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.
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
0
5
10
15
20
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
0
20
40
60
80
100
120
140
160
180
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
StressAmplitude x 2.5
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
Vertical Bolt .4 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
0
5
10
15
20
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
StressAmplitude x 2.5
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 .2 Friction
Vertical Bolt .2 Friction
45 Deg. Bolt .4 Friction
Vertical Bolt .4 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 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
0
20
40
60
80
100
120
0 20 40 60 80 100 120 140 160
Mean Stress (kpsi)
Fa
tig
ue
Str
es
s A
mp
litu
de
(k
ps
i)
StressAmplitude x 2.5
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