Central Burst

Hi Sir,

Greetings

I am working in copper rod plant as a supervisor.Can you please tell me about the central burst defect in wires during drawing process.

What are all the causes for the defect like raw material defects,machine or die or lubricant problems?

How to identify the defect which is because of raw material (copper rod) or from drawing process?

Please help me sir.

hello again,

The WAI Nonferrous Wire Handbook Volume 2 has a few pages dedicated to this (Pages 21 - 24 inclusive). You should have the following book set in the company's technical library. www.wirenet.org/waistore/productdetail.cfm?productid=12

The central burst phenomenon is quite rare and generally has to do with the drawing die cone angle. Often it occurs when an operator actually puts a die in backwards.

For simplicity, think of it this way:

Somewhere between drawing solid wire and breaking the wire, there is a spot where a very rough internal surface tube is drawn.

If you actually find a sample of central burst wire, you will see that it is quite brittle and will snap like a twig if bent between your fingers.

Please get your technical library going. Here is the nonferrous directory, www.wirenet.org/waistore/prodlist.cfm?categoryid=11

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212

When a piece of material is bent, the outside of the bend will be under tension and the inside of the bend will be under compression. If it is bent while being subjected to tension the compression will be reduced and the tension will be increased.

As a piece of wire is leaving the wiredrawing die, it is being bent outwards. It is also under significant tension - it has to be to pull it through the die. The bending effect means that the centre of the wire will be subjected to a significantly higher tension than the surface. Tension at the centre of the wire may be high enough to cause fracturing along the centre line even if the average tension is not sufficient to cause a wire break.

Steeper die angles (particularly if a die is installed backwards) make the bend sharper and increase this effect, but are not the only cause. Any thing that increases the overall tension will also increase the tension peak at the centre and thus increase the tendency to form this type of defect.

Its good to see some other particpants from Down under and especially with the experience of Rebboc.

Hello Rebboc and welcome,

I found your explanation above interesting but I beg to disagree with some of it. We generally try to stay away from going too deeply into very complex issues because WAI books are available and it wastes a lot of our time but I cannot leave it the way you have described.

Firstly, if the wire is bent at the exit of the die, there are, as you pointed out, two bending stresses generated from the center or neutral axis of the wire. These stresses start at zero and radially increase as one moves outwards towards the outer curved surface of the wire A compressive stress is on the inner side of the curve and the tensile stress is on the outer side of the curve.

The stresses are additive (Compression negative.) to the overall drawing (pulling) tensile stress created by the downstream capstan pulling on the wire. Thus on the compressive side of the bend, the total stress is slightly reduced and slightly increased on the tensile side of the bend. It is my understanding that the highest tensile stress is at one outside surface at the die and that is where surface damage called 'chevrons' or "crows feet" are created. It is the point of highest external to the drawing die tensile stress.

Central bursting however occurs while the copper wire is actually inside the drawing die and this is a much more complex situation. If you want the technical details I suggest you purchase the WAI book I referenced above because it requires graphs and drawings which we cannot recreate here.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212

Hello vettecoupe,

I suspect that the "disagreement" is due to ambiguity and lack of a diagram rather than a disagreement in principle.

I did not mean to imply that internal defects were caused by bending of the wire as it exits the die.

The bending I referred to was the bending of the material as it moves from the conical reduction section of the die to the parallel section. The outside surface of the material conforms to the die surface, effectively bending it outwards. As you say, on the inside of the bend (the outside surface of the wire) the longitudinal tension will be reduced slightly whilst on the outside of the bend (the centre of the wire) (although, at the very centre, there is no bending) the tension will be increased slightly. At this transition point in the die, the stress will be a complex mix of the longitudinal drawing stress, the geometric effect described above, the radial and tangential compression from the axisymetric reduction in section area and localised stresses due to microstructural variations and any inclusions present. If the resultant stress at any point exceeds the strength of the material at that point, then an internal rupture will occur.

The main point I was trying to make in my original comment was that a large number of factors are involved in determining both the stress state and the material strength at the centre of the wire. A small change in any one of them might push the stress over the limit and cause a rupture. Looking only at die angles as the potential cause can waste a lot of time. (Perhaps I should have just said this and referred to the textbooks for the explanation.)

I take your point about level of detail, and obviously my original response was not unambiguous and, without a diagram, was easily misunderstood.

When dealing with technical issues in production, there is no substitute for a sound understanding. I have found the WAI publications to be an excellent reference source over many years and would highly commend them to anyone who wants to understand rather than just "know about" technical issues.

I will keep your comments in mind when making future responses.