Transcript Slotted_Interface_Stiffness_Project_KLeach_4.9_RP+

Evaluating and Accounting for Stiffness Differences
between 2D Axisymmetric and 3D Cyclic Symmetric
Models at Slotted Interfaces
Kurt Leach
3/21/2012
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Problem: The 2D Stiffness using Plane Stress
Elements is Different than the True Stiffness at
Slotted Interfaces
•
2D Model uses plane stress with thickness elements to model the tab
•
•
Thickness applied = (Number of Tabs) X (Thickness of One Tab)
Does not differentiate between thick low count tabs, or thin high count tabs
– i.e. 2 tabs 10” thick is accounted for the same way as 100 tabs 0.2” thick
•
Interface load differences can be as high as 15-20% in some cases
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This page contains no technical data.
Two Main Objectives: Evaluate 2D vs. 3D Stiffness
Differences, and Account for the Stiffness in 2D
• Use 2D and 3D models to evaluate
stiffness differences
• Evaluate the effect of:
• Number of Tabs
• % Material Removed by the Tab
• Come up with a way to accurately
account for the stiffness in the 2D model
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Model Parameters are Typical of a High Turbine
Rotor
1.500”
Θ 21.000”
Tab Height
Θ 20.000”
0.400”
Tab Length
2.500”
Θ 12.000”
Θ 12.000”
1.250”
1.500”
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Model Parameters are Typical of a High Turbine
Rotor
Common contact stiffnesses (KN = 1e10) were used between 2D and 3D models
(2D uses CONTAC12s, 3D uses CONTA174s)
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Number of Tabs
Four main variables are captured: % Material
Removed, Tab Length, Tab Height, Number of Tabs
% Material Removed
25%
50%
75%
4
4
4
8
8
8
16
16
16
32
32
32
64
64
64
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% Material Removed does not drive 2D vs. 3D
difference, but number of tabs does
• % Material Removed is not a large driver
• The number of tabs has the greatest effect here
• As number of tabs decreases:
• Tabs get wider
• 3D Interface Load decreases
• As tabs get wider, plane stress for the tab is no longer
applicable
For this portion of the study, Plane Stress with Thickness elements were used in 2D to model the tab.
Tab Height was held constant at 0.400” and Tab Length was held constant at 1.000”
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There are a number of ways to alter the 2D model to
achieve the true stiffness of the tab
• Ideas to alter the 2D model to more accurately account for
stiffness:
• Use Plane Strain Elements for Tab
• Use Axisymmetric Element with Zero Out-of-plane Modulus for Tab
• Modify Axial Modulus of Tab to reduce bending stiffness
• Maybe use simple shell model to evaluate what the 3D stiffness
should be
• Modify Full Hoop Region Adjacent to tab to reduce bending
stiffness of full hoop region
• Open to more suggestions!!!
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Plane Strain Elements at Tab Provide More
Accuracy at Low Tab Counts, but Still Not within
10% in some cases
Plane Strain Elements were used at Tab for this Study
Using Plane Strain Elements at the does not accurately
capture the stiffness of very low tab counts (4 tabs), and
can be too soft for high tab counts.
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Axisymmetric Elements with Zero Hoop Modulus at
Tab are No Better than the Plane Stress with
Thickness Elements
Axisymmetric Elements with Zero Hoop Modulus were used at Tab for these results
These results are very similar to the plane stress with
thickness results, and do not provide any better capability
than the Plane Stress with Thickness Elements
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Reducing the Axial Modulus of the Tabbed Region
Does Not Significantly Effect the Stiffness of the
Flange
Plane Stress with Thickness Elements were used at the tab, with reduced axial modulus
to reduce the bending stiffness
% Mat'l Rmvd # of tabs
25
25
25
25
25
4
8
16
32
64
Tab Height
# of tabs
0.200
0.200
0.200
0.200
0.200
4
8
16
32
64
Tab Length
# of tabs
0.500
0.500
0.500
0.500
0.500
4
8
16
32
64
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
158085.5
112420.7
28.9%
158959.3
130019.8
18.2%
159750.3
145715.3
8.8%
160097.5
152931.1
4.5%
160263.7
156520.8
2.3%
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
62454.8
43113.8
31.0%
62186.1
49972.0
19.6%
61963.3
55692.3
10.1%
61845.6
58776.2
5.0%
61792.0
60193.0
2.6%
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
109447.0
68764.9
37.2%
109775.0
84573.0
23.0%
109977.6
98209.6
10.7%
110167.9
104676.7
5.0%
110217.8
107533.7
2.4%
% Mat'l Rmvd # of tabs
50
50
50
50
50
4
8
16
32
64
Tab Height
# of tabs
0.400
0.400
0.400
0.400
0.400
4
8
16
32
64
Tab Length
# of tabs
1.000
1.000
1.000
1.000
1.000
4
8
16
32
64
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
116426.0
86353.7
25.8%
117028.0
98060.2
16.2%
117560.8
107928.0
8.2%
117835.9
112940.9
4.2%
117972.1
115524.5
2.1%
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
116426.0
86353.7
25.8%
117028.0
98060.2
16.2%
117560.8
107928.0
8.2%
117835.9
112940.9
4.2%
117972.1
115524.5
2.1%
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
116426.0
86353.7
25.8%
117028.0
98060.2
16.2%
117560.8
107928.0
8.2%
117835.9
112940.9
4.2%
117972.1
115524.5
2.1%
% Mat'l Rmvd # of tabs
75
75
75
75
75
4
8
16
32
64
Tab Height
# of tabs
0.800
0.800
0.800
0.800
0.800
4
8
16
32
64
Tab Length
# of tabs
2.000
2.000
2.000
2.000
2.000
4
8
16
32
64
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
74696.5
49563.8
33.6%
74863.7
56179.7
25.0%
75152.7
64959.0
13.6%
75360.0
70943.6
5.9%
75435.8
73475.4
2.6%
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
47090.5
-#VALUE!
48088.7
195040.9
-305.6%
49069.9
217548.1
-343.3%
49505.4
228461.5
-361.5%
49687.5
233131.1
-369.2%
2D Interface Load 3D Interface Load % Diff from
[lbs]
All Tabs [lbs]
2D Model
138296.2
-#VALUE!
138373.6
129640.4
6.3%
138601.9
135033.2
2.6%
138676.8
137077.2
1.2%
138706.5
137910.1
0.6%
Reducing the axial modulus does not significantly reduce
the bending stiffness of the flange.
Something else other than the tab is driving stiffness
difference.
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