Modeling Of Deformation And Failure In Wire Drawing And Its Applications

As an important structural material, PC strands have widely used in modern construction, like skyscraper, bridge, and so on. High performance and low broken frequency in drawing (BFD) are basic requirement of high-carbon wire for PC strand to ensure the uninterrupted and highly efficient producing process. High-carbon steel wire depended on importing because of homemade wire with a high BFD. Therefore, the drawing process is investigated in details in this thesis, and the initiation and propagation of flaws during wire drawing is modeled and numerically analyzed with the finite element method and theory of fracture mechanics.Based on the information on status and failure in wire drawing collected from the product line, the assumption of single crack in the center of wires or with eccentricity is made, and two modes of finite element models are proposed in the thesis, that is, central flaw model and eccentricity flaw model.The numerical analysis is carried out from two sides of mechanical behavior in wire drawing. Firstly, the stress field in formation zone and stress singularity on the tip or around the front of the crack can be displayed with stress contour directly and show the possible trend of crack propagation. Secondly, quantitative analysis on the deformation and failure of drawing process in carried out, in which fracture parameter J-integral is used and therefore the J-integral criterion is introduced.An axisymmetrical numerical model is proposed on the basis of the assumption of the central axisymmetrical flaw, and the effects of parameters (die angle and friction coefficient etc.) on the stress field near the crack tip during the wire drawing are analyzed with the developed axisymmetrical finite element model. It can be concluded that the angle between the stress singularity zone near the crack tip and the axes of the steel (i.e. a greater gradient of the nibbed pencil fracture and a longer neb) is decreased with the increase of the angle of drawing die and the decrease of the friction coefficient between drawing die and wire. Finally, the quantitative analysis of the effects of die angle, friction coefficient and the size of the flaw on the value of J-integral is carried out by the calculation with ANSYS. It is shown that: the value of J-integral increases with increasing of the angle of drawing die, the friction coefficient between drawing die and wire and the initial dimension of the flaw. When friction coefficient equals 0.1, J-integral value round the crack tip with the same flaw decrease with decreasing of the angle of the die. J-integral value changes slightly and tends to be a constant value when the angle reached to 8°.The non-even stress field is still the primary character during wire drawing with an eccentricity flaw from centerline and stress singularity occurs around the front of the crack. Though the crack is axisymmetric, stress field along the crack front is uneven and this phenomenon is more prominent with increasing excursion ration. Stress concentration occurs on wire surface near the crack obviously and aggravate with increasing excursion ration, which suggests abrasion on this side is more serious. It is shown that steel surface endures non-even stress on the side near the crack and the opposition side, whichmay causes outburst of the steel bar, that is, brittle fracture, when excursion ration is enough high.The results of quantitative analysis on the effects of eccentricity ratio of cracking on the value of J-integral show that, with the same flaw, J-integral values are different along three given planes and is larger on 90°plane than on the other two planes, which suggests crack propagation may incident on this point. J-integral values decreases with the increase of excursion ration, however this decrease is slighter and slighter. With increasing excursion ration, the relative space of the flaw changes, causing different fracture mode, the stress state of formation and fracture morphology also are different.The calculated results are then applied to improve the optimization of the technology for wire drawing.