Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
1998-12-14
2001-02-27
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
C073S594000
Reexamination Certificate
active
06192758
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to fault detection, in load bearing structures, and more particularly concerns improvements in methods and apparatus for determining locations of damage or faults in such structure, one example being bridges.
1. Introduction
To safeguard the safety performance of a civil infrastructure, such as a bridge, regular inspections are essential. At the present time, the inspection method is primarily visual. An experienced engineer or technician has to go through a bridge to examine each member and certify its safety. This method is subjective and flawed, for lack of rigorous standards. For example, for a bridge deteriorating from fatigue or aging, the damage is usually not clear-cut. Therefore, any call is judgmental. Furthermore, it is not feasible to use this visual method for complicated bridge structures; there might be members located at positions too awkward to access; there might be too many members that would require too much time to inspect; and there might be damage too subtle to detect visually. Because of these limitations, the visual inspection results are known to be incompletely reliable; yet, inspectors are forced to rely on it, today.
The safety of the bridges, however, is too critical and urgent a problem to be left in the present state for long; and it is a crisis of gigantic proportions considering the aging of many thousands of bridges. Since the greatest highway construction period of the US was in the late nineteen fifties and in the nineteen sixties, many bridges are reaching their service limitation at this time. In a recent survey by the Federal Highway Administration (Chase and Washer, 1997), 37% of all bridges in service were found to have some degree of structural deficiency. That percentage is increasing fast with the aging of the bridges; therefore, a reliable strategy has to be devised so that limited financial resources can be effectively employed in response to this national infrastructure crisis.
Ideally, any inspection method will have to satisfy the following conditions:
1. To be robust, objective, and reliable.
2. To be able to identify the existence of damage.
3. To be able to locate the damage.
4. To be able to determine the degree of the damage.
Visual inspection methods have clearly failed the first requirement, and thus have introduced uncertainties in the rest of the requirements. This requires the conclusion that non-destructive inspection methods should employ precise scientific sensors coupled with rigorous data analysis. That approach has been the central theme of research in the Bridge Management Program, Turner-Fairbank Highway Research Center, Federal Highway Administration. A large research program of research and development in new technologies for the nondestructive evaluation of highway bridges has been initiated. The objectives are locating, quantifying, and assessing the degree of damage of the bridges in supporting of the bridges. Although various technologies have been developed, such as Infrared Thermography, Ground-Penetrating Radar, Acoustic Emission Monitoring, Eddy Current Detection and others, none of them are practical. The difficulties of such systems are due, mostly, to their limited field of view. One must locate the damage first before he can use sophisticated imaging devices to examine the damage, in detail. For a complicated structure, locating the damage is more than 90% of the job. As a result, even with the advances of these esoteric techniques, the data used in bridge management today is still based almost entirely upon unreliable visual inspection.
SUMMARY OF THE INVENTION
A viable alternative approach lies in the structure damage identification and health monitoring through use of changes in structure vibration characteristics.
The general topic of monitoring the health of structure through vibration has been a subject for extensive reviews by Doebling et al (1996) and Salawu (1997), and it has also been the topic for large Symposia (Natke, Tomlinson and Yao, 1993, and Chang, 1997). For bridges in particular, the subject has been reviewed by Salawu and Williams (1995a and b) and Blandford (1997). The present invention provides improvements in non-destructive damage detection in general, and for bridges in particular. The new approach is characterized in only a Nondestructive Instrument Bridge Safety Inspection System (NIBSIS) Using a Transient Load, which demonstrates its feasibility and practicality through numerical modeling. An extension of this method is applicable to other types of structural damage detection, such as in building damages and mechanical system faults.
2. The Present State-of-the Art, A Review
The approach of using dynamic response and vibration characteristics of a structure to detect damage is the theoretical foundation of instrumental safety inspection methods. It has also been the mainstream of research for more than thirty years. Doebling, et al. (1996), has reviewed the available literature of this approach. The practical problems associated with this approach have also been reviewed by Farrar and Doebling (1997) and Felber (1997). The basic idea is straightforward: in principle, each structure should have its proper frequency of vibration under dynamic loading. The value of this proper frequency can be computed based on well established formula (see for example, Clough and Penzien, 1993). For a general single degree of freedom beam as in a bridge, the stiffness of the beam, K, can be computed as follows:
K
=
∫
0
L
k
(
x
)
ϕ
(
x
)
2
ⅆ
x
+
∫
0
L
E
I
(
x
)
ϕ
′′
(
x
)
2
ⅆ
x
+
∑
j
k
j
ϕ
j
2
-
∫
0
L
N
(
x
)
ϕ
′
(
x
)
2
ⅆ
x
,
(
1
)
in which k(x) and kj are the distributed and discrete external spring support; E is the Young's modulus of the beam; I(x) is the moment of inertia of the member, N(x) is the axial force, and &phgr;(x) and &phgr;j are the generalized displacement function defined as
D(
x,t
)=&phgr;(
x
)
d
(
t
), (2)
where D(x,t) is the true displacement, and d(t) is the generalized coordinate. By the same argument one can also compute the effective mass, M, of the member given as
M
=
∫
0
L
m
(
x
)
ϕ
(
x
)
2
ⅆ
x
+
∑
j
m
j
ϕ
j
2
+
∑
j
n
j
ϕ
j
′2
,
(
3
)
in which m(x) is the distributed mass of the member, nj is the external mass inertia.
In most instances, the situation is much simpler. For example, for a simply supported bridge, we only have the second and third terms of Eq. (1) and only the first term of Eq. (3) being non-zero. If we assume that support to be a perfect bridge, we would only require the second term of Eq. (1) to be non-zero. Therefore, the stiffness is determined only by the integrated value of the moment of inertia, which, in turn, depends on the effective cross section. With both the stiffness and effective mass known, the proper frequency of the vibration, w, is calculated as
ω
=
(
K
M
)
1
/
2
.
(
4
)
Sound as this argument is, the instrument inspection system has never worked successfully. The reasons are many: first, there is the lack of precision sensors to measure the detailed dynamic response of the structure under loading. Secondly, there is a lack of sensitivity of the structure in response to local damage, because of the large safety factor built in. A damage up to 50% of the cross-section can only result in a few percentages vibration frequency changes. Such a small frequency shift, when processed with conventional methods, would be totally lost in the inevitable noise in all real situations.
Although use of sensors presented real problems, along with the recent advances in sensor technology, the presently available sensors are sensitive enough for the detection of minute changes in displacement and acceleration. On the other hand, the
Haefliger William W.
Moller Richard A.
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