4.1.2 COMPRESSIVE PROPERTIES Compressive properties of thin composite laminates are difficult to measure owing to sidewise buckling of specimens.A number of test methods and specimen designs have been developed to overcome the buckling problem [12].Three of these test methods are described as follows. Celanese test:This was the first ASTM standard test developed for testing fiber-reinforced composites in compression;however,because of its several deficiencies,it is no longer a standard test.It employs a straight-sided specimen with tabs bonded at its ends and 10 tapered collet-type grips that fit into sleeves with a matching inner taper(Figure 4.20).An outer cylindrical shell is used for ease of assembly and alignment.As the compressive load is applied at *-3.99mm Collet 63.5mm grip Tapered sleeve Specimen 12.7mm Cylindrical shell 63.5mm 63.5 mm (a) diameter FIGURE 4.20(a)Celanese test specimen and fixture for compression testing. (continued 2007 by Taylor&Francis Group.LLC
4.1.2 COMPRESSIVE PROPERTIES Compressive properties of thin composite laminates are difficult to measure owing to sidewise buckling of specimens. A number of test methods and specimen designs have been developed to overcome the buckling problem [12]. Three of these test methods are described as follows. Celanese test: This was the first ASTM standard test developed for testing fiber-reinforced composites in compression; however, because of its several deficiencies, it is no longer a standard test. It employs a straight-sided specimen with tabs bonded at its ends and 108 tapered collet-type grips that fit into sleeves with a matching inner taper (Figure 4.20). An outer cylindrical shell is used for ease of assembly and alignment. As the compressive load is applied at Specimen 3.99 mm (a) Collet grip Tapered sleeve Cylindrical shell 63.5 mm diameter 63.5 mm 12.7 mm 10 63.5 mm FIGURE 4.20 (a) Celanese test specimen and fixture for compression testing. (continued) 2007 by Taylor & Francis Group, LLC.
(b) FIGURE 4.20(continued)(b)Celanese compression test fixture.(Courtesy of MTS Systems Corporation.With permission.) the ends of the tapered sleeves,the grip on the specimen tightens and the gage section of the specimen is compressed by the frictional forces transmitted through the end tabs.Strain gages are mounted in the gage section to measure longitudinal and transverse strain data from which compressive modulus and Poisson's ratio are determined. IITRI test:The IITRI test was first developed at the Illinois Institute of Technology Research Institute and was later adopted as a standard compres- sion test for fiber-reinforced composites (ASTM D3410).It is similar to the Celanese test,except it uses flat wedge grips instead of conical wedge grips (Figure 4.21).Flat wedge surfaces provide a better contact between the wedge and the collet than conical wedge surfaces and improve the axial alignment.Flat wedge grips can also accommodate variation in specimen thickness.The IITRI test fixture contains two parallel guide pins in its bottom half that slide into two roller bushings that are located in its top half.The guide pins help maintain good lateral alignment between the two halves during testing.The standard specimen length is 140 mm,out of which the middle 12.7 mm is unsupported and serves as the gage length.Either untabbed or tabbed specimens can be used; however,tabbing is preferred,since it prevents surface damage and end crush- ing of the specimen if the clamping force becomes too high. Sandwich edgewise compression test:In this test,two straight-sided speci- mens are bonded to an aluminum honeycomb core that provides the necessary 2007 by Taylor Francis Group,LLC
the ends of the tapered sleeves, the grip on the specimen tightens and the gage section of the specimen is compressed by the frictional forces transmitted through the end tabs. Strain gages are mounted in the gage section to measure longitudinal and transverse strain data from which compressive modulus and Poisson’s ratio are determined. IITRI test: The IITRI test was first developed at the Illinois Institute of Technology Research Institute and was later adopted as a standard compression test for fiber-reinforced composites (ASTM D3410). It is similar to the Celanese test, except it uses flat wedge grips instead of conical wedge grips (Figure 4.21). Flat wedge surfaces provide a better contact between the wedge and the collet than conical wedge surfaces and improve the axial alignment. Flat wedge grips can also accommodate variation in specimen thickness. The IITRI test fixture contains two parallel guide pins in its bottom half that slide into two roller bushings that are located in its top half. The guide pins help maintain good lateral alignment between the two halves during testing. The standard specimen length is 140 mm, out of which the middle 12.7 mm is unsupported and serves as the gage length. Either untabbed or tabbed specimens can be used; however, tabbing is preferred, since it prevents surface damage and end crushing of the specimen if the clamping force becomes too high. Sandwich edgewise compression test: In this test, two straight-sided specimens are bonded to an aluminum honeycomb core that provides the necessary FIGURE 4.20 (continued) (b) Celanese compression test fixture. (Courtesy of MTS Systems Corporation. With permission.) 2007 by Taylor & Francis Group, LLC
Flat wedge grip o°Unidirectional Guide pin composite specimen (a) (b) FIGURE 4.21 IITRI compression test fixture.(Courtesy of MTS Systems Corporation. With permission.) support for lateral stability(Figure 4.22).Compressive load is applied through the end caps,which are used for supporting the specimen as well as preventing end crushing.The average compressive stress in the composite laminate is calculated assuming that the core does not carry any load.Table 4.5 shows representative compressive properties for carbon fiber-epoxy and boron fiber- epoxy laminates obtained in a sandwich edgewise compression test.The data in this table show that the compressive properties depend strongly on the fiber type as well as the laminate configuration. Compressive test data on fiber-reinforced composites are limited.From the available data on 0 laminates,the following general observations can be made. 1.Unlike ductile metals,the compressive modulus of a 0 laminate is not equal to its tensile modulus. 2.Unlike tensile stress-strain curves,compressive stress-strain curves of laminates may not be linear. 2007 by Taylor Francis Group.LLC
support for lateral stability (Figure 4.22). Compressive load is applied through the end caps, which are used for supporting the specimen as well as preventing end crushing. The average compressive stress in the composite laminate is calculated assuming that the core does not carry any load. Table 4.5 shows representative compressive properties for carbon fiber–epoxy and boron fiber– epoxy laminates obtained in a sandwich edgewise compression test. The data in this table show that the compressive properties depend strongly on the fiber type as well as the laminate configuration. Compressive test data on fiber-reinforced composites are limited. From the available data on 08 laminates, the following general observations can be made. 1. Unlike ductile metals, the compressive modulus of a 08 laminate is not equal to its tensile modulus. 2. Unlike tensile stress–strain curves, compressive stress–strain curves of 08 laminates may not be linear. Guide pin 08 Unidirectional composite specimen Flat wedge grip (a) FIGURE 4.21 IITRI compression test fixture. (Courtesy of MTS Systems Corporation. With permission.) 2007 by Taylor & Francis Group, LLC
Aluminum honeycomb Composite specimens End cap FIGURE 4.22 Sandwich edgewise compression testing specimen. 3.The longitudinal compressive strength of a 0laminate depends on the fiber type,fiber volume fraction,matrix yield strength,fiber length-diameter ratio,fiber straightness,fiber alignment as well as fiber-matrix interfacial shear strength.The effects of some of these variables on the compressive properties of unidirectional fiber-reinforced polyester composites have been studied by Piggott and Harris and are described in Ref.[4]. TABLE 4.5 Compressive Properties of Carbon and Boron Fiber-Reinforced Epoxy Composites Carbon-Epoxy Boron-Epoxy Strength,MPa Modulus,GPa Strength,MPa Modulus,GPa Laminate (ksi) (Msi) (ksi) (Msi) [0I 1219.5(177) 110.9(16.1) 2101.4(305 215.6(31.3) [15] 799.2(116 95.8(13.9) 943.9(137) 162.9(23.65) [±45) 259.7(37.7) 15.62.27) 235.6(34.2) 17.4(2.53) [901 194.3(28.2) 13.1(1.91) 211.5(30.7) 20.5(2.98) [0/90] 778.6(113) 60.6(8.79) 1412.4(205) 118.3(17.17) [0/±45/901 642.8(93.3) 46.4(6.74) 1054.2(153) 79.0(11.47) Source:Adapted from Weller.T..Experimental studies of graphite/epoxy and boron/epoxy angle ply laminates in compression,NASA Report No.NASA-CR-145233,September 1977. 2007 by Taylor Francis Group,LLC
3. The longitudinal compressive strength of a 08 laminate depends on the fiber type, fiber volume fraction, matrix yield strength, fiber length–diameter ratio, fiber straightness, fiber alignment as well as fiber–matrix interfacial shear strength. The effects of some of these variables on the compressive properties of unidirectional fiber-reinforced polyester composites have been studied by Piggott and Harris and are described in Ref. [4]. End cap Composite specimens Aluminum honeycomb FIGURE 4.22 Sandwich edgewise compression testing specimen. TABLE 4.5 Compressive Properties of Carbon and Boron Fiber-Reinforced Epoxy Composites Carbon–Epoxy Boron–Epoxy Laminate Strength, MPa (ksi) Modulus, GPa (Msi) Strength, MPa (ksi) Modulus, GPa (Msi) [0] 1219.5 (177) 110.9 (16.1) 2101.4 (305) 215.6 (31.3) [±15] 799.2 (116) 95.8 (13.9) 943.9 (137) 162.9 (23.65) [±45] 259.7 (37.7) 15.6 (2.27) 235.6 (34.2) 17.4 (2.53) [90] 194.3 (28.2) 13.1 (1.91) 211.5 (30.7) 20.5 (2.98) [0=90] 778.6 (113) 60.6 (8.79) 1412.4 (205) 118.3 (17.17) [0=±45=90] 642.8 (93.3) 46.4 (6.74) 1054.2 (153) 79.0 (11.47) Source: Adapted from Weller, T., Experimental studies of graphite=epoxy and boron=epoxy angle ply laminates in compression, NASA Report No. NASA-CR-145233, September 1977. 2007 by Taylor & Francis Group, LLC
4.Among the commercially used fibers,the compressive strength and modulus of Kevlar 49-reinforced composites are much lower than their tensile strength and modulus.Carbon and glass fiber-reinforced composites exhibit slightly lower compressive strength and modulus than their respective tensile values,and boron fiber-reinforced compos- ites exhibit virtually no difference between the tensile and compressive properties. 4.1.3 FLEXURAL PROPERTIES Flexural properties,such as flexural strength and modulus,are determined by ASTM test method D790.In this test,a composite beam specimen of rectangular cross section is loaded in either a three-point bending mode(Figure 4.23a)or a four-point bending mode (Figure 4.23b).In either mode,a large span-thickness(L/h)ratio is recommended.We will consider only the three- point flexural test for our discussion. The maximum fiber stress at failure on the tension side of a flexural specimen is considered the flexural strength of the material.Thus,using a homogeneous beam theory,the flexural strength in a three-point flexural test is given by 3Pmax L OUF= 26h2 (4.10) where Pmax=maximum load at failure =specimen width h =specimen thickness L =specimen length between the two support points Flexural modulus is calculated from the initial slope of the load-deflection curve: L/2 一L2 (a) (b) FIGURE 4.23 Flexural test arrangements in (a)three-point bending and (b)four-point bending modes. 2007 by Taylor&Francis Group.LLC
4. Among the commercially used fibers, the compressive strength and modulus of Kevlar 49-reinforced composites are much lower than their tensile strength and modulus. Carbon and glass fiber-reinforced composites exhibit slightly lower compressive strength and modulus than their respective tensile values, and boron fiber-reinforced composites exhibit virtually no difference between the tensile and compressive properties. 4.1.3 FLEXURAL PROPERTIES Flexural properties, such as flexural strength and modulus, are determined by ASTM test method D790. In this test, a composite beam specimen of rectangular cross section is loaded in either a three-point bending mode (Figure 4.23a) or a four-point bending mode (Figure 4.23b). In either mode, a large span–thickness (L=h) ratio is recommended. We will consider only the threepoint flexural test for our discussion. The maximum fiber stress at failure on the tension side of a flexural specimen is considered the flexural strength of the material. Thus, using a homogeneous beam theory, the flexural strength in a three-point flexural test is given by sUF ¼ 3Pmax L 2bh2 , (4:10) where Pmax ¼ maximum load at failure b ¼ specimen width h ¼ specimen thickness L ¼ specimen length between the two support points Flexural modulus is calculated from the initial slope of the load–deflection curve: h L P P/2 P/2 (a) (b) L /2 L/2 h L b h FIGURE 4.23 Flexural test arrangements in (a) three-point bending and (b) four-point bending modes. 2007 by Taylor & Francis Group, LLC