The concept of acoustic emission analysis is explained in scientific terms. The detection of acoustic events, their localization, damage discrimination, and event summation curves are discussed. A block diagram of the concept of damage-free testing of fiber-reinforced synthetic materials is depicted. Prospects for application of the concept are assessed

Block, J.

English

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NASA TECHNICAL MEMORANDUM
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ACOUSTIC EMISSION ANALYSIS AS A NON-DESTRUCTIVE
TEST PROCEDURE FOR FIBER COMPOUND STRUCTURES
NASA TM- 77011
J. Block
1983
RECEIVED
311
ACCESj
Translat°.on of "Schallemissionsanalyse als zerstorungs-
freies Prufverfah°en fur Faserverbundstrukturen",
(Deutsche Fopschungo and Versuchsanstalt fur Luft-und
Raumfahrt, Institut fur Strukturmechanik Brunswick,
West Germany) DFVLR-Nachrichten, June 1962, PP. 34-37•
(NASA-T d-77011) ACOUSTIC EMISSION ANAL ISIS
AS A NON-DESTRUCTIVE TEST PROCEDURE FOh
FIBER C09PUUND STRUCTURES (national
Aeronautics anti Space Adoinistration) 13 p
HC A02/MF A01	 CSC L 14D 63/38
U83-2 1372
Uuclas
09336
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 20546	 JANUARY 1983
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TIC EMISSION ANALYSIS AS A NON-
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Translation of "Schalleinissionsanal,yse als zerscorungsfreies
Prufverfahren fur FaserverL.undstrukturen", (Deutsche Forschu
and Versuchsanstalt fur Luft-und Raumfahrt, institut fur
St.rukturmechanik, Brunswick, West Germany) DFVLR-Nachrichtein
June 1982, pp 34-37 (A82-37527)
1& Ab~*
Acoustic emission analysis is explained in scientific terms.
The detection of acoustic events, their localization,
damage discrimination, and event summation curves are dis-
cussed. A block diagram of the concept of damage-free
testing of fiber-reinforced synthetic materials is depicted.
Prospects for application of the concept are assessed.
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ACOUSTIC EMISSION ANALYSIS AS A NONDESTRUCTIVE TEST
PROCEDURE FOR FIBER COMPOUND STRUCTURES
0 4
J. Block
Fiber reinforced materials are becoming more and more import-
ant, especially in aviation construction. The following contribu-
tion will present sound emission analysis as a method which can be
used for non-destructive testing of fiber composite structures,
and can be a useful complement to other methods.
Non-destructive testing of fiber composite structures
Technological use of the speci'_'ic properties of fiber-reinforced
materials is not a new procedure. For many thousands of years,
humans have used it to manufacture components,to build vehicles and
to build buildings by processing the wood found in nature. The easy
availability of wood, its exceptional mechanical properties, its
relatively easy workability and the fact that it has a small speci-
fic weight in spite of its high strength, made it the "lest
used material up to recent times. i!etal was only used for a long
time for special products, such as weapons or tools. It is only
since the Industrial Revolution in the previous century that metal
products began to look like the material of the future.	 j
However, today,considering the general availability of plastics
and the fact that the ground raw materials are becoming scarce, all
metals will be more expensive and less available in the future. In the
Ideal case, all components, both the fibers and the resin of the
imbedding mass (matrix) would be manufactured from basic materials
which are easily available and therefore cheap and could be combined
to form laminate structures depending on the application. In addi-
tion, fiber materials are specifically lighter than metals, which in
Numbers in margin indicate naginat{nn of foreign text
E
at ions
OR101NAL PAGE IS
Of POOR QUALITY
JJ— I
I	 I
se 1 ection -t—e5
1 1 1rure 1. Concept of the non-destructive testing of
I'lber • -reinforced plant ios .
r,oneral, is ;in advant age if the te:-hMeal product is to have buoyancy
or ha:• to be acceler•ite,i.
This nwans that fiber composite materials again are interesting
for aircraft Construction. In practice, lwwever, there are a number
of Problems. One of these 1s the problem of non-destructive test 111F
whleh is mere difficult than in the case of metals and alleys because
t he i nt er • nal struet ur y is I111wh more complex.
Tliv damzFe which c , an occur under loads in the structure 1::
mi-Ai more complicated, for ovimple, fiber and matrix fracture, separa•-
t lon lwiwccn the fiber and the matrix or between entire layers
t1
0RIGI M PAGE IS
OF POOR QUALITY
(delaminaLl=n). Non-destructive testing is supposed to give an
exact picture of all of these defects if possible during testing,
that is in real time. In addition, the testing should not produce
any more damage (no falsification of measurement results due to the
measurement process) (Figure 1). It is obvious that none of the
known methods can satisfy these criteria in an optimum way at the
same time. Several test techni;ues have to be combined for basic
investigations.
Sound emission analysis
Sound emission analysis (SEA) is one of the non-destructive test
methods and very often is simply called "sound emissions" (English
"acoustic emission"). Strictly speaking, this * is false. In
contrast to ultrasonic testing, the test apparatus transmits nothing
at all.
Instead, it has a completely passive function. It records
noises which are created in the investigated sample, if stored energy
is given off in the form of elastic waves there.
Sound emission of solid bodies is a well known phenomenon. All
possible physical processes in metals, alloys, composite structures
as well ac amorphous solids produce noise in some form. One well
known example of this is "tin screaming", a phenomenon in the twin
formation in a tin crystal. It is only spectacular because it occurs
with a relatively high intensity in the audible range of the spectrum.
Most natural sound emission processes have too high a frequency or 135
are too weak to be recorded by ear. There is another possibility
that they are not recognized because they occur so often, for example,
the noises in the settling of wood.
The first sound emission phenomena where analysis was carried
out was done in a frequency range which can be considered esoteric
from the point of view of technical SEA. These were the earthquake
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Figure 2. Signal scanning of a sound event
waves, infrasound emissions of the earth's crust and the earth's
cover. All of the seismic processes occur in the range between
about 10-2 Hz to 10 Hz. For non-destructive testing of materials
and components, frequencies on the order of 100 kHz are used.
Up to recent times, the teeanical applications of SEA were res-
tricted drastically. Essentially, it was restricted to the monitor-
ing of extensive metal structures (tanks, refinery installations,
reactor pressure vessels, pipelines, etc.) where the safety require-
ments were high and the amount of information process is small.
Usually, the SEA facility only had to determine the positions of
suspicious activities in the investigated objects. The analysis was
done using other methods. It was only recently that advanced systems
for sound emission analysis were developed which allow domestication
of complicated structures as the laminates made of composite fiber
materials mentioned above. In the investigations of the Institute
for Structural Mechanics, all samples were made of carbon reinforced
plastic (see CFK).
4
ORKi U PAGE Is
OF ppplt QUALITY
Sound event scanning
When elastic energy is suddenly released, sound waves are pro-
duced whose frequency distribution can be very wide. They propagate
through the sample and are recorded by the eensors of the SEA install-
ation. These are piezoceramic transducers which give off a voltage
signal corresponding to the sound wave. It is then amplified and
then can be analyzed. It is important that the sound emission can
be resolved in time into individual events. If this decays into a
continuous noise, t1lell it is lot p,-)ssiL)le to obtain meaningful correla-
tions of measured parameter values. Then either the load rate has
to be reduced or if this cannot be done because the load rate corres-
ponds to the conditions of the test, the sensitivity has to be re-
duced. Figure 2 gives a picture of the scanning of a voltage signal
produced by an individual sound event scan.
All signals which exceed a voltage threshold clearly above the
noise it-v-e1 are converted to digital pulses. At the same time, the
amplitude is scanned so that the rise time up to peak amplitude, the
decay time as well as the total duration of the event can be deter-
mined.
The fixation of the events in sn absolute time scale is also
very important because the entire position determination technology
is based on the possibility of correlating the damage processes in
the sample with a load/time variation.
Neither X-ray nor ultrasonic technologies have this advantage.
They enly give instantaneous pictures of the sample with all of the
damage which has accumulated up to the time the photograph is made.
Position determination technique of SEA
aa
The simplest method of locali:-ing a sound source with a sound
	
s
emission analysis is linear position determination. Two sense: are
5
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Figure 3. Linear position determination on a flat
isotropic plate.
used for this. The electronics, of course, has to be designed for
two simultaneously operating channels. Any acoustic event in the
sample is then recorded in one sensor somewhat earlier than in the
other. The system measures the time difference and relates them to
the running time of the sound between the two sensors which were
determined previously by calibration. The event is output as a frac-
tion of the path between the sensors.
This method only is reliable when the sample is not only flat
but also is elongated and narrow. Even if we consider the simple	
s
case of a flat plate, where the speed of sound is independent of
direction (isotropic) and constant (which is never true for fiber
composite materials), that is, when the running time and the path
difference of the sound are equivalent, one obtains hyperbolas as the
loci of equal running time difference. The sensors are then located
at the foci of the hyperbolas (Figure 3).
6
O1MUL MW
OF POOR QUMM
The deviations in the position result from the true location and
( referred to the axis) are only sufficientl;i small in the vicinity of
the connection axis . This also is true for real laminates with high
anisotropy (direction dependence) with respect to the speed of sound.
This is because in the one-dimensional special case, the deviation
of the position lines from the hyperbolic shape plays no role. The
axis is intersected at the correct point.
This mer,ns that as long as the sensors are applied at the end
of a long sample with not excessive damping, the position results are
correct for composite structures as well. Figure 4 shows several
event histograms over the length of a CFK Semple with a (slit running
through one-half of the sample width in the center. The sample was
loaded by increasing amounts in the longitudinal Direction.	 The	 116
distribution of the sound events was output for various load levels.
The maximum in the center is caused by cracks which occur in the cor-
ners of the slot to an increasing degree. In addition, one can see
a general growth of damage over the right sample half with increasing
loads.
In the case of measurements with two-dimensional components and
also with components which have a curvature in space, one can no
longer disregard an isotropy if the planar position determination is
done by triangulation, that is, with path-time measurements for a
known geometry.	 We will see that this is not the only possibility.
The transfer of the method of applying sensors in the corners of the
network made of equilateral triangles and the valuation of the measure-
ment data (running time differences) is difficult using suitable tri-
angulation programs.	 it is difficult to transfer this to the SEA
method.	 In contrast to measurements of a large metal tank, for
example, it is necessary to consider the complicated sound trans-
F
r mission properties of the fiber complex.	 The determination of these
transmission functions by measuriig the damping, dispersion and othert
effects here also gives physical data about the 	 sample	 structure
and does not have to be interpreted only as an auxiliary aid for
planar position determinaticn.
	
However, up to the present time,
7
u PAN a
OF POOR QUAMY
Figure 4. Load test with a long CFK sample. Position
determination of damage with sound event histog:_.ns
:here are nc concrete results for this method.
The position determination using triangulation is made more
complicated by the fact that the geometry of the sample can be very
complicated (large c.irvatures, holes, edges). Another approach can
be taken as shown in Figure 5. Then one does not become involved
with the unknown transmission functions. Before the test, a large
number of regions are defined where sound events are to be determined.
Using a calibration sensor, sound signals of a suitable frequency are
8
ORNWM POM a
Of POOR QUALM
injected into the sample. The measurement centers respond to them
after a certain running time. Figure 5 shows how sensor 3 is
affected first which starts a clock which again then measu,-es the
running time differences to the other sensors. This time pattern
is stored. In a real test, then,every sound event which corresponds
to the sample within large or small tolerance limits (shown in
bright and dark colors in the figure) is associated with the corres-
ponding zone around the calibration point. The certainty and unique-
ness of this method, of course, increases with the number of sensors
used.
Damage discrimination
By means of SEA, damage is not only located but should be class-
ified according to various damage types. The main emphasis here is
on the amplitude distribution of the sound event.
In samples of rods made of various CFK laminates at the Insti-
tute for Structural Mechanics, it was found that the signals corres-
ponding to fiber fracture had a substantially higher amplitude than
those corresponding to pure matrix fracture. Up to the present time,
It has not been possible to distinguish among the various weaker sig-
nals and the correspondi:,.g damage types. Crack edge friction and
other effects have a disturbing influence here.
Event sum curves
I
Plots of sound events summed as
give interesting resul'. g . The begins
failure processes can be represented
determination of the felicity effect
history of the sample and under some
future load capacity (Figure 6).
a function of time, loud or load cycle
ling and variation of critical
directly. In particular, the
giver information about the load
conditions, can also predict its
The felicity effect is one variation of the known Kaiser effect.
In metals, the Kaiser effect produces sound emissions only above the
Figure 5. Planar position
determination by running
time difference pattern
recognition.
1
time
O
i4Rt MM PAGE IS
OF POOR QUALITY
Figure 6. The sample was loaded
and unloaded several times with
continuous increase of the maximum
load. The felicity curve is the
sound evert sum plotted over the
load. It gives information for
evaluating the state of damage of
the sample.
maximum load to which the sample was subjected up to that time. In
the case of composite structures, the emission starts before this.
There are indications that the ratio of the load where this occurs
and the maximum load will allow predictions about the fracture loads.
In particular, by using the felicity curve of an unknown sample, one
can determine how much it has been previously loaded.
The future
The use of SEA as a non-destructive test method for fiber com-
posite structures is still in the beginning stages. However, many
i
1	 10
LA
ORIGINAL PAGE IS
OF POOR QVA1.1TV
bi
Figure 7. Programmable four-channel measurement unit
for sound emission analysis. Two piezoceramic sensors
with preamplifiers on the housing.
people are developing th_ method and standard test, procedurc3 Pre
being developed. The device shown i:i Figure 7 at the Institute for
Structural Mechanics is a tyrical prototype of a new generation of
sound emission measurement installatlons which can b p used for this
today. Previous results indicate that sound emission analysis will
In the futt.re complement other test techniques in an effect.'ve manner
here we have only outlined the poszibilitiPs.
11


Acoustic emission analysis as a non-destructive test procedure for fiber compound structures

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