Pretension in a bolted joint

[kotawsu]

Hello everybody. I am trying to analyse a bolted joint and i have defined pretension to the bolts. But i see that the pretension section shrinks and the bolt head has negative stresses even though the preload is a tensile load which i didnt understand why. I have defined pretension in step 1 using cload and in the second step i have applied the external load using cload,op=new and i have also used boundary,fixed in the second step. is this right or am i missing somethin? looking forward for your reply. Thanks.

Kota

 

[corus]

If the section of the bolt has shrunk then it will be in tension, relatively. Look at the displaced shape of the object that is being bolted. This must show a compressed area under the bolt head. op=new will ignore the previous loads. Surely the second step is where you are adding additional loads to the bolt load?
The bolt head will be in compression generally as it is being loaded by a tensile load acting downwards, and an upward load from the object surface. These two forces apply compression.

corus

 

[kotawsu]

Thanks corus..but i kinda couldn't figure out when you said,"The bolt head will be in compression generally as it is being loaded by a tensile load acting downwards, and an upward load from the object surface. These two forces apply compression". The bolt head gives a compressive force to the plates which in turn gives a tensile force to the bolt head right?

and also i had one more question. To check the bolt load i check the contact force since i have defined contact between the upper bolt surface and the upper plate hole and the lower bolt surface and the lower bolt hole. But is this contact force what i get is the force exerted by the slave on the master. 'cause i have defined the bolt as the slave and i am getting the contact force for the upper bolt surface negative( which means the force is acting downards according to the co-ord system i have chosen)..Thanks for ur help. i really appreciate it.
Kota

 

[corus]

Consider the bolt head at the top. There are tensile stresses in the bolt shaft. Take the upper half of the bolt and look at the forces. There is a tensile load acting upwards on the shaft. This is opposed by a downward load acting on the bolt head. This is opposed by an upward reaction force at the contact surface. These two upward and downward loads oppose each other and give compressive stresses, generally. As they're offset you'd get bending in the bolt head too.
If you don't believe the results then simply look at the deformed shape of the surface underneath the bolt head. This should show a compressed surface.
Another way of checking the bolt load is to look at the stresses in the bolt shaft. Multiply the average stress by the area.

corus

 

[kotawsu]

hey corus..thnx once again..i shall check the average stress and shll let u know what u get. happy new year.

 

[kotawsu]

corus, i computed the average nodal stress in the bolt shaft and multiplied by the cross sectional area of the shaft. but when i checked the CFT in the transverse direction it shows 1122 lbs .and when i did the averaging stuff it is giving me 1222 lbs.i haev 1167 lbs as my pretension load and 100 lbs as the external axial load on the bolt head. but in reality isnt the bolt load supposed to increase and the joint load is supposed to decrease.becuase if i consider the CFT values both my bolt load and the joint load is decreasing.wonder why?
here are the details:
CFT(BOLT)----1122 LBS
CFT(JOINT)----1122 LBS
EXTERNAL LOAD-- 100 LBS(TENSILE)
AVERAGING METHOD(BOLT SHAFT)--1222 LBS

I always thought in reality the bolt load shld increae and the joint load shld decrease.if i follow the averaging method then it proves my above thought.But if i juss go for the CFT values then both bolt and joint load is decreasing.Do u know why this could be happening? or should i juss go for the averaging method considering the bolt shaft and not the bolt head to get the bolt load and consider the CFT value for the joint load. do u think its reasonable. looking forward for your reply. Thanks.