Cyclic Stress?? 2

[wertyu1980]

I need help with the concept of cyclic stress.

Suppose a shaft(...or drill string)is drilling a hole. The shaft is basically rotating with the drillbit in contact with the substance being drilled through. The shaft is made up of number of connections along it's length. Now while drilling, the drilling conditions are difficult. Say the drillbit is having trouble drilling through the substance. This will induce stress in the shaft. The shaft is vibrating in a lateral direction.

Now, due to stress/vibration, one of the connections is fractured...meaning shaft breaks off. Now, every connection is torqued to a certain value. Due to the drilling condition it is torqued further to a point it breaks.

How would you define this fracture? Was it due to cyclic bending stresses, torsional loading, or fatigue???

I would really appreciate if you can explain these terms.
Thank you.

 

[Cockroach]

In a drilling application, probably torsional loading. You would see this as the bit stalls, yet energy in the form of work is being applied to the top of the string. This is the primary source for string parting.

Bending stress is something like a canilever, you hold one end fixed and apply a load at the other end. Consider for example, a guy on a diving board. He loads the free end up by jumping with his weight under the influence of gravity, thus causing a deflection. Up at the far end, the fixed end, the maximum moment occurs and therefore the maximum stress. This would be a bending stress.

Cyclic loading is the same phenonema except the load varies as a bend from tensile to compressive stresses. This happens in a rotating drive shaft, for example, the outside fibers go from tension to compression and then back to tension as the shaft rotates through a complete revolution. Clearly this introduces fatigue into the metal from cold working. It is in the cold working of material that eventually cracks or other imperfections will occur that will result in a failure.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[IceStationZebra]

You are getting some terms mixed up.
-Any kind of load (bending, torque, axial, etc.) that is repeatedly applied and released (or reversed) is cyclic stress.
-Apply enough cyclic stress cycles and you have fatigue. There is no minimum number of cycles to be considered fatigue, but for steel you are generally looking at at least 1,000 stress cycles. If you have significant plastic deformation it is not fatigue but tearing. For steels to fatigue the stress has to get past the fatigue limit - which is ~1/2 the yield strength. (aluminum does not have a fatigue limit) One tell tale sign of fatigue is fretting at the fracture.

"Now, due to stress/vibration, one of the connections is fractured...meaning shaft breaks off. Now, every connection is torqued to a certain value. Due to the drilling condition it is torqued further to a point it breaks."
I don't get what you mean. If one shaft breaks off how can another connection get torqued to the point it breaks?

ISZ

 

[wertyu1980]

"Now, due to stress/vibration, one of the connections is fractured...meaning shaft breaks off. Now, every connection is torqued to a certain value. Due to the drilling condition it is torqued further to a point it breaks."

Sorry for the confusion. The drilling string (..or the rotating shaft)is made up of number of tools joined together. Some tools have more one connection. When these tools are assembled on the shop floor, they are basically torqued to a certain value to make connection. There are make up and break up torque values for every connection.

When this tool used as a part of the drill string, i.e. it's a part of the rotating shaft downhole, it is subjected to stress and lateral vibration. Sometimes due to harsh drilling conditions, a further make up torque is applied to the connections downhole.

Now, due to the added make up torque, one of the connections of one of the tools parted/fractured.

A cyclic bending stress acting on the tool may be responsible for the fatigue fracture. Now, I don't really understand this statement/process clearly.

Can somebody talk more about how fracture are caused by bending stresses and ductile tensile stresses? What is a fatigue fracture?

 

[IceStationZebra]

All fractures are caused by stress - its just a matter of how high the stress is compared to the material's yield strength. Low cycle - overloaded with localized stress at or above yield, high cycle - fatigue with localized stress 50-90% of yield. The easiest thing to do is inspect the crack - look for how rough the surface is. Do you see any beachlines? See below for more details and pics.

http://www.tech.plym.ac.uk/sme/Inte...ailureAnalysis/Images/Fractography/Shaft2.JPG

In this picture you can see that the fracture started in the lower right corner of the keyway. Those lines you see are called beachlines - typical of high cycles fatigue failures. The surfaces are fairly smooth because the fracture happened slowly and the two halves tend to polish each other as the sides move ever so slightly relative to each other. The rough texture on the bottom of the shaft indicates a near instantaneous failure. (Essentially the shaft was so over designed that it lost almost 80% of its cross section before it failed!)

http://www.tech.plym.ac.uk/sme/Inte...s/Images/Fractography/Fatigue_Beachmarks4.JPG

This picture is similar to first. 1 is the crack initiation point, 2 shows beachlines, 3 rapid fracture.

http://www.tech.plym.ac.uk/sme/Inte...lysis/Images/Fractography/Ratchet_Smeared.JPG

This is typical of a bolt. Initiation - upper left, C - crack growth, D - rapid fracture.

Often times you will see rust in the crack initiation area on parts that have taken weeks or longer to fail.

ISZ

 

[Newmanite]

I am guessing that your rods are breaking below the thread point to where the insert is welded to the rod?
If this is the case, as I have seen many times the welding preparation could be of poor quality, this includes the preheat into the rod before welding it into position,the amount of weld gone into the rod and the final finish (this has more to do with hardfacing though)

 

[IceStationZebra]

To add to Newmanite's comment - a proper weld joint should fail along the toe of the weld. This area usually has the most brittle material. If the crack started in the middle of the weld it was weak in some way. As previously mentioned some of the candidates include insufficient/missing preheat (depending on parent metal), lack of filler metal, porosity - but can also include wrong filler metal, improper joint preparation (no chamfer, etc), contamination, wrong shield gas (for MIG), cold starts (poor penetration) at start of bead, poor penetration in general, focusing penetration on one half of the joint, etc. Unfortunately welders will often hide these sins by grinding the bead after they finish the weld.
ISZ

 

[BillBirch]

You mention lateral vibration. What other posters are trying to explain is cyclical bending stress leading to fatigue. As you know, the drill string will not be suspended perfectly straight. if the drill string is rotating, a point on the outer fibre will undergo a complete compression - tension cycle in one revolution.

 

[hydtools]

Here is a Drill Engineering book review that explains loading on drill strings. It is not complete, but does have some good discussion and use of terms.

http://books.google.com/books?id=es...SA59il8VmRg8t-ArYt4ULKKOE&hl=en#PRA3-PA333,M1

Ted

 

[IceStationZebra]

Bill and Hydtools - I agree 100%. ISZ

 

[rudylyon57]

Drill strings can be used on all sorts of drilling tools which can impart different types of loads on the tool joints.

Percussion drilling (DHD) is low torque and generally a string for a deep hole (greater than 300 feet) is in tension due to lower WOB needs. However, there can be considerable axial stress pulses coming from the percussive device.

Rotary drilling (tricone) requires higher torque, RPM and WOB which generally develops more axial, torsional and lateral string vibration.

My experience has always indicated that tool joint failures are caused by improper preload torque to accommodate the drilling application. We all know that preloading is a means to reduce the effect of changing loads which results in fatigue.