The build and analysis of a simple truss structure was required in order to obtain information on locked-in strain of a simple truss structure made of acrylic members. In order to calculate the locked-in strain, masses were loaded and unloaded in various patterns while measuring the strain when the structure was upright and flipped 180 degrees. Data was collected in order to calculate the locked-in strain by subtracting the downward strain from the upward strain.

1. PRECISION STRUCTURE ANALYSIS AND BUILD
Gabriela Alatorre, Leonardo Bonilla, Garret Dunn, Johann Franco-Ortiz
University of Rochester, Hajim School of Engineering and Applied Sciences
Department of Mechanical Engineering
Problem Statement
The build and analysis of a simple truss structure was required
in order to obtain information on locked-in strain of a simple
truss structure made of acrylic members. In order to calculate
the locked-in strain, masses were loaded and unloaded in
various patterns while measuring the strain when the structure
was upright and flipped 180 degrees. Data was collected in
order to calculate the locked-in strain by subtracting the
downward strain from the upward strain.
Requirements and Specifications
Requirements:
● The structure must be made out of low cost materials.
● Strain gauges must be utilized.
● The masses must mount to the structure with at least 3
points of contact.
● The structure must be modular and be able to support at
least 3 different configurations of loading and unloading
the masses.
● The structure and test procedure must be made/ executed
such that the process can be repeated.
● The structure must be able to be rotated 180 degrees,
while maintaining the same boundary conditions, in order
to average out the strain readings.
● The structure must have some way to lock in the strain due
to the masses.
Specifications:
● The material used for the structure must be rigid but still
easily deformed (Modulus between 1-200 GPa).
● Deformations must stay within linear region of material but
be large enough to get significant results with strain
gauges. Strain Levels must be at least in the microstrain
regime for strain gauge to read results.
A rectangular truss structure design was chosen mostly because of its ability to meet the desired
requirements, its simple design that helped with ease of manufacturing, and several others reasons shown in
the Pugh Matrix (Table 1).
Results/ Challenges
Evaluation of
Requirements and Specifications
The current design has satisfied the established requirements
and specifications. Acrylic was selected as the construction
material due its low modulus (2 GPa). Acrylic also produced
strain results within the microstrain regime. The support
structure for the truss was constructed with a rotating bar in
order to sustain 180 degree rotation. This allowed for the truss
structure to maintain the same boundary conditions once it
was rotated. 4 loading locations exceeded the requirement of
3 loading configurations. Spherical bearings between the
masses and horizontal beams allowed for multiple points of
contact even when the structure was rotated.
Analysis/Manufacturing/Testing
Concept Description
Acknowledgements
Future Work
There has been a minimal amounts of research looking into
locked-in strain. Future tests with various modifications could
be done to investigate locked-in strain further, including;
different shaped truss structures, structures made from
different materials, use of more than three masses during
testing, and different methodologies to replicate the locked-in
strain.
Design Strength* Ease of
Manufacturing
Ability to easily be mounted
and rotated
Customizability
(Different loading pattern options)
Triangular -1 +1 -1 -1
Rectangular - - - -
Hexagonal -1 -1 -1 +1
Hexagonal(2) -1 -2 -1 +2
TABLE 1
PUGH MATRIX
*In this case a less resilient structure is preferred.
This project would not have been possible without the initial proposal and continued
guidance of Leslie Johnson, the counsel of Professor Muir, and the advice of Scott
Russell. The manufacturing portion of the project, which was pertinent to the project’s
completion,was made possible by the ordering assistance of Tal Haring and the
fabrication assistance of Jim Alkins and John Miller. Professor Ellis, Professor Genberg
and Mike Pomerantz also provided initial advice on alternative strain measurement
methods which was crucial in determining which method was used.
Figure 2. Truss Structure
Rotated 180 degrees
Figure 1. Truss
Structure Upright
Locked-In Strain Gauge 1 Gauge 2 Gauge 3 Gauge 4 Gauge 5 Gauge 6 Gauge 7 Gauge 8
Loading
One Weight 0.000192 -0.000382 0.000122 0.000386 -0.000113 -0.000432 -0.000009 -0.000066
Two Weight 0.000693 -0.000502 0.000593 0.000585 -0.000257 -0.000611 -0.000002 -0.000097
Three Weights 0.000747 -0.000555 0.000740 0.000675 -0.000377 -0.000707 0.000008 -0.000076
Unloading
One Weight -0.065484 -0.060015 -0.063757 0.000384 0.000000 -0.000178 0.000056 0.000123
Two Weights -0.065484 -0.060015 -0.063758 0.000549 0.000000 -0.000406 0.000052 0.000089
Three Weights n/a n/a n/a n/a n/a n/a n/a n/a
TABLE 3
CALCULATED LOCKED-IN STRAIN VALUES
All eight strain gauges were wired into an eight channel data
acquisition device. This device was plugged into a computer
that was running a LABVIEW program. Using the program,
100 data points were taken three times for each test
configuration. The structure was tested using 1-3 weights, in
an up orientation and down, and in different loading
sequences. The resulting data files were manipulated in
MATLAB which outputted figures of the data as well as
average values for the eight channels. These average values
were taken and manipulated further in excel. In particular
baselines were subtracted out from the data to help zero the
channels. From there, the Up configurations were subtracted
from the Down configurations for the Loading and Unloading
patterns. The locked-in strain was the calculated in Table 3.
Only minor changes occurred during testing. The ends of the
vertical legs experienced some minor cracking which was
easily remedied. One of the strain gauges also seemed to fail
during testing as the data was inconsistent. This was
unfortunate but not viewed as an issue since six gauges
were required and the team placed eight.
The structure which was fitted with eight uni-axial strain
gauges was designed such that deflections when the
structure was loaded were easily measured by the
gauges. The structure was loaded with ~1 lb weights in
different locations and loading patterns both upright and
flipped 180 degrees down. By obtaining this strain data,
gravity could be “averaged out” and a correlation between
the FEA model and the physical model could hopefully be
made which would help the aerospace industry better
understand locked-in strain.
The test structure, seen to the right, was made entirely
out of acrylic. This material was chosen for its low
modulus of elasticity and ease of manufacturing. Rod end
bearings were used at the ends of all the vertical legs to
make the forces in the legs be more axial and more easily
read by the gauges. The two side support structures seen
in Figures 1 and 2 had a main pivot pin and a clamping
bar in order to rotate the structure. The clamping bar
rotated on the main pivot pin and two locking pins were
put into it at either end. The base plate would be inserted
onto all three pins on both sides and clamps would be
added to hold it from rotating. By clamping in the direction
normal to gravity, no external forces would be added to
the structure in the desired direction.
Figure 1. Linear Buckling FEA Analysis
Figure 1. Three masses
loaded