1775

Critical Void Content for ThermosetComposite Laminates

ZHAN-SHENG GUO,1,* LING LIU,1 BO-MING ZHANG2AND SHANYI DU

2

1School of Aerospace Engineering and Applied Mechanics

Tongji University, Shanghai 200092, China2Center for Composite Materials, Harbin Institute of Technology

Harbin, 150001, China

(Received September 25, 2005)(Accepted January 10, 2006)

ABSTRACT: An experimental program to characterize the effect of voids on thestrength of composite laminates is presented. The adequacy of a fracture criterion torepresent the experimental data for the effect of voids on the flexure strength, tensilestrength, and interlaminar shear strength of composite laminates is assessed. Theexperimental program investigates the effect of different pressures and dwell timeson the critical void content. Laminates produced with carbon fiber/epoxy resinunidirectional prepreg have been produced with an intentionally high void content.Short beam shear, three-point flexure, and tensile testing are used for mechanicalevaluation and the results correlate to void volume fraction and ultrasonicabsorption coefficient. The ultrasonic absorption coefficient is measured for allthe specimens and its variation is approximately linear with the void content,corroborating previous experimental results. The effects of these factors on thestrength of the composite laminates are discussed in terms of the fracture parametersinvolved in the fracture criterion. The critical void content is estimated for each caseboth in terms of void content and ultrasonic attenuation.

KEYWORDS: polymeric composites, voids, water absorption, mechanical strength,failure criteria.

INTRODUCTION

ACOMMON PROBLEM in the manufacturing of polymer composites is the formationof defects such as resin-rich regions, crimped and distorted fibers, foreign inclusions,

and voids. Among those defects, voids are arguably the greatest problem because theyare difficult to avoid, particularly at the corners of composite components, and aredetrimental to the mechanical properties [1–11]. Voids are formed because of a numberof reasons, including the formation of bubbles from volatile byproducts produced during

*Author to whom correspondence should be addressed. E-mail: [email protected] 1, 3–6, 10 and 11 appear in color online: http://jcm.sagepub.com

Journal of COMPOSITE MATERIALS, Vol. 43, No. 17/2009 1775

0021-9983/09/17 1775–16 $10.00/0 DOI: 10.1177/0021998306065289� 2009 SAGE Publications

the cure reaction of the polymeric matrix, the use of a high-viscosity resin combinedwith closely packed fibers that are not completely wetted by resin, the entrapment of airin the material system, and fabrication mishaps such as a leaking vacuum bag or a poorvacuum source [1–10].

Most aircraft composite parts are inspected after fabrication with nondestructivetechniques such as ultrasound either by pulse echo or through transmission. Thesetechniques are able to detect defects that cause ultrasonic attenuation such as voids,delaminations, interlaminar cracks, inclusions, foreign object damage, resin-rich regions,and others [3–5,9].

The behavior of a composite laminate with voids under different types of mechanicalloading has been widely studied. Most works consider the interlaminar shear strength[3–5,9–15] but the interlaminar fracture toughness [12], bending strength under static andfatigue loading [9], and compressive strength [3–5] have also been studied. However, thereis no general agreement over the magnitude of the effect of porosity on the mechanicalproperties of composites. The difficulty lies in the large number of parameters involvedin the problem.

Different types of prepreg materials used in the manufacture of the laminates affectthe material toughness; the processing parameters (such as temperature and pressure) andtype of reinforcement affect the distribution, location, shape, and size of the voids in thelaminate.

All of these factors, in turn, produce different effects on the laminate strength. The typeof mechanical loading, its nature (static or fatigue), and inspection technique used are alsosignificant factors.

For example, the use of different frequencies in the ultrasound equipment resultsin different values of attenuation. Therefore, only the results obtained from thecharacterization of composites produced and tested in a similar way can directly becompared. The experimental studies mentioned aim at correlating the void content tothe laminate strength for a specific type of loading. In those studies, it is implicitlyassumed that the void content is uniform at least over the critical section of the specimen.However, in practice, voids are not uniformly distributed but are random in nature.Note that void content is a measurement associated with a finite volume of material ratherthan measurement to a point. The void content measurement by either matrix digestionor ultrasonic inspection captures some sort of average value over a given volume,without retaining the information on the shape, size, and distribution of the voids.However, these features play an important role in determining the effect of voids on themechanical behavior of laminate and are primarily controlled by matrix material,type of reinforcement, and manufacturing problem that originated the defects. Theimportant issue of the effect of the size of the area of the laminate affected by voidsis not addressed in this work. The voids are assumed to be uniformly distributed overthe laminate.

A fracture criterion that correlates fracture stress with void content or, alternatively,to ultrasonic attenuation, is needed to establish an acceptance level for the inspection.Establishing the acceptable level of defects is a critical issue in designing compositestructures. An overly conservative acceptance criterion causes many parts that couldperform satisfactorily to be unnecessarily discarded, increasing the manufacturing cost.On the other hand, if the deleterious effects of defects are underestimated, in-service failureof some parts may occur. Both situations are avoided by a judicious choice of acceptablelevel of defects in the part. This should be based on reliable fracture criteria supported

1776 Z.-S. GUO ET AL.

by extensive experimental characterization and an in-depth understanding of the effectof defects on the mechanical behavior of the laminate [3–5].

Almeida and Neto [9] proposed a fracture criterion that presented good correlationwith the experimental data on the bending strength of composite laminates with voids.The same idea was successfully applied to predict the interlaminar shear strength byCosta et al. [3–5], Jeong [7], and Almeida and Santacreu [8]. The fracture criterionproposed by Almeida and Neto [9] is based on ultrasonic attenuation rather than directlyusing the void content. The rationale for this is that, in practical applications, theacceptable level of defects must be established in terms of a nondestructive technique.However, laminate thickness, type of matrix, and reinforcement affect the shapeand size of the voids and, consequently, ultrasonic attenuation. Therefore, thevoid content corresponding to a given ultrasonic attenuation level depends on thosefactors; as a consequence, the fracture parameters obtained from the application ofthe fracture criterion will reflect the dependence of the ultrasonic measurements onthose factors.

The purpose of this work is (1) to obtain an optimal cure cycle as well as to evaluatethe effects of pressure conditions on the void contents and mechanical properties,(2) to present the results of an experimental program to investigate the effect of voids onthe mechanical strength of composite laminates, (3) to assess the adequacy of the fracturecriterion to represent the experimental data for the effect of voids on the flexure, tensile,and shear strength of composite laminates, and (4) to discuss the effect of the typeof loading (flexure vs. interlaminar shear) on the critical void content of compositelaminates both in terms of volume fraction and ultrasonic attenuation. Moreover,the fracture parameters are estimated by the fracture criterion.

EXPERIMENTAL PROCEDURE

Fabrication of the Specimens and Void Content Measurements

The material under research was T700/TDE85 carbon fiber reinforced epoxy prepreg.The initial fiber volume fraction Vf is 60� 2%.

The viscosity was measured on an improved NDJ-7 rheometer. The viscosity wasmeasured at the manufacturer’s temperature cure cycle. The temperature was increasedup to 120�C at a rate of 2�C/min and was held for 2 h. Then it was increased to 160�Cat the same rate and was held for 4 h . Finally, it was cooled down to room temperature.

Polymer composites with high void content were manufactured using a procedure basedon the rheological analyses of the wet prepregs [3–5]. Laminates presenting intentionallyhigh porosity levels were produced, combining the technique proposed by Almeidaand Neto [9] and Olivier et al. [16]. This procedure involved the control of effectivepressure on liquid resin during cure with the simultaneous introduction of moisturebetween layers during layup, as suggested by Gurdal et al. [17]. Moisture was introducedby spraying water finely and uniformly to produce laminate plates with homogeneousporosity. The effective pressure on the liquid resin and the amount of moisture dispersedinto the laminate were used to control the void content [5]. All specimens were cured in anautoclave at 160�C with different pressures.

Two groups of cure pressure cycles were selected to assess the influence of voids on themechanical properties. One group kept the first isothermal time as 120min and changed

Critical Void Content for Thermoset Composite Laminates 1777

the autoclave pressures as 0.0, 0.1, 0.2, 0.4, and 0.6MPa. The other group selected thetemperature held at 120�C for 0, 30, 90, and 120min. The autoclave pressure was setas 0.2MPa. The time for which the pressure was applied was chosen with respect to theminimum viscosity and gelation points. The pressure inside the vacuum bag is 0.1MPa andremains so throughout the entire cycle for all experiments. A reference specimenwas produced to represent the behavior of low void content laminates. All laminateswith 12 plies, [0/90]3S, were manufactured as described above for each of the cure routesconsidered respectively. The size of each plate was about 300� 300� 2mm.

All plates were ultrasonically inspected to assess the resulting distribution of porosities.Areas of uniform porosity within each plate were identified, and different levels of voidand fiber content specimens and at least 10 interlaminar shear and flexure specimenswere cut from each of those areas.

C-scan Ultrasonic Inspection

All plates were inspected by double-through-transmission technique (Figure 1). Thehost machine was SONIC—138VFD and the probe diameter was 25mm. The scanningwas implemented with a 5MHz ultrasonic failure detector by an automation M400Dsystem to generate an actual size map of the plate, associating a color with eachattenuation level. This feature was used to identify areas of constant porosity level.

The measured ultrasonic attenuation is the result of three factors: front surface loss,transmission loss, and back surface losses. The front and back surface losses do notdepend on the condition of the panel, apart from its surface finish, and would beexpected to be independent on plate thickness [18]. On the other hand, transmissionloss depends on the defects present in the laminate. A calibration procedure was usedto compensate for the front and back surfaces, such that the measured attenuationcorresponds to transmission losses through the specimen only [3]. Three independentscans of each plate were performed to measure the absorption coefficient of selectedareas with approximately uniform porosity level. The average value of thesemeasurements is considered as the absorption coefficient of the samples respectively.

Figure 1. Ultrasonic double-through-transmission technique.


The ultrasonic absorption coefficient � may be defined as

� ¼At

tð1Þ

where � is measured in decibels per millimeter and depends on the internal condition ofthe laminate, particularly on void content, and where transmission loss At (measuredin decibels) is assumed to increase linearly with plate thickness t.

Three independent scans of each plate were performed to measure the absorptioncoefficient of selected areas with approximately uniform porosity level.

Characterization of Void Content

After the C-scan inspection, specimens were cut and their densities were determinedin order to estimate void content. The density of each composite sample was determinedusing the water displacement method by measuring its weight in air and in water. The voidcontent and volume fiber content of each specimen were measured by matrix digestionaccording to ASTM D3171. The void content measurement was made with five specimenson each plate and the average was taken as the normal void content associated witheach porosity level. Specimens with different porosity levels ranging from 0 to 3.5%were obtained for these carbon/epoxy laminates. The fiber and resin density used forcomputations were provided by the manufacturer.

Microstructural Analysis

Microscopic image analysis is reported as one of the best methods to measure voidcontents [1]. In addition, this technique provides detailed information on other vitalparameters such as void location, shape, and size that cannot be assessed by othermethods. The void morphology in composite laminates is usually assessed by microscopicimage analysis. A ZEISS MC80DX Microscope equipped with a camera and an imageprocessing system was used for all analyses. Void features are obtained from imagesacquired at 200� magnification using a PC-based CCD camera mounted on a MEIJIoptical microscope. The selected magnification of 200� enables the assessment of voids assmall as the radius of a single fiber of 7 mm. Consequently, all identifiable voidsthroughout the entire composite samples were included in the analysis of void contentand morphology.

Mechanical Tests

INTERLAMINAR SHEAR STRENGTH TESTSThe results of the interlaminar shear strength (ILSS) tests were measured according

to short beam shear (ASTM D2344). There were 10 specimens, with dimensions25� 8� 2mm, tested to assess the effect of void content on the ILSS. A nominal spanlength of 20mm and a 6.4mm-diameter loading nose were used. The diameter of thesupports was 3.2mm. The tests were performed in an Instron mechanical testing machineusing a test speed of 1mm/min.


FLEXURE STRENGTH TESTSThe flexure strengths were obtained according to three-point flexure (ASTM D790).

There were 10 specimens, with dimensions 40� 15� 2mm, tested to assess the effectof void content on the flexure strength. The tests were also performed in an Instronmechanical testing machine using a test speed of 1mm/min.

TENSILE STRENGTH TESTSThe tensile strengths were obtained according to tensile testing (ASTM D3039).

There were 10 specimens, with dimensions 180� 12� 2mm, tested to assess the effect ofvoid content on the tensile strength. The tests were also performed in an Instronmechanical testing machine using a test speed of 0.5mm/min.

RESULTS AND DISCUSSION

The Viscosity Curve of TDE85 Epoxy Resin

Rheological tests provide important information used to determine the time duringwhich the pressure must be applied. The results of viscosity are plotted against timeand temperature in Figure 2. It shows that there exists one minimum viscosity plateau(from Point A to B) in the viscosity versus time curve. At the end of the plateau, theviscosity increases quickly with time or temperature owing to the initiation of curereaction. In those cure temperature cycles, a—b—c—d is the cure cycle recommendedby the prepreg’s manufacturer.

Ultrasonic Attenuation Properties of Polymeric Composites

Figure 3 shows a C-scan of the studied laminates in a double-through-transmissiontechnique with pressures 0.0MPa (a) and 0.6MPa (b). It shows the distribution andlocation of porosity. Figure 3(c) gives the presentation of grey levels associating to each

0 30 60 90 120 150 180 2100

2

4

6

8

10

12

40

80

120

160

200 Viscosity

-------- Temperature

Vis

cosi

ty (

Pa

s)

Time (min)

a

bc

d

A B

Tem

pera

ture

(°C

)efg

Figure 2. Plot of viscosity vs time and temperature.


attenuation level, denoted as the echo amplitude. The area porosity corresponding to echoamplitude is also shown. It was calculated from the results of ultrasonic attenuation andvoid contents through experimental method. Hence, area porosity levels are affectedby frequency and echo amplitude of ultrasonic inspection, material type such as the fiberand the matrix, and the thickness of composite laminates, and so on. This methodis a quantitative grading evaluation criterion that will be very useful in engineeringapplication.

Figure 4 correlates the void content determined by acid digestion (ASTM D3171) to themeasured absorption coefficients � for all laminates studied. As expected, the smallest

Figure 3. C-scan showing areas with different void contents: (a) cure pressure is 0.0 MPa; (b) cure pressureis 0.6 MPa; and (c) presentation of grey level and its porosity.

Figure 4. Correlation between void contents and absorption coefficient.


absorption coefficient corresponds to the low porosity laminates. This suggested thatgreater void content causes increased attenuation and a linear correlation between theporosity and the absorption coefficient can be observed for laminates with a porosityrange from 0 to 3.5%. Few measurements could be made of very high void contentlaminates because of the difficulty in consistently obtaining laminates with a uniformdistribution of voids for high void contents. However, the results of this work demonstratethat the range of void content obtained for all specimen types sufficed to determine themaximum allowable void content.

Ultrasonic attenuation depends on a number of factors [3–5,9]: laminatethickness, type of reinforcement, matrix material, fiber content, and the internal conditionof the material, which includes the void content. Moreover, the shape, size, anddistribution of voids in the laminate and parameters such as the ultrasound frequency,size of the probe, and calibration procedure also affect the absorption coefficientmeasurements. Therefore, it is difficult to compare ultrasonic attenuation resultsobtained by other researchers without full knowledge of all the factors that affect themeasurements.

Optical Image Analysis

As illustrated in Figure 5, voids are seen at two different locations within the laminates.The first location is defined as areas rich in matrix away from fibers. Voids encounteredin this location are completely surrounded by the epoxy matrix. The shapes of voids aremostly circular (Figure 5(a)). The second location is defined as areas rich in interface,where the area is primarily composed of reinforcing fibers. The shapes of voids are moreelliptical or of different irregular geometry (Figure 5(b)). A photomicrograph of thesample of low porosity laminate is shown in Figure 5(a), which has a void content of 0.6%with a curing pressure of 0.6MPa. In this view, the voids occur mostly at the resin-richregion. Figure 5(b) shows the occurrence of much larger, flattened, and elongated voidsdistributed in the samples with a cure pressure of 0.0MPa. As can be seen, the voids occur

Figure 5. Photomicrograph of voids with different cure pressures: (a) 0.6 MPa and (b) 0.0 MPa.


mostly at the ply interface. The differences in the morphology of the voids are determinedby the process parameters and physical properties of the resin.

Cure Cycles and Void Contents

Physically, void removal by pressurization is feasible that pressurization resultsin void dissolution. Efforts were made to describe the effects of cure pressureconditions on void contents. A set of cure cycle pressures, 0.0, 0.1, 0.2, 0.4, and0.6MPa, were used. The void content of each type of laminate is plotted in Figure 6 asa function of the cure pressure. It shows that an exponential decrease fitting curve canbe well characterized by the relationship between void content and cure pressure.This result is similar to the works of Tang et al. [10], Boey et al. [11], and Olivier et al. [16].Hence, for a particular application, one can determine an appropriate magnitude ofcuring pressure according to void levels required when other process parameters areunchanged.

In order to determine the optimum time of first dwell time, some experimentswere carried out. The pressure within the range of minimum viscosity was appliedfor 30, 60, 90, and 120min, respectively. Figure 7 shows the results of void contentsvarying with the first dwell time. It shows that there exists one time span for applyingpressures, and if the level of porosity limits to less than 1.5%, the time span chosenbetween the 50 and 90min is better. If pressure is applied too early, a resin-rich region mayappear in the laminates. It usually resulted in high void content and low mechanicalstrength (Figures 7 and 8). Of course, pressure must not be applied too late because of theexisting resin-starved region which also resulted in high void content and low mechanicalstrength (Figures 7 and 8). Hence, the optimum time of applying pressure is not 120minbut 90min. The resin viscosity affects the resin flow and also affects the transport of voids,void formation, and growth. Experimental results show that the resin can flow fully in the

Figure 6. Void contents as a function of cure pressures.


time span from 50 to 90min. An improved cure cycle is obtained as a—b—e—d (Figure 2).The time was shortened to 30min.

Fracture Criterion

A fracture criterion that correlates fracture stress with void content or, alternatively,to ultrasonic attenuation, is needed to establish an acceptance level for the inspection.Criteria to predict the laminate strength under certain loading conditions in the presenceof voids are scarce in the literature. Almeida and Neto [9] proposed a fracture criterionthat presented good correlation with experimental data on the bending strength ofcomposite laminates with voids. The same idea was successfully applied to predict theinterlaminar shear strength by Costa et al. [3–5], Jeong [7], and Almeida and Santacreu [8].

0 30 60 90 12075

80

85

90

95

100

Shear strength Flexural strength Tensile strength

Str

engt

h fr

actio

n (%

)

Dwell time (min)

1.2% 0.9%

2.0%(Void content)2.2%

Figure 8. Mechanical strength vs the first dwell time.

0 30 60 90 120 1500.0

0.5

1.0

1.5

2.0

2.5

3.0

Voi

d co

nten

t (%

)

Dwell time (min)

Figure 7. Void contents vs the first dwell time.


Almeida and Neto [9] also took advantage of the form of the equation to estimate thecritical void content defined as the void content below which the strength of the laminate isnot significantly affected by the presence of the voids. A similar approach was used bySoriano and Almeida [19] to analyze the fracture strength data of composite laminateswith circular notches.

The considered fracture criterion for the strength of composite laminates containingvoids is given by

�f ¼ Hð�Þ�m ð2Þ

where �f is the fracture stress, H is the laminate toughness, � is the ultrasonic absorptioncoefficient in decibels per millimeter, and m is the slope parameter.

Equation (2) provides a good fit to experimental results for specimens with voids[3–5,7–9,20]. However, it predicts infinite fracture stress for void-free laminates. To avoidthis inconsistency, the fracture criterion assumes that, for low void content, fractureoccurs according to classical fracture mechanisms with no influence of void content(fiber microbuckling for compression and shear failure for ILSS tests). Therefore, for lowvoid content, fracture stress �f is assumed to be given by

�f ¼ �f0 ð3Þ

where �f0 is the fracture stress of a laminate with low void content.Equations (2) and (3) yield a critical value for the ultrasonic absorption coefficient �cr,

below which the void content does not affect the laminate strength. The value of �cr canbe computed from

logð�crÞ ¼ �1

mlog

�f0H

� �ð4Þ

where �cr is the critical value of the ultrasonic absorption coefficient. Note that thedefinition of the critical ultrasonic absorption coefficient provides a systematic approachto establish a maximum allowable value for void content. The approach is derived fromthe mathematical form of the fracture criterion, which, in turn, is a consequence of thebasic assumptions described earlier. Therefore, the critical ultrasonic absorptioncoefficient �cr should be interpreted as a reference value for the minimum value of voidcontent that affects laminate strength rather than as a physical characteristic of thelaminate.

The proposed criterion in logarithmic form becomes

logð�fÞ ¼logð�f0Þ � � �cr

logðH Þ �m logð�Þ � > �cr

�ð5Þ

Therefore, this criterion implies that the logarithmic plot of fracture stress �f,as a function of ultrasonic absorption coefficient �, should be approximately linearfor �>�cr and constant for ��cr. Logarithmic plots of experimental results are presentedto assess the validity of this hypothesis for all cases considered in the present work (Figures9–11). A straight line obtained from a best-fit procedure is included in all plots. Note that,


to be consistent with Equation (5), the best-fit procedure must not include the referencelaminate that corresponds to the fracture stress of a laminate with low void content, �f 0.

To verify the robustness of the fracture criterion, it was also applied to experimentaldata obtained independently. Figure 12 shows the application of the proposed fracturecriterion to results obtained by Costa et al. [3–5] for the interlaminar shear strength ofcarbon tape/epoxy laminates with voids. Figure 13 shows the application of the proposedfracture criterion to the results obtained by Stone and Clarke [18] for the interlaminarshear strength of carbon tape/epoxy laminates with voids.

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

420

440

460

480

500

520

540

560

Fle

xura

l str

engt

h (M

Pa)

Absorption coefficient (dB/mm)

σ = 609(α)−0.365

Figure 10. Flexure strength vs ultrasonic absorption coefficient.

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.632

34

36

38

40

42

44

Inte

rlam

inar

str

engt

h (M

Pa)


σf = 47.3(α)−0.364

Figure 9. Interlaminar shear strength as a function of ultrasonic absorption coefficient.


The results shown in Figures 9–13 demonstrate that a good agreement withthe experimental results was obtained from the application of the fracture criterion tofit the fracture stress as a function of the absorption coefficient � for all thecases considered. Note that the experimental data include the effect of load type(flexure and interlaminar shear) and experimental data from other researchers. Therefore,the proposed fracture criterion yields good estimates of laminate fracture stress for a wide

Figure 12. Logarithmic plot of interlaminar shear strength as a function of ultrasonic absorption coefficient forcarbon tape/epoxy laminates by Costa et al. [3–5].

1100

1150

1200

1250

1300

1350

1400

1450

Ten

sile

str

engt

h (M

Pa)

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6


σ =1536(α)−0.310

Figure 11. Tensile strength vs ultrasonic absorption coefficient.


range of situations, provided that adequate values for fracture parameters are used in eachcase. The fracture parameters are the laminate toughness H, slope parameter m, andcritical void content, either in terms of the absorption coefficient �cr (decibels permillimeter), or in terms of volume fraction Vcr (percent).

The values of the fracture parameters which are listed in Table 1 are obtained from thelinear regression procedure based on Equation (5). The value of m is about 0.365 eitherunder flexure or interlaminar shear loading, 0.310 under tensile loading for all specimens.It seems that the slope parameter m depends on the loading type. The laminate toughnessH depends on the loading type as demonstrated by the results in Table 1.

An important fracture parameter is the critical void content, which establishesan acceptance criterion for the nondestructive inspection of composite laminates.When expressed in terms of the critical absorption coefficient, it depends on the type ofloading. For laminates under ILSS and tensile loading, the critical absorption coefficientis 1.45 dB/mm; for flexure strength measurements, the critical absorption coefficientis 1.38 dB/mm. These results indicate that the type of loading has a significant effect onthe critical absorption coefficient.

When the critical void content is expressed in terms of critical volume fraction Vcr, theload type has little influence on the critical void content. Note, however, that from

Figure 13. Logarithmic plot of interlaminar shear strength as a function of ultrasonic absorption coefficient forcarbon tape/epoxy laminates by Stone and Clarke [18].

Table 1. Fracture parameters for all laminates under interlaminar shear,flexure, and tensile strength tests.

m H (MPa) acr (dB/mm) Vcr (%)

ILSS 0.364 47.3 1.46 1.11Flexure strength 0.365 609 1.38 1.05Tensile strength 0.310 1536 1.45 1.10


the point of view of nondestructive inspection, the critical void content must be establishedin terms of the parameters of ultrasonic inspection, that is, the critical absorptioncoefficient �cr.

CONCLUSIONS

An experimental program aiming at establishing acceptance levels for the attenuationlevel in the ultrasonic inspection of composite laminates is described. Laminates producedwith carbon tape/epoxy were produced with intentionally high void content. Theabsorption coefficient was measured for all specimens and shown to vary approximatelylinearly with the void content, corroborating previous results. The magnitude and time ofapplied pressure have a significant effect on void content. If they were selected properly,the entrapped air, water vapor, and excessive resin can be squeezed out from the laminatesand the first dwell time can be shortened. Microscopic image analysis was utilized toexamine void content, location, shape, etc.

A fracture criterion was applied to the experimental data to correlate the interlaminarshear strength, tensile strength, and flexure strength of the laminates to the absorptioncoefficient. The theory presented good correlation with the experimental data for all cases,including experimental data independently obtained by other researchers. Therefore,it was demonstrated to be a useful nondestructive inspection aid in certifying the structuralintegrity and safety of composite laminates containing porosity. The application of thefracture criterion to the experimental data establishes a systematic approach to estimatean inspection acceptance criterion based on a critical void content below which thestrength of the laminate is not significantly affected by the presence of the voids.The critical void content may be described either in terms of the absorption coefficientin the ultrasonic inspection or in terms of the volume fraction. It is shown that, whenexpressed in terms of the critical absorption coefficient, the critical void content dependson the type of loading. However, the resulting values are also dependent on parametersof the ultrasonic inspection, namely, the frequency, diameter of the probe, calibrationprocedure, etc. Therefore, the estimated critical absorption coefficient may only be usedas an inspection criterion for the same conditions used in this work. These data, however,cannot directly be used as an acceptance criterion in a nondestructive inspection ofcomposite laminates.

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