JOM, Vol. 66, No. 2, 2014
DOI: 10.1007/s11837-013-0825-7 Ó 2013 The Minerals, Metals & Materials Society
Hybrid Three-Dimensional (3-D) Woven Thick Composite Architectures in Bending MARK PANKOW,1,3 ASHIQ QUABILI,2 and CHIAN-FONG YEN2 1.—North Carolina State University, Raleigh, NC, USA. 2.—Army Research Laboratory, Aberdeen, MD, USA. 3.—e-mail:
[email protected]
In this study, three 3-dimensional (3-D) woven composite materials were examined to determine how yarn tow configurations affect the flexural response of the structure. Woven fabric preforms were manufactured with a Z-fiber architecture in 2–3 in. thicknesses. These preforms contained S-2 Glass (AGY, Aiken, SC, USA), carbon, and Twaron (Teijin Aramid, Arnhem, The Netherlands) yarns in different architectures creating a hybrid material system. Due to the thickness of the material, these samples required a significant span length (30 in.). The results showed a change in the strength and degradation after failure with the addition of carbon layers in tension.
INTRODUCTION Composite use has grown steadily in the past few decades in aerospace, marine, and military industries for applications in lightweight, high-strength structures. With their widespread adoption comes a limitation in strength under complex loading conditions different from simple uniaxial tension and compression. The failure mode in bending and other types of transverse loading is dominated by delaminations in traditional layered composites. There are many methods to provide reinforcement through the thickness to prevent delamination. Three main methods have been used in the past: stitching, z-pins, and three-dimensional (3-D) weaving. The last method shows high potential for large-scale applications (such as beams and vehicle panels) due to the low amount of damage and integral member of the fabric and out of autoclave processing. The investigation into 3-D woven composites was first evaluated by Cox et al.,1 who investigated the pure compression response of 3-D woven panels and found them to have architecture-dependent strain localizations. They also investigated the failure mechanisms present in these new types of materials finding kink band formation in the compressive side with fiber failure on the tensile side.2 The bending response of 3-D woven sandwich structures was investigated by Bannister et al.,3 who found a large increase in shear strength through the addition of the binding fibers. Chou et al.4 investigated the effect of weaving structure on the response of woven (Published online November 23, 2013)
composites, finding that there is critical value for the amount of binding fiber to add, after which there is no longer a benefit. Others have followed, showing similar results and adding rate dependence.5 Bending and fatigue were investigated to understand the response of the panels subjected to repeated loading, changing the core material in the bending process.6 Walter et al.7 investigated the response of various architectures in the short beam shear test, additionally looking at energy dissipation. Impact was investigated by Wang et al.8 to determine the effect of fiber woven architecture on the dissipation of energy. They found that by using a combination of various fibers, the amount of energy dissipated could be increased. Walter et al.9 investigated the penetration response and distribution of damage of composites, finding an architecture-dependent mechanism that contributed to energy dissipation. Overall, researchers have found that 3-D woven composites increase the delamination resistance and increase the energy absorption, with minimal impact on in-plane properties. This investigation will focus on hybrid 3-D woven composites (H3DWC), where the hybrid refers to the multiple types of yarns that will be used within the same architecture. Hybridization has previously been shown to provide benefits such as increased energy absorption10,11 and an increase in failure strength.12 The tailoring of hybrid structures was investigated by Hufenbach et al.,13 where it was shown to have the ability to design composite 255
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architectures for structural applications. Through a combination of different yarn tows, the authors were able to localize the delamination that occurred in the specimens. They further suggested that the use of multiple materials can be tailored to structural applications through the weave type and yarn choice. In this article, three different architectures of 3D-woven material systems were evaluated in threepoint bending to understand the flexural response of the beams and the impact from thin layers of carbon on the outer surfaces. The effective homogenized properties were determined as a comparison of the different material systems as a way of comparing them. However, the results suggest trends that occur in the material and a means of differentiating between the different samples. MATERIALS Preforms were woven in a simple orthogonal woven architecture with a Twaron z-yarn (Teijin Aramid, Arnhem, The Netherlands). Warp and weft yarns were mainly S-2 Glass (AGY, Aiken, SC, USA) yarns with the addition of carbon (IM7; Hexcel Corporation, Stamford, CT, USA) layers on the top and bottom. Three configurations were woven: an all glass panel (G), a carbon–glass (CG) sample, and a carbon–glass–carbon (CGC) symmetric sample. These configurations were chosen to make the best use of the carbon by placing it on the outer surface due to its better tensile response compared with the S-2 Glass. After weaving, the samples were infused with SC-15 matrix material through a vacuum-assist resin-transfer mold (VARTM) system.14 These preforms are unusually thick compared to traditional composite material applications, which are relatively thin. Details of the preform can be seen in Fig. 1, which shows the variation of architectures with the alternating Z-fiber material. MECHANICAL TESTING Linear Elastic Compression Modulus Three sample cubes were cut out of the material to determine the linear elastic compression modulus. These samples were 1.5-in. squares by the thickness of the material and contain about 25 units cells of the material. Because of the relatively large cross section of the cubes necessary to obtain accurate results, only the elastic constants were determined, and failure was not analyzed due to load limitations of the testing frame. The average strain obtained from digital image correlation (DIC) measurements is used because it will be comparable to using an extensometer or strain gauge to obtain strain measurement. DIC measurements showed
some variation in strain measurements in the carbon and glass material. The in-plane and out-ofplane moduli are presented in Table I. The results show that the addition of the carbon layers has very little impact on the modulus of the material. Three-Point Bending Three-point bend tests were carried out in accordance with ASTM Standard D790.15 The specimen dimensions along with a schematic of the test can be seen in Table II and Fig. 2. Center point deflection along with end point displacement were monitored using three separate linear variable differential transformers (LVDTs) to account for any bending of the fixture. ASTM Standard D790 does not account for such thick samples; therefore, the ratio of db and dl has been preserved so that the data analysis will remain consistent with ASTM standards. One parameter that can be calculated from the tests is the effective strength at failure, which is given as r¼
3PL 2bd2
(1)
The results have been plotted to provide a comparison of the results; see Fig. 3. The data have been normalized to account for variations in thickness so that direct comparisons can be made due to the difference in manufacturing. The normalization of the load can be achieved by the following relation: normalized load ¼
P bd3
(2)
The deflection is normalized by the span that is held constant in this case, and the results can also be seen in Fig. 3. The beam with carbon in compression had similar characteristics and failure loads to those observed with the all-glass composite. Composites with carbon in tensions showed much higher effective strength and a different type of failure as observed from the load deflection curve. Both have load drops after their peak load of over 2000 lbs, while the glass samples showed a plateau load that did not exhibit a large drop in load. The effective secant modulus was also plotted as a function of the center deflection in Fig. 4. The effective secant modulus is the effective modulus based on an isotropic material in three-point bending. These results are then calculated as a function of center deflection. These results show that the symmetric material had the highest effective stiffness of all the materials, followed by the nonsymmetric sample with the carbon in tension. The carbon in compression produced similar results to the all-glass samples.
Hybrid Three-Dimensional (3-D) Woven Thick Composite Architectures in Bending
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Fig. 1. Schematic of the architectures using fabricated preforms. Note that the schematic should have many more layers of glass inserted to make the total thickness. Corresponding photo of the cross section of an actual sample, showing the multiple layers and multiple Z-fibers that have been inserted: (a) glass architecture, (b) carbon glass architecture, (c) carbon glass carbon architecture, (d) glass architecture, (e) carbon glass architecture, and (f) carbon glass carbon architecture.
b
P
Table I. Compressive modulus (MSI) of different hybrid architectures
CGC CG G
Warp
Weft
3.59 3.54 3.56
3.76 3.58 3.56
d
L
y
x z
Table II. Three-point bend test parameters
Fig. 2. Schematic of the three-point bend tests configurations.
CGC CG G
L
b
d
30 30 30
3.0 3.0 3.0
1.74 1.74 1.55
All units are in inches
The materials had a range of roughly 1 in. of center deflection at a span of 30 in. where they were still in the linear elastic range before noticing a drop in the effective linear elastic modulus. The glass sample provided the largest linear elastic range.
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(a)
(b)
(c)
Fig. 3. Load versus deflection for three-point bend tests comparing the different layups: (a) load versus deflection, (b) normalized load versus normalized deflection, and (c) CGC composites to show variation between samples.
Fig. 4. Comparison of effective modulus for different architectures. The deviation from the horizontal dashed line indicates the location when damage will start occurring in the samples. It happens the earliest for the symmetric sample.
Load deflection paths are shown in Fig. 5, which shows a representative curve for the cyclically loaded beams to understand the damage that occurs in the beam. These tests help to identify the residual strength in the beams. The loading rate was held constant, while the unloading ran at a faster rate. The deflection point at which unloading commenced
was chosen randomly to determine the progression of damage. This monitored progressive damage in the material and determined the type of failure model that could be used. The progressive damage can be characterized through a secant modulus approach with some amount of damage accumulation. The secant modulus is a measure of damage accumulation in the material by bisecting the area under the load–deflection curve into two distinct parts: the damage accumulation and the reduced linear elastic region. This is done by taking the secant modulus at a given point. If the point is chosen during linear elastic loading, then the secant modulus is equal to the elastic modulus of the material. Once the material reaches the proportional limit, the secant modulus begins to decrease as damage accumulates in the material. The strength along with the absorbed energy is presented in Table III. The energy absorbed is the calculation of the total area under the load versus deflection curve. It can be seen that all the properties increase with the addition of thin layers of carbon material. This results in increased effective modulus, flexural strength, and energy absorption of the material. In addition, an increase in all the effective properties is also observed when the reinforcement is switched from carbon in the compression side to carbon in the tension side of the panels.
Hybrid Three-Dimensional (3-D) Woven Thick Composite Architectures in Bending
(a)
(b)
(c)
(d)
259
Fig. 5. Load versus deflection for three-point bend tests where black represents a loading portion and red represents unloading. It should be noted that the unloading occurred at a much faster rate and, therefore, shows a different trend when being reloaded. (a) glass, (b) carbon glass with carbon in compression, (c) glass carbon with carbon in tension, and (d) carbon glass carbon.
Table III. Flexural modulus
Flexural modulus (MSI) Flexural strength (KSI) Energy (lbf-in.)
G
CG
GC
CGC
2.969 62.88 23991
3.272 67.90 26413
3.736 76.57 27887
4.688 74.878 34253
CONCLUSION Three-dimensional woven hybrid composites were examined in bending to provide insight into how hybrid architectures affect the flexural response of thick 3-D woven composites. The in-plane mechanical properties did not show large variations in properties with the addition of carbon. However, the bending results change dramatically as we add carbon layers. The largest benefits can be seen when the carbon layers are put in the tension side of the bending panels. The addition of carbon provides an increase in bending stiffness and strength along with a change in the post-peak behavior of the material.
ACKNOWLEDGEMENTS The authors would like to thank ARL for their continued financial support. REFERENCES 1. B.N. Cox, M.S. Dadkhah, R.V. Inman, W.L. Morris, and J. Zupon, Acta Metall. Mater. 40, 3285 (1992). 2. B.N. Cox, M.S. Dadkhah, W.L. Morris, and J.G. Flintoff, Acta Metall. Mater. 42, 3967 (1994). 3. M.K. Bannister, R. Braemar, and P.J. Crothers, Compos. Struct. 47, 687 (1999). 4. S. Chou, H.-C. Chen, and H.-E. Chen, Compos. Sci. Technol. 45, 23 (1992). 5. J. Liu and H. Jiang, J. Ind. Text. 41, 174 (2011). 6. H. Judawisastra, J. Ivens, and I. Verpoest, Compos. Struct. 43, 35 (1998).
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Pankow, Quabili, and Yen 12. S.C. Khatri and M.J. Koczak, Compos. Sci. Technol. 56, 181 (1996). 13. W. Hufenbach, M. Gude, and C. Ebert, Compos. Sci. Technol. 69, 1422 (2009). 14. R. Chen, C. Dong, Z. Liang, C. Zhang, and B. Wang, Polym. Compos. 25, 146 (2004). 15. ASTM D 790, Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials (West Conshohocken, PA, ASTM, 2010).
