NCSX Modular Coil Joint Load/Stress Calculation

NCSX Modular CoilJoint Load/Stress

Calculation

ByLeonard Myatt

Myatt Consulting, Inc.

2 January 2007 MC Joint Analysis 2

The Big Picture Estimates of Bolt Loads with Bonded

Flange Interfaces need to be checked. MC Global model already quite big. Need to develop a simple bolt model with

properties (shear & tensile stiffness) similar to the reference joint designs.

Apply approximation to MC Global model to determine bolt load distribution.


Process Overview Import Joint Models from ORNL into ANSYS:

“Joint1” (Double nut on Stud) “Joint2” (Bolt & tapped flange hole)

Apply symmetry, add 30 mil Stycast gap, add general contact at flange-shim interface

Loading: Impose lateral deformation (parallel to flange face) Impose axial deformation (parallel to bolt axis)

Response: Lateral force and joint stiffness v. imposed deflection. Axial joint stiffness (single-value per joint

configuration).


Process Overview, cont’d FYI: Report stresses for 25 k-lb lateral

load. Develop simplistic equivalent bolt model

for Global MC simulations. Incorporate simplistic bolted connection

model into Global MC simulation. Analysis Units:

Bolted Joints use English units. Global MC uses SI units.


Reference Design(Courtesy D. Williamson)

Joint1

Joint2Expected 30 mil Annular Gap atFlange Through-Hole Not Shown


Set Preload to ~73 k-lbf

Fastener Torquebolt tensile yield strength Fty 85000 psi

yield criteria - 0.7nominal diameter D 1.375 in

thread pitch p 0.167 inbolt tensile area At 1.155 in2

nut factor K 0.2applied torque T 18897 in-lb

preload uncertainty factor u 0.25min bolt preload Po 51537 lb

max bolt preload Po 85894 lb

NASA Technical Memorandum 106943 used to define Preload Spechttp://gltrs.grc.nasa.gov/reports/1995/TM-106943.pdf


ANSYS Model, Joint1


ANSYS Model, Joint2


Joint1 Bolt PreloadNominal Preload ~50 ksi


Joint2 Bolt PreloadNominal Preload ~50 ksi


Joint1 Force, Deflection & Stiffness

Preload + Transverse MotionJoint1 Transverse Load, Stiffness v. Motion

Zero Friction at Shim Interface

0

3000

6000

9000

12000

15000

18000

21000

24000

27000

30000

33000

0 5 10 15 20 25 30

Transverse Motion, in/1000

Tran

sver

se L

oad,

lbf

0

150

300

450

600

750

900

1050

1200

1350

1500

1650

Inst

anta

neou

s St

iffne

ss, k

-lb/in

Force

Stiffness


Joint2 Force, Deflection & Stiffness

Preload + Transverse MotionJoint2 Transverse Load, Stiffness v. Motion

Zero Friction at Shim Interface

0

10000

20000

30000

40000

50000

60000

70000

0 5 10 15 20 25 30Transverse Motion, in/1000

Tran

sver

se L

oad,

lbf

0

500

1000

1500

2000

2500

3000

3500

Inst

. Stif

fnes

s, k

-lb/in

Force

Stiffness


Joint1 Contact Stress (S3)G11 & Stycast Insulating

Parts

Stress & Force Vectors, 25000 lb Shear Load


Joint1 Bolt Tension Stress (S1)

Stress from 25000 lb Shear Load


Joint2 Contact Stress (S3)G11 & Stycast Insulating

Parts

Stress & Force Vectors, 25000 lb Shear Load


Joint2 Bolt Tension Stress (S1)

Stress from 25000 lb Shear Load


Tensile Stiffness of Joints 1&2

Apply axial loading where bolt hardware interfaces with flange face.

Calculate tensile stiffness of each joint. Stiffness values to be used as basis for

developing simplistic joint model for more precise Global simulation of MC structure.


Joint1 Tensile Stiffness~5.4 M-lb/in

Check: k(bolt)=AE/L~(1.48in2)(27.6Msi)/6”~7 M-lb/in


Joint2 Tensile Stiffness~8.8 M-lb/in

Check: k(bolt)=AE/L~(1.48in2)(27.6Msi)/3.5”~12 M-lb/in


Simple Model of Bolted Joint1 for Equivalent Stiffness

Approx.

Stiffness Match Achieved with 2.9” diameter Bolt


Simple Model of Bolted Joint2a for Equivalent

Stiffness Approx.

Stiffness Match Achieved with 2.75” diameter Bolt


A-B Joint Bolt Definition

Bolt layout drawing shows 26 bolts at this A-B flange.


A-B Bolt Types(M. Cole FP-STUDS.PPT)


Bolts 1-9 Defined by Project (M. Cole FP-

STUDS.PPT)



STUDS.PPT)



STUDS.PPT)



STUDS.PPT)


Application of Equivalent Stiffness Bolted Joints to Global Model at A-B

Flange

Solid PIPE16 elements, with dimensions to match calc’d Joint stiffness, are added to each A-B Bolted connection.


Top 16 Bolted Connections

Bolt #11 is questionable


Bottom 10 Bolted Connections

Would-be Bolt#16 is missing.


Global Model Analysis Notes

The global model bolt #11 does not appear in the bolt numbering drawing or M. Cole’s pictorial layout.

The global model appears to be missing a bolt (#16 in the bolt numbering drawing).

If global model bolt #11 is eliminated, and a bolt is added to hole #16, then the model would be consistent with other references.


Analysis Notes, cont’d A significant effort was made to simulate both

zero and finite friction A-B joint behavior. Bolt Preload Load-Step converged OK. But excessive computer run-time (4+ days

for 4% of EM load) lead to an alternative approach for evaluating max bolt shear loads.

The interface is modeled as sliding and always in contact (KEYOPT(12)=4) with zero friction.


Analysis Notes, cont’d The model converges nicely and produces

conservative shear loads for evaluating preload and friction requirements.

However, it does not report accurate tensile loads on the bolts.

We may ultimately need to find a way to run the model with friction or at least open/closed contact behavior (keyopt(12)=0) to confirm preload levels and bolt stresses. (This requires more thought.)


Global Model Results Bar graph and contour plot of shear

load on each A-B bolt. A-B Interface pressure from EM loads,

neglecting bolt preload effects. A-B Interface relative motion from EM

loads, neglecting bolt preload effects.


A-B Bolt Load Distribution(ORNL reference: fsum_dl-

em1.xls)A-B Flange Bolt Shear Loads (Without Friction)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

2625242322212019181716151413121110987654321

Bolt #

Shea

r For

ce, l

b

global16 (d_bolt=2.75")

ORNL: EM without IL Bolts


A-B Bolt Shear Load [N per bolt]

Contour Plot Visual


Shear Load Distribution Observations

The missing bolt (would-be #16) forces the nearby bolt (#17) to carry a larger portion of the loading.

The approach used to integrate shear stresses from a bonded analysis over regions of the flange interface (referred to here by the “ORNL” data set) underestimates the shear force distribution compared to this quasi-nonlinear approach.


Shear Load Distribution Observations, cont’d

Adding bolt #16 will surely drop the max shear load on bolt #17, possibly to 12 k-lbf.

However, the load on bolt #26 will likely be unaffected by the change and must carry almost 16 k-lbf.

Applying a reference preload of 73 k-lbf and using friction to carry the shear force will require a minimum friction coefficient, μ, of 16k/73k or 0.22. This correctly assumes that EM loads do not diminish the contact pressure around this bolt (see next slide).


A-B Interface Pressure The influence of EM Loads on

flange face pressure: increase compression stress in

red areas decrease compression stress in

blue areas Blue means reduction in

preload which will diminish frictional capacity of bolted joint.

Notice inboard leg interface predominantly in compression and may carry shear stress (analysis TBD).


A-B Interface Slip (No Friction)

Plot shows influence of EM Loads on flange face relative motion (slip).

In bolt region, relative motion <8 mils, and higher near #24.

In unbolted Inboard Leg region, relative motion 37 mils max.


Bolt Stress Observations If the stabilizing effects of friction are conservatively

ignored, then the transverse joint loads (F) must be carried by the bolting hardware & insulation.

Stresses can be estimated by scaling the bolt tension and insulation compression by (F/25k) from the plots presented above.

F=16k-lbf at #24 would result in: A bolt tensile stress of (124ksi)(16/25)=79 ksi plus a thread

concentration factor. An insulation stress of (-45ksi)(16/25)=29 ksi in Stycast (which

has an ultimate strength of <20 ksi, ref. Freudenberg test data) Therefore, the present design will need to rely on

friction.


Conclusions Two prototypical MC bolted connections are

modeled, analyzed and characterized. A simplistic approximation is developed and

incorporated into the global MC model. Complexities of modeling a friction interface

are compounded by the size of the global model and force a more efficient approach: Contact at A-B only with zero friction & zero

separation.


Conclusions This approach produces conservative

shear loads on each A-B bolt (not diminished by friction), and provides a benchmark for earlier calculations which assumed a bonded interface condition.

These earlier results appear to be non-conservative by as much as 2x, and miss a peak A-B shear load of ~16 k-lbf.


Conclusions Unprotected by the isolating effect of a

preloaded frictional joint, the bolt and Stycast would exceed reasonable stress limits with such a shear force.

Assuming a modest friction coefficient of 0.22, the friction developed by the preloaded joint would completely isolate the bolt and Stycast from this shear load. Other bolts should also be checked as local normal stresses may reduce the interface pressure and diminish the joint’s ability to carry the shear in friction.


Conclusions A more thorough analysis of the interface still

requires a traditional [nasty] contact analysis where flange separation can occur.

Such an analysis will do a better job at determining the variation in bolt tension stresses and interface pressure with EM loading.

Analyses by others indicate that A-A may be a more heavily-loaded connection, and therefore should be evaluated ASAP.

Documents

NCSX Modular Coil Joint Load/Stress Calculation