Fasteners / Joint Design Michael Kalish NSTX TF FLAG JOINT REVIEW 8/7/03.

Fasteners / Joint Design

Michael Kalish

NSTX TF FLAG JOINT REVIEW 8/7/03

Outline

• Assembly Overview

• Discussion of Preload

• Review of Design with respect to Cyclic and Static Loading for:

– Flag Inserts

– Flag Studs

– Shear Key Threads

– Shear Key Bolts

• Acceptance Criteria

Flag Hardware Exploded View

Stud Preload

• Maintaining the preload on the stud is critical for maintaining contact pressure and contact resistance

• Using a long narrow stud results in a much higher stud elasticity relative to the flag

• Stud elasticity keeps fatigue loading for the stud and insert low relative to the applied loading

0

1000

2000

3000

4000

5000

6000

7000

0 10 20 30

Load vs Deflection for Stud and Cu Flag

Stud Extension Flag Compression

AppliedLoad3,300 lbs

AdditionalBolt Loading300 lbs

Slope representsElasticity of Stud

Slope RepresentsElasticity of Cu Flag

Forcelbs

Inches x 10^-3

Preload Continued

• Belleville washers guarantee maintenance of preload in the event of unanticipated strain– The washers have a stiffness equivalent to that of the stud– Total washer deflection = .026”– With a strain as high as .010” washers will prevent preload from

dropping below 3,900 lbf. (The washer and the stud each relax .005”)

• Studs to be pre-tensioned with a hydraulic puller to eliminate stored torque along the length of the stud.

Flag Hardware

INSERT

TapLok Threaded Inserts

• A “TapLok” 3/8-16 “Medium Length” insert is used (OD into copper is .50”)

• Loading:– The stud preload of 5,000 lbf results in 11,800 psi in shear at

the outer threads of the insert into the copper.– Thermal + Mechanical loading adds a cyclic load of 1,800 psi

• Per the inspection certification the Cu Tensile strength = 38 kpsi and Yield strength = 36 kpsi.

• Values of 34 kpsi used for yield to account for observation of slight degradation to hardness after thermal cycling

Flag Stud Insert Loading & Stress Summary

Stud Nominal Loading

Stud Nominal Loading + Thermal

Insert Cyclic Test

Insert Cyclic Test 2x Stress

Axial Preload lbf 5000 5000 5000 5000

Cyclic Axial Loading lbf 200 755 1000 2360

Maximum Axial Load 5200 5755 6000 7360

Cu Thread Preload Stress 11765 11765 11765 11765

Cu Thread Cyclic Axial Max Stress 12235 13541 14118 17318

Cu Thread Static Factor of Safety 1.60 1.45 1.39 1.13

Modified Goodman Diagram for Insert in Copper Conductor

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25

0 5 10 15 20 25

Mean Stress (kpsi)

Fa

tig

ue

Str

es

s A

mp

litu

de

(k

ps

i)

Ultimate Shear

.577x Tensile Yield

StressAmplitude x 2.5

Mean Stress

Beneath Bold Lines RepresentsAcceptance Criteria

Pull Testing Indicated Failureat 27,000 psi

Nominal Stress Values(50,000 Cycles)

Stress Values with Flag Thermal (1,000 Cycles)

As Tested at 50,000 Cycles

As Tested >2x stress at 50,000 Cycles

.577xYield

10^6 Cycles

20,000 Cycles

`

Testing Reinforces Analysis

• Ultimate shear strength used in analysis is 22 kpsi, lowest pull out force of 11,000 lbf from testing is equivalent to 27 kpsi shear. Testing verifies values in analysis are conservative

• Cyclic testing at 50,000 cycles did not result in any degradation to pull out strength for the test samples

• Samples survived tests at 2x fatigue stress at 50,000 cycles

Flag Hardware

STUD

Flag Stud Loading

• A preload of 5,000 lbf is applied with an equivalent stress of 64,900 psi

• Thermal loading after ratcheting of the flag temperature applies an enforced deflection of .0043 inches

• Thermal ratcheting +mechanical loading adds 9,800 psi

Flag Stud Loading & Stress Summary

• With the 5,000 lbf preload and the thermal loading applied the stud sees a max stress of 74.7 ksi

• The ultimate tensile strength for the Inconel 718 stud is 210 ksi and the yield strength is 185 ksi

Stud Nominal Loading

Stud Nominal Loading + Thermal

Axial Preload lbf 5000 5000

Cyclical Axial Loading lbf 200 755

Maximum Axial Load 5200 5755

Stud Preload Stress 64935 64935

Stud Cyclical VonMises Stress 67532 74740

Stud Static Factor of Safety VonMises 2.74 2.48

Modified Goodman Diagram For Stud

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200

0 50 100 150 200 250

Mean Stress (kpsi)

Fat

igu

e S

tres

s A

mp

litu

de

(kp

si)

Ultimate Tensile

S.e Infinite life

Yield Strength


Mean Stress

Mean Stress = 69,840 psiStress Amplitude = 4,900 psi

Beneath Bold Line RepresentsAcceptance Criteria

YieldStrength

Shear Key Hardware

Threads

Shear Key, Threaded Copper

• Unlike the Flag studs the Shear Key bolts have more depth of copper available (and less width) so no inserts are used

• Testing indicated similar pullout strength for the deeper tapped holes without inserts

• Analysis indicates adequate shear area for both cyclic and static loading

• Analysis indicates strength is adequate to survive off normal conditions

Shear Key Thread Load & Stress Summary

45 Deg. Bolt Frictionless Case

Vertical Bolt Frictionless Case

45 Deg. Bolt .2 Friction

Vertical Bolt .2 Friction



45 Deg. Bolt .4 Friction No Preload

Cyclic Test Conditions

Axial Preload lbf 4500 4500 4500 4500 4500 4500 0 5000

Resultant Cyclic Axial Load lbf 1014 828 779 600 556 389 2390 1000

Maximum Axial Load lbf 5514 5328 5279 5100 5056 4889 2390 6000

Cu Thread Preload Stress psi 10204 10204 10204 10204 10204 10204 0 11338

Cu Thread Cyclic Axial Max Stress psi 12503 12082 11971 11565 11465 11086 5420 13605

Cu Thread Static Factor of Safety 1.57 1.62 1.64 1.70 1.71 1.77 3.62 1.44

Modified Goodman Diagram for Shear Key Bolt in Copper Conductor

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5

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15

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25

0 5 10 15 20 25

Mean Stress (kpsi)

Fat

igu

e S

tres

s A

mp

litu

de

(kp

si)

Ultimate Shear

.577xYield

.577xTensile Yield


Mean Stress

10^6 Cycles

Worst CasePull Testing Indicated Failureat 24,670 psi

KEY

45 Degree ThreadsVertical Threads

Design Point, .4 FrictionLoss of FrictionLoss of Preload .4 FrictionTest Point 5,000 -6,000 lbf

50,000 Cycles

Endurance Limit

Shear Key Hardware

Bolts

Shear Key, Bolts

• Custom compression washers are used to maintain preload. Provides .007” deflection

• Use of Inconel 718 Bolts ensures large safety margins even with loss of preload and without friction

Shear Key Bolt Loading & Stress Summary

45 Deg. Bolt Frictionless Case

Vertical Bolt Frictionless Case





45 Deg. Bolt .4 Friction No Preload

Cyclic Test Conditions

Axial Preload lbf 4500 4500 4500 4500 4500 4500 0 5000

Resultant Cyclic Axial Load 1014 828 779 600 556 389 2390 1000

Maximum Axial Load 5514 5328 5279 5100 5056 4889 2390 6000

Bolt Preload Stress 58442 58442 58442 58442 58442 58442 0 N/A

Bolt Cyclic Axial Max Stress 71610 69195 68558 66234 65662 63494 31039 N/A

Bolt Static Factor of Safety (Von Mises) 2.58 2.60 2.69 2.73 2.82 2.87 5.96 N/A

Modified Goodman Diagram for Shear Key Bolt

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120

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Mean Stress (kpsi)

Fa

tig

ue

Str

es

s A

mp

litu

de

(k

ps

i)


Mean Stress

Beneath Bold Line RepresentsAcceptance Criteria

KEY45 Degree Bolt Vertical Bolt

Design Point, .4 Friction

Loss of Friction

Loss of Preload, .4 Friction

Loss of Preload No Friction

A286 Bolt

Inconel 718 Bolt

`

Summary, Acceptance Criteria

• As plotted against the 20x life fatigue curve nominal design points fall within acceptable fatigue stress limits for inserts and copper threads.

• Copper threads and Inserts were tested at 2x nominal design cyclic stress values at 50,000 cycles (1 x life) or greater with no failure

• Static stress values fall at about 2/3 yield for stress in the copper threads and inserts and at less than half of the tested mean shear failure strength

• Analysis and testing predict no failure in the copper threads for the off normal conditions presented

• Stress values for the Inconel Shear Key Bolts and Flag Studs have much larger margins of safety for all of the above criteria

Fasteners / Joint Design Michael Kalish NSTX TF FLAG JOINT REVIEW 8/7/03.

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Transcript of Fasteners / Joint Design Michael Kalish NSTX TF FLAG JOINT REVIEW 8/7/03.