Differences in use Bolt Connectors and External forces 1

[estebanruiz]

Hi Guys,

I am doing a simulation analysis with Solidworks 2012, of a screw joint, trying to understand the effect of the torque to the material (ultem-1000) which is below the screw head and the washer, because the material is cracking just when the torque is applied.

I am using a bolt connector made of 316 SS, and a washer which uses the same material, so when I run the simulation, the results are very high using the bolt connector. However, if I utilized a compressed axial force of 133.33 lbf and a torque of 1.6 in-lbs respectively, the results are very low. I am concerned about the difference of using bolt connector and applying external forces.

Could you please help me to understand the differences?

Thanks in advance.

Esteban

 

[Sparweb]

Is the Ultem part made with the glass-reinforced variety? Depending on the quantity of glass, it gets very brittle. Like, only a few % elongation (or contraction if the compressive modulus is similar) before break. I have no trouble believing that a metal screw can crack a plastic part. Depending on the size of that screw it could be installed with more than 10x the torque you're trying to use.

-datasheet-


STF

 

[estebanruiz]

Hi SparWeb,

The material does not have any glass. Here's the datasheet I received it from the material directly. Also, The size of the screw is #0-80, the torque is 1.6 in-lbs
However, my concern is about the compression force which is been applied when the torque is applied. By hand I am using the formula T=0.2*D*F to calculate the axial force, which is around 133.33 lbf, using this value the stress is around 39 Ksi (133.33 lbs/3.389x 10-3 in^2), because the area I am assuming is under stress is the circular ring behind the flat washer. So, according to this, the stress is exceeding the compressive force and I got it, which is only 22 ksi.

However, my effort is to reference my hand calcs with the static FEA simulation from solidworks. I started using threaded connectors and the results are 52 ksi for Von Mises Stress, which is higher than my values with hand calcs. Thus I tried using a compressive force equal to the axial force on the washer surface and is giving me higher values (84 Ksi) and then I tried the torque as an external force and it goes down but higher than using connectors (64 Ksi).

I am not sure which one I have to believe. Also as this is a very brittle material, I thought in using non-linear would help and I did it, so I could define the stress-strain curve, but my results change with the time I applied the torque, so I do not know what is the best time which could give me realistic results.

Thanks.