Acoustic testing of complex multiple segment structures

The present invention relates to a method for acoustic testing of multiple segment complex structures for detecting changes in the integrity of such structures to anticipate failure. The method of the present invention in its simplest form requires providing each segment of a complex multiple segment structure with at least one acoustic sensor, recording the intensity and frequency distribution of the acoustic waves sensed by such acoustic sensor; and finally comparing the acoustic waves that are sensed either against a standard, over time, and/or from one segment of the complex multiple segment structure to another segment of the complex multiple segment structure.

DESCRIPTION 
1. Field of Invention 
The present invention relates to acoustic testing of complex multiple 
segment structures for the purpose of detecting time dependent and/or 
structure dependent changes in the integrity of such structures. 
2. Background Art 
Each year many complex multiple segment mechanical structures fail. These 
failures can cause economic loss, schedule delays and may even cause death 
as was the case in June of 1983 when a portion of a bridge which carried 
Interstate 95 traffic failed. 
In an effort to avoid the failure of complex multiple segment structures 
the structures are initially designed with generous safety factors; during 
use and/or exposure to deteriorating environmental conditions the complex 
multiple segment structures are frequently inspected and tested; and/or 
the complex multiple segment structures are replaced at intervals deemed 
to be sufficiently often to assure that the structures will not fail in 
service. In spite of the current precautions taken to assure that failure 
of complex multiple segment structures will not occur, some do fail. 
Additional precautions can and should be taken to further reduce the 
occurrence of such failures. 
It is an object of the present invention to provide a method for 
anticipating the impending failure of a complex multiple segment 
structure. 
An object of the present invention is to provide a method for monitoring 
the structural integrity of complex multiple segment structures that will 
supplement existing visual inspection procedures and thereby reduce the 
incidence of unanticipated catastrophic failures. 
Another object of the present invention is to provide a method for 
identifying which of a series of complex multiple segment structures is 
most likely to fail in a catastrophic and/or premature manner. 
These and other objects of the present invention will become apparent from 
the following description, figures and claims. 
SUMMARY OF INVENTION 
The present invention relates to a method of testing, monitoring and 
inspecting complex multiple segment structures, and in particular the 
method of the present invention relates to acoustic testing of such 
complex multiple segment structures to predict the structural integrity of 
such structure by either identifying differences in the acoustic emissions 
of different segments of similar construction, or by determining changes 
as a function of time and/or usage in the complex multiple segment 
structures. 
The method of the present invention in its simplest form requires providing 
each segment of a complex multiple segment structure with at least one 
acoustic sensor, recording the intensity and frequency distribution of the 
acoustic waves sensed by such acoustic sensor; and finally comparing the 
acoustic waves that are sensed either against a standard, or over time 
and/or from one segment of the complex multiple segment structure to 
another segment of the complex multiple segment structure.

BEST MODES FOR CARRYING THE INVENTION INTO PRACTICE 
The methods of the present invention can be used to inspect, monitor and 
test any of a variety of complex multiple segment structures including but 
not limited to bridges, airplanes, storage tanks and/or crane booms. 
The present invention relates to a method of testing and inspecting complex 
multiple segment structures, and in particular the method of the present 
invention relates to acoustic testing of such complex multiple segment 
structures to determine structural integrity; changes in structural 
integrity from one segment of the complex multiple segment structure to 
another segment of the complex multiple segment structure; and/or 
deterioration as a function of time and usage of such complex multiple 
segment structures. 
The method of the present invention in its simplest form requires providing 
each segment of a complex multiple segment structure with at least one 
acoustic sensor, recording the intensity and frequency distribution of the 
acoustic wave sensed by the acoustic sensor, and finally comparing the 
acoustic waves that are sensed against a standard, over time, and/or from 
one segment of the complex multiple segment structure to another segment 
of the complex multiple segment structure. 
FIG. 1 is a schematic representation of an acoustic monitoring and 
recording system. The multiple segment structure 1 shown in FIG. 1 has 
three segments 12, 14 and 16 of similar construction. These segments are 
rigidly connected by fastening means 18 such as rivets or bolts. The 
structure is supported by a first support 20 and a second support 22. The 
acoustic emission from each of the segments is detected by acoustic 
sensors, such as piezoelectric transducers, which convert the acoustic 
emission into electric potentials. The electric potentials may be recorded 
and/or processed. In FIG. 1 two transducers are attached to each segment. 
The first segment 12 has a first transducer 24 positioned in close 
proximity to the first support 20 and a second transducer 26 positioned 
near the first junction 28 of the first segment 12 and the second segment 
14. 
A third transducer 30 is located on the second segment 14 near the first 
junction 28 while a forth transducer 32 is positioned on the second 
segment 14 near the second junction 34 where the second segment 14 joins 
the third segment 16. 
A fifth transducer 36 is positioned on the third segment 16 near the second 
junction 34 while a sixth transducer 38 is positioned on the third segment 
16 near the second support 22. 
Each of the transducers are electrically connected to a multiple channel 
recorder 40. The multiple channel recorder 40 records the acoustic 
emissions from each of the transducers. 
FIG. 2 is a schematic representation of a system for analyzing broad 
spectrum acoustic emissions, white noise, in accordance with the present 
invention. The white noise is sensed by sensors positioned in a manner 
such as depicted in FIG. 1. The acoustic emissions generated at each of 
the sensors can be fed to the system shown in FIG. 2 where the generated 
signals are summed for each segment of the structure. 
The output signal 42 of the recorder 40, which is the recorded signal from 
the first transducer 24, is added to the output signal 44 of the recorder 
40 which is the recorded signal from the second transducer 26, to produce 
a time dependent summed output signal for segment 12. This summed output 
signal for segment 12 is schematically represented by the signal 46. 
In a similar manner the output signals 48 and 50, which are respectively 
the recorded signals from transducers 30 and 32 located on the second 
segment 14 of the complex multiple segment structure are summed. Summing 
signals 48 and 50 provides a summed output signal for segment 14 which is 
schematically represented by the signal 52. 
The same summing procedure is done for the output signals 54 and 56 from 
the transducers located on the third segment 16 of the complex multiple 
segment structure. This provides a time dependent summed output signal for 
the third segment which is schematically represented by 58. 
The summed output signals 46, 52 and 58 can be visually compared and 
differences in the patterns noted. These differences over time will be a 
result of changes in the structural integrity of the individual segments 
of the complex multiple segment structure. 
The comparison of the output signals can be done automatically by 
processing pairs of the summed output signals in such a manner that the 
output signals are subtracted and their difference recorded. For example, 
signal 46 and 52 can be fed into an analyzer 60 which would then subtract 
signal 46 from signal 52 to produce a difference output signal 64. In a 
similar manner summed output signals 52 and 58 can be processed by 
analyzer 66 to produce a difference signal 68, and finally summed output 
signals 46 and 58 can be fed to analyzer 70 where they may be subtracted 
to produce a difference signal 72. The difference signals can be compared 
over time to determine deterioration in the structural integrity of the 
complex multiple segment structure. 
Testing in accordance with this embodiment of the present invention will 
provide an integrated acoustic signal that will be most sensitive to 
global defects such as missing bolts and fractured support members. 
FIG. 3 is a schematic representation of a system for analyzing filtered 
noise in accordance with a preferred embodiment of the present invention. 
When the complex multiple segment structure is monitored in accordance 
with this preferred embodiment the white noise is filtered and the 
emissions monitored are principally the acoustic emissions associated with 
the generation of internal defects such as cracks forming within the 
structure. 
With this technique differences in the paired output signals of each 
segment are analyzed after filtration. For the structure of FIG. 1 the 
signals 42 and 44 are combined in an analyzer 74 to provide a difference 
signal which is passed through a filter 76 which eliminates those 
frequencies below 100 KHz. This filtration would remove the lower 
frequency acoustic emissions which result from traffic and mechanical 
interaction of the individual segments. The time dependent output signal 
represented schematically by 78 may be reviewed. This filtered signal 78 
will grow over time if cracks are initiated and grow. The initiation of 
growth of the output signal over time will indicate that the first stage 
of failure has occurred. 
In a similar manner output signals 48 and 50 can be combined in analyzer 80 
and then filtered by a 100 KHz filter 82 to produce a time dependent 
filtered signal 84. Output signals 54 and 56 can be combined in analyzer 
86 and filtered with a 100 KHz filter 88 to produce a filtered output 
signal 90. 
Inspecting a structure in accordance with the embodiment described above 
and depicted in FIG. 3 will provide information on changes in the internal 
structure. Changes over time in the acoustic wave generated in accordance 
with this method will indicate the time dependent change of internal 
defects such as the dislocation structure. Filtered signal 78 as depicted 
in FIG. 3 shows a time dependent growth indicating crack initiation and 
growth in the first segment 12 of the complex multiple segment structure. 
Any of the above methods of the present invention can be used to analysis 
paired output signals. For example if one were interested in inspecting 
the junction 28 of the complex multiple segment structure 10 shown in FIG. 
1 the difference in the output signals of acoustic sensors 26 and 30 could 
be taken. This output signal could then be analyzed in the manners 
described above. Likewise the acoustic emissions sensed by acoustic sensor 
126 shown in FIG. 1 could be combined with the acoustic emissions sensed 
by acoustic sensor 26 and this combined acoustic output signal could be 
analyzed and compared with similar acoustic output signals in the manners 
described above. 
Complex multiple segment structures can be monitored in accordance with yet 
another embodiment of the present invention by first generating an 
acoustic wave at one location and sensing the acoustic wave at another 
location. This method allows one to observe the attenuation of an acoustic 
wave. A change in the attenuation of an acoustic wave indicates a change 
in the path of the acoustic wave and therefore indicates a difference in 
the connectivity of the structure. This change in the connectivity can be 
the result of the absence, or fracture of an element. For testing in 
accordance with this embodiment the acoustic wave should be generated and 
sensed when the structure is not being subject to external forces which 
could generate additional acoustic waves. Therefore it is preferred when 
testing a structure such as a bridge in accordance with this embodiment to 
do so at times when traffic flow is at a minimum. 
FIG. 4 is a schematic representation of the embodiment of the present 
invention in which the inspection is carried out by monitoring a generated 
acoustic signal in the complex multiple segment structure 10 of FIG. 1. In 
this embodiment the first transducer 24 is driven by a signal generator 
92, this causes an acoustic signal 94 to be generated and transmitted to 
the complex multiple segment structure. The acoustic signal 94 is 
transmitted through the complex multiple segment structure and sensed by 
the transducer 38. The difference between the generated signal 94 and the 
signal 96 sensed by the transducer 38 is a function of the acoustic 
conductivity of the multiple segment structure 10. Initially the acoustic 
wave sensed by the sensor 38 can be compared to a standard to determine 
connectivity of the complex multiple segment structure. If during assembly 
of such a complex multiple segment structure a connecting pin or other 
such connecting element is omitted such would be indicated by the 
inability of the complex multiple segment structure to transmit the higher 
frequency acoustic waves. 
Changes in the structural integrity of the complex multiple segment 
structure 10 can be detected by periodically generating a standard signal, 
recording the transmitted signal, and comparing the recorded transmitted 
signals over time. If changes over time in the transmitted signal are 
detected the location of the changes can be more closely ascertained by 
reducing the spacial separation of the signal generating transducer and 
the acoustic sensors. For example, examination of the first junction 28 
could be accomplished by employing the transducer 26 to transmit a 
generated acoustic wave and the transducer 30 could be used to record the 
transmitted signal.