompanied by its large plastic deformations [1, 2]. The production of bimetallic wire requires compliance with technical specifications for the thickness and quality of coatings. It is known that for each design of a bimetallic wire there are critical drawings (minimum and maximum), beyond which the uniform plastic deformation of the coating and the core is violated during drawing [3].

Currently, there are practically no data on the assessment of the critical degree of plastic deformation, which causes a violation of the continuity of the steel-copper wire, in particular, delamination of the shell and core.

Statement of the problem and research methods.

In this work, the task is set to determine the degree of plastic deformation, at which delamination of the core and sheath of steel-copper wire occurs, which worsens its operational properties [4, 5, 6].

Samples of steel-copper wire with a diameter of 6 mm were subjected to deformation, stretching on a tensile testing machine MM-10 and torsion on an installation equipped with grippers.

The torsion test of the samples was carried out in 2 series: 1 — with monotonic loading of the sample until destruction and delamination of its components and 2 — step loading of the samples with a loading break of 24 hours to a similar state in 4 stages.

Results.

It has been established that delamination of the core and shell during tension occurs if the degree of plastic deformation exceeds 12–18%, and during torsion, the twisting angle exceeds 1.36–1.39 radians. Large values ​​of the angle of twist correspond to the total stepped loading of the samples and is associated with the phenomenon of stress relaxation of the wire components during the daily break time.

The greatest stratification is observed in the zones adjacent to the areas of its destruction. For example, when twisting, cracks are observed, up to several tenths of a millimeter wide and up to 4 mm long. More remote zones from the fracture site are characterized by contact between the components without discontinuity. Some areas were characterized by a shell with cracks perpendicular to the contact surface of the components.

Findings.

            Critical values ​​of the degree of plastic deformation during tension (exceeds 12-18%) and twisting (twisting angle exceeds 1.36 — 1.39 radians) of steel-copper wire, which cause discontinuity, are established. The results obtained are recommended to be used in the design of technological operations associated with plastic deformation of steel-copper wire during its tension and twisting.