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Batman
(Each presentation has been given an
unique name so as to hide the identity of
the presentation author from the Reviewer,but known to the Instructor)
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Finite Element Validation of Low
Impact Response on a Lab-Scale
Space Frame Structure
Jagadeep Thota, Mohamed Trabia & Brendan OTooleDepartment of Mechanical EngineeringHoward R. Hughes College of Engineering
University of Nevada, Las Vegas
Las Vegas, NV, USA.
ASME 2012 Verification & Validation SymposiumMay 2nd4th, 2012, Las Vegas, NV, USA
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Background Space frame structures have been commonly used in
vehicles to enhance their structural strength while reducing
the overall weight.
When a vehicle, with an internal space frame structure, is
subjected to an impact load, the individual frames and
joints of the space frame play a critical role in mitigating
the generated shocks.
In order to effectively design the space frame structure, it
is important to predict the propagation of these shocks
through the space frame members.
While performance of space frame structures under staticloads is well-understood, research on space frame
structures subjected to impact loading is minimal
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Literature Review Gaul and Lenz (1997) showed that nonlinear shock transfer
performance of joints has substantial influence on the dynamics
of the structure as they induce large amount of damping.
Sandia National Laboratory (2001) conducted FE studies for
investigating energy dissipation due to micro-slip in the bolted
joints.
Song et al. (2002, 2004) developed a beam element, which can
simulate the non-linear behavior of bolted joints on a vibrating
frame.
Ibrahim and Pettit (2005) suggested that friction in bolted joints
is a main sources of energy dissipation in mechanical structures.
Thota et al. (2011) conducted computational studies on a military
vehicle space frame subjected to high impact load.
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Objective
Develop a lab-scale space frame structure
having bolted joints.
Conduct a low impact experiment on the space
frame structure and measuring the resulting
acceleration (shock) response.
Propose a FE method that can predict the shock
response measured in the experiment.
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Lab-Scale Space Frame To consider the shocks within a 3-D structure, a lab-scale space frame
is designed.
The overall length of the cube shaped structure is 482.6 mm. The frame members are hollow, having square cross-section with wall
thickness being 3.175 mm.
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Joint Design The joint halves are C-shaped sections, which are bolted together through
the frame members.
The joint has two orthogonal branches of 114.3 mm, and the combinedwidth is 50.8 mm.
The angle joint houses the ends of the longer frame members while the
shorter frame member ends are enclosed by the joint halves.
The angle joint legs are 100 mm long, and width is 50.8 mm.
The wall thickness of the joints (including angle) is 6.35 mm.
The length of frame members outside
the joints are:
Horizontal members are 254 mm
long, Vertical members are 381 mm long.
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Joint Halves
To eliminate noise in the acceleration (shock)
signal, the faces of the opposing joint halves are3.175 mm apart.
This arrangement ensures a more homogenous
contact between the joint and the frame members.
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Space Frame Sections
D
W
tD
W
t
D
W
t
Frame member Angle Joint Joint half
Al l dimensions are in mm
Type D W t
Frame member 38.1 38.1 3.2
Angle joint 88.9 88.9 6.4
Joint 50.8 25.4 6.4
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Material All components of the lab-scale space frame, except the bolts, is made
of Aluminum 6061 alloy.
The structure approximately weighs 11.4 kg.
Density
(kg/m3)
Youngs
Modulus
(GPa)
Poissons
Ratio
Yield
Strength
(MPa)
Tangent
Modulus
(MPa)
2700 68.9 0.33 276 562
Strain
Stress
Yield
Point Tangent
Modulus
Elastic
Modulus
Failure
Point
MAT_PLASTIC_KINEMATIC
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Bolt Tightening Grade 8 bolts were used in the space frame structure .
The bolts are tightened to reduce the noise in the output signals that can result
from loose connections. Applying the same tightening torque on all bolts ensures the repeatability of
the results.
The bolts are tightened to a preload of 10.8 kN and a torque of 12.5 Nm.
These values are computed from the standard design equations:
= 0.9 = 10.8
= 0.21 = 12.5
Sp= Proof stress of the bolt material = 586 MPa
At= Tensile stress area of bolt = 2.1e-5 m2
dp= Pitch diameter of the bolt threads = 5.525e-3 m
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Impact Experiment Low velocity, non-destructive, impact experiment was carried out.
The structure is placed on an aluminum support during the experiment.
An upper frame member is impacted at the mid-member location with a forcehammer.
Acceleration is recorded, through an accelerometer, in the middle of the
opposite frame member location.
Fast Fourier Transform (FFT) of the resulting acceleration signal is conducted.
FFT is used to determine the natural frequencies of the space frame.
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Finite Element Model All components of the lab-scale structure are modeled as beam elements.
Common elements between the different components of the structure are merged to
obtain contact.
The angle joint and the bolts are not structurally modeled, but their masses are
accounted for by adding mass-elements at each corner of the cube.
Preprocessor: Altair-HyperMesh (v 9.0)
Solver: LS-DYNA (v 971)
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Finite Element Model
(3-Drepresentation
is for
illustration
purpose only)
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Finite Element Model The joint is modeled as two parts:
The first part (blue) comprises of
combined cross-sections of the frameand joint.
The second part (red) is the cross-
section of the joint.
The length of the beam elements are
maintained at 3.2 mm, resulting in a
total of 1,832 elements.
The loading condition and output
similar to the experiment are
mimicked.
Total simulation run time: 8 ms CPU (3 GHz Intel Xeon processor
with 2 GB RAM) time: Approximately
6 minutes.
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Results The unfiltered force signal obtained from the experiment is used to define the
impact curve for FE analysis.
The acceleration signals from experiment and simulation are filtered usingButterworth low-pass filter with cutoff frequency of 10,000 Hz.
The sampling rate for the experiment and FE analysis is 1 mega-
sample/second.
Typical Force
Signal
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Results: Acceleration Signal The predicted acceleration signal captures the first peak of the experiment.
Most of the subsequent acceleration peaks for the FE model are smaller than the
experimental result. The frequency of the predicted signal matches well with the experimental signal.
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Results: FFT The first predicted natural frequency is very close to the experimental one.
The rest of the natural frequencies of the cube, including the predominant one, 1500 Hz,are predicted by the FE model.
There is an additional frequency, 810 Hz, predicted by the FE model:
This may be due to the absence of some structural components such as angle joints and bolts,and holes in the FE model which might have suppressed this additional frequency.
The acceleration amplitude of this frequency is small.
Overall, this a very good match for a space frame structure such as the cube comprising ofa total of 48 bolts, 36 structural components, and 8 joint locations.
Experimental Computational
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Conclusions
Shock propagation though a space frame with bolted
joints is not well-understood. An approach using finite element analysis for
predicting shock transmission within such structures
is proposed.
The proposed approach is verified using a lab-scale
space frame structure.
Comparing experimental and finite element results
lead to the following observations: The initial peak of the acceleration signals match closely.
The FE model is able to predict all the experimental
natural frequencies.
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Future Work
Incorporate bolts in the FE model. Explore ways for mitigating shocks by optimizing
the joint design variables and include shockabsorbing material.
Expand current research to model shocks resultingfrom high velocity impact.
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Acknowledgments
We would like to thank Mr. Ami Frydman, U.S.
Army Research Laboratory, for interacting withthe authors.
We are grateful to Dr. Douglas Templeton, U.S.Army TACOM-TARDEC, for being helpful in
developing the ideas of this research. This work was funded through a cooperative
agreement with the U.S. Army ResearchLaboratory under contract DAAD19-03-2-0007.
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Thank you
&
Questions?
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Robins Comments
(type your name in place of Robin!)
6 comments on what is good in this
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6 comments on what is good in this
presentation
The model looks awesome
This is a bad comment!
The comment is too abstract
No explanation is given for why the reviewer thinks the
model is awesome
Good comments:
The quality of mesh is quite good as all the elements
comprise of the same element size and even the
smallest of the components is finely meshed. The description regarding why the joint halves should
not touching each other is informative and useful for
other researchers.
8 comments on what the presentation is
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8 comments on what the presentation is
lacking OR not clear
I dont like the figures
Bad comment
Too abstract: all figures? particular figure? Why?
Sounds personal than technical (avoid using I, we, this
person, this student.)
Good comments:
Not clear on why static material properties were used for a
dynamic analysis.
Not mentioned anywhere what type of beam elements wereused, i.e., will the space frame members acting as beams take
into effect the shear in the beams.
The natural frequency values, and the axis values on the FFT
plots on slide 18 are not clear and too small to read.
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Do not include this slide when submitting
The deadline for the review comments is
14thMay 2013(by 5:00 pm).
Emailme back the entire presentation with
your review comments at the end of the
presentation. 8 %of your project grade will be based on
how good/bad you have reviewed the
assigned project. The review comments for a project will not
have an effect on the grade of that project.
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Do not include this slide when submitting
Project grade breakdown:
Structural model: 10%
Heat transfer model: 8%
Modal analysis model: 4%
Presentation (including the results shown in thepresentation): 15%
Review comments: 8%
Total Project grade: 45% Total HW grade: 50%
Inclass:5%