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Makine Teknolojileri Elektronik Dergisi Cilt: 7, No: 4, 2010 (13-23)

Electronic Journal of Machine Technologies Vol: 7, No: 4, 2010 (13-23)

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Bu makaleye atıf yapmak için Aktaş M., Deniz M.E., “ Kompozit Tabakaların Düzlemdeki Kayma Özelliklerinin Belirlenmesi“ Makine Teknolojileri Elektronik Dergisi 2010, (7) 13-23 How to cite this article Aktaş M., Deniz M.E., “Determination of In-Plane Shear Properties of Composite Laminates” Electronic Journal of Machine Technologies, 2010, (7) 13-23

Makale (Article)

Determination of In-Plane Shear Properties of Composite Laminates

Mehmet AKTAS*, Mehmet Emin DENIZ**

*Usak University, Department of Mechanical Engineering, 64200, Usak, Turkey **Dokuz Eylül University, Department of Mechanical Engineering, 35100, Izmir, Turkey

[email protected]

Abstract

This study focuses on the investigation of in-plane shear properties of glass/epoxy laminated composite plates by using V-notched rail shear test fixture. V-notched shear test, described in ASTM Standard D 7078, incorporates the attractive features of Iosipescu and two-rail shear tests. The effects of the material orientation and notch angle of V-notched rail shear specimens on in-plane shear properties are investigated.

Keywords : V-notched rail shear test fixture, in-plane shear properties, composite laminate, notch angle, material orientation.

Kompozit Tabakaların Düzlemdeki Kayma Özelliklerinin Belirlenmesi Özet

Bu çalışmanın amacı cam elyaf/epoksi esaslı tabakalı kompozit plakların düzlemdeki kayma özelliklerinin V-çentikli kayma test aparatı kullanılarak belirlenmeye çalışmaktır. V-çentikli kayma testi ASTM Standard D 7078’de tanımlanan Iosipescu and 2 çentikli kayma testlerini kapsayan modife edilmiş halidir. Bu çalışmada, V-çentikli kayma test numunelerin düzlemdeki kayma özelliklerine malzeme oryantasyon açısının ve çentik açısının etkileri araştırılmıştır. Anahtar kelimeler: V-çentikli kayma test aparatı, düzlemdeki kayma özellikleri, kompozit tabaka, çentik açısı, malzeme oryantasyon açısı.

1.INTRODUCTION In nowadays technology, fiber reinforced composite materials are widely used in various engineering applications including automotive, aviation, and civil engineering structures, etc. due to its lower weight, high specific stiffness (stiffness/density), specific strength (strength/density), and damping characteristics [1-3]. Therefore, the mechanical properties of fiber-reinforced composite materials have to be well known. One of the important properties of composite materials is the shear strength and its characterization. Accordingly, a variety of test methods have been developed to measure the shear properties of composite materials. The most popular of these shear test methods are short beam shear test [4], two-rail shear test [5], ±45o tensile shear test [6], Iosipescu shear test [7] and the V-notched rail shear test [8]. However, the most commonly used test method is the V-notched rail shear test due to its attractive features and standardization as ASTM D 7078. A few researchers have carried out studies in order to obtain shear properties of composite materials.

Teknolojik Araştırmalar: MTED 2010 (7) 13-23 Determination of In-Plane Shear Properties …

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Hussain and Adams [9, 10] have measured the in-plane shear stress of tabbed and untabbed unidirectional and cross-ply glass/epoxy and carbon/epoxy composite plates, which have trapezoidal and rectangular geometries, by using a modified two-rail shear test fixture. Results showed that the shear stress distribution was more uniform for trapezoidal specimen, but the local stress concentration was larger than the rectangular one. Tapering of the specimen did not favorably influence the stress distributions; in fact it reduced the stress concentrations. A new V-notched rail shear test fixture has been developed for measuring the in-plane shear modulus and shear strengths of multidirectional composite laminates and textile composites [11, 12]. Yeow and Brinson [13] have used various test methods to discuss the shear stress-strain characterization of composite laminates for their advantages and limitations, experimentally and numerically. These test methods are the off-axis tensile test, ±45o tensile test and the (0o over 90o) symmetric rail shear test methods. Different shear behaviors were determined depending on the nature of the glass/epoxy in off-axis tensile test method and other two methods. Boller [14] has conducted a series of tests to characterize the stress-strain behavior of composite laminates. The shear distortion is measured by a detrusion gage located midway between rails and midway along the 5-inch dimension. The size of the apparatus and the nature of the detrusion gage are convenient for automatically evaluating composites in abnormal environments. Aktas and Karakuzu [15] have determined the tensile, compressive and shear properties of unidirectional glass/epoxy composite plates under room (approx. 20°C) and high temperatures (40°C, 60°C, 80°C and 100°C). Results show that the mechanical properties of glass/epoxy composites are reduced by increasing temperature. Chang et al. [16] have investigated the effect of testing methods on the in-situ shear stress distribution in cross-ply graphite/epoxy laminated composites. A small difference is obtained between the rail shear and Iosipescu fixtures for the measurement of the in-plane shear strength. Tarnopol’skii et al. [17] have investigated the shear properties of 3D textile composites and 2D glass/epoxy with a lay-up of ±45o by using Iosipescu and Asymmetrical Four-Point Bending (AFPB) tests. Results showed that the Iosipescu and AFPB tests are suitable for the determination of shear strength of the composites. But, 2D glass/epoxy with lay-up ±45o specimens necessitated the use of metal tabs to prevent crushing of loaded surfaces. In this study, in-plane shear properties of glass/epoxy composite specimens were determined by using a standard V-notched rail shear test fixture. For this purpose, eight-ply glass/epoxy composite laminates were produced. The material orientation and notch angle effects on the shear properties of composite specimens were investigated, experimentally and numerically. 2. MATERIAL and METHOD 2.1. Material 2.1.1. Manufacturing of test fixture The V-notched rail shear test fixture is commonly used due to the uniform distribution of in-plane shear stress inside of the specimen gage section and reduced cross-sectional area to make contact with in-plane shear stresses nearby the grips. Therefore, this test method is described in ASTM Standard D7078. The most important aspect for this test fixture is to provide the force current through the upper and lower notch tips. The specimen should not be sliding from the clamped apparatus. Hence, the other important feature is the surface of the clamped apparatus. Surface finishing was carried out to increase the roughness of the clamping surface. In addition, some required zones were chamfered for minimizing the notch effect. According to ASTM standard, the material of the V-notched rail shear test fixture was chosen as AISI E8620. Modulus of elasticity and Poisson’s ratio for this steel are 205 GPa and 0.29, respectively. Heat treatment was applied on all components for surface hardening after the manufacturing

Aktaş M., Deniz M.E. Teknolojik Araştırmalar: MTED 2010 (7) 13-23

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process (Fig. 1).

Figure 1. Standard V-notched rail shear test fixture 2.1.2. Manufacturing and preparation of test specimens The fiber reinforced composite material used in this study is manufactured from unidirectional E-glass fabric having a weight of 509 gr/m2 and epoxy resin CY225 with HY225 hardener. Unidirectional glass/epoxy composite plate with eight plies was produced at Izoreel Firm by using hand lay-up technique. A hot lamination press was used for fabrication of composite plates. For the curing process, laminated plates were retained at a constant pressure (250 kPa) and 120°C for 2 hours. Afterwards, the composite plate was cooled to room temperature at the same pressure. The volume fraction and the nominal thicknesses of composite plate were obtained as 65 % and 3 mm, respectively.

According to ASTM D 7078 the V-notched rail shear specimens must be 76 mm long and 56 mm wide with a minimum of 3 mm thickness. The depth and length of notch must be 12.5 mm and 25 mm, respectively. The distance between the notches (c=31 mm) is constant. For this purpose, test specimens were cut from the manufactured plates as illustrated in Fig. 2(a).

(a) (b)

Figure 2. V-notched rail shear test specimen with, (a) dimensions and geometric parameters (b) 45° strain-gage

2.2. Method 2.2.1. Experimental method In this part of the study, two composite specimens, which have 0° and 90° material orientations, are tested


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to determine the material orientation effects on the shear properties by using the V-notched rail shear test fixture. Six specimens are used for each experiment and the mean values are calculated. Most of the shear test methods carried out on unidirectional composites have been performed with fibers parallel to the longitudinal axis of the specimen; such specimens are designated as 0° specimens. Less frequently, unidirectional specimens are tested with fibers perpendicular to the loading axis; these are designated as 90° specimens. The test specimen is mounted between two gripping plates within each L-shape fixture as in a manner of clamped boundary conditions (Fig. 3). Afterwards, tensile loads are applied along the line joining the two tips of notches in order not to create any bending moments using INSTRON 1114 tensile testing machine of 50 kN load capacity. For loading, the V-notch rail shear test fixture is placed between two extremely stiff machine heads, of which the upper one is fixed during the test, whereas the lower head is moved downwards by a mechanical screw system. All specimens are loaded at constant cross-head speed of 1 mm/min. The load displacement diagrams for all specimens are plotted.

Figure 3. V-notched rail shear test fixture with installed specimen

According to ASTM D 4255/D 4255M-01 [7], by assuming a uniform stress distribution throughout the specimen, in-plane shear strength of [0°]8 and [90°]8 composites are determined as,

ct

PS f

(1)

where, Pf is the applied failure force and is equal to shear force, c and t denote the distance between the upper and the lower notch tips and thickness of the specimen, respectively. Multiplying them gives the effective area subjected to only shear force. In-plane shear modulus is also obtained by using a 45° strain-gage, located between the notches with respect to the longitudinal axis in the V-notched specimen as shown in Fig. 2(b): ε is measured by using strain-gage and G12 is obtained as 4.13 GPa by using the following equations,

2εγ12 A

Pτ12

ctA

12

1212

γ

τG

(2)

For the elastic stress analysis, other mechanical properties of glass/epoxy composite determined by using standard test methods are also given in Table 1.


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Table 1. Mechanical properties of glass-epoxy composite plate [15]

E1 (GPa) E2 (GPa) ν12

40.51 13.96 0.22

2.2.2. Numerical method In order to investigate the material orientation and notch angle effects on the in-plane shear stress, an elastic stress analysis is carried out. Seven different fiber angles as q= 0°, 15°, 30°, 45°, 60°, 75°, 90° and four different notch angles as y=30°, 60°, 90°, 120° are selected. The material or fiber orientation (q) and notch angle (y) of composite specimens are illustrated in Fig. 2(a). In the elastic stress analysis, ABAQUS 6.6-1 commercial software is used. Each model of V-notched rail shear specimen has eight plies (Fig. 4); the dimensions of the plies are given in Figure 2. The thickness of each ply is of 0.375 mm. The rail shear test fixture is meshed by using four node tetrahedral solid elements (C3D4) due to geometric complexity. On the other hand, the V-notched rail shear test specimen is meshed by using eight-node solid brick elements with reduced integration (C3D8R) as illustrated in Fig. 5(a-b). The number of elements is 37543 and 16224 for each part of rail shear fixture and shear specimen, respectively. Surface to surface contact is applied between the test specimen capture surface of fixture and specimen test surface. Laminated specimens are clamped (UX=UY=UZ=0 and ROTX=ROTY=ROTZ=0) along both short sides. The sides with notches are left free. While one edge of the fixture is fixed, the load obtained from V-notched rail shear test of [0°]8 specimen is uniformly applied to the other edge of the fixture in the axial direction for all cases.

Figure 4. Laminate with eight plies and material directions


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(a)

(b)

Figure 5. The illustration of mesh for (a) assemble of V-notched rail shear test fixture and test specimen,

(b) test specimen

3. RESULTS and DISCUSSION This study is composed of two sub steps; the first one includes experimental investigation of in-plane shear stresses for two different material orientations as 0° and 90° in glass/epoxy specimens, which have 90° notch angle. The second one is to perform numerical analysis to investigate the material orientation and notch angle effects on in-plane shear stress of glass/epoxy composite specimens. In-plane shear strength and numerical maximum in-plane shear stresses for specimens having [0°]8 and [90°]8 material orientations and 90° notch angle are presented in Table 2. In both analyses, the loads obtained from the experiments are used. It is clearly seen from the table that the results of numerical analysis and experimental study are close to each other.


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Table 2. Comparison of the experimental and numerical maximum shear stresses [MPa] for y = 90°

Orientation Experimental Numerical

[0°]8 76.8 78.2

[90°]8 68.2 72.3

To investigate the material orientation and notch angle effects on in-plane shear stresses, numerically, seven different material orientations as 0°, 15°, 30°, 45°, 60°, 75°, 90° and four different notch angles as 30°, 60°, 90° and 120° were used. The variation of in-plane shear stresses depending on notch angle for various material orientations is illustrated in Fig. 6. It is seen from this figure that the lowest three in-plane shear stresses are obtained in specimens with 45°, 60° and 30° material orientations for all notch angles, respectively. However, the maximum in-plane shear stress is obtained from the specimen with 0° material orientation in the range of 30°-90° notch angles. For 120° notch angle, the maximum in-plane shear stress is obtained from the specimen with 90° material orientation. The notch angle does not drastically influence the specimen with 15° material orientation. For the specimens with 75° and 90° material orientations, in-plane shear stress increases by increasing notch angle except for 30° and 60°. As a result, in general, in-plane shear stresses have high or low values when angle θ-β is high or low, where, θ-β is the angle between the material

orientation angle and notch angle with respect to longitudinal axis and 22

y as shown in Fig. 2.

The curves of in-plane shear stresses versus material orientation for various notch angles are plotted in Fig. 7. It can be easily seen from the figure that in-plane shear stress distributions with respect to material orientation show similar behavior except for 30° notch angle. In-plane shear stress decreases by increasing material orientation from 0° to 45° for all notch angles. However, in the range from 45° to 90°, in-plane shear stresses increase with material orientation. In addition, the minimum in-plane shear stress is 15.2 MPa in the specimen with 90° notch angle and 45° material orientation, while the maximum in-plane shear stress is acquired as 85.2 MPa in the specimen with 120° notch angle and 90° material orientation.

0

10

20

30

40

50

60

70

80

90

100

30 60 90 120

Notch angle, y ( o

)

In-p

lan

e sh

ear

stre

ss (

MP

a)

0° 15° 30° 45° 60° 75° 90°

Figure 6. The variation of in-plane shear stress versus notch angle for various material orientations


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0

10

20

30

40

50

60

70

80

90

100

0 15 30 45 60 75 90

Material orientation, q ( o

)

In-p

lane

she

ar s

tres

s (M

Pa)

30°

60°

90°

120°

Figure 7. The variation of in-plane shear stress depending on material orientation for various notch

angles In-plane shear stress distribution for V-notched rail shear test system and the specimen with 0° material orientation and 90° notch angle, which is obtained from numerical analysis, is illustrated in Fig. 8. It is seen from the figure that symmetrical in-plane shear stress distribution has occurred in the test specimen.

(a)


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(b)

Figure 8. The in-plane shear stress distribution in (a) V-notched rail shear test fixture and specimen, (b)

the half specimen with 0° material orientation and 90° notch angle In order to show damage modes for the specimens with 0° and 90° material orientations, two typical photographs of damaged specimens are shown in Fig. 9. It can be clearly seen from the figure that the damage distribution directly depends on the material orientation of laminated composite specimens. The damage zone of the specimen with 0° material orientation is larger than the specimen with 90° one. In the specimen with 0° material orientation, damage starts at the tip of the notch, then develops at the surface between fiber and matrix. This type of damage mode influences the fiber breakage and matrix cracking. On the contrary, in the specimen with 90° material orientation, although damage also starts at the tip of the notch, it develops from one notch to the other one. Only matrix cracking is observed in this type of damage mode.

(a) (b)

Figure 9. Two typical photographs of damaged specimens (a) 0° material orientation, (b) 90° material orientation

4. CONCLUSIONS In this study, material orientation and notch angle effects on in-plane shear properties of glass/epoxy laminated composite plates are investigated experimentally and numerically by using a standard V-notched rail shear test fixture. The following conclusions can be drawn from the results obtained;


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Material orientation has more effect on in-plane shear properties of glass/epoxy composite specimens than notch angle.

In general, in-plane shear stresses have high or low values when the angle θ-β is high or low. The shear damage distribution directly depends on the material orientation of composite

specimens. 5. REFERENCES 1. Aktas, M., Karakuzu, R., 2007, “Effect of orientation angle of elliptical hole on residual stresses and

expansion of plastic zone in thermoplastic composite plates under in-plane loading”, Indian Journal of Materials and Sciences, 14, 103-111

2. Karakuzu, R., Taylak, N., İçten, B.M., Aktas, M., 2008, “Effects of geometric parameters on failure

behavior in laminated composite plates with two parallel pin-loaded holes”, Composite Structures, 85, 1-9

3. Karakuzu, R., Çalışkan, C.R., Aktaş, M., İçten, B.M., 2008, “Failure behavior of laminated composite

plates with two serial pin-loaded holes”, Composite Structures, 82, 225-234 4. Standard test method for short-beam strength of polymer matrix composite materials and their

laminates, ASTM D 2344/D 2344M-00, American Society for Testing and Materials, West Conshohocken, PA, 2000

5. Standard test method for shear properties of composite materials by the V-notched beam method,

ASTM D 5379/D 5379M-98, American Society for Testing and Materials, West Conshohocken, PA, 1998

6. Standard test method for in-plane shear response of polymer matrix composite materials by tensile

test of a ±45o laminate, ASTM D 3518/D 3518M-94, American Society for Testing and Materials, West Conshohocken, PA, 2000

7. Standard test method for in-plane shear properties of polymer matrix composite materials by the rail

shear method, ASTM D 4255/D 4255M-01, American Society for Testing and Materials, West Conshohocken, PA, 2001

8. Standard test method for shear properties of composite materials by the V-notched rail shear method,

ASTM D 7078/D 7078M-05, American Society for Testing and Materials, West Conshohocken, PA, 2005

9. Hussain, A.K., Adams, D.F., 2004, “Analytical evaluation of the two-rail shear test method for

composite materials”, Composite Science and Technology, 64, 221-238 10. Hussain, A.K., Adams, D.F., 2004, “Experimental evaluation of the Wyoming-modified two-rail

shear test method for composite materials”, Experimental Mechanics, 44(4), 354-364 11. Adams, D.O., Moriarty, J.M., Gallegos, A.M., Adams, D.F., 2007, “The V-notched rail shear test”,

Journal of Composite Materials, 41, 281-297 12. Adams, D.F., Lewis, E.Q., 1994, “Current status of composite material shear test methods”, SAMPE

Journal, 31(1), 32–41 13. Yeow, Y.T., Brinson, H.F., 1978, “A comparison of simple shear characterization methods for


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composite laminates”, Composites, 9(1), 49–55 14. Boller, K.H., 1969, “A method to measure intralaminar shear properties of composite laminates”,

Technical report AFML-TR-69-311.Dayton (OH): Air Force Materials Laboratory 15. Aktas, M., Karakuzu, R., 2009, “Determination of mechanical properties of glass-epoxy composites in

high temperatures”, Polymer Composites, 30(10), 1437-1441 16. Chang, F.K., Tang, J.M., Peterson, D.G., 1987, “The effect of testing methods on the shear strength

distribution in laminated composites”, Journal of Reinforced Plastics & Composites, 6, 304–318 17. Tarnopol’skii, Y.M., Arnautov, A.K., Kulakov, V.L., 1999, “Methods of determination of shear

properties of textile composites”, Composites Part A: Applied Science and Manufacturing Volume, 30 (7), 879–885

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