Transcript Presentation.ppt

The Effect of T-Stiffener Web and Flange Tilt on Frame
Stress Evaluated using Finite Element Analysis
by
Dean Pasquerella
MASTER OF ENGINEERING
Major Subject: MECHANICAL ENGINEERING
Project Adviser: Ernesto Gutierrez-Miravete
Rensselaer Polytechnic Institute
Hartford, Connecticut
December, 2014
Brief Overview
Goal:
Use finite element analysis to determine at what angular offset a T-stiffener will exceed yield
when subjected to uniform loading from submergence pressure in a submarine application. It
is unavoidable during the fabrication and construction process of a plate/stiffener assembly to
create geometrical imperfections. These imperfections result in an increase in stress and
deflection of the stiffener due to a modified load path. However, during typical initial design,
these deficiencies are not taken into account, as the assembly is analyzed as having perfect
geometry. Therefore, the effect on specification requirements for stress due to tilt in the web
and flange of a T-stiffener is unknown.
Evaluation:
The evaluation will be performed on a cylinder supported by T-stiffeners, which is externally
pressurized to simulate a submarine under sea pressure. Initial engineering design will be
performed on a T-stiffener in which the web and flange are perpendicular to one another and
the web and cylinder are perpendicular to one another. This will serve as the baseline model.
The vertical web and horizontal flange pieces of the T-stiffener will then be offset to certain
angular imperfections resulting in increased stress and deflection in the T-stiffener. Varying
offsets of the web and flange will be evaluated to determine what construction tolerances are
required to assure that the design does not result in stresses exceeding specification
tolerances, which in turn could lead to failure in the design.
Introduction/Background
Schematic Showing the Portions of a T-Stiffener
Elevation View of a Submarine Pressure Hull with Internal Stiffeners
Cross Section View of Different Web and Flange Tilt Combinations
Theory/Methodology
Assumptions:
Material: HY-100 Steel
Loading: 1100 psi uniformly distributed external pressure
Stress Allowable: 95% of the material yield strength
Geometry: stiffener size = 10.0” x 6.0” x 0.5” x 0.7”, shell thickness = 1.5”, outside
diameter = 360”, frame spacing = 30”
Closed Form Solution:
Circular cylindrical shell reinforced by uniformly space ring fames
Finite Element Analysis:
Elements: 2-dimensional, four noded, quadrilateral general shell elements
Model Type: Sub-model of only 4 stiffeners total
FEM Loading and Boundary Conditions
Submarine Pressure Hull and Sub-Model Location
Depiction of the Location of End Load and Displacement
Boundary Conditions in Relation to Frame Locations
Depiction of the Location of End Load and
Displacement Boundary Conditions
Section View at Cut Plane Showing Location of
Displacement Boundary Conditions
Preliminary Evaluation
Purpose: Perform a boundary condition evaluation and mesh convergence study to
determine the most accurate baseline model to use in the evaluation.
Convergence Analysis:
Frame # Frame Stress (ksi) % Change
1
-92.56
2
-94.21
1.78
3
-94.35
0.15
4
-94.32
0.03
5
-94.32
0.00
6
-94.32
0.00
Results
Section View Showing Frame Locations
Mesh Convergence Study:
Model Name
Converge1.cdb
Converge2.cdb
Converge3.cdb
Converge4.cdb
Shell
# of Elements
Circumferentially Longitudinally Per Bay
120 = 3 degrees
4
180 - 2 degrees
4
270 - 1.5 degrees
8
360 = 1 degrees
8
Web Height
Flange Width
Size (in.) # of Elements Length (in.) # of Elements Length (in.)
9.4 x
6.3 x
4.2 x
3.1 x
7.5
7.5
3.8
3.8
2
3
3
4
Description of Mesh Density
5.2
3.5
3.5
2.6
2
2
4
4
3.5
3.5
1.8
1.8
Results
Results
Closed Form Solution:
Peak compressive stress on the internal face of the flange: 94.79 ksi < 95.0 ksi allowable
Baseline Model:
Peak compressive stress on the internal face of the flange: 94.69 ksi, 0.11% difference
from closed form solution
Model Iterations:
Peak compressive stress on the internal face of the flange exceeds 100 ksi with a 2.0
degree web and flange tilt
Peak compressive stress on the internal face of the flange with a 1.5 degree web and
flange tilt: 99.38 ksi < 100 ksi maximum limit
Web and Flange
Tilt (Degrees)
0.0
0.5
1.0
1.5
2.0
Peak Compressive
Stress in Frame (ksi)
-94.69
-96.36
-97.87
-99.38
-101.42
Web and Flange Tilt Model Iteration Results
Stress and Deflection Plots
FEM Results for 2.0 Degree Web and Flange Tilt Condition
Conclusions
•
Using 2-dimension quadrilateral elements, the initial design was constructed using FEA to
validate the closed form solution and allow for modifications of the stiffener geometry. The
baseline FEM resulted in a peak compressive stress in the frame of -94.69 ksi, which has a
percent difference from the closed form solution of 0.11%. A difference of 0.11% is considered
adequate for the level of accuracy required for pressure hull structure design.
•
The baseline FEM was then modified by varying the angular offset of the web to the pressure
hull and the flange to the web until a peak compressive stress exceeded the 100 ksi allowable
limit. Using this approach, a web and flange tilt of 2.0 degrees resulted in a peak compressive
stress of -101.42 ksi, which exceeded the allowable by 1.42 ksi, while a web and flange tilt of 1.5
degrees resulted in a peak compressive stress of -99.38 ksi, which is within the allowable limits.
Therefore, for the pressure hull design evaluated herein, the specification documentation shall
require a web and flange tilt of less than or equal to 1.5 degrees to assure yield of the material is
not exceeded, which could result in failure of the design.
•
The 1.5 degree requirement for web and flange tilt is within the capabilities of welding and
assembly of typical pressure hull submarine structure. Knowing the allowable web and flange tilt
tolerance permits engineering, design and trades personnel to make informed decisions about
construction of the pressure hull. During measurement of these geometrical properties, any
value below 1.5 degrees for web or flange tilt can be accepted without requiring rework of the
structure. Additionally, steps can be taken in the fabrication and assembly processes to utilize
this tolerance to construct the submarine while taking into account quality, time and cost.