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CHAPTER - 5
EVALUATION OF VIBRATION TEST FIXTURES
Two vibration test fixtures for testing advanced launch
vehicle sub assemblies are designed and realized with AA 6061-T6
material. After design and realization of vibration test fixtures, it is
necessary to evaluate the performance of test fixture. Performance
evaluation is carried out by conducting pre resonance search,
sinusoidal vibration test, random vibration test and post resonance
search. Evaluation shows the shortcomings of the fixtures and
suggests ways to improve them. After changes are made, re-
evaluation shows improved behavior. This process teaches astute
designers more about fixture design than they can learn from any
other source. Each designer conducts his own evaluations, but the
next best is to receive the results of an evaluation done by others.
Accelerometers are mounted on the fixture at critical locations to
know the dynamic behavior. Shaker motion is investigated in the
direction of test axis that gives the ratio of input vibration to the
fixture vibration. During testing of fixture orthogonal response is
also measured. Preliminary investigation with the fixture unloaded
gives some useful information, but main emphasis should be on
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the fixture‟s response when it is loaded by a test item or dummy
load.
5.1 PERFORMANCE EVALUATION
The dynamic tests are carried out in all axes [85] to evaluate the
following characteristics of the test fixtures:
Identification of the natural frequencies of the fixture in the
test axis.
Evaluation of the cross axis motion.
Determination of the damping characteristics.
Evaluation of the isolation characteristics.
Validation of analytical methods used for predicting the
dynamic characteristics.
To evaluate the suitability of the fixture for the intended use.
To determine the best locations for vibration control on the
fixture.
For the determination of the natural frequencies,
transmissibility, damping and isolation characteristics, the
excitation is applied at the base and the responses are measured
at the top flange of the fixture at different locations. The cross axis
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response of the fixture is determined by mounting the
accelerometers in orthogonal directions to the excitation axis. The
ability to control uniformly at the fixture and test specimen
interface is demonstrated by conducting the sine and random
vibration tests with control accelerometers mounted on the fixture
flange.
5.1.1. Validation of analytical methods:
In this thesis two analytical methods are used for predicting
the dynamic characteristics of the fixtures which are modal
analysis and static analysis. From the Modal analysis different
mode resonance frequencies are identified. The modal analysis
results are validated by conducting sine resonance search test.
Sine resonance search test is conducted by sweeping the
frequencies from 10 to 2000Hz with 0.5 g constant input energy
level through frequency band. The static analysis results are
validated by conducting sine and random vibration tests for full
capacity. During this full level testing, strain gauges are used to
measure the critical strains as per the specified energy levels.
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5.2 PERFORMANCE EVALUATION TESTS
5.2.1 Sine Resonance Search Test
This test is conducted to find the different modal resonant
frequencies, transmissibility at resonant frequencies and damping.
For this test, control accelerometers are fixed at the base of the
fixture and monitoring accelerometers are fixed on the top plate of
the fixture. Sine resonance search test is conducted by sweeping
the frequencies from 10 to 2000Hz with 0.5 g constant input
energy level through frequency band.
5.2.2 Sine vibration test
Sinusoidal Vibration is a special class of vibration. The
structure is excited by a forcing function that is a pure tone with a
single frequency. Sinusoidal vibration is not common in nature,
but it provides an excellent engineering tool that enables us to
understand complex vibrations by breaking them down into simple
one tone vibrations. This test is conducted from frequency band of
10 to 100 Hz. The motion of any point on the structure can be
described as a sinusoidal function of time. When performing a sine
test, one frequency is exited at each time. During sine vibration
test each part of a complex structure is resonate at a different
frequency. From this test sub components resonant frequencies
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and its impact on other components are identified. This test can be
performed for the maximum capacity of the test equipment. During
this test strain gauges are bonded to measure the critical strain
values. With this data, the static analysis data can be validated.
5.2.3 Random Vibration Test
Random vibration test simulates the realistic dynamic
Environment to the structures. Unlike sine vibration test, in
random vibration test all frequencies in the test frequency band
are present at all times. The random vibration test has a non-
deterministic nature ie the vibration levels cannot be predicted at
any time in the future. In random vibration testing, the vibration
has a Gaussian distribution, which means it can have peaks three
or more times the rms of the vibration level. From this test all the
sub components resonant frequencies are exists at a time and its
impact on the test article can be understood.
5.2.4 Post Resonance Search Test
The post resonance search test is similar to pre resonance
search test in specifications. This test is used to identify any
potential weakness in the system because of full level sine and
random tests, by identifying the shift in resonance frequencies and
amplifications at resonances and isolations at other frequencies.
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5.3 Affect of no. of drilled holes and size on dynamic
characteristics of the fixture:
Drilled holes in the fixture top and bottom flanges are used
to fix test article and vibration shaker. Vibration shaker table is
having standard PCDs and tapped holes to fix the test fixture. The
existing test setup is having a maximum PCD of 1200mm and M12
tapped holes in vertical axis whereas in horizontal axis, a
maximum PCD of 2800mm and M12 tapped holes are available.
Accordingly in the same PCDs drilled holes (14mm size) are
reproduced in the bottom flanges of both the fixtures. The top
flange of the fixtures is used for fixing the test article. The
advanced launch vehicle sub systems are having drilled holes in
4030mm PCD. Therefore, top flanges of both the fixtures are
provided with drilled holes in 4030mm PCD to fix the test articles.
Hence the drilled holes and its sizes are not chosen arbitrarily and
the no. of holes and size does not affecting the vibration
characteristics of the fixture.
5.4 VERTICAL AXIS TEST FIXTURE
The vibration test fixture for longitudinal axis is configured
as an expanding conical structure stiffened with radial ribs. In
view of the large dimensions and weight, the fabrication method
chosen is by welding. It has a flange at the top with 13 mm
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diameter holes on 4030 mm PCD and 3240mm to match the
interface of the gsLVM3 inter stages. The base and the flange face
of the fixture are machined. The dimensional details of the fixtures
are as follows:
Table 5.1 Dimensional Details of vertical axis test Fixture
Weight of the fixture 1500 kg
Top Flange diameter 4100 mm.
Flange thickness 25 mm.
Base diameter 1350 mm
Base thickness 50 mm
The fixture is drilled with counter bored holes of size 24x13 mm for
mounting it on to the shaker. The pattern of the counter bored
holes is given below:
6 holes equi-spaced on 550 mm PCD
14 holes equi-spaced on 800 mm PCD
22 holes equi-spaced on 1000 mm PCD
19 holes equi-spaced on 1200 mm PCD
20 holes equi-spaced on 1400 mm PCD
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5.5 PERFORMANCE EVALUATION OF VERTICAL
AXIS FIXTURE
A 30t force rating dual Shaker system is used for evaluation
of this fixture in vertical axis. Two 16t capacity Electrodynamic
shakers positioned vertically. A load bearing platform is assembled
to the shaker tables which acts as a common bare table for fixing
the fixture in vertical axis. After switching on of all the sub
systems related to the shaker systems, handled the fixture with the
help of Electric crane and wire ropes and positioned over the load
bearing platform. The fixture is assembled to the load bearing
platform by using 12.9 grade high tensile allen head bolts on 8”,
16”, 22”, 800 mm, 1000 mm, 1200 mm PCDs. Nine vibration
measurement channels are fixed on the fixture top flange (at 3
locations 1200 apart ,three tri axial) and used during the vibration
tests for measurement. Two Accelerometers are fixed at fixture
base to control during resonance search. During the evaluation of
the fixture, strains at critical locations of the fixture are measured
to ascertain the stresses in the fixture during various loadings.
This strain measurement is used to verify the design calculations.
The test set up in vertical axis is shown in the Fig. 5.1(a) & 5.5(b)
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5.5.1 Performance evaluation test specifications in vertical
Axis
The sequence of tests conducted is pre sine resonance
search test, sine vibration test, random vibration test and post sine
resonance search test [115]. Table 5.2 gives the pre sine resonance
search test specifications, Table 5.3 shows sine vibration test
specifications, Table 5.4 shows the random vibration test
specifications and Table 5.5 shows the post resonance search test
specifications.
Table 5.2 Pre sine resonance search test specifications
Frequency(Hz) Test level Duration/sweep rate
10-2000 Hz <0.5g(sine) 4 oct/min
Table 5.3 Sinusoidal Vibration test Specifications
Frequency(Hz) Test level Duration/sweep rate
5-10 Hz 10 mm DA
2 oct/min
10-20 Hz 1.5g
20-100 Hz 1.05g
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Table 5.4 Random Vibration Test Specifications
Frequency(Hz) Test level
PSD(g2/Hz)
grms Duration(s)/axes
20 0.01125
11.86
120
200 0.1125
600 0.1125
2000 0.046
Table 5.5 Post Resonance Survey Specifications
Frequency(Hz) Test level Duration/sweep rate
10-2000 Hz <0.5g(sine) 2 oct/min
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Fig 5.1Test setup in vertical axis
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5.2 Accelerometers used for vibration measurement
5.3 Strain Gauges used for measuring strain values
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5.6 HORIZONTAL AXIS TEST FIXTURE
The vibration test fixture for lateral axis is configured as
cylindrical structure stiffened with internal and external radial
ribs. In view of the large dimensions and weight, the fabrication
method chosen is by welding. It has a flange at the top with 13 mm
diameter holes on 4030 mm PCD and 3240mm to match the
interface of the gsLVM3 inter stages. The base and the flange face
of the fixture are machined. The dimensional details of the fixtures
are shown in Table.5.6
Table 5.6 Dimensional Details of horizontal axis test fixture
Weight of the fixture 1000 Kg
Top Flange diameter 4080 mm
Top Flange thickness 16 mm
Base diameter 2900 mm
Base thickness 55 mm
The fixture is drilled with counter bored holes of size 24x13 mm for
mounting it on to the large slip table. The pattern of the counter
bored holes is given below:
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60 holes equi-spaced on 2600 mm PCD
60 holes equi-spaced on 2800 mm PCD
5.7 PERFORMANCE EVALUATION OF HORIZONTAL
AXIS FIXTURE
A 30-ton force rating dual Shaker system with large slip table
of 3.4 x 3.0 size is used for evaluation of this fixture in horizontal
axis. Two 16t capacity Electrodynamic shakers positioned
horizontal with connecting bars. A large slip table is assembled to
the dual shakers through drive bars. After switching on all the
sub systems related to the dual shaker systems and large slip
table, the fixture is handled with the help of Electric crane and
wire ropes and positioned over the large slip table. The fixture is
assembled to the large slip table by using 12.9 grade high tensile
Allen head 120 bolts on 2600mm and 2800mm PCDs. Twelve
vibration measurement channels are fixed on the fixture top flange
(at 4 locations 900 apart, three tri axial) and used during the
vibration tests for measurement. Four Accelerometers are fixed at
fixture base for controlling purpose. During the evaluation of the
fixture, lateral strains at critical locations of the fixture are
measured to ascertain the stresses in the fixture during various
loadings. This strain measurement is used to verify the design
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calculations. The test set up in horizontal axis is shown in the
following figures.
5.7.1 Performance evaluation test specifications in horizontal
axis
The sequence of tests conducted is pre sine resonance
search test, sine vibration test, random vibration test and post sine
resonance search test. Table 5.7 gives the pre sine resonance
search test specifications, table 5.8 shows sine vibration test
specifications, table 5.9 shows the random vibration test
specifications and 5.10 shows the post resonance search test
specifications.
Table 5.7 pre sine resonance search test specifications
Frequency(Hz) Test level Duration/sweep rate
10-2000 Hz <0.5g(sine) 4 oct/min
Table 5.8 Sine vibration test specifications
Frequency(Hz) Test level Duration/sweep
rate
5-10 Hz 15 mm DA
2 oct/min
10-20 Hz 1.5g
20-100 Hz 1.05g
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Table 5.9 Random vibration test specifications
Frequency(Hz) Test level
PSD(g2/Hz)
grms Duration(s)/axes
20 0.01125
12.6
120
200 0.1125
600 0.1125
2000 0.05175
Table 5.10 post resonance search test specifications
Frequency(Hz) Test level Duration/sweep rate
10-2000 Hz <0.5g(sine) 2 oct/min
Fig 5.4 Test setup in horizontal axis
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Fig. 5.5 Accelerometers used for vibration measurement
Fig 5.6 Strain Gauges used for measuring strain values
