An acoustical-ultrasonic technique was used to demonstrate relationships existing between changes in attenuation of stress waves and tensile stress on an eight ply 0 degree graphite-epoxy fiber reinforced composite. All tests were conducted in the linear range of the material for which no mechanical or macroscopic damage was evident. Changes in attenuation were measured as a function of tensile stress in the frequency domain and in the time domain. Stress wave propagation in these specimens was dispersive, i.e., the wave speed depends on frequency. Wave speeds varied from 267,400 cm/sec to 680,000 cm/sec as the frequency of the signal was varied from 150 kHz to 1.9 MHz which strongly suggests that flexural/lamb wave modes of propagation exist. The magnitude of the attenuation changes depended strongly on tensile stress. It was further observed that the wave speeds increased slightly for all tested frequencies as the stress was increased

Baaklini, G. Y.

Hemann, J. H.

English

NASA Technical Reports Server

EFFECTOF STRESS ON ULTRASONICPULSES IN FIBER REINFORCEDCOMPOSITES
3ohn H. Hemann
ClevelandState University
Cleveland,Ohio 44115
George Y. Baakllnl
NationalAeronauticsand Space Administration
Lewis ResearchCenter
Cleveland,Ohio 44135
An Acoustical-UltrasonicTechniquewas used to demonstratethat relationships
exist betweenchangesin attenuationof stress waves and tensilestress for an
8-ply "0" degree graphite-epoxyfiber reinforcedcomposite. All tests were con-
ducted in the linear range of the materialfor which no mechanicalor macroscopic
damagewas evident. Changesin attenuationwere measuredas a functionof tensile
stress in the frequencydomain and in the time domain. Stress wave propagationin
these specimenswas dispersive,i.e., the wave speed dependson frequency. Wave
speeds varied from 2700 m/sec to 6800 m/sec as the frequencyof the signal was
varied from 150 KHZ to 1.9 MHZ which stronglysuggeststhat flexural/Lambwave
modes of propagationexist. The magnitudeof the attenuationchangesdepended
stronglyon tensilestress. It was furtherobservedthat the wave speeds increased
slightlyfor all tested frequenciesas the stress was increased.
INTRODUCTION
Non-Destructive Evaluation (NDE) techniques are being widely used today to
characterize the physical state of solid materials. Vary, et.al., (refs. I-6) have
demonstrated that correlations do exist between ultrasonic measures and mechanical
properties of metals, ceramics, and fiber composites. Williams, et.al., (refs.
7,8) have shown that ultrasonic attenuation is an indicator of fatigue life for
graphite-epoxy composites. Ultrasonic measurements in the area of composite
materials have provided a nondestructive means for measuring variations in strength
related properties, predicting interlaminar shear strength of fiber composite
laminates, and ranking composite structures according to strength.
This research presents evidence on how well (NDE) techniques describe stress
related effects. An ultrasonic attenuation study of a stress wave propagating
through a "0" degree graphite-epoxy fiber reinforced composite specimen subjected
to tensile loads was performed. This paper will provide not only qualitative
data, but also quantitative data, showing the relationships that exist between
changes in attenuation of stress waves and tensile stress for the 8-ply "0" degree
specimen under consideration. Tensile testing was done in the linear range of the
material, where linearity is defined by constant slope of the stress-strain curve
in the range of the test conducted. Wave speeds were also measured showing that
stress wave propagation is dispersive.
*This research was supported by a grant from the NASA-Lewis Research Center,
NAG3-106.
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The demonstratedconclusionsare:
I. Dispersiondoes occur, i.e., wave speeds are frequencydependent.
2. The attenuationis stronglydependenton tensilestress.
3. The wave speed is weakly dependenton tensilestress.
EXPERIMENTALPROCEDURE
Material
The specimenused was an eight-ply"0" degree angle-plygraphite-epoxyfiber
reinforcedcomposite. An AS graphitefiber and a PR-288exposy resin have been
used to constructan AS/PR-288preimpregnatedfiber-resinply. The fibers were
coated with a polyvinylalcoholto enhancefiber-matrixinterfacestrength
properties. A schematicof the unidirectionalcompositeis shown in Figure I.
The physical propertiesof the specimenhave been determinedby Vary, et.al.,
(ref. 3).
Apparatus
A schematicof the specimenand transducersin ultrasonicthroughtransmission
testingwas shown in Figure 2. Broadbandtransducersof variouscenter frequencies
were used to gather all of the data over the variousfrequencies.
Aluminum semicylindricalbuffersof 3.157 mm (,125 in) diameterwere attached
to the transducersin order to:
a. Providea line contactof length 12.7mm (.5 in) which is the width
of the specimen.
b. Prevent bendingfrom occurringas pressurewas appliedto the
transducerholders.
c. Place the sendingand the receivingtransducerat specifiedlocation.
Strain gages were mounted on the specimenon both faces to measurethe axial
and transversestrains. Strain gages were 0o and 900 orthogonalback-to-back. Also
strain gages were installedonto the transducerholdersto measure the pressure
appliedto the transducers. Williams,et.al., (ref. 8) definedand measured a
"saturationpressure"which is requiredfor reproduciblecouplingof transducerto
specimen;this pressurewas monitoredby the strain-gagedtransducerholders.
An Ultragel II ultrasoniccouplantwas used betweenthe aluminumbuffer and
the specimen. A very thin coatingof couplantwas used where effectsof thickness
of the couplantwere neglected. A felt materialwas used on the backsideof the
transducerholder to insurethat energy did not enter the transducerholder from
the specimen.
The ultrasonicequipmentand the tensiletestingsystem are schematically
shown in Figure 3.
182
Experimental Loading
An ultrasonicwave was transmittedinto the specimenby a sendingtransducer.
Two inches away in the longitudinaldirection,and on the same face, a receiving
transducerwas placed to sense the stresswave energy traversingthe specimen.
The receivingtransduceroutputwas displayedon the oscilloscopefor real time
analysisand on the spectrumanalyserfor frequencyanalysis.
The specimen,mountedin a tensilemachine where care was taken to minimize
off-axisloadingor twisting,was pulled at an initialloadingrate of approximately
17 _m/sec (.04 in/min)to set the grips. After this initialloadingto about 445N
(lO0 Ibf) which was the referenceload for attenuationmeasurementsand change in
attenuationmeasurements,furtherloadingwas done at mean rate of 4.2 um/sec
(.Ol in/min).
The load was increasedto 3560N (800 Ibf), 6675N (1500 Ibf), and in some
experimentsto 13350N (3000 Ibf) successively. Then the load was decreasedto
6675N, 3560N,and 445N. At the same time strain-loaddata was generated,photo-
graphs of the stresswave output and the frequencyoutput were taken.
The loading-unloadingcycle was repeatedfive to seven times for each setting
of independentvariables;the test usuallywas completedwithin a two-hourperiod.
The number of cycles that were requiredwas dictatedby the reproducabilityand
stabilityof the attenuationchanges.
Typical output signalsare shown in Figure 4 and Figure 5 for an entire
loading-unloadingcycle.
RESULTS
In this experimentalresearchmany observationswere made concerninghow
variousparameterseffectedthe ultrasonicoutput;only a few were examined and are
presentedin this paper becauseof time, equipment,and experimentalsystem limita-
tions. This chapterwill presentthe dispersionphenomenonand the'wave type, the
attenuationas relatedto tensile load, and the wave speed as relatedto tensile
load; the specimenwas "0" degree graphite-epoxyfiber reinforcedcomposite.
Dispersion
Dispersionis understoodto mean that wave speeds are frequencydependent.
From the data collectedand the measurementstaken, a graph shown in Figure 6 was
plottedshowing speed versus frequency. The wave speed was determinedby measuring
the delay time betweenthe outputwave front and input wave front over a fixed
distance. The graph shows that the wave velocityalong_thelongitudinaldirection
is frequencydependent. Wave speeds varied from 2.7xI0a cm/sec to 6.8xi05cm/sec
as the frequencyof the signalwas varied from 150KHZ to 1.9 MHZ.
There are two types of dispersion. First is viscoelasticdispersiondue to
viscoelasticpropertiesof the material, Second is a geometricdispersiondue to
the dimensionsof the material. Williams (ref. 8) demonstratedthat there is no
viscoelasticdispersionin the graphite-epoxycompositefor longitudinaland shear
IB3
wave velocities in the range tested. Shear wave velocities were tested in the range
of 0.48 to 3.0 MHZand longitudinal wave velocities were tested in the range of
0.225 MHZto 5 MHZ. The frequency range for the results presented in this paper
is 0.15 to 1.9 MHZ, which is very close to the range of Williams' result.
For the wave propagating in the xI direction and having a particle motion in
the x I direction Williams (ref. 8) measured the longitudinal wave speed and foundit to be 8.5 to _0 cm/sec. This wave speed CL is plotted in Figure 6 and is used
as a comparison since the material used in Williams' research is very similar to
the material used in the research reported here.
By comparing CL to the velocities obtained in this experiment as shown in
Figure 6, one can see that the wave velocities measured were not longitudinal wave
velocities. The sending transducer inputs a longitudinal wave perpendicular to
specimen which, because of many reflections, becomes a wave propagating along the
length of the specimen. During this propagation shear waves are formed due to
the reflections and interact with longitudinal waves. This interaction of waves
propagating along a thin specimen produces Lambwaves (ref. 9). Dispersion does
occur in the propagation of flexural/Lamb waves (ref. I0).
Lamb wave propagation may exist in a multimode state at a given frequency
producing a finite number of wave speeds to exist simultaneously. Because of
experimental equipment limitations, this research could not define which mode
(fig. 6) and how many modes were superimposed upon each other, but this paper
recognized the possible existence of different modes simultaneously in the stress
wave output.
Attenuation vs. Tensile Stress
The relationship between attenuation and tensile stress was investigated by
measuring the pulse amplitude change in the time domain and in the frequency domain.
The tensile load applied to the specimen was in the linear range of the material
where linearity is demonstrated by the constant slope of the stress-strain curve
shown in Figure 7. Percent change in amplitude vs. tensile load in the frequency
domain is shown in Fiugre 8 for each of the six frequencies used in the experiment.
At 150, 180, 250, and 1900 KHZ, a negative attenuation with increasing load is
observed whereas at 850 and 1700 KHZ the attenuation is positive. The slope of
these lines remains constant with cycling, but a translation of the lines occurs in
the first few cycles, after which the lines stabilize for subsequent cycles. The
data needed to plot these lines come from direct measurements of amplitude changes
from photographs of which Figure 4 is typical.
Figure 9 shows percent change in amplitude vs. tensile load in the time domain.
At 150, 180, 250, and 1900 KHZ a negative attenuation with increasing load is
observed whereas at 850 KHZ a positive attenuation occurs. The slopes and transla-
tion of these lines with cycling behaves similarly to the frequency lines.
Theoretically Figures 8 and 9 should be identical, but as one can see from Figure 5
the energy response in the time domain is stretched out into many _mall
immeasurable spikes. The data plotted in Figure 9 reflects only the main part of
the pulse. The 1700 KHZ response could not be clearly separated from the 1900 KHZ
response in the time domain and hence was not plotted.
The data shown in Figures 8 and 9 behave in a generally similar manner. The
184
data clearlyindicatesthat the attenuation,both positiveand negative,of the
ultrasonicpulse dependson tensilestress.
Wave Speed vs. TensileLoad
The data at all frequenciesclearlyshows that the wave speed increaseswith
increasingstress. This can be clearlyseen in Figure 5 where all pulses are
translatedto the left (decreasingarrivaltime) with increasingload. The data
showed wave speed changesas large as 8% at stress levels 30% of the ultimate
stress.
CONCLUSION
It has been shown that dispersiondid occur, i.e., wave speeds were frequency
dependent. Moreover,it was found that flexural/Lambwaves existed, and a super-
positionof differentmodes existedin the stresswave outputof the signal.
It has been proven that attenuationdependedstronglyon tensilestress. Also
it was observedthat attenuationwas frequencydependent.
Finally,it was demonstrated_thatwave speedswere weakly dependenton tensile
load.
Furtherresearchis needed using a pulsingsystemproducingnarrow band
frequenciessince all effectsmeasuredwere frequencydependent.
REFERENCES
I. Vary, A., and Bowles,K. J.: UltrasonicEvaluationof Strengthof Unidirec-
tional Graphite-PolyimideComposites. NASA TechnicalMemorandumTM-73646,
April 1977.
2. Vary, A., and Bowles,K. J.: Use of an Ultrasonic-AcousticTechniquefor
NondestructiveEvaluationof Fiber CompositeStrength. NASA Technical
MemorandumTM-73813,February1978.
3. Vary, A., and Lark, R. F.: Correlationof Fiber CompositeTensileStrength
with the UltrasonicStress Wave Factor. NASA TechnicalMemorandumTM-78846,
April 1978.
4. Vary, A.: CorrelationsAmong UltrasonicPropagationFactorsand Fracture
ToughnessPropertiesof MetallicMaterials. MaterialsEvaluation,Vol. 36,
No. 7, June 1978, pp. 55-64.
5. Vary, A.: QuantitativeUltrasonicEvaluationof MechanicalPropertiesof
EngineeringMaterials. NASA TechnicalMemorandumTM-78905,June 1978.
6. Vary, A.: CorrelationsBetweenUltrasonicand FractureToughnessFactorsin
MetallicMaterials. NASA TechnicalMemorandumTM-73805,June 1978.
185
7. Williams,Jr. J. H., and Doll, B,: UltrasonicAttenuationas an Indicatorof
FatigueLife of Graphite/EpoxyComposites. NASA CR-3179,1979.
8. Williams,Jr. J. H., Nayeb-Hashemi,H., and Lee, S. S.: UltrasonicAttenuation
and Velocity in AS/3501-GGraphite/EpoxyFiber Composite, NASA Contractor
Report 3180, December1979.
9. Lamb, H.: On Waves in an ElasticPlate. Proc. Rog. Soc. A.93, I14-28 (1917).
lO. Meeker,T. R., and Meitzler,A. H.: GuidedWave Propagationin Elongated
Cylindersand Plates. PhysicalAcoustics,edited by Mason,Warren P.,
Academic Press, New York and London,1964. Part A, pp. I12-166.
186
aluminum
tabs
transducer
holder
IX3(through thickness direction) sendingtransducer
J
_" ____ receiving
transducer
aluminum
buffer
°
X2
strain
gagesrse direction)
/X 1 (longitudinal direction)
Figure 1. Principal directions of the "0" degree Figure 2. Schematic diagram specimen
angle-ply specimen and transducers
Machine Riehle 90000N Met _ V _"
Capacity Pul,ersi __
Digital _ IQ ,)__ ?ektronicsScope
Load Input I.
Dlgi tal Output
Voltmeter
Strain
Indicator * Dunegan I f Krohn-Hite ITotalizer
Tensile 301
Specimen
SYSTEM FOR TENSILE Transducers Is _m 1
TESTING MEAS_S
SYST_ FOR VEb0CITY AND
ATI_VJATI(ANMEAS_
Figure 3. Experimental system
187
a. 507N b. 3658N
c. 6826N d, 3617N
e. 525N
Figure 4. - Ultrasonic pulse attenuation versus load of 150 kHz in frequency domain
188
a. 507N b. 3658N
c, 6826N d. 3617N
e525N
Figure 5. - Ultrasonicpulse attenuationversus load of 150 kHz in time domain.
189
8,5 ...................................... CL
6.8 •
6,3 •
t
"_ 5.0 •E
x
(J 3.6 •
qo
2.7 •
U_
180
I11 I I I
150 250 850 1700 1900
Frequency f, KHZ --_
Figure 6. Wave speed vs, frequency
8
t.
%
x
<.J
0 I I I I I I
1000 2000 3000 4000 5000 6000
Strain _-, tx m/m
Figure 7. Axial stress vs. axial strain
190
60 • 160 khz
Z_ 180 khz
=) Q 250 khz
¢ 40 • 850 khz
•_ II 1700 khz
¢ C] 1900 khz
"o
o
= t _._,_B TENSILE LOAD,
(1.
-20
445 3560 6675 13350
Figure 8. Percent change in amplitude vs.
tensile load in the frequency domain
• 150 khz
60 z_ 180 khz
0 250 khz
o_ Q 850 khz
t-(0 40 • 1700 khz
[] 1900 khz
'10
2
=_. 20
TENSILE LOAD, N
-20
445 3560 6675 13350
Figure 9. Percent change in amplitude vs.
tensile load in the time domain
191


Effect of stress on ultrasonic pulses in fiber reinforced composites

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