27 Replies Latest reply on Jul 15, 2009 12:14 PM by Poovaiah Devenira

    Non-linear Analysis of Bolt Preload

    Prashanth Srinivasan
      Hello all,

      This is my first post on these forums, so I would like to say hello to everyone :-)

      So far at our company we have done FEA (Linear Static) of structural assemblies involving bolts & nuts tightened to a specified preload torque - which is converted into tensile contact forces using the thread equation F=T/(0.2*D) and applied at the contact areas between each bolt with the contacting plate & each nut with the contacting plate.

      However, I recently read in an internet article that the best way to simulate bolt preload in FEA is to perform a Non-Linear Analysis which takes into account thermal stresses induced in the members due to friction/tightening. I am interested in learning more about this, so could someone please tell me what it exactly means & which books or internet articles would you recommend us to study about non-linear themal analysis in detail?
        • you need to be a lot mosre sopecifcNon-linear Analysis of Bolt Preload
          Bill McEachern
          You would need tobe alot more specific about exactly what the object of such an analysis would be. here is somehting to think about: The way your post is written it sort of implies that the heat generation of tightening the bolt maye relevant. Hard to really believe that one. Another possibility is that in some codes an NL analysis maybe required to obtain the pre stress state. the fist step in the analysis would be to apply a thermal load to shrink the solid element model of the bolt to obtain the response of the structure due to the bolt tightening. The next step would be to apply the external loads.
          Solidworks simulation does all this for you in the static analysis.
            • you need to be a lot mosre sopecifcNon-linear Analysis of Bolt Preload
              Prashanth Srinivasan
              Thanks Bill, yes you're right on the part that a thermal load needs to be applied to shrink the bolt & nut to pre-stress or pre-strain the assembly. I would just like to know how this can be done & basically learn more about thermal loads (as I haven't studied it much in my degree).

              The real objective of the analysis is that we're trying to achieve better correlation between the FEA & real-time testing results. The method which we're following at our company (as I mentioned in my first post) does not give us good correlation with real-time stresses & displacements. Hence we're trying to explore the possibility of Non-linear analysis so as to get good correlation with real-time results.

              BTW, I would like to know how Solidworks can do this in the Static Analysis itself, as you have mentioned? Because the way we've been doing so far is merely assembling the CAD models of the nuts and botls & applying equal and opposite pressure forces at the contact areas. We aren't applying thermal loads anywhere in our analysis.
              • modeling of bolts
                Bill McEachern
                You can apply temperature as a load in a static analysis. You should avoid the word thermal as it implies a heat transfer calc. If you apply a temperature as a load the program shrinks or expands the component it is applied to as per the coefficient of linear expansion defined in the material definition. The delta T used is based on the zero strain reference temp defined in the properties of the study (RMB under the top of the analysis tree).
                It does nto sound like you are aware of the bolt connector available in the program. You might want to try using it as it automates the procedure outlined in my last post and avoids you have to specify the temperature method to get the pre-load. If your adherent in the bolted connection ae not very stiff the bolted connector can lead to overly conservative pediction of fastener loads. IF you want to to used solid elements to represent the fastener then the temperature option is what you have to use and again a non-linear analysis procedure must be used to get the sequence effects.
                  • modeling of bolts
                    Prashanth Srinivasan
                    Thanks again for your valuable advice, Bill. Just a couple of questions here:

                    1. Can I use the "bolt connector" option in my Linear Analysis itself, or should I go for Non-linear Analysis for getting the desired accurate results?

                    2. Where does this temperature load come from? What's the physics behind it? Is it because of the friction between the bolt-head or nut-head with the plates, or is it because of the high temperature that our assembly is subjected to during regular operation?

                    I apologize beforehand if my questions seem too basic :-) But this is a new area we're venturing into as we haven't had much exposure on it during our educational degrees. We feel this will turn out to be very useful and profitable for our company in the long term.

                    • modeling of bolts
                      Bill,
                      I cannot find the manner to specify in SolidWorks the temperature of the bolt connector. In case of non-uniform temperature of the bolted bodies (thermoelastic static analysis made over a thermal analysis) which assumption makes SolidWorks on the temperature of the bolt connectors?
                      Thank you for your information
                      Leonardo
                      • modeling of bolts
                        Poovaiah Devenira
                        Hi Bill,

                        I am intrested in knowing more about applying the temperature load for simulation a pre-tension in an anchor bolt. I was'nt sure on how excatly to apply this load. Do i apply the temperature on the cylindrical face of the bolt. Colu u give me more information on how to exactly simulate this. The anchor bolts are 10 ft long and a pretension load of 187.5 kips is to be applied
                          • modeling of bolts
                            Bill McEachern
                            Guys you need to re-read my original post. The bolt connectrs apply a pre load - so the temperature thing is irrelavent in this context. If you want to have the bolt geometry in the model then you can use a NL analysis ( non-linear ananlysis) and run a sequence of loads to get the job done. First you apply a temperature to the bolts to shrink them and develop the bolted state prestress - then you apply the external loads after that in another load sequence (ie using the existing state of stress as a starting point for the external loads). If the problem is stable you could do it in a static analysis all at the same go assuming the friction forces due to the bolt loads are not required to support other loads.
                              • modeling of bolts
                                Poovaiah Devenira
                                Hi Bill,

                                I want to model the bolts as I am more intrested in studying the deflection of the plate to which these bolts are connected. So i plan to use the temperature method.
                                I wasnt sure if i understood how to do it excatly. Do I apply the temperature on all the faces including the bolt head and nuts or only on the bolt shank. could you also tell me on how to get the required temperature i have to use.

                                Thank you
                                  • modeling of bolts
                                    Bill McEachern
                                    Pick the bodies filter and then select the body and it will apply a uniform temperature through out the bolt. Or just pick the body formthe solid bodies folder.
                                      • modeling of bolts
                                        Poovaiah Devenira
                                        Hi Bill,

                                        a = 6.5 * 10^-6
                                        Strain = alpha * delta(t)
                                        F/A = E*Strain

                                        For A = 3.14 in^2
                                        E = 29,000,000 psi
                                        F= 185 kip

                                        delta(t) = 312 F
                                        Assuming ambient temp to be 82 F ( I set it as 82 in thermal effect, reference temp)
                                        Do I apply (-230) F on the body of the bolt to simulate the pre-tension load
                                      • Non-linear Analysis of Bolt Preload
                                        Peter Biggert
                                        If you want to model the bolt in a bolted connection, there are two ways to apply a preload.

                                        Apply a temperature Load/Restraint to the BODY of the bolt (not its surfaces). In the Study's properties you can set the reference temperature at zero strain. The temperature Load/Restaint you apply to the bolt will be smaller. The value will depend on the size of the bolt, its coefficient of thermal expansion and the desired preload. Be sure and apply the reference temperature to the other bodies.

                                        An alternative is to model your bolt with an initial interference and use the shrink-fit contact set between the two surfaces. It helps to apply splitlines on the surfaces where your boltheads contacts. The magnitude of the initial interference depends upon the desired preload.

                                        If you want to apply the results from a thermal study then the shrink fit option of applying bolt preload is the way to go.
                                      • modeling of bolts
                                        dear Bill,
                                        this is not my point. Let me try to explain better. The usage of the bolt connector on an isotherm structure is clear to me. But I want to use the same method to deal with a non isotherm structure, in which the bolts have a temperature different from the compressed members. Or even a case in which the temperatures are all uniform, but different from the zero strain temperature. Now my point: it is possible to specify for the bolt material a coefficient for thermal expansion, but, seemengly, not a bolt temperature: this is illogic. The program should not have any difficulty to keep in consideration the thermal inelastic strain of the bolt, when appling the preload (which is for sure modeled in turn as another inelastic strain).
                                        Insted, the software gives incorrect results even in the simple case of temperature uniform throughout the model but different from the zero starin temp. Do you avail an explanation?
                                        Thank you
                                        Leonardo
                                • Non-linear Analysis of Bolt Preload
                                  Chad Schmidt
                                  This doesn't sound like a non-linear analysis. This is more of a non-linear dynamic analysis. The heat is being caused by friction with time? You have to know more about how fast the pre-load happens. You have to consider velocity and acceleration of the turn at a given time to generate pre-load. It's this acceleration that generates the heat (power). You probably need to consider things that are well beyond the scope of this discussion.