EFFECTS OF ACCUMULATED TORSIONAL STRAIN AND DAMAGE ON THE MAGNETIC PROPERTIES OF STEEL

Transcription

1 EFFECTS OF ACCUMULATED TORSIONAL STRAIN AND DAMAGE ON THE MAGNETIC PROPERTIES OF STEEL E.S. Gorkunov, S.V. Smirnov, D.I. Vychuzhanin, S.Yu. Mitropolskaya The Urals Branch o the Russian Academy o Sciences, Yekaterinurg, Russia. Astract: The eect o shear strain and accumulated damage on magnetic properties was studied on cylindrical steel rods sujected to torsion. The correlations otained make it possile to evaluate the shear strain and current damage o metal, as well as to estimate the residual lietime o an article under torsion, provided its magnetic parameters are availale. Introduction: It is known that during plastic deormation, particularly in course o torsion, deects o metal continuity are generated (e.g. micropores, microcracks, vacancies, etc). The deects are oserved as early as at early stages o plastic deormation. The ormation o discontinuities is accompanied y partial relaxation o elastic energy [1], which leads to changes in the magnetoelastic energy o erromagnetic material in the regions adjacent to the deects. This phenomenon is likely to aect the magnetic parameters o materials. Results: The inluence o shear strain rate and torsion-induced damage on the magnetic parameters o metals was investigated. Analytical relationships were otained or magnetic coercivity (rom oth major and minor hysteresis loops) as a unction o strain and damage accumulated y a material. Commercial steel with 0.45% caron content was studied. Measurements were made on hot-rolled rods 5mm in diameter. The test unit utilized made it possile to take magnetic parameters in the run o loading. Magnetic properties were determined rom oth major and minor cycles o magnetic hysteresis. As distinct rom [2], all the magnetic properties were measured as a unction o internal magnetic ield. Measurements were made o the ollowing magnetic parameters: maximum magnetic permeaility µ max, coercivity H c (h c ) and residual induction B r ( r ) rom major (Н max = 60 ka/m) and minor hysteresis loops during magnetization in medium ( max =0.4 T) and low ( max =0.1 T) magnetic ields. The degree o shear strain Λ under torsion was estimated under the assumption that the sample cross-section radius is not sujected to distortion in course o loading [3], thus the shear strain degree on the surace o the sample is Λ = tgφ, (1) where φ is the angle etween the printed mark on the sample surace and its generating line. The average degree o shear strain along the cross-section o the sample was calculated as [3] 2π R ~ 1 r 2 Λ = tgϕ r dϕ dr = Λ, (2) 2 πr 0 0R 3 where R is the radius o the sample, r is its current radius ranging rom 0 to R. The average integral degree o accumulated shear strain Λ int was calculated y taking the summation over all the previous values o the strain degree. The parameter o material damage ω is used in mechanics to indicate the development o deormational microdeects. According to the phenomenological theory o racture [4], ω = 0 eore deormation, whereas ω = 1 when a racture crack emerges. Since the state o strain was not sujected to changes in course o deormation, the parameter o material damage ω was calculated in accordance with the linear model y Kolmogorov or damage accumulation: [3] as Λ dλ Λ 0 ω =, (3) where Λ is the degree o shear strain at the moment o racture. As the strain degree rises, the magnetic hardness o steel increases, the coercive orce and residual induction grow, while magnetic permeaility decreases (Fig. 1 to 3).

2 a c d e a Fig. 1

3 c d e Fig. 2 a

4 Fig. 3 This is indicative o hampered magnetization and magnetization reversal due to higher values o the critical ields o interaction etween the domain oundaries and the deects in the metal structure. Discussion: A numer o actors inluence magnetization reversal. A notale change in the magnetic characteristics occurs as early as at small values o strain degree. According to the model representations o the eect o dislocations on magnetization reversal, the coercive orce is associated with the dislocation density as H c N. Thereore the coercive orce increases as the dislocation density grows at the initial stage o deormation. The urther increase in the values o magnetic characteristics results rom the evolution o the dislocation and domain magnetic structures, which is associated with the ormation o dislocation walls, as they are the eective areas or domain oundary pinning. Near the inclusions and in the regions o high microstress gradients there appears a disperse structure o 90 o magnetic domains, which is typiied y critical ields approximately 1.5 times as great as those or magnetic structures with 180 o domains. As this occurs, these regions may e quite large in volumes. Figures 1 to 3 show magnetic parameters as a unction o strain degree. It is ovious that magnetic parameters measured under various magnetic ield strengths make it possile to estimate the degree o plastic strain, as well as material damage associated with accumulation o microdeects. The results otained reveal analytical relationships or magnetic characteristics as a unction o the calculated parameter o material damage: 0,384 h = 0,04 + 0,0917 Λ 0, 0055 с 0,1 h с 0,4 0,485 = 0, ,2 Λ + 0, 009 0,41 Н = 0,24 + 0,35 Λ + 0, 09 (4) с r 0,1 r 0,4 0,177 = 0, ,0107 Λ 0, ,593 = 0,294 0,039 Λ + 0, ,33 B r = 1,025 0,3 Λ + 0, 124 0,33 µ max = ,7 Λ + 215, 962. Conclusions: The result o the study perormed demonstrates that it is possile to monitor torsioninduced shear strain and the deormation-related metal damage y means o magnetic

5 characteristics measured on oth minor and major loops o magnetic hysteresis. Knowing the parameters Λ and ω, one can calculate the remaining lietime o an article using corresponding mechanical models. This work has een conducted with the inancial support rom the RFFR, grants and Reerences: 1. Low, J.R. Brittle racture as related to microstructure. In: The Structure o Metals and Properties. M., National Science and Technology Pulisher or Literature on Ferrous and Non- Ferrous Metallurgy. 1957, p Gorkunov, E.S., Smirnov, S.V., Rodionova, S.S. The eect o plastic deormation under hydrostatic pressure on the damage and magnetic characteristics o low-caron 3sp steel. Physical mezomechanics, 2003, vol. 6, N5, p Kolmogorov, V. L. Stress, strain, racture. M., Metallurgia, 1970, 230 p. 4. Kolmogorov, V. L. Mechanics o metal orming. M., Metallurgia, 1986, 688 p. Captions Fig. 1. Coercivity measured on minor (a, : at max = 0,1 Т; c, d: at max = 0,4 Т) and major (e, ) cycles o magnetic hysteresis as a unction o shear strain degree and the parameter o damage or 0.45% C steel samples. Fig. 2. Residual induction measured on minor (a, : at max = 0,1 Т; c, d: at max = 0,4 Т) and major (e, ) cycles o magnetic hysteresis as a unction o shear strain degree and the parameter o damage or 0.45% C steel samples. Fig. 3. Maximum magnetic permeaility as a unction o shear strain degree and the parameter o damage or 0.45% C steel samples.