ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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fraction, and void content have to engineering property development in composite systems. This article focuses,

thus, exclusively on measuring the engineering and structural performance of laminates and composite

structures.

Mechanical properties of a composite material refer to the elastic and strength properties of the material under

tensile, shear, or compression loading. Other properties, such as fracture toughness and flexural strength and

stiffness, are also useful in characterizing the performance of a composite material. Finally, thermomechanical

and hydromechanical properties are of importance under changing temperature and moisture environments. The

homogeneity assumption that microstructural features of the material are small enough to be inconsequential to

the average behavior of the material on a macroscale may not apply to composites, especially when strength

and fracture are considered. Fabrics and laminates are very inhomogeneous in character. The scale of

homogeneity of a composite system must be taken into account for fixture design, instrumentation decisions,

and in data analysis.

The fundamental description of the engineering properties for a lamina under tension, compression, and shear

loading is given in terms of the lamina coordinate system shown in Fig. 1. The strength and stiffness properties

are defined in Table 1. If the material is transversely isotropic, then the indicated properties need not be

determined. The fracture toughness is sometimes measured as part of durability assessment of a material

system. These properties are the mode I and II critical strain energy release rates (G

and G

IIc

, respectively).

Flexural properties are also determined routinely and result from bending the material to produce tension,

compression, and shear stresses. The result is more a structural property than an intrinsic material property, but

it is very useful in materials screening and quality control.

Table 1 Listing of mechanical properties typically determined for composite materials

Symbol

Property

Tensile modulus in the fiber direction

Tensile modulus transverse to the fiber

Tensile modulus transverse through the thickness

(a)

Compression modulus in the fiber direction

Compression modulus transverse to the fiber

Compression modulus transverse through the thickness

(a)

Shear modulus in the 1–2 plane

Shear modulus in the 1–3 plane

(a)

Shear modulus in the 2–3 plane

Tensile strength in the fiber direction

Tensile strength transverse to the fiber

Tensile strength through the thickness

(a)

Compression strength in the fiber direction

Compression strength transverse to the fiber

Compression strength through the thickness

(a)

Shear strength in the 1–2 plane

Shear strength in the 1–3 plane

(a)

Shear strength in the 2–3 plane

Poisson's ratio in the 1–2 plane

Poisson's ratio in the 1–3 plane

Poisson's ratio in the 2–3 plane

Ultimate tensile strain in fiber direction

Ultimate tensile strain transverse to the fiber

Ultimate tensile strain through the thickness

(a)

Ultimate compression strain in the fiber direction

Ultimate compression strain transverse to the fiber

Ultimate compression strain through the thickness

(a)

(a) This property does not need to be determined if the 2–3 plane is transversely isotropic.

Fig. 1 Lamina and plate coordinate designation system for composites. Plate coordinates are labeled x, y,

and z. Lamina coordinates are labeled 1, parallel to the fiber axis; 2, perpendicular to the fiber axis; and

3, normal to the fiber plane.

Properties of laminated composites are defined similarly to those for the lamina, except a laminate coordinate

system (x, y, z) is employed. The subscripts 1, 2, 3 on the properties defined in Table 1 are respectively changed

to x, y, and z, and the properties become the effective laminate properties. The word “effective” is very

important because it signifies that the measured response is an average response through the material thickness.

In reality, the stresses in a composite are nonuniform. It should also be noted that composites may behave

differently in compression and tension; the elastic and strength properties must be characterized in both tension

and compression to fully characterize the material.

Role of Specimen Fabrication

The test results from any characterization are critically dependent on material and specimen integrity. Material

processing and specimen machining strongly influence the quality and reproducibility of test results. The

recently issued ASTM D 5687, “Guide for Preparation of Flat Composite Panels with Processing Guidelines for

Specimen Preparation,” provides descriptions of current practices for autoclave processed composites.

Machining of test specimens influences the cost of characterization, and trade-offs must often be weighed

between machining tolerance (program costs) and the requirements on the end use of that data.

Specific specimen geometry and laminate configuration requirements are defined for each test type and are

associated with the discussion of the test methods. Materials for the test specimens are either cast, molded to

shape, pultruded, filament wound, or machined from plaques or plates fabricated using autoclave or other

processing methods. For laminates, a reference edge must be established, and each ply must be oriented

accurately with respect to the reference edge. Before cure, the laminate reference edge is accurately scribed

with a reference line that is used to maintain alignment when the cured plate is trimmed. The fiber volume

fraction, void content, and the uniformity of fiber wetout in the part are controlled by the processing of the test

panel or specimen. All of these factors strongly influence the mechanical properties of the material.

Relationships between processing conditions and material microstructure must be understood and controlled to

produce valid test specimens that are representative of the actual material being characterized (i.e., specimens

that have the same microstructure and properties that the material will have in a structure). The specimen must

be fabricated or machined so that the material axes align properly with the test axes.

Specimen Machining. When fabricated from panels, specimens are normally machined using a diamond wafing

saw. A cut is first made along the reference edge, and then all subsequent cuts are made relative to the reference

edge to preserve the accuracy of the fiber orientation in the panel. When machining specimens to final

geometry, make allowances for scrap, and machine specimens from the heart of the material away from edges.

A diamond saw blade produces a very smooth surface along the cut edges, and no further finishing is required

usually. In specimens requiring holes, a diamond core bit provides satisfactory hole quality. Other methods

include ultrasonic drilling, or drilling with special drill bits in conjunction with templates to guard against

punch-through delamination.

If a specimen is designed to have tabs, the tabs are bonded into place before the specimen is machined to its

final shape. The tab material is typically 3.2 mm (0.13 in.) thick [0/90°] glass/epoxy or a woven fabric

glass/epoxy material, although steel or aluminum can be used. When required, bevels are machined onto the

tab, and then the tab is bonded onto the test panel using special jigs to ensure alignment of the tabs with the

specimen reference edge. Individual specimens are cut from the tabbed panel, again taking care to maintain

alignment with the reference edge (test axis).

Good quality composite specimens should be of uniform dimensions, have a precise fiber alignment, and

possess high-quality finish on machined edges. There should be no evidence of delamination along machined

edges. The laminate should contain no dry fiber regions, voids, or other obvious flaws. If available, ultrasonic

C-scan should be employed to nondestructively evaluate composite panels for flaws prior to specimen

fabrication and testing. Flawed panels or flawed regions within panels should be discarded.

Test Equipment and Fixturing Considerations

The availability of suitable, well-maintained, and accurately calibrated testing equipment is essential for

reliable characterization of composite materials. Standard test instrumentation is used for load introduction and

strain measurement of composite materials, but test fixturing must be specially designed to meet the specific

requirements of the composite tests. The drawings and specifications for standard composite test fixtures are

available for most test standards, and many fixtures are now available commercially. Testing is usually

performed in a screw-driven or a servo-hydraulic universal test machine.

The test machine must have sufficient stiffness and load capacity to insure accurate load application and

deformation measurement. A universal test machine with a load capacity greater than or equal to 110 kN (25 ×

lbf) is recommended to test composite materials. Longitudinal tension and compression properties of some

composite specimens require this capacity. Fiber tests, transverse tension, and flex properties require much

lower load capacity. Universal test machines allow interchangeability of load cells to accommodate different

testing requirements, and, when needed, small capacity load cells can be used in a 110 kN frame. The load cell

must be properly matched to the loading requirements of the specimen in order to ensure required levels of

accuracy and sensitivity.

Fixturing Issues. Proper fixturing is critically important to composite testing. The special fixtures for each test

are designed to perform two important functions: (a) to transfer loads or displacements from the test machine to

the test specimen, and (b) to achieve load introduction such that the desired stress state and deformation are

produced in the specimen test section. The quality of test results is governed by proper fixture design, accurate

machining to design specifications, and meticulous maintenance of the fixture.

No fixture functions perfectly in generating required states of uniform stress in test specimens; good tests

closely approximate desired stress states and minimize stress concentrations in the test section. Mechanically,

fixtures must provide reproducible alignment of the specimen in the test machine, and specimens should be

easy to insert and remove after testing. The fixture must be strong and stiff enough not to change the

characteristics of the state of stress in the specimen during the test. It must also be constructed of hardened

materials that will not wear excessively with repeated use. Mating surfaces designed to slip relative to each

other must be polished to stringent flatness-finish requirements to minimize friction binding during testing.

Fixture functionality must conform to specifications in all required test environments.

All fixtures should be inspected routinely before testing to ensure that they are not worn or damaged in any way

that will affect the test results. With use, all fixtures wear and eventually decrease the reliability of test results.

Electronic Transducers for Strain Measurement. Strain and deformation measurements are performed on

composites using methods and instruments similar to those used for metals (Ref 4, 5, 6). A few issues must be

addressed when using standard, bondable-foil strain gages on composites.

Gage heating is a problem that must be addressed when using bondable-foil strain gages on polymeric matrix

composites because the polymer does not conduct heat very well. This allows heat to build up in the gage, and

the resulting temperature change causes a resistance change, which is falsely recorded as an apparent strain.

The use of 350 Ω (or greater) strain gages is recommended for composite testing, and the excitation voltage

should be between 2 and 5 V. Measurement sensitivity is related to the excitation voltage, and 2 V is the lowest

voltage that will ensure sufficient sensitivity.

Composites, especially woven fabrics and braided structures, have very coarse microstructure. The size of the

strain-gage grid area must be large enough to average deformation over a representative area of the specimen. If

the material is heterogeneous, such as a woven fabric, the grid must be large enough to cover at least one unit

cell of the structure. A comprehensive discussion of this issue is covered by Masters and Ifju (Ref 6), based on

their extensive survey of experimental results using strain gages of varying size on different types of fabric and

braided composite structures.

The third problem that can occur when using bondable-foil strain gages for testing composites is that transverse

strain sensitivity can influence strain measurements. This is especially prevalent for certain types of angle-ply

lay-ups, such as the [±45°] laminate. Correction factors must be applied to achieve accurate longitudinal

readings measured under such conditions.

Environmental Conditioning

Environmental conditioning is perhaps one of the most controversial aspects of composites testing. It is not just

the temperature at which the material is to be tested that is important. The entire temperature and moisture

histories of the specimen influence the properties. Specimens should be preconditioned before testing by

exposure to the specified temperature and humidity conditions. Specimens to be tested under standard

laboratory conditions (21 ± 1.0°C, 50% ± 20% relative humidity) can be conditioned in the laboratory for a

period of 24 h prior to testing. Specimens designed to evaluate effects of environmental exposure must be

conditioned by using conventional environmental chambers or temperature-controlled baths using the

procedures outlined below.

Moisture conditioning is covered under a relatively new method (ASTM D 5229) and is usually specified in

one of three ways:

• Exposure at a specific temperature and relative humidity or in a water bath to attain a target percentage

weight gain

• Exposure for a specified time duration

• Exposure to attain equilibrium weight gain

Diffusion constants for composites are often very small, and sometimes accelerated conditioning practices are

employed. A specimen can be soaked in water for short durations and attain a certain moisture content instead

of being conditioned for months at 95% relative humidity to attain the same moisture content. Increase of

temperature greatly increases the diffusion rate.

Accelerated conditioning is usually not equivalent to normal conditioning. The aging of the material is a rate

process that depends on path; thus, specimens may not yield the same properties when conditioned using

accelerated processes. Accelerated conditioning is considered a conservative approach, because accelerated

conditioning typically yields larger degradation of properties than tests that are more representative of actual

service conditions.

Prior to conditioning, polymeric matrix composites should ideally be fully characterized using infrared

spectroscopy, thermomechanical analysis, and differential scanning calorimetry to determine the state of cure,

moisture content, and glass transition temperature (T

) of the matrix material. This information defines the

preconditioned state of the material. The material can then be subjected to the specified environmental

conditioning regimen. Once conditioned, specimens should again be characterized by analytical equipment.

This is best done on travelers, which are dummy specimens conditioned along with the test specimens for this

purpose. The test specimen cannot be used, since the analytical tests will influence the conditioning of the

specimen and render it useless for testing.

Metal matrix and ceramic matrix composites also have requirements for environmental conditioning. The key

difference is that these types of composites have less sensitivity to moisture uptake, because metallic or ceramic

materials do not exhibit moisture-induced “aging” phenomena.

Analysis of Test Results

Statistical principles should be employed in test program design and analysis of test results (Ref 7). Variability

is normal in all testing, and proper statistical treatment of data ensures that variability is handled properly in

deducing conclusions about the meaning and trends of the measured data. To start, typical variability of a given

test should be used to determine the sample sizes needed to meet statistical significance requirements. Sample

means, medians, and standard deviations are useful but may not be sufficient if the population is not normally

distributed or if variance is large. Analysis of variance techniques may need to be employed to determine

significance in comparisons of results.

Many investigators conduct parametric experiments changing one variable at a time to determine relationships

among experimental variables. This approach results in huge, costly test matrices and often produces poor

results. Design of experiments is a rigorous, statistically based method for testing the relationship of

experimental variables. The design minimizes the number of tests necessary to determine the relationships

between the test variables to a desired level of statistical significance. Consultation with an expert is advised,

since this can save money and produce more reliable results. Several software packages are available to support

statistical analysis and design of experiments.

In any experimental test program, failure modes must be carefully noted. Specimen failures should be the

proper mode for the test being conducted and be consistent. Improper failures are an indication that the test is

poorly designed, the specimen is flawed, or the test fixture or setup is improper. Failed specimens should

always be saved, and typical failures should be photographed and documented.

Footnote

The section “Interlaminar Shear Properties of Fiber-

Reinforced Composites at High Strain Rates” was written

by John Harding and Stephen Hallett, Oxford University. The section “Fatigue Testing and Behavior of Fiber-

Reinforced Composites” was written by W. Steven Johnson and Ramesh Talreja, Ge

orgia Institute of

Technology.

References cited in this section

1. R.M. Jones, Mechanics of Composite Materials, 2nd ed., Taylor and Francis, Philadelphia, 1999

2. S.W. Tsai, and H.T. Hahn, Introduction to Composite Materials, Technomic, Lancaster, 1980

3. J.R. Vinson, and T.W. Chou, Composite Materials and Their Use in Structures, Halstead Press, Applied

Science Publishers, Barking, 1975

4. J.M. Whitney, I.M. Daniel, and R.B. Pipes, Experimental Mechanics of Fiber Reinforced Composite

Materials, revised ed., Society for Experimental Mechanics; Prentice-Hall, Englewood Cliffs, 1984

5. R.B. Pipes, Test Methods, Delaware Composites Design Encyclopedia, Vol 6, (L.A. Carlsson and J.W.

Gillespie, Jr., Ed.), Technomic, Lancaster, 1990, p 3

6. J.E. Masters and P.G. Ifju, Strain Gage Selection Criteria for Textile Composite Materials, J. Compos.

Technol. Res., Vol 19 (No. 3), 1997, p 152

7. H.M. Wadsworth, Handbook of Statistical Methods for Engineers and Scientists, McGraw-Hill, New

York, 1990

Mechanical Testing of Fiber-Reinforced Composites

Dale Wilson, The Johns Hopkins University, Leif A. Carlsson, Florida Atlantic University

Characterization of Mechanical Properties

The basic building block of laminated composites is the unidirectional lamina (Fig. 1). Specific test methods are

available to measure lamina mechanical properties. The unidirectional lamina is highly anisotropic, which

complicates mechanical testing. Alignment of the test specimen in the test frame is an important requirement

for obtaining adequate test results, and undesirable transverse failures are a common occurrence in such

materials. For these reasons, a number of investigators propose to back out unidirectional properties from

laminate tests, but no acceptable standards have yet emerged, except for the [±45°] laminate coupon subjected

to uniaxial tension for the generation of lamina shear stress-strain response (ASTM D 3518). There exists a host

of test fixtures, specimen geometries, and test procedures for the generation of mechanical property data for

composite materials. Reviews and further information on the subject are provided in Ref 4, 5, and 8–10. Space

limitations prohibit discussion of each test method. Only test methods accepted by the community as ASTM

standards or as candidates for ASTM standardization are considered in this article. In cases where more than

one method is discussed, the differences are clearly brought out, and guidelines are given about the use of each

method.

Tension Testing

The most basic mechanical test is the tension test. For most structural materials, the tensile properties are

essential elements of the material design allowables. The tension test is used to measure Young's modulus,

Poisson's ratio, tensile strength, and ultimate strain to failure for composites. The properties reduced from

tension tests on composite materials are effective (averaged) properties. The test method applies to

unidirectional composites but can also be performed on laminates, woven fabrics, or discontinuous fiber

composites. For asymmetric and/or unbalanced laminates, extension/bending coupling and extension/shear

coupling effects produce nonuniform stress states in the test section. Under these conditions, effective

properties cannot be accurately determined from the test results using the standard data reduction methods.

The adequate gripping of the test specimen is the major issue in tension tests. Any tension test specimens (Fig.

2) require gripping regions where loads are introduced through the specimen surfaces, a transition region, and a

gage section region that may be of reduced cross-sectional area to promote failures away from the grips.

Sufficient volume should be involved in the gage section to achieve adequate sampling of the material being

tested.

Fig. 2 Generic tension test specimen

The widthwise tapering popular with metals (Fig. 2) usually leads to splitting failures of highly anisotropic

composites in the gripping region prior to ultimate failure of the material in the gage section. This problem is

avoided by using uniform width (rectangular) test specimens (Ref 10). The grips of the tension test frame

introduce large clamping forces that can cause splitting failures or surface damage in the gripped region. These

forces, coupled with normal stress concentrations induced by load introduction, can lead to anomalous failures.

Tabs with tapered (beveled) ends, therefore, are bonded on each side of the specimen. The load is transferred

into the specimen test section through shear (see Fig. 3). When tabs are used, the properties of the adhesive

must be carefully chosen to meet the strength and elongation requirements of the composite under the

temperature and moisture conditions imposed by the test. Figure 4 shows the geometry of the ASTM D 3039

tension test specimen. Stress analysis of tabbed specimens (Ref 11, 12) indicates that intense out-of-plane peel

and shear stress exist at the tip of the bevel and that the axial tensile stress in the specimen is increased.

Consequently, tabbed specimens may fail at the tab ends or inside the tabs, but low bevel angles will reduce the

stress concentration (Ref 11). Typically, bevel angles in the range of 15 to 30° are used because tabs with small

taper angles occupy too much of the gage section.

Fig. 3 Load transfer in gripping region of tension test specimen through end tabs

Fig. 4 Specimen for tension testing of composites as defined in ASTM D 3039, L

= gage length; L

= tab

length; θ = tab bevel angle; w = width

To characterize the tensile response of the unidirectional lamina, 0° and 90° specimens are employed to

determine longitudinal and transverse properties. The [±45°] laminate tension test measures shear properties of

the lamina and is discussed under shear testing. When characterizing multidirectional laminates, the tabs are

sometimes replaced by emery paper inserted between the grips and specimen surface to avoid slippage and

minimize surface damage, but in composite laminates with 0° surface plies, fiber damage is likely to occur in

the gripping region if tabs are not used.

Specimen Machining and Instrumentation. For unidirectional composites of 0° fiber orientation, a specimen

width of 12.7 mm (0.5 in.) and a thickness of 6 plies are common. Unidirectional 90° specimens are typically

25 mm (1 in.) wide and 8 to 16 plies thick. Laminates and sheet molding compound use the same geometry as

the 90° specimen and have the specimen thickness defined by laminate configuration or fundamental sheet

thickness. Loading eccentricity may arise due to variations in tab and specimen thickness. As proposed in

ASTM standard D 3039, tolerances for tab and specimen thicknesses are ±1 and 4%, respectively (Ref 10).

Tabs should be made from [±45°] or [0/90°] glass/epoxy or woven fabric composites. Printed circuit board

(NVF Co., Kennett Square, PA) is often used because of its tight thickness tolerances. Laminates and sheet

molding compound can be tested with or without tabs, although tabs are recommended for thin specimens.

Gage length (L

), (Fig. 4) is commonly 125 to 150 mm (5 to 6 in.).

It is common to bond continuous end tabs on the panels prior to machining the specimens. After careful surface

preparation of the bonding surfaces of the specimens and end tabs (Ref 8), the end tabs are attached with an

adhesive, typically Hysol 9309, 934 or 929 (Hysol Division, The Dexter Corporation, Pittsburg, CA) or similar

epoxy adhesive appropriate to the test conditions specified. The tab length, L

in Fig. 4, should be at least 38

mm (1.5 in.), and the tab material should be 1.6 to 3.2 mm (0.06 to 0.13 in.) thick. As pointed out in Ref 10 and

13, strips of beveled tab material of similar or slightly larger length than the width of the uncut composite panel

should be bonded on both ends of the composite panel prior to machining the specimens. It is desirable to use a

special tab-fixturing jig to symmetrically secure the position of the four strips of tabs on the composite panel to

maintain positive alignment between the tabs and composite panel. Such fixtures are available commercially,

(e.g., Ref 14).

Resin matrix composites are typically machined using a slitting saw or a water-cooled diamond saw (Ref 8).

Polishing of the edges has been found to increase the strength, but the finish produced by diamond sawing

meets the requirements of ASTM methods and is the commonly accepted industry practice. Alignment of the

specimen axis with respect to the fiber direction is an important issue in machining of composites (Ref 8, 15).

Hart-Smith (Ref 15) found that specimens cut with 1° of misalignment may cause as much as a 30% decrease in

strength due to reduced effective width of the specimen. The variations of the specimen width should not

exceed 1% (ASTM D 3039). If Poisson's ratio is desired, a 0/90° strain gage rosette should be bonded in the

center-gage-section region of the specimen. If only Young's modulus and strength are desired, a longitudinal

strain gage or an extensometer attached to the specimen can be used. When load eccentricity is of concern,

back-to-back strain gages may be used to detect bending of the specimen. When using strain gages on woven

fabric materials, one must select the strain gage size to average deformation over a representative portion of the

fabric structure. Failure to use a sufficiently large strain gage will result in large variability in the measured

strain.

The specimen geometry and dimensions discussed so far are strictly valid for polymeric resin matrix

composites but also apply to other types of composites, after some modifications. Johnson et al. (Ref 16) for

example, who studied metal matrix composites containing unidirectional silicon-carbide fibers, found that

laminates containing 0° fibers required reduction of the gage-section cross-sectional area (as shown in Fig. 5)

so that the specimen would fail in the gage section without slipping or failing in the grips.

Fig. 5 Dog-bone specimen used for tension strength testing of fiber-dominated metal matrix composites.

All dimensions are in millimeters. Source: Ref 16

Test Procedure. Use of standard wedge-action grips with hardened steel serrated jaws is the common practice.

With such grips, the clamping pressure increases in proportion to the axial load acting on the specimen.

Hydraulic grips provide means to adjust the clamping pressure to avoid crushing of the specimen ends at high

loads. Alignment of the test specimen is especially important for unidirectional composites. The specimen is

tested monotonically to failure while recording load, crosshead displacement, and strain. Normally, the test is

run at a crosshead speed of 2 mm/min (0.08 in./min). Failure mode and location should be noted for each test

along with ultimate failure load. A failure located outside the test section justifies rejection of the result. Figure

6 shows acceptable and common failure modes for 0 and 90° carbon fiber composites. Some 0° specimens

literally explode. Safety glasses and a protective shield are recommended during tension testing.

Fig. 6 Commonly observed, acceptable failure modes of (a) 0°, and (b) 90° carbon/epoxy unidirectional

composites

Figure 7 shows a representative example of stress-strain curves for a 0° glass/polyphenylene sulfide (PPS)

composite, that is, stress σ

versus longitudinal and transverse strains, strains ε

and ε

. The stress σ

is defined

as load divided by cross-sectional area in the test section. Based on the data collected from the test, the modulus

was reduced using a least squares linear fit to the linear initial portion of the curve σ

versus ε

. Poisson's

ratio, ν

, was determined from the ratio of the initial slopes of σ

versus ε

and σ

versus -ε

, with ν

= -ε

/ε

The ultimate strength in tension = is the maximum value of σ

. Values of E

, ν

and are given in

Fig. 7.

Fig. 7 Stress-strain response for a unidirectional [0]

glass/polyphenylene sulfide (PPS) specimen

Compression Testing

Compression testing is performed by subjecting a test specimen to an increasing compressive load until the

specimen fails in a failure mode that is representative of that in an actual structure. As discussed in Ref 10,

however, compressive failure is triggered by phenomena on the microlevel that are very difficult to observe,

and detailed study is required to reach a clear definition of valid failure modes. It is clear, however, that

strength degradation due to stress concentrations in the specimen arising from load introduction or slight

eccentricities in load-specimen alignment should be minimized and that failure caused by global specimen

buckling must be suppressed. Buckling and kinking of the fibers within the composite are features regarded as

representative for the material and should not be inhibited.

To avoid buckling instability, relatively short gage lengths are necessary, but short gage lengths generally tend

to amplify sensitivities due to clamping. Thus, for very short gage lengths, the apparent compressive strength

tends to decrease (Ref 17, 18, 19). It is likely that nonuniformities in specimen thickness or in the bond or tab

thicknesses result in nonuniform loading in the short gage section, leading to premature failure. Figure 8 shows

some typical failure modes. The failure modes shown in Fig. 8(a–c) are acceptable, but the column buckling

failure (Fig. 8d) is clearly unacceptable, although difficult to visually observe. Because of the stress

concentration at the tab ends, it is common to observe failure at the tabbed region (Ref 20). Such failures are

not acceptable in tensile testing but are usually accepted in compression testing, because they rarely can be

avoided (Ref 10).

Fig. 8 Typical failure modes for composite compression specimens

Several compression test methods have emerged during the past twenty years, and much confusion exists on

their relative virtues. The methods may be grouped into three categories based on load introduction and

specimen design: shear loading, end-loading, and sandwich beam specimen testing (Fig. 9). The sandwich beam

specimen (Fig. 9c), may be tested in flexure or axial compression. Chatterjee et al. (Ref 10) presented a

thorough review of currently available compression test methods and rated the methods according to problems

associated with load introduction, uniformity of stress field, sensitivity to imperfections, simplicity,

acceptability of failure modes, adequacy of data reduction, specimen preparation and fixture requirements, and

consistency of results. Table 2 summarizes the various test methods and their ratings (1, 2, and 3). The test

methods included in Table 2 are essentially based on polymer matrix composites but should be applicable to

metal matrix and ceramic matrix composites as well. According to Ref 10, the Illinois Institute of Technology

Research Institute (IITRI) test method (ASTM standard D 3410) is the most reliable and versatile. Because of

space limitations, only the methods with the highest rating (1) are described here with the exception of the

Boeing-modified ASTM D 695 method, which has been adopted as a recommended method (SRM 1-88) by the

(Suppliers of Advanced Composite Materials Association) (SACMA). It should be emphasized that while all

the methods provide adequate measures of modulus, the “true” composite strength may not be determined.

Table 2 Compression test methods for fiber-reinforced composites

Method Description Rating

(a)

Shear-loaded specimen test methods

Celanese (ASTM D 3410) Long-established ASTM standard. Results are very sensitive

to accuracy of fixture and test procedure.

Wyoming-modified Celanese Cone grips replaced by tapered cylindrical grips. Post and

bearing alignment replaces sleeve. Reduced fixture cost.

Wider specimen

IITRI (Illinois Institute of

Technology Research Institute)

(ASTM D 3410)

An ASTM standard since 1987. Tapered flat wedges. Post and

bearing alignment. Massive, relatively expensive fixture.

Wide, thick specimen can be tested.