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97

6Single-Fiber Tensile Testing andMatrix Characterization Tests

6.1 Introduction

Fiber tests are used extensively for quality control purposes and evalua-tion of the effect of potentially detrimental fiber surface treatments. Theyare also used to evaluate environmental degradation of fibers and to deter-mine mechanical properties required for input in micromechanical analysis(Chapter 2). Most fiber test methods are designed to determine the modulus

and strength of the fiber in uniaxial tension, although compression tests suchas the recoil test have been approached (see Allen 1987 and Kozey et al. 1995).

6.2 Single-Filament Test

The single-filament test of ASTM (American Society for Testing and

Materials) D3379 (1989) (withdrawn in 1998) provides methods to determinefiber tensile modulus and strength. For the test, a single filament is separatedfrom a dry tow of fibers and mounted on a slotted cardboard tab as shownin Figure 6.1. The tabbed specimen is then inserted in a testing machine; thesupporting tab carefully cut on both sides of the fiber or burned away usinga hot wire, a soldering iron, or something similar; and the unsupported sin-gle filament tested to failure. The test requires a very low force load cell, suchas a 5- to 20-N (0.5- to 2-kg) capacity load cell. Typically, there is large scatterin fiber modulus and strength data (Van der Zwaag 1989). Hence, a large

number of specimens must be tested to obtain statistically significant results.One source of variability in the test data is the fiber cross-sectional areaused to determine modulus and strength. The fiber cross-sectional area isdetermined on the dry tow from which the fibers are selected for specimenmounting. This means that an average cross-sectional area is used, not theone specific for each fiber.

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98 Experimental Characterization of Advanced Composite Materials

6.2.1 Single-Filament Test Procedure

Choose a suitable single filament from a strand bundle. Care must be exer-

cised to ensure that coalesced fibers are not chosen. Each fiber should beinspected under an optical microscope prior to specimen preparation. Centerthe filament over the tab opening with one end taped to the tab. Lightlystretch the filament and fix the free end to the tab using tape. Carefully placesmall dots of adhesive on the filament at the slot edges to bond the filamentto the tab (Figure 6.1). The specimen gage length is defined by the lengthof the open slot in the end tab, nominally 25 mm as shown in Figure 6.1.Determine the gage length to within 0.1 mm.

Mount the specimen in the grips of a properly calibrated testing machine.

Visually check for alignment of the test specimen between the grips. Cutboth sides of the mounting tab or burn the tab away at the midgage length.Use care not to damage the filament. Set the crosshead speed to achieve fail-ure in about 1 min. Ramirez (2008) used a crosshead speed of 0.5 mm/min.

Apply load to the specimen while recording load and crosshead displace-ment until the filament fails. Fibers used as reinforcements of compositestypically display linearly elastic behavior up to failure; examples of stressstrain curves for carbon and glass fibers are shown in Figure 6.2.

6.2.2 Data Reduction for Modulus and StrengthThe fiber elastic modulus is defined as follows

E =

(6.1)

12 mm

Cement or wax dot

Paper tab

Filament25mm

60mm

AA

FIGURE 6.1Single-filament test specimen.

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99Single-Fiber Tensile Testing and Matrix Characterization Tests

where is the fiber stress defined by

P

Af = (6.2)

where P is the load applied, and Af is the fiber cross-sectional area. Thestrain is defined as the crosshead displacement () divided by the gagelength L:

L =

(6.3)

Notice that Hookes law, Equation (6.1), demands strain in the linearregion of material response, that is,

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where C, Ca, and Csare the true specimen compliance, the apparent speci-men compliance, and the system compliance, respectively. The true speci-men compliance is given by

CL

EAf= (6.5)

where Lis the gage length. ASTM D3379 suggests conducting tension testing(as described previously) on a material similar to the filament (or the actualfilament) over a range of gage lengths at small loads to determine the appar-ent compliance Ca. Plotting Caversus gage length should result in a graphsimilar to the sketch shown in Figure 6.3.

The system compliance Cs, defined as the intercept of the best-fit straightline at L =0 is obtained by extrapolation. The fiber axial modulus Emay bedetermined from the slope mof the line in Figure 6.3 as

1E

mAf= (6.6)

where it is assumed that the fiber cross-sectional area is known.The ultimate tensile strength of the fiber f is defined as the maximum

stress before failure [Equation (6.2)].

6.3 Matrix Test Methods

Mechanical properties of matrix materials are mostly determined for perfor-mance evaluation and specification of matrix properties for product data sheets.Such properties can be used in micromechanics models that predict composite

00

Gage Length, L

Comp

liance,

Ca

m

Cs

FIGURE 6.3Typical graph of compliance versus gage length for a single filament.

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properties. It should always be remembered that neat resin properties are notnecessarily equivalent to those that will be achieved by the resin in a composite.

The fiber/matrix interface region in particular may differ from the bulk matrix.The mechanical properties of the matrix material are normally measured

in tension, compression, and shear. Since the materials are usually very brit-tle, extreme care should be exercised in preparation of specimens and in car-rying out the test.

As described in Chapter 4, neat resin test specimens are often prepared bymolding the resin. Some resins can be cast into void-free thick bars or dog-

bone test specimens. Others are best cast into sheets. The presence of voidsin casting will cause problems with premature failures of the test specimen.

The quality of specimens is important. To the extent possible, specimen designshould follow the standards or guidelines specified for the test procedureused.

Strain measurement should be accomplished through the use of extensom-eters or strain gages. If an extensometer is used, care must be taken so thatthe sharp knife edges of the extensometer do not promote failure. Typically,the specimen surfaces are smooth and glossy, and the knife edges of theextensometer may slip. A dot of White-Out correcting fluid has been foundto enhance friction. If strain gages are used, they should be 350-resistance

(or greater) gages. Polymers do not dissipate heat very fast, and gage heatingcan be a problem. Excitation voltages should therefore be kept as low as pos-sible. Strain gages may not perform properly for high-elongation polymers

because the gage/gages tend to debond at moderate-to-large strains. This is,however, not a problem for most thermoset matrices used in advanced com-posites since they tend to be relatively brittle.

Polymers are often hydrophilic and sensitive to temperature changes.Specimen conditioning and the laboratory environment should be carefullycontrolled and monitored both prior to and during testing.

ASTM D638 (2010) specifies conditioning of the specimens at 23 2

C and50 5% relative humidity for at least 40 h unless otherwise specified. Testingshould be conducted under the same conditions. Because polymers are vis-coelastic materials and strain rate sensitive, testing speeds must be welldefined and controlled for valid comparison of data.

6.3.1 Matrix Tensile Testing

The standard tension test is described by ASTM D638 (2010). Several types ofdog-bone specimen geometries are specified by this standard, depending onsheet thickness and polymer rigidity. Figure 6.4shows a typical type I (rigidor semirigid polymer 7 mm or less thick) neat resin tensile specimen geom-etry and dimension. For modulus determination, a longitudinal strain gageor extensometer is required. Measurement of Poissons ratio requires the useof two strain gages, a biaxial strain gage (0/90rosette), or a biaxial exten-someter to measure longitudinal and transverse strains simultaneously.

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All specimens should be inspected to ensure that voids and surface flawsare not present. ASTM D638 specifies tolerance of thickness and gage sectiondimensions of 0.4 and 0.1 mm, respectively.

ASTM D638 stipulates a crosshead speed of 5 mm/min. To conform tomost composite tests described in this text, however, a crosshead rate of12 mm/min is recommended. Test at least five replicate specimens.

Figure 6.5 shows a typical stressstrain curve recorded for a vinylestermatrix in tension. The specimen fails after displaying a short nonlinear

165

99.3

57

19

R76 13

FIGURE 6.4Tensile test specimen geometry (type I). All dimensions are in millimeters.

75

50

8084 Tensile

25

0 0 1 2

Strain (%)

3 4

Stress(MPa)

FIGURE 6.5Stressstrain curve in tension for vinylester 8084 resin. (After Figliolini, A.M., 2011, Degradationof Mechanical Properties of Vinylester and Carbon Fiber/Vinylester Composites Due to EnvironmentalExposure, master thesis, Florida Atlantic University.)

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response region at a strain of 3.78%. From this curve, it is possible to deter-mine Youngs modulus E, the yield stress (ys), defined by a 0.2% offset strain

(see Gere and Timoshenko 1997), and the ultimate tensile strength t. Forthis particular specimen, E=3.01 GPa, ys=52.9 MPa, and t=66.1 MPa.

6.3.2 Matrix Compressive Testing

Compression testing of the matrix material is conducted for purposes ofresearch and development and quality control. Compressive propertiesinclude modulus of elasticity, yield stress, and compressive strength. TheASTM D695 (2010) standard describes test specimens of various forms, such

as sheet and solid cylindrical rod. Although ASTM D695 includes testing ofthin sheet-like materials of thickness under 3.2 mm, testing of such requiresspecial support fixtures, and it is recommended here to test relatively short,solid, cylindrical rod specimens to avoid instability failures.

The standard specimen is 12.7 mm in diameter and 25.4 mm long. Suchspecimens may be prepared by molding longer cylinders (see Chapter 4)and machining the specimen in a lathe to achieve smooth and parallel endsurfaces. At least five replicate specimens should be prepared and tested.Figure 6.6 shows a photo of an actual test specimen.

FIGURE 6.6Compression test specimen. (After Figliolini, A.M., 2011, Degradation of Mechanical Properties ofVinylester and Carbon Fiber/Vinylester Composites Due to Environmental Exposure, master thesis,Florida Atlantic University.)

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Prior to testing, the diameter and length of the specimen are to be mea-sured to the nearest 0.01 mm at several points. Make sure the end surfaces

are flat and parallel. Place the specimen between the parallel platens ofthe testing machine, centering the specimen to avoid introduction of load-ing eccentricities. Since the specimen is quite short, it may not be feasibleto record strain using an extensometer or a strain gage. Hence, in manycases strain is defined as crosshead displacement divided by the originalspecimen length, which does not produce reliable results. As for the single-filament testing described in Section 6.2, improved modulus determina-tion is obtained if the system (machine) compliance is subtracted from theapparent specimen compliance. The machine compliance for the compres-

sion test may be determined in a way similar to the procedure suggested forthe single-filament test.

Before the actual test, the position of the crosshead of the test frameshould be adjusted until it just contacts the specimen top surface. Set thecrosshead speed at 12 mm/min and start the test while recording load andcrosshead displacement. If the specimen does not fail catastrophically butcontinues to yield, the test may be interrupted.

Figure 6.7 shows a typical compressive stressstrain curve for a vinylesterresin. Based on this curve, it is possible to determine an apparent elastic modu-

lus Ea, yield stress ys, and compressive strength c.The apparent elastic modulus Ea,is calculated based on an apparent strainmeasure (crosshead displacement/specimen length). From the stressstrain

120

90

60

Stress

(MPa)

510A Compression

30

00 2 4

Strain (%)

6 8

FIGURE 6.7Compressive stressstrain curve for a 510 vinylester specimen. (After Figliolini, A.M., 2011,Degradation of Mechanical Properties of Vinylester and Carbon Fiber/Vinylester Composites Due toEnvironmental Exposure, master thesis, Florida Atlantic University.)

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curve in Figure 6.7, the following data were reduced: Ea=4.00 GPa, ys=95.2MPa, c=115 MPa.

6.3.3 Matrix Shear Testing

One of the most important functions of the matrix in a composite is to transferload into the stiff and strong reinforcing fibers. Such load transfer, discussedin more detail in Chapter 7, occurs through shear stresses at the fiber/matrixinterface. Analyses of several loading situations of structural parts madefrom composite materials reveal transfer of load by shear. Consequently, theshear response of the matrix is of prime importance.

Although there is no ASTM standard for the determination of the shearstressstrain response of matrix materials, it is widely recognized (Adams1990, Sullivan et al. 1984) that the most promising shear test of the matrix is theIosipescu shear test method (ASTM D5379 2005). This standard, developedfor composite materials (Chapter 10), is schematically illustrated in Figure 6.8.

As shown in Figure 6.8, the V-notched test specimen is held in the fixture,which is loaded in compression to produce a fairly uniform state of shearstress in the center region of the specimen, between the two notches.

A detail of the test specimen is shown in Figure 6.9. The specimen may

be molded or machined from a flat plate. The top and bottom edges mustbe carefully machined to be flat and parallel to each other to avoid bend-ing and twisting deformations when load is applied. The standard fixtureallows specimens up to 12.7 mm thick, although neat resin plates typi-cally are about 3 to 5 mm in thickness. Details of specimen preparation areprovided in Chapter 4. The specimen is instrumented in the same way asthe composite specimen (Chapter 10), typically with a two-element straingage rosette configured to measure strain in the 45 and 45 directions(Figure 6.9).

P

Fixture base

Adjustable

wedge

Specimen

Fixture attachedto guide rod bylinear roller bearing

Fixtureguide rod

Wedgeadjusting

screw

Specimen

adjustingpin

FIGURE 6.8Schematic of Iosipescu shear test.

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Prior to testing, the specimen dimensions are measured at several locationsto the nearest 0.01 mm. Insert the specimen in the test fixture. Apply load at acrosshead rate of 12 mm/min while recording load and strain until the speci-men fails. Failure of brittle-matrix specimens tends to initiate at the notches andpropagate in the 45 plane, where the principal stress is maximum.

For reduction of the load and strain gage readings, it is recognized that theshear stress is given by

P

A = (6.7)

where Pis the load applied, andAis the cross-sectional area of the specimenbetween the notches. The shear strain is given by

= + | (45 )| | ( 45 )| (6.8)

50

40

30

ShearStress

(MPa)

20

10

00 2 4

Shear Strain (%)

8084 Shear

6

FIGURE 6.10Shear stressstrain curve for vinylester 8084. (After Figliolini, A.M., 2011, Degradation of

Mechanical Properties of Vinylester and Carbon Fiber/Vinylester Composites Due to EnvironmentalExposure, master thesis, Florida Atlantic University.)

76.2

11.5 19.1

R1.3

38.1

90.0

FIGURE 6.9Details of matrix Iosipescu specimen. All dimensions are in millimeters.

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This formula assumes that a 45 strain gage rosette, or two 45 gages, isused. If a single gage oriented at 45or 45is used,

= 2| (45 )| (6.9)

Figure 6.10 shows a representative shear stressstrain curve for a vinyl-ester Iosipescu specimen. Data reduction allows determination of the shearmodulus G, shear yield stress ys, and shear strength u. Based on the curveshown in Figure 6.10, the following properties were reduced: G=0.93 GPa,ys =33.3 MPa, and u=42.2 MPa.

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