interpretation of mc interface analysis results

Interpretation of MC InterfaceAnalysis Results

Wayne Reiersen

April 11, 2007

2

Joint strength design criteria

• A design handbook for bolted joints was developed to provide a unified approach for the project

• Design criteria for joint strength include the following…– The separation of a preloaded joint must not occur due to an

external load– The minimum bolt preload shall be used to calculate the friction

force – The allowable coefficient of friction (a) must always be determined

in a conservative manner. Testing under representative conditions should be performed in order to determine the allowable coefficient of friction.

– Friction coefficient extremes must be considered as anticipated upset conditions in the design. For allowable coefficients of friction above 0.45, use amin=2/3 a and amax=4/3 a.

– In shear-loaded joints, with members that slide, the joint members transmit shear loads to the bolt and the minimum preload must be sufficient to hold the joint members in contact and without additional sliding during the stress cycle.

3

Gettelfinger’s friction test resultsAlumina coated samples

• Results indicate a favorable dependence for COF v. contact pressure

• A “design curve” can be characterized by an equation of the form COF = 0.4 + 0.02P where P is the contact pressure in ksi

• The average contact pressure through which the bolt preload is transmitted at the shim/flange interface is approximately 10 ksi (based on Fan’s FEA analysis and Viola’s Fuji film test) resulting in an effective COF of 0.6

• Additional tests are being performed to investigate the sensitivity to flange surface finish and alumina coating (with and without binder)

y = 2E-05x + 0.4

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Ap

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Brooks/Fan linear analysisRegion identification

cc2t

cc2b

cct

cc2int

ccb

ccint

bc2tlbc2tru

bc2trl

bc2bru

bc2int

bc2brlbc2bl

bcint

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bctrl

bctrl

bcbrlbcbl

aaint

aatl

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aatr

aabr

ab2int

ab2tl ab2tru

ab2trl

ab2br

ab2bl

abint

abtl

abtr

abbru

abbrlabbl

-60 deg

+60 deg

0 deg

cc2inbbc2inb

ab2inb aainb

ccinbbcinb

abinb

5

Brooks/Fan linear analysis Shear Load - Inboard Regions

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2int

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Joint-Location

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ar L

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6

Brooks/Fan linear analysis

Normal Load w/o Bolt Preload - Inboard Regions

-350000

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7

Brooks/Fan linear analysisCoefficent of Friction Needed to Prevent IL Slip

Combined Coil Model (Mod/TF/PF)Inboard Regions With and Without Added Bolts - Dead Load+EM+CD

0.000

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cc2i

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Joint-Location

mu

74 kips/bolt preload

8


Shear Load Per Bolt - Outboard Region

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cc2b

cc2t

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Joint-Location

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ar L

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Normal Load Per Bolt w/o Bolt Preload - Outboard Regions

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Joint-Location

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ce L

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10

Brooks/Fan linear analysisCoefficent of Friction Needed to Prevent Slip

Combined Coil Model (Mod/TF/PF)Outboard Regions Dead Load+EM+CD

0.000

0.050

0.100

0.150

0.200

cc2b

cc2t

bc2b

l

bc2b

rl

bc2b

ru

bc2t

rl

bc2t

ru

bc2t

l

ab2b

l

ab2b

r

ab2t

rl

ab2t

ru

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aabl

aabr

aatr

aatl

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u

abtr

abtl

bcbl

bcbr

l

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bctr

l

bctr

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bctl

ccb

cct

Joint-Location

mu

74 kips/bolt preload

11

Brooks/Fan linear analysisConclusions

• In order to prevent slip on the inboard C-C interface, an average coefficient of friction (COF) of 0.4 is required.

• In order to prevent slip on the other inboard interfaces (B-C, A-B, and A-A), the required COF is 0.3 with the added bolts preloaded to 74 kips per bolt. Without the added preload, the required COF was 0.74.

• On the outboard region, the normal load per bolt is small relative to the bolt preload of 74 kips, ranging from -15 to +15 kips – flange separation should not be an issue

• In order to prevent slip on the outboard interfaces, the required COF is a maximum of 0.15 and typically, 0.1.

12

Fan single bolted joint analysisShear load only

• Shear load of 15 kips• Lateral deflection small w/o slip – 1.3 mils• Lateral deflection large w/slip – 10.7 mils

Bonded Frictionless

13


Large lateral deflections result in large bolt bending and contact stresses(34 ksi) in frictionless case. Stresses are a maximum near where bolt bears on the end of the bushing.

Small lateral deflections in bonded case result in low bolt stresses (1 ksi)

Bonded Frictionless

Sz SzSeqv Seqv

14


Contact pressure on bushing peaks near shim interface and is two orders ofmagnitude higher in the frictionless case (34 ksi) than in the bonded case (0.4 ksi)

Bonded Frictionless

15

Fan single bolted analysisShear load only

high local stress at edge is the combination of the bearing stress and shear stress primarily in the y direction

Bonded Frictionless

16

Fan single bolted joint analysisCombined 60 kip preload, 15 kip shear

Bonded Frictionless

In bonded case, dominant deformation is bolt shortening due to preload (6 mils)

In frictionless case, lateral deflection is dominant (12 mils)

17


SeqvSzSeqvSz

Average tensile stress due to 60kip preload is 40ksi

Peak stresses occur at the faces of nuts due to the assumption that stud and nut are bonded

Peak stresses in frictionless case are 27% higher

BondedFrictionless

18


No contact pressure on bushing due to Poison’s effect of the preload on bolt and small shear displacement

Bonded Frictionless

19


high local stress located at the washer bearing surface primarily due to preload

Bonded Frictionless

20

Fan single bolted joint analysisG11 v. SS bushings

Frictionless withG11 bushing

Frictionless withSS bushing

15 kip shear load appliedReplacing the G11 bushing with a SS bushing cuts lateral displacement in half

21

Fan single bolted joint analysis G11 v. SS bushings

Frictionless w/G11 bushing Frictionless w/SS bushing Bolt stresses due to shear loads are significantly reduced with SS bushings

22


Contact pressures with SS bushings are substantially higher

Frictionless w/ G11 bushing Frictionless w/ SS bushing

23


Loads at flange interface are also higher w/SS bushing

Frictionless w/G11 bushing

Frictionlessw/SS bushing

24

Fan single bolted joint analysis Imperfect fit-up

• For baseline case, perfect fit-up between the stud and bushing and between the bushing and flange was assumed

• In order to assemble the bolted joint, there needs to be finite assembly gaps to insert the bushing into the hole and to insert the stud into the bushing

• Poisson effect also opens a gap between the stud and bushing when the stud is preloaded

• The limiting case of no contact between the stud and bushing was modeled assuming a 15 kip shear load

• The no contact case is compared to the tight-fitting G11 bushing case (with frictionless shims)

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The joint is markedly less stiff with imperfect fit-up, deflecting 61 mils (4 mils/kip)versus 10.7 mils (0.7 mils/kip) with perfect fit-up

26


Sz Seqv

Sz Seqv

Bolt stresses increase by an order of magnitude, from 34 ksi to 364 ksiLocation of peak stresses moves from bearing contact area to nut/stud interface

27

Fan single bolted joint analysisConclusions

• A friction joint in which shear loads are transferred through the shim via friction is a better joint than one in which shear loads are transferred via bolt bending and contact pressure on the bushing

– If friction is adequate to prevent slip, the incremental stresses and deflections due to a 15 kip shear load are small relative to the stresses and deflections due to the 60 kip bolt preload

– If friction is inadequate to prevent slip and the shear load are transferred to the bolt, deflections due to a 15 kip shear load will be dominant. Bolt stresses will be substantially higher than from a 60 kip preload alone.

– With imperfect fit-up between the stud and bushing, markedly higher bolt stresses result until contact is made with bushing

• It would be prudent to set the preload as high as possible to maximize the shear capability of the friction path. This leaves essentially little or no headroom for bolt stresses should slippage occur. De-rating the preload to allow more shear to be put into the bolts is contraindicated.

• Design philosophy should change from providing “belt and suspenders” to providing a stronger “belt”

– The “suspenders” are inherently weaker than the “belt”– The neighboring “belt” may well pick up excess shear loads before the

“suspenders” do any work. A lateral displacement of only 1 mil loads the friction joint to 15 kips. A lateral displacement of 6 mils is required to load the bolt (through hole assuming a perfect fit and a SS bushing). Tapped holes will be somewhat stiffer. Assembly gaps will make the load path through the bolts much softer.

28

Fan single bolted joint analysisSteps toward providing a stronger belt

• Increase the preload– Add bolts where possible, even on the inbd/outbd feet if appropriate– Use low CTE washers and shims– Measure the preload with high accuracy– Re-tension the bolts after allowing them to relax

• Increase the COF– Optimize and control preparation of the alumina coating and flanges to ensure a

high COF– Provide small contact areas that result in high but manageable contact pressures

(because of the apparent favorable trend of COF v. contact pressure)• Provide good fit-up • Ensure the preload is not lost

– Monitor preload during operation. Re-tension bolts where needed.– Positively secure nuts so they do not back off.– Avoid overstressing bolts

• Bushings serve no apparent purpose (except perhaps to register adjacent coils to each other) if we have no slippage. Where we have through bolts, additional preload could be provided by eliminating the bushings and going to a larger (1.5”) bolt size, especially on the C-C interface.

29

Myatt analysis on 1/2/07Frictionless single bolted joint

Joint1 Transverse Load, Stiffness v. MotionZero Friction at Shim Interface

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Stiffness

15 kip lateral load results in 12 mil lateral deflection with zero friction. Consistent with Fan’s later calculations.

30

Myatt analysis on 1/2/07 Frictionless single bolted joint

Joint2 Transverse Load, Stiffness v. MotionZero Friction at Shim Interface

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Tapped holes are stiffer than through holes. 15 kip shear load results in 7 mils lateral deflection v. 12 mils.

31

Myatt analysis on 1/2/07 Frictionless single bolted joint

25 kip lateral loadMax bearing stress is 67ksi2.68 ksi/kip

15 kip lateral loadMax bearing stress is 35ksi2.35 ksi/kip

Bearing stresses on the bushings are also consistent with Fan’s later calculations.Provides confidence in Fan/Myatt models of single bolted joint.

32

Myatt analysis on 1/2/07Frictionless model of A-B joint

• Interface modeled as sliding (frictionless) but always in contact because of numerical difficulties with gaps opening

33


• Concluded that ORNL results (aka Brooks/Fan no slip calculations) underestimates the shear force distribution compared to this quasi-nonlinear approach

– Not clear that the comparison is meaningful• Max shear per bolt is calculated to be 19 kips on Bolt 17, but may be lower if

“missing bolt” added to model. Max would then be 16 kips on Bolt 26.

A-B Flange Bolt Shear Loads (Without Friction)

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2625242322212019181716151413121110987654321

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Sh

ea

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orc

e, l

b

global16 (d_bolt=2.75")

ORNL: EM without IL Bolts

34


• Observed that max shear per bolt seems to peak on end and isolated bolts• Concluded that 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

35

Myatt analysis on 3/22/07Model of A-A joint with friction

• Added 5 bolts on inboard side. Middle bolt was found to carry no shear so effectively only 4 bolts were added.

• Bolt preload of 81 kips with a COF of 0.3 were used

36


• Max shear loads were on the added inboard bolts with a range of 12-15 kips/bolt

37


• Concluded that contact slippage would occur and that friction would not suffice to prevent slippage on each cycle

• Cyclic loading of the bolt in bending was flagged as a concern

38

Myatt analysis on 3/22/07Conclusions

• We should eliminate the bolt on the midplane and add two more off the midplane on the A-A flange

• We should add as many bolts as possible on the A-B flange. The number was upped to six.

• We already added as many bolts as possible (4) on the B-C flange.

• Assuming a COF of 0.6 (representative of a 10ksi contact pressure under the shims), the 2/3 criterion would result in a minimum COF of 0.4. Lowering the preload to 74kips and increasing the minimum COF to 0.4 would increase the shear capability per bolt by 5 kips. This reduces the max shear carried by the bolts in the outboard region to near zero. The shear carried by the four inboard bolts would be reduced to 7-10 kips/bolt.

• Adding the two additional inboard bolts would increase the shear reacted by friction by 30 kips per bolt. This will likely reduce the max shear carried by the six inboard bolts to zero.

39

Summary of bolted joint analysis

• All of the analysis is driving us in the same direction – do whatever we can to ensure that slippage does not occur– Shear loads transmitted by friction results in low

incremental stresses. The opposite is true for shear loads transmitted by bolt bending and bushing contact.

– Assembly gaps between the bolt and bushing have no effect when shear loads are transmitted by friction. They have a very detrimental effect when shear loads are transmitted by bolt bending and bushing contact.

– Slippage in the inner leg increases shear loads on the end bolted joints, reducing our structural margins

– Adding bolts on the inner leg reduces the COF required to prevent slippage to 0.4 which is significantly below what is being achieved in the tests.

40

Managing shear loads in the unbolted regions

• We do not have a workable design for managing the shear in the unbolted regions

• This is recognized as a major cost and schedule risk in addition to being a major technical risk

• The history is as follows…– Friction relying on conventional SS shims found to be

inadequate– Studs welded to flanges with potted shear plate had assembly

and performance issues– Shear pucks mounted in plates welded to the flanges

proposed. Weld stresses and assembly fit-up are lingering concerns. Analysis and weld trials underway.

– High friction (alumina coated) shims suggested as an alternative

41

Freudenberg analysis of shear puckBasic model

• Puck Size = 1”• Holder shim = 1.5” diameter• Weld = 3/16” perimeter.• Weld connects shim to flange

only (no contact under shim to flange face except at welds, small 5 mil gap under shim)

• Sliding contact between two shims (mu = 0)

• Initially bonded contact between shim and puck.

• All materials have Stainless steel properties at LN2.

Front view with 1/8” weld and 1” puck, 1.5” long shim

Fixed on face

42

Freudenberg analysis of shear puckShear loads

Running shear Load

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Flange

Sh

ea

r L

oa

d (

lb/i

n)

Current models use 4.2 kips/inConsistent with running shear load from Brooks’ calculations (in theabsence of bolts)

43

Freudenberg analysis of shear puckDeflections

Max deflections in flanges are 1.7 mils Comparable to stiffness of bolted joints in no-slip condition

44

Freudenberg analysis of shear puckStresses

Weld size of 3/16”Peak stresses of 42 ksi in weldAllowable stresses are TBD

45

Freudenberg analysis of shear puckAlternate model of weld region

• Changing the weld profile from curved to triangular reduces the peak stresses from 42 ksi to 33 ksi

• Changing the plate from square to circular did not reduce peak stresses

46

Shear puck optionWeld distortion trials

• A substantial 3/16” perimeter appears needed to transmit the shear loads from the plate to the flange

• The concern is that applying the weld will bow the plate and open/distort the hole due to weld shrinkage

• Dudek has initiated a weld trial to be performed– Circular plate– Reliefs cut on top/bottom of the plates to minimize

distortion of the circular hole– First results should be available this week

47

Shear puck option Requirements and performance

• Provide structural continuity in the toroidal direction so radial centering forces can be reacted by hoop compression– Requires good fit-up, either by machining to fit or using thin shim material– There is a risk that bowing of the plates after welding could degrade toroidal fit-

up• React shear loads to prevent relative motion and keep the bolted joints

from being overloaded– The stiffness of the shear puck design is comparable to the stiffness of the

bolted joint in the no-slip condition so in principle, shear loads could be shared– There is a risk that the weld stresses will be too high. Allowables are TBD.

• Provide for manageable assembly– This concept requires simultaneous blind fit-up of dozens of pucks on the three

C-C interfaces into tight fitting holes – major technical risk– Assembly tolerances between the puck and the mating hole are needed

because of potential distortion from welding plus the need to simultaneously fit dozens of pucks. If a puck does not fit, it will be virtually impossible to determine which one is the problem.

– Dudek has estimated that annular gap of perhaps 30 mils will be required between the shear puck and the hole that receives it.

– Our ability to measure and position is likely to be in the 20 mil range at best– If the assembly tolerances are indeed large relative to the deflections under

load (1.7mils), then the shear pucks will not pick up initial shear loads and will not share the load evenly. Individual shear pucks plus the bolted joints will likely be overloaded. THIS LOOKS LIKE A SHOWSTOPPER.

48

Friction puck optionDescription

• An alternative to having a positive shear connection (a la the shear puck option) in the unbolted region is to rely on friction

• One concept would be to provide constant thickness shims like we are providing in the bolted region attached to one flange via a stud welded to the flange– The primary purpose of the stud is to retain the friction shim

• The shim would be alumina coated on both sides, providing high COF surfaces and electrical isolation

49

The friction puck optionDescription

• Compressive loads on the C-C interface are about 6.1 kips/in. Shear loads are about 2.5 kips/in.

• With 1.75 in OD pucks on 2.75” centers featuring a 1” ID hole for attaching a retaining nut to the stud, the compressive stress due to EM loads should be in the neighborhood of 10 ksi with shear loads of 4 ksi

• Compressive stresses are the same as for the shims under the bolts

• The expected COF is 0.6 and the required COF is 0.4, providing a factor of safety of 50%

• We may want a retaining plate around the friction pucks to ensure they never fall out

• A special arrangement might be required for the narrow band near the midplane on the C-C interface

50

Requirements and performanceFriction puck option

• Provide structural continuity in the toroidal direction so radial centering forces can be reacted by hoop compression– Requires good fit-up. Using alumina coated shims, we should

have almost a continuum of thicknesses to choose from.• React shear loads to prevent relative motion and keep the

bolted joints from being overloaded– The stiffness of the friction puck design should be directly

comparable to the stiffness of the bolted joint in the no-slip condition so in principle, shear loads could be shared

– Shear loads and compressive loads come from the same coil currents. Net radial centering forces HAVE TO be reacted by hoop compression – there is no other load path. In a friction joint, hoop compression provides shear capability. You cannot have add shear loads w/o added shear capability – analogous to wedged TF coils.

• Provide for manageable assembly– In this option, the coils will be pre-fit, the gaps will be

measured, and shims of the appropriate thickness installed– Assembly risks are minimal

51

Managing shear loads in unbolted regions – a path forward

• The shear puck option has serious risks– Simultaneous assembly of dozens of shear pucks into blind holes

on three interfaces is a daunting prospect– Not clear it will work if assembly gaps are large (as expected)

relative to shear puck and bolted joint deflections under shear loads

• The friction puck option is a simple extension of the shim design in the bolted joint– The assembly challenge will be to provide good fit-up– Compressive loads will provide added shear capability even if they

are not ideally distributed– Assembly and performance risks appear much lower

• Recommend pursuing the friction puck option as the baseline– Determine what, if any, development activities are appropriate– Wrap up development activities on the shear puck option– What are the risks in going down this path?

interpretation of mc interface analysis results

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