Impact Test Technique for Large-Scale Structure
H.Q. Ying S. Shen M.Ying J.M. Liu
China Orient Institute of Noise & Vibration
Room 516, Technology & Trade Bldg. Shangdi, Beijing 100085,P.R.China
[email protected]
ABSTRACT: Large-scale structure, such as bridge or spacecraft mobile launcher, if only excited by ambient force,
their complete modal parameters can't be gotten. Except the small rocket equipment or energy-accumulating
elastic hammer are used as the impact exciting force. Using small rockets, the Qiangtang River railway and
highway bi-function bridge modal test was completed successfully. For rockets are expensive, the
energy-accumulating elastic hammer was designed. This hammer can extend the force working time by almost 2
times, thus increasing the total energy and concentrate the energy in low frequency band. Great structure's
dominant modal frequencies are low, so the low sampling frequency is needed for FRF calculation and the
sampling time interval is long, which cause the impact force is not accurately sampled. To overcome this
disadvantage, we put forward the VTB (varied time base) FRF method, which using high sampling frequency to
acquire force data and low frequency to response data. Using elastic energy-amassing hammer as impact force
and VTB FRF method, many large-scale structure modal tests were completed, such as Wuhai Yellow railway
bridge, the spacecraft mobile launcher which weighted more than 700 tons, and spinning machine for astronaut's
overweight training.
1 Introduction
For large-scale structure, ambient force is often used in modal test because of its large weight and size. By this
means, the excitation signal cannot be measured, so not all modal parameters can be obtained, and the precision
is lower. In the impacting modal test, the whole modal parameters can be obtained because the excitation can be
measured.
Obviously the common hammer with small excitation energy cannot complete the impact test for large-scale
structure. The small rocket equipment, which is special designed, and energy-accumulating elastic hammer are
fit to excite the large-scale structure such as railway bridge, large machinery and so on. They can produce
large impact force on the structure, and the impact pulse can last a long time so that the energy is concentrated
into low frequency band. It is beneficial because the dominant natural frequencies of large-scale structures are
low.
Besides of the excitation equipment, a special FRF calculate method named Varied-time-base (VTB) is used in the
impacting test. Commonly, the impact force signal need high AD sample rate, but the response signal need low AD
sample rate. VTB can solve this problem effectively.
2 Small Rocket Excitation Equipment
Early in 1988, COINV(China Orient Institute of Noise & Vibration) designed small rocket excitation equipment for
Qiantang river bridge modal test. It is shown in figure 1. The bridge is a railway and highway double deck steel
frame bridge in Hangzhou city of China. The rocket equipment is special designed for impact excitation . Its axial
direction is orthogonal to the surface of structure, and its head is fixed against the structure. When fired, it
generates a large impact force on the structure, though it don’t move
In the small rocket equipment, solid fuel is used as the propellant. According to the requirement of experiment, by
designing the rocket’s propellant, the rocket can produce perfect excitation force. The pulse parameters, including
the shape (such as sine or rectangle), magnitude and lasting time, can be designed.
[1]
With the small rocket equipment, COINV complete the modal test of Qiantang river bridge
and large boxcar. In
the bridge test, the small rocket equipment produces a rectangular pulse force of 1 ton to excite one span of the
bridge weighting 280 tons. On the other hand, the pulse lasts 100 ms and the excitation energy are concentrated in
a low frequency band below 10 Hz for the bridge’s lowest two natural frequencies are 0.99Hz and 1.13Hz.
But the small rocket equipment has some disadvantages. The process of manufacture is complex, the propellant
can only be used one time, and the operation would be restricted by the environment and local statute.
Fig. 1 small rocket equipment
Fig. 2 elastic hammer
Fig. 3 force waveform and spectrum
3 Energy-Accumulating Elastic Hammer
The most convenient impact method is still the hammer. But the common hammer can’t produce enough energy to
excite the low frequency vibration of a large-scale structure. Then, the energy-accumulating elastic hammer is
specially designed by COINV, which is shown in figure 2.
The hammer is heavy. DFC-1 type of this hammer weights 18 kg. If several steel blocks were added, the total
weight can reach to 30kg. So the hammer can produce a large impacting force ranges from 1000kg to 10,000kg.
Moreover, there has special elastic connect equipment between the hammerhead and the handle. The elastic
connect equipment can extend the pulse lasts time to 20 ms so that the exciting energy is greatly increased and
concentrated into low frequency band below 70 Hz. Figure 3 shows an example of the impact force waveform and
power spectrum.
With the energy-accumulating elastic hammer, COINV completed the modal test in horizontal direction of Wuhai
Yellow River Bridge and Shenzhou spacecraft mobile launcher which weights 750 tons. In Wuhai bridge modal
[2]
test , the bridge’s weight of one span is about 90 ton and the length is 28 meters. The impact force generated by
the hammer is shown in figure 3. It can be seen that the maximum magnitude is about 1600 kg, and the last time is
about 21ms. Exciting with this hammer, the bridge’s vibration of low natural frequency is large enough to be
measured.
4 Varied-Time-Base (VTB) FRF Analysis Method
As for impact test, the FRF in low frequency has a conflict between time precision and frequency precision. A high
sampling rate is necessary to ensure that the pulse force signal can be sampled correctly. But the response of the
large-scale structure has a low and narrow frequency band. So a low sampling rate is needed to ensure its
frequency precision.
If a low sampling rate is used in impact test, the sampling interval is large. After the AD convert , the force digital
signal is not perfect which is shown in figure 4. Then the precision and the repetitiveness of FRF are bad.
To solve this problem, Varied-Time-Base (VTB) FRF analysis method is used. In VTB method, the sampling rate for
force signal is many times higher than the sampling rate for response signal. Because the sampling rates are
different, the common method in calculating FRF is unsuitable. A corresponding algorithm is described in reference
[3]. The advantage of VTB lies in its good low frequency characteristic, especially for large-scale structure with low
nature frequency. Figure 5 shows the VTB sampling result. The sampling rates are 1600Hz and 50Hz for force
signal and response signal.
Fig. 4 use low sample rate for force signal
Fig. 5 example of VTB sampling result
5 Results and Conclusions
The partial modal result of Qiantang River Bridge is shown in figure 6, which is excited by small rocket equipment.
The foremost two horizontal modes and two vertical modes are given.
The first four horizontal modes of Wuhai Yellow River Bridge are shown in figure 7, which is excited by
energy-accumulating elastic hammer.
Fig. 6 results of Qiantang bridge modal test
Fig. 7 results of Wuhai bridge modal test
Using impact test for modal analysis, the whole modal parameters can be obtained, and the precision of modal
identification can be improved. The key to complete the impact test towards large-scale structure lies in three
aspects:
(1) The impact force must be large enough to excite a large structure;
(2) The impact force lasts long time, so the energy can concentrated in low frequency band;
(3) The VTB method not only ensures the time precision of force signal, but also ensures the frequency precision
of response signal.
References
1 Ying H.Q., Liu J.M. Ao Q.B. and Yang J.X. Small Rockets Excting Qian-Tang Great Bridge For Modal Analysis, Proceeding of
th
17 IMAC, pp 2056-2060, 1999.
th
2 Ying H.Q., Shen S., Lei S.H. and Liu J.M. Yellow River Railway Bridge’s Modal Test With Hammer, Proceeding of 17 IMAC,
pp 1089-1092, 1999.
3 Ying,H.Q., Varied-Time-Base Sampling and Analysis Method of Transfer Function Calculation for Force Impulse and
Structure Vibration Response, National Invention Patent of China, 1990.
4 Ewins,D.J., Modal Testing: Theory and Practice, Research Studies Press Ltd. 1984.