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phenolic braided composites at high strain rates
Materials Science and Engineering A 526 (2009) 134–139
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Longitudinal compressive behavior and failure mechanism of three-dimensional five-directional carbon/phenolic braided composites at high strain rates Dian-sen Li a,b,∗ , Zi-xing Lu b , Dai-ning Fang a a b
FML, School of Aerospace, Tsinghua University, Beijing 100084, PR China School of Aeronautical Science and Technology, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2009 Accepted 4 July 2009
Keywords: 3D braided composites Dynamic properties High strain rate Compression Failure mechanism Split Hopkinson pressure bar
a b s t r a c t The uniaxial compressive stress–strain response of three-dimensional (3D) five-directional carbon/phenolic braided composites are experimentally investigated at high strain rates from 350 to 1600 s−1 using the split Hopkinson pressure bar technique. The compressive loads are applied in a longitudinal direction (along the braiding direction). The macro- and micro-fracture morphology examinations are conducted using a scanning electron microscope (SEM) to understand the deformation and dynamic failure mechanism. The results show that the dynamic stress vs. strain curves exhibit a linear response followed by a load-drop at the on-set of matrix failure. The composites clearly demonstrate the strain rate strengthening effects and dynamic toughness phenomenon. The fracture morphology of the composites is varied under different loading rates. The damage is in the form of matrix cracking yielding and falling off, the interface debonding, and the migration, local buckling and shear fracture of the fibers. In addition, the ideal aspect ratio for the composites under dynamic compression is in the range of 0.75–1.25. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The traditional laminated composites have relatively low toughness against in-plane splitting and are prone to interlaminar delamination. In recent years, three-dimensional textile composites from braiding, weaving and knitting have received tremendous attention. In particular, three-dimensional (3D) braided composites were recognized as the most promising materials because of their low delamination tendency. Furthermore, the near-net-shape manufacturing, out-of-plane stiffness, strength, impact tolerance and ablation resistance recommend these materials [1]. A large number of the applications for composite structures involve high strain rate loading during their service life, especially in primary aerospace structures. Hence, it is of special importance to characterize the high-strain-rate behavior of 3D braided composites and then uses the high-strain-rate properties to design composite structures which will be applied under dynamic or impulsive loadings. Much effort has been spent on performance characterization of 3D braided composites under quasi-static loading conditions [2–13]. But comparatively little work has been done on the dynamic properties and the understanding of the dynamic failure
∗ Corresponding author at: FML, School of Aerospace, Tsinghua University, Beijing 100084, PR China. Tel.: +86 10 51779179; fax: +86 10 51779179. E-mail address: [email protected] (D.-s. Li). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.07.009
mechanism of 3D braided composites. Gu and Xu [14] presented ballistic perforation test results of 3D four-directional Twaron/epoxy braided composites and numerically simulated the ballistic perforation based on the ‘Fiber inclination model’ equivalent to 3D braided composite. Shen [15] carried out the high-speed symmetrical impact hurtle and pill perforated target experiments by means of a 57 hydric-gun. Tang et al. [16] analyzed the damage and fracture mechanism of 3D four-directional carbon/epoxy and glass/epoxy braided composites during explosive impact by using an optical microscope and field emission gun scanning electron microscope. Recently, Sun et al. [17,18] studied 3D four-directional E-glass/epoxy braided composites. They obtained the uniaxial tensile and compressive properties of composites at high strain rates with the help of a split Hopkinson pressure bar apparatus. However, the experiments they carried out did not fully reveal the dynamic compressive properties with respect to different braiding parameters. To our knowledge, the compressive properties of 3D fivedirectional braided composites at high strain rates have not yet been reported. In this paper, the high-strain-rate compressive properties of the 3D five-directional carbon/phenolic braided composites with different braiding angles and fiber volume fractions in the longitudinal direction (along braiding direction) are tested on a split Hopkinson pressure bar (SHPB). The longitudinal compressive stress vs. strain curves, compressive stiffness, peak stress and corresponding failure strain at high strain rate loading conditions have been compared and then the effects of strain rate on the longitudi-
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135
Table 1 Details of longitudinal dynamic compression specimens. Specimen
Braiding structure
Braiding angle (◦ )
Fiber volume fraction (%)
Loading mode
DLC1 DLC2 DLC3
Five-directional Five-directional Five-directional
20◦ 40◦ 20◦
50 50 55
Longitudinal Longitudinal Longitudinal
Fig. 1. The fiber architecture of 3D five-directional braided composites.
nal compression properties are discussed. The damage and fracture morphology of the composite specimens after longitudinal compressive failure are observed from the macroscopic and microscopic point of view and the dynamic failure mechanism of composites is demonstrated. 2. Composites and specimens Fig. 1 illustrates the fiber architecture of the interior unit of the 3D five-directional braided composites used in this study. It is noted that the interior braiding angles have a relation to the braiding angle and the details can be referenced in the literature [19,20]. The high strength carbon yarns used for 3D preforms consist of 12 K T300 carbon fibres for braiding yarn and 6 K T300 carbon fibres for axial yarn, respectively. The phenolic resins are injected into the 3D braided preforms by the resin transfer moulding (RTM) process and then consolidated to produce the 3D five-directional carbon/phenolic braided composite specimens. The composite specimens are cut, and the in-plane size of the composite coupon for compressive tests at high strain rates is 13.5 mm (length) × 10 mm (width) × 10 mm (thickness). The photograph of a longitudinal compression specimen is given in Fig. 2. A series of specimens with different braiding parameters have been considered in the present study. Details of the specimens tested are summarised in Table 1. 3. Apparatus for high strain rate compression tests A split Hopkinson pressure bar (SHPB) is used to test the compressive properties of composite specimens at high strain rates. Fig. 3 gives a photograph of the overall experimental device. A schematic drawing of the SHPB can be seen in Fig. 4. The split Hop-
Fig. 2. A photograph of longitudinal dynamic compressive specimen.
Fig. 3. The photograph of overall experimental device.
kinson pressure bars used in the present investigation are equipped with a momentum trapping device and a gas gun with an inner diameter of 14.5 mm. A small specimen is sandwiched between two elastic bars with the same cross sectional area and modulus, called the incident bar and the transmission bar. The striker and pressure bars with a diameter of 14.5 mm are made of aluminium in order to ensure the impedance resistance of elastic bars matches that of the composite materials. The gas gun operates with compressed nitrogen and can fire striker bars with different lengths. The length of the striker bar used for this work is 200 mm and the pressure bars are 1500 mm. The impact velocity is controlled by changing the gas pressure in the gas tank so that the strain rate can be adjusted. The impact of the striker bar on the incident bar generates an elastic stress wave, which passes through the specimen and deforms it. When the elastic wave reaches the incident bar-specimen interface, part of it returns and part of it transmits through the specimen and passes through the transmission bar. To measure the direct incident pulse, the reflected pulse and the transmitted pulse, strain gages are mounted on both the incident and transmission bars. The strain gages are linked and symmetrically affixed on both sides of the incident and transmission bars, in order to reduce the influence of the bending wave and avoid the superposition of the waves onto the two bars. The positions of strain gages in the incident and transmission bars are shown in Fig. 4. The signals from the strain gages are amplified with a super-dynamic strain amplifier, recorded with a high speed data acquisition card, and later analyzed on a PC. The data-sampling rate is 10 MHz for all experiments in the present work. One-dimensional wave propagation is assumed in the analysis of the strain signals from the strain gages. In this study, we use the incident pulse and the transmitted pulse to compute the strainrate, strain and stress based on the assumption that the dynamic forces on both sides of the specimen are equal, that is, εi + εr = εt . Supposing the modulus, cross section area and density of the bar are denoted by E, A and and the initial length and cross-sectional area of the specimen are l0 and A0 , the equations for the strain-rate
136
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Fig. 4. The schematic of split Hopkinson pressure bar. .
(ε), strain (ε), and stress () of the specimen are given by [21]: ˙ ε(t) =−
2C (ε (t) − εt (t)) l0 i
ε(t) = −
2C l0
(t) =
(1)
t
(εi (t) − εt (t))dt
(2)
0
A Eεt (t) A0
(3)
the stress vs. time in the process of impact. The duration of the transmitted pulses is similar to the incident pulses, but the length of the transmitted wave from zero to the maximum is different. It decreases with the increasing amplitude of the incident wave. Furthermore, the transmitted wave has a certain degree of elongation, which is the response of viscoelastic materials. The input signals are converted to the average stress of the specimen by multiplying the stress scale factor. Then the strain rate vs. time curves and stress vs. strain curves can be calculated with Eqs. (1)–(3) at each strain rate.
E/ is the longitudinal wave velocity in the bar, and where C = εi (t) and εt (t) are the strain gage signal of the incident and the transmitted pulses, respectively. Eqs. (1)–(3) give the average stress and strain in the specimen as a function of time. They also show that the strain can be obtained by integrating the difference between the incident pulse and the transmitted pulse, and that the stress in the specimen is gained from the transmitted pulse. From the above formula, the dynamic stress vs. strain curve of the composites can be obtained by expunction the time. 4. Experimental procedure The compression tests on composite specimens with different braiding parameters in the longitudinal (braiding direction) direction at high strain rates are conducted with the SHPB apparatus described in Fig. 4. The different strain rates are obtained by changing the air pressure from 0.1 to 0.7 MPa. In order to uniform the stress state on the contact surface of the specimen, Vaseline lubrication oil is used on both ends to reduce the friction. Then the 3D five-directional braided composites are imbedded between the bottom surface of the incident bar and the top surface of the transmission bar. Three specimens are tested to obtain average stress vs. strain curves at each strain rate range. The typical strain waves detected by the strain-gages mounted on the incident and transmission bars for DLC2 specimen at the strain rate of 900 s−1 are presented in Fig. 5. It can be seen that the incident wave is similar to a square wave to ensure the specimen deformation under a constant strain rate. The amplitude of the incident wave is a function of impact velocity, which reflects the stress value in the incident bar and the width of the incident wave reflects the duration of the collision course. The reflected pulses depict the strain rate vs. time curve. The reflected pulse is not constant over time; rather, it increases from zero to a maximum value in a short period of time, then fluctuates within a certain constant value and finally drops to zero. The amplitude of the transmitted wave reflects
5. Results and discussion 5.1. The effect of aspect ratios on dynamic properties The size of different specimens may have a great effect on the experimental results. In order to determine the proper size of the specimens, the longitudinal dynamic compression experiments are performed for the specimens with aspect ratio (l0 /d0 , where d0 is the equivalent diameter of specimen) of 0.4, 0.75, 1, 1.25 and 1.53, respectively. In the course of the whole test, the same nitrogen pressure is exerted on the specimens.
Fig. 5. Typical signals on the incident and transmission bars for DLC2 specimen under 900 s−1 .
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Fig. 8. The stress vs. strain curves for DLC2 specimen at different strain rates.
Fig. 6. The dynamic stress vs. strain curves of composites with different aspect ratios.
Fig. 6 gives longitudinal dynamic compression stress vs. strain curves of the specimens with five different aspect ratios. There is a huge difference of the shape and changing tendency between the curves with aspect ratio of 0.4 and other aspect ratios. The peak stress and strain of the curve with aspect ratio of 1.53 is the lowest and the strain rate is also the smallest among all materials. For the composite with aspect ratio of 0.75, the curve is evident on the high side when it reaches the peak stress and the strain rate remains the highest. The curve with aspect ratio of 1 shows a similar change trend with that of 1.25. By comparison, it can be concluded that the curve with the aspect ratio of 1.25 is the best among them all. So, it is chosen as the preferred specimen in this study with 13.5 mm, 10 mm, 10 mm in length, width, and thickness respectively as mentioned in Section 2. As a result, the ideal aspect ratio of 3D braided composites for dynamic compression is 0.75–1.25 and the diameter of the specimen must be slightly smaller than that of the pressure bar during the whole experiments. 5.2. Dynamic properties for the composites with different braiding angles Fig. 7 gives the experimental stress vs. strain curves of the DLC1 specimen (for braiding angle 20◦ ) which were compressed in the
Fig. 7. The stress vs. strain curves for DLC1 specimen at different strain rates.
longitudinal direction at different strain rates. The stress strain response of the specimen types tested reveals that the materials behave almost in a linear manner up to failure and have no clear yield in the whole loading process; on the other hand, the curve shows a non-linear trend whereas approaching the peak. The Young’s modulus of composites basically changes little with increasing strain rate and is about 36.43 GPa, showing that the longitudinal Young’s modulus is not sensitive to the change of the strain rate. At the same time, it is found that the stress vs. strain curves go up gradually with increasing strain rate. The curve with the lowest strain rate (378 s−1 ) is at the bottom, whereas the highest response rate (1355 s−1 ) is on the top. The strain rates of 378, 582, 1090 and 1355 s−1 , correspond to the peak stress of the curves of 137.01, 166.94, 185.61 and 224.16 MPa and the failure strains of 0.0048, 0.0062, 0.0084 and 0.0093, respectively. It can be concluded that the peak stress and the corresponding failure strain of the curves increase gradually with that of the strain rate. The composites have an obvious strain rate effect and dynamic fracture toughness. In addition, there has been a marked softening effect for four sets of specimens when the stress reaches the peak value. It is mainly because the destruction of the resin matrix and the debonding between fiber and matrix, fibers break down and weaken the carrying capacity. The curve with a low strain rate (378 s−1 , 582 s−1 ) does not drop rapidly but fluctuates back and forth when it reaches the peak. Whereas at the high strain rate (1090 s−1 , 1355 s−1 ), the curve begins to decline rapidly after it reaches the peak. The higher the strain rate is, the faster the curve declines.
Fig. 9. The stress vs. strain curves for DLC3 specimen at different strain rates.
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Fig. 10. The fractographs of DLC1 specimen at different strain rates.
From the above mentioned, we conclude that DLC1 specimen is a strain-rate sensitive material under longitudinal compressive loading. Fig. 8 shows the longitudinal compression stress vs. strain curves of DLC2 specimen which has a bigger braiding angle (40◦ ) at different strain rates. It can be seen that the shapes of DLC2 specimen curves are basically the same as those of the DLC1 specimen. The curves rise up with the increase of strain rate; the area surrounded increases as well, and the materials show sensitive strain rate effects. Moreover, the peak stress of composites increase significantly and the failure strain also tends to increase with that of the strain rate. When the strain rates are 901, 1227, and 1560 s−1 , the corresponding peak stresses of the curves are 104.77, 137.14 and 150.83 MPa and the failure strain are 0.0073, 0.0128 and 0.0142, respectively. All of these results demonstrate that the 3D braided composites are strain rate sensitive despite the braiding angle when subjected to the longitudinal compression. 5.3. Dynamic properties for the composites with different fiber volume fractions Fig. 9 shows the stress vs. strain curves of DLC3 specimen (Vf = 55%) in the longitudinal compression at different high strain rates. It is found that the composites are strain-rate sensitive in spite of different fiber volume fractions. As seen from Fig. 9, the curve with a lower strain rate goes towards the bottom line. It surrounds a smaller area and the peak stress diminishes. The failure strain has a trend of increase with that of the strain rate. The strain rates of 675 and 1020 s−1 , correspond to the peak stress of the curves of 145.85 and 211.16 MPa and the failure strains of 0.0066 and 0.0076,
respectively. A fluctuation can also be observed at a low strain rate level during the softening phase. 5.4. Fracture morphology and dynamic failure mechanism Fig. 10 gives the fractographs of DLC1 specimen compressed at different strain rates. It is clear that the damage of composites is rate-sensitive. At the lower strain rate (378 s−1 ), only a small amount of tiny cracks spread irregularly on the surface. As the strain rate increases (582 s−1 ), the damage area of the specimen becomes larger. The surface is rougher and the broken fibers expose, showing the buckling fracture of the fibers and the matrix cracking. At a higher strain rate (1090 s−1 , 1355 s−1 ), stress wave propagation effects are more visible and the specimen is completely broken up into bunches of carbon fibers attached with a little resin. Fig. 11 shows the SEM Micrographs of damage morphology of DLC1 specimen compressed in the longitudinal direction at 582 s−1 . The matrix peels off sharply and serious interface damage occurs (see Fig. 11(a)). Fig. 11(b) shows the matrix deformed and yielded into rows of tile-shaped pieces with a larger crimp. At a higher strain rate (1355 s−1 ), the matrix has more serious damage and shows cleavage fracture (see Fig. 12(a) (b)). The size, shape and degree of convex are affected by the micro-stress, spacing between fibers and the interfacial bond strength. Fig. 12(b) shows the damage of the interface becoming more serious with plenty of matrix shocked off and with fibers broken off and debonded. In short, 3D braided composites have defects such as microcracks and porosity during manufacture. When the incident bar strikes on the surface of the specimen, the pressure wave produced will bring a local deformation around the defects due to the weak thermal conductivity of the resin matrix. Heat generated by the
Fig. 11. SEM micrographs of DLC1 specimen under 582 s−1 .
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Fig. 12. SEM micrographs of DLC1 specimen under 1355 s−1 .
local deformation under high strain rate loading cannot spread out instantly and thus results in a sharp increase of temperature in the local area softening the matrix. As a result, the bond ability of the interface declines and forms a large scale damage zone. Meanwhile, a stress wave against the impact direction transfers from fibers to the matrix and then to the nearby fibers producing a great pressure which debonds the fiber and matrix. The fibers migrate and expand along this direction and continuously absorb energy until the fiber structure becomes loose and destroyed. In addition, when the pressure wave propagates to the back of the specimen, the reflected wave will also rapidly produce the micro-cracks. Due to the short impact, the crack expands and merges fast and finally damages the specimen greatly. At a relatively low strain rate, where the intensity of the stress wave is not more than the local dynamic failure strength, only a number of small cracks can be seen on the surface of the specimen. If the intensity of the stress wave exceeds the local dynamic failure strength of the specimen, the composites will break up in local areas. When the strain rate is relatively high, due to very fast impact speed, a higher temperature and pressure is generated, resulting in more serious damage, and eventually causing overall specimen expansion and fragmentation. Therefore, the damage of 3D braided composites in the longitudinal dynamic compression is in the form of matrix cracking, yielding and falling off, the interface debonding, and the migration, local buckling and shear fracture of the fibers. 6. Conclusions The high strain rates (350–1600 s−1 ) compression properties of 3D five-directional carbon/phenolic braided composites in the longitudinal direction are determined with a split Hopkinson pressure bar (SHPB) apparatus. The dynamic compressive stress vs. strain curves exhibit the linear feature and a marked softening phenomenon. The composites show a high-strain-rate sensitivity. The peak stress and the corresponding failure strain increases as the strain rate increases; while the compressive modulus is not sensitive with the change of the strain rate.
The fracture morphology examinations indicate that the damage and failure patterns of composites vary with high strain rates. The composites exhibit more serious damage and fail in a more brittle mode with increasing strain rates. The damage and fracture of composites under the longitudinal dynamic compression is in the form of matrix cracking, yielding and falling off, interface debonding, together with the migrating, local buckling and shear fracture of fibers. Acknowledgements The authors of this paper acknowledge the financial supports of the Aeronautical Science Foundation of China (No. 04B51045), the Common Construction Project of the Education Committee of Beijing (No.XK100060522) and the Doctor Innovation Foundation of Beijing University of Aeronautics and Astronautics. The authors would like to thank the editors and anonymous reviewers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
W. Li, M. Hammad, S.A. El, J. Text. Inst. 81 (1990) 491–514. J.M. Yang, C.L. Ma, T.-W. Chou, J. Compos. Mater. 20 (1986) 472–484. H.Y. Sun, C.H. Wu, E.R. Bian, Acta Mater. Compos. Sin. 15 (1998) 102–106. Z.X. Tang, R. Postle, Compos. Struct. 51 (2001) 451–457. Z.X. Tang, R. Postle, Compos. Struct. 55 (2002) 307–317. T. Zeng, D.N. Fang, L. Ma, L.C. Guo, Mater. Lett. 58 (2004) 3237–3241. W.W. Zuo, L.Y. Xiao, D.X. Liao, Compos. Sci. Technol. 67 (2007) 2095–2102. D.S. Li, Z.X. Lu, W.S. Lu, J. Appl. Math. Mech. 29 (2008) 163–170. D.L. Wu, Compos. Sci. Technol. 56 (1996) 225–233. D.S. Li, Z.X. Liu, Z.X. Lu, J Beijing Univ, Aeronaut. Astronaut. 34 (2008) 891–895. D.S. Li, Z.X. Lu, Z.X. Liu, J. Aerosp. Power 23 (2008) 1455–1460. D.S. Li, Z.X. Lu, L. Chen, J.L. Li, Acta Mater. Compos. Sin. 23 (2006) 112–118. D.S. Li, Z.X. Lu, L. Chen, J.L. Li, Acta Mater. Compos. Sin. 23 (2006) 135–141. B.H. Gu, J.Y. Xu, Compos. B: Eng. 35 (2004) 291–297. H.R. Shen, J. Natl. Univ. Def. Technol. 21 (1999) 8–12. G.Y. Tang, Y.J. Yan, X.H. Chen, Mater. Des. 22 (2001) 21–25. B.Z. Sun, F. Liu, B.H. Gu, Compos. Part A 36 (2005) 1477–1485. B.Z. Sun, L. Yang, B.H. Gu, AIAA J. 43 (2005) 994–999. D.S. Li, L. Chen, J.L. Li, J. Tianjin Polytech. Univ. 22 (2003) 7–11. D.S. Li, Z.X. Lu, L. Chen, J.L. Li, J. Int. J. Solids Struct. 46 (2009) 3422–3432. M.C. Meyers, Dynamic Behavior of Materials, Wiley, New York, 1995.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Longitudinal compressive behavior and failure mechanism of three-dimensional five-directional carbon/phenolic braided composites at high strain rates Dian-sen Li a,b,∗ , Zi-xing Lu b , Dai-ning Fang a a b
FML, School of Aerospace, Tsinghua University, Beijing 100084, PR China School of Aeronautical Science and Technology, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2009 Accepted 4 July 2009
Keywords: 3D braided composites Dynamic properties High strain rate Compression Failure mechanism Split Hopkinson pressure bar
a b s t r a c t The uniaxial compressive stress–strain response of three-dimensional (3D) five-directional carbon/phenolic braided composites are experimentally investigated at high strain rates from 350 to 1600 s−1 using the split Hopkinson pressure bar technique. The compressive loads are applied in a longitudinal direction (along the braiding direction). The macro- and micro-fracture morphology examinations are conducted using a scanning electron microscope (SEM) to understand the deformation and dynamic failure mechanism. The results show that the dynamic stress vs. strain curves exhibit a linear response followed by a load-drop at the on-set of matrix failure. The composites clearly demonstrate the strain rate strengthening effects and dynamic toughness phenomenon. The fracture morphology of the composites is varied under different loading rates. The damage is in the form of matrix cracking yielding and falling off, the interface debonding, and the migration, local buckling and shear fracture of the fibers. In addition, the ideal aspect ratio for the composites under dynamic compression is in the range of 0.75–1.25. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The traditional laminated composites have relatively low toughness against in-plane splitting and are prone to interlaminar delamination. In recent years, three-dimensional textile composites from braiding, weaving and knitting have received tremendous attention. In particular, three-dimensional (3D) braided composites were recognized as the most promising materials because of their low delamination tendency. Furthermore, the near-net-shape manufacturing, out-of-plane stiffness, strength, impact tolerance and ablation resistance recommend these materials [1]. A large number of the applications for composite structures involve high strain rate loading during their service life, especially in primary aerospace structures. Hence, it is of special importance to characterize the high-strain-rate behavior of 3D braided composites and then uses the high-strain-rate properties to design composite structures which will be applied under dynamic or impulsive loadings. Much effort has been spent on performance characterization of 3D braided composites under quasi-static loading conditions [2–13]. But comparatively little work has been done on the dynamic properties and the understanding of the dynamic failure
∗ Corresponding author at: FML, School of Aerospace, Tsinghua University, Beijing 100084, PR China. Tel.: +86 10 51779179; fax: +86 10 51779179. E-mail address: [email protected] (D.-s. Li). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.07.009
mechanism of 3D braided composites. Gu and Xu [14] presented ballistic perforation test results of 3D four-directional Twaron/epoxy braided composites and numerically simulated the ballistic perforation based on the ‘Fiber inclination model’ equivalent to 3D braided composite. Shen [15] carried out the high-speed symmetrical impact hurtle and pill perforated target experiments by means of a 57 hydric-gun. Tang et al. [16] analyzed the damage and fracture mechanism of 3D four-directional carbon/epoxy and glass/epoxy braided composites during explosive impact by using an optical microscope and field emission gun scanning electron microscope. Recently, Sun et al. [17,18] studied 3D four-directional E-glass/epoxy braided composites. They obtained the uniaxial tensile and compressive properties of composites at high strain rates with the help of a split Hopkinson pressure bar apparatus. However, the experiments they carried out did not fully reveal the dynamic compressive properties with respect to different braiding parameters. To our knowledge, the compressive properties of 3D fivedirectional braided composites at high strain rates have not yet been reported. In this paper, the high-strain-rate compressive properties of the 3D five-directional carbon/phenolic braided composites with different braiding angles and fiber volume fractions in the longitudinal direction (along braiding direction) are tested on a split Hopkinson pressure bar (SHPB). The longitudinal compressive stress vs. strain curves, compressive stiffness, peak stress and corresponding failure strain at high strain rate loading conditions have been compared and then the effects of strain rate on the longitudi-
D.-s. Li et al. / Materials Science and Engineering A 526 (2009) 134–139
135
Table 1 Details of longitudinal dynamic compression specimens. Specimen
Braiding structure
Braiding angle (◦ )
Fiber volume fraction (%)
Loading mode
DLC1 DLC2 DLC3
Five-directional Five-directional Five-directional
20◦ 40◦ 20◦
50 50 55
Longitudinal Longitudinal Longitudinal
Fig. 1. The fiber architecture of 3D five-directional braided composites.
nal compression properties are discussed. The damage and fracture morphology of the composite specimens after longitudinal compressive failure are observed from the macroscopic and microscopic point of view and the dynamic failure mechanism of composites is demonstrated. 2. Composites and specimens Fig. 1 illustrates the fiber architecture of the interior unit of the 3D five-directional braided composites used in this study. It is noted that the interior braiding angles have a relation to the braiding angle and the details can be referenced in the literature [19,20]. The high strength carbon yarns used for 3D preforms consist of 12 K T300 carbon fibres for braiding yarn and 6 K T300 carbon fibres for axial yarn, respectively. The phenolic resins are injected into the 3D braided preforms by the resin transfer moulding (RTM) process and then consolidated to produce the 3D five-directional carbon/phenolic braided composite specimens. The composite specimens are cut, and the in-plane size of the composite coupon for compressive tests at high strain rates is 13.5 mm (length) × 10 mm (width) × 10 mm (thickness). The photograph of a longitudinal compression specimen is given in Fig. 2. A series of specimens with different braiding parameters have been considered in the present study. Details of the specimens tested are summarised in Table 1. 3. Apparatus for high strain rate compression tests A split Hopkinson pressure bar (SHPB) is used to test the compressive properties of composite specimens at high strain rates. Fig. 3 gives a photograph of the overall experimental device. A schematic drawing of the SHPB can be seen in Fig. 4. The split Hop-
Fig. 2. A photograph of longitudinal dynamic compressive specimen.
Fig. 3. The photograph of overall experimental device.
kinson pressure bars used in the present investigation are equipped with a momentum trapping device and a gas gun with an inner diameter of 14.5 mm. A small specimen is sandwiched between two elastic bars with the same cross sectional area and modulus, called the incident bar and the transmission bar. The striker and pressure bars with a diameter of 14.5 mm are made of aluminium in order to ensure the impedance resistance of elastic bars matches that of the composite materials. The gas gun operates with compressed nitrogen and can fire striker bars with different lengths. The length of the striker bar used for this work is 200 mm and the pressure bars are 1500 mm. The impact velocity is controlled by changing the gas pressure in the gas tank so that the strain rate can be adjusted. The impact of the striker bar on the incident bar generates an elastic stress wave, which passes through the specimen and deforms it. When the elastic wave reaches the incident bar-specimen interface, part of it returns and part of it transmits through the specimen and passes through the transmission bar. To measure the direct incident pulse, the reflected pulse and the transmitted pulse, strain gages are mounted on both the incident and transmission bars. The strain gages are linked and symmetrically affixed on both sides of the incident and transmission bars, in order to reduce the influence of the bending wave and avoid the superposition of the waves onto the two bars. The positions of strain gages in the incident and transmission bars are shown in Fig. 4. The signals from the strain gages are amplified with a super-dynamic strain amplifier, recorded with a high speed data acquisition card, and later analyzed on a PC. The data-sampling rate is 10 MHz for all experiments in the present work. One-dimensional wave propagation is assumed in the analysis of the strain signals from the strain gages. In this study, we use the incident pulse and the transmitted pulse to compute the strainrate, strain and stress based on the assumption that the dynamic forces on both sides of the specimen are equal, that is, εi + εr = εt . Supposing the modulus, cross section area and density of the bar are denoted by E, A and and the initial length and cross-sectional area of the specimen are l0 and A0 , the equations for the strain-rate
136
D.-s. Li et al. / Materials Science and Engineering A 526 (2009) 134–139
Fig. 4. The schematic of split Hopkinson pressure bar. .
(ε), strain (ε), and stress () of the specimen are given by [21]: ˙ ε(t) =−
2C (ε (t) − εt (t)) l0 i
ε(t) = −
2C l0
(t) =
(1)
t
(εi (t) − εt (t))dt
(2)
0
A Eεt (t) A0
(3)
the stress vs. time in the process of impact. The duration of the transmitted pulses is similar to the incident pulses, but the length of the transmitted wave from zero to the maximum is different. It decreases with the increasing amplitude of the incident wave. Furthermore, the transmitted wave has a certain degree of elongation, which is the response of viscoelastic materials. The input signals are converted to the average stress of the specimen by multiplying the stress scale factor. Then the strain rate vs. time curves and stress vs. strain curves can be calculated with Eqs. (1)–(3) at each strain rate.
E/ is the longitudinal wave velocity in the bar, and where C = εi (t) and εt (t) are the strain gage signal of the incident and the transmitted pulses, respectively. Eqs. (1)–(3) give the average stress and strain in the specimen as a function of time. They also show that the strain can be obtained by integrating the difference between the incident pulse and the transmitted pulse, and that the stress in the specimen is gained from the transmitted pulse. From the above formula, the dynamic stress vs. strain curve of the composites can be obtained by expunction the time. 4. Experimental procedure The compression tests on composite specimens with different braiding parameters in the longitudinal (braiding direction) direction at high strain rates are conducted with the SHPB apparatus described in Fig. 4. The different strain rates are obtained by changing the air pressure from 0.1 to 0.7 MPa. In order to uniform the stress state on the contact surface of the specimen, Vaseline lubrication oil is used on both ends to reduce the friction. Then the 3D five-directional braided composites are imbedded between the bottom surface of the incident bar and the top surface of the transmission bar. Three specimens are tested to obtain average stress vs. strain curves at each strain rate range. The typical strain waves detected by the strain-gages mounted on the incident and transmission bars for DLC2 specimen at the strain rate of 900 s−1 are presented in Fig. 5. It can be seen that the incident wave is similar to a square wave to ensure the specimen deformation under a constant strain rate. The amplitude of the incident wave is a function of impact velocity, which reflects the stress value in the incident bar and the width of the incident wave reflects the duration of the collision course. The reflected pulses depict the strain rate vs. time curve. The reflected pulse is not constant over time; rather, it increases from zero to a maximum value in a short period of time, then fluctuates within a certain constant value and finally drops to zero. The amplitude of the transmitted wave reflects
5. Results and discussion 5.1. The effect of aspect ratios on dynamic properties The size of different specimens may have a great effect on the experimental results. In order to determine the proper size of the specimens, the longitudinal dynamic compression experiments are performed for the specimens with aspect ratio (l0 /d0 , where d0 is the equivalent diameter of specimen) of 0.4, 0.75, 1, 1.25 and 1.53, respectively. In the course of the whole test, the same nitrogen pressure is exerted on the specimens.
Fig. 5. Typical signals on the incident and transmission bars for DLC2 specimen under 900 s−1 .
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Fig. 8. The stress vs. strain curves for DLC2 specimen at different strain rates.
Fig. 6. The dynamic stress vs. strain curves of composites with different aspect ratios.
Fig. 6 gives longitudinal dynamic compression stress vs. strain curves of the specimens with five different aspect ratios. There is a huge difference of the shape and changing tendency between the curves with aspect ratio of 0.4 and other aspect ratios. The peak stress and strain of the curve with aspect ratio of 1.53 is the lowest and the strain rate is also the smallest among all materials. For the composite with aspect ratio of 0.75, the curve is evident on the high side when it reaches the peak stress and the strain rate remains the highest. The curve with aspect ratio of 1 shows a similar change trend with that of 1.25. By comparison, it can be concluded that the curve with the aspect ratio of 1.25 is the best among them all. So, it is chosen as the preferred specimen in this study with 13.5 mm, 10 mm, 10 mm in length, width, and thickness respectively as mentioned in Section 2. As a result, the ideal aspect ratio of 3D braided composites for dynamic compression is 0.75–1.25 and the diameter of the specimen must be slightly smaller than that of the pressure bar during the whole experiments. 5.2. Dynamic properties for the composites with different braiding angles Fig. 7 gives the experimental stress vs. strain curves of the DLC1 specimen (for braiding angle 20◦ ) which were compressed in the
Fig. 7. The stress vs. strain curves for DLC1 specimen at different strain rates.
longitudinal direction at different strain rates. The stress strain response of the specimen types tested reveals that the materials behave almost in a linear manner up to failure and have no clear yield in the whole loading process; on the other hand, the curve shows a non-linear trend whereas approaching the peak. The Young’s modulus of composites basically changes little with increasing strain rate and is about 36.43 GPa, showing that the longitudinal Young’s modulus is not sensitive to the change of the strain rate. At the same time, it is found that the stress vs. strain curves go up gradually with increasing strain rate. The curve with the lowest strain rate (378 s−1 ) is at the bottom, whereas the highest response rate (1355 s−1 ) is on the top. The strain rates of 378, 582, 1090 and 1355 s−1 , correspond to the peak stress of the curves of 137.01, 166.94, 185.61 and 224.16 MPa and the failure strains of 0.0048, 0.0062, 0.0084 and 0.0093, respectively. It can be concluded that the peak stress and the corresponding failure strain of the curves increase gradually with that of the strain rate. The composites have an obvious strain rate effect and dynamic fracture toughness. In addition, there has been a marked softening effect for four sets of specimens when the stress reaches the peak value. It is mainly because the destruction of the resin matrix and the debonding between fiber and matrix, fibers break down and weaken the carrying capacity. The curve with a low strain rate (378 s−1 , 582 s−1 ) does not drop rapidly but fluctuates back and forth when it reaches the peak. Whereas at the high strain rate (1090 s−1 , 1355 s−1 ), the curve begins to decline rapidly after it reaches the peak. The higher the strain rate is, the faster the curve declines.
Fig. 9. The stress vs. strain curves for DLC3 specimen at different strain rates.
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Fig. 10. The fractographs of DLC1 specimen at different strain rates.
From the above mentioned, we conclude that DLC1 specimen is a strain-rate sensitive material under longitudinal compressive loading. Fig. 8 shows the longitudinal compression stress vs. strain curves of DLC2 specimen which has a bigger braiding angle (40◦ ) at different strain rates. It can be seen that the shapes of DLC2 specimen curves are basically the same as those of the DLC1 specimen. The curves rise up with the increase of strain rate; the area surrounded increases as well, and the materials show sensitive strain rate effects. Moreover, the peak stress of composites increase significantly and the failure strain also tends to increase with that of the strain rate. When the strain rates are 901, 1227, and 1560 s−1 , the corresponding peak stresses of the curves are 104.77, 137.14 and 150.83 MPa and the failure strain are 0.0073, 0.0128 and 0.0142, respectively. All of these results demonstrate that the 3D braided composites are strain rate sensitive despite the braiding angle when subjected to the longitudinal compression. 5.3. Dynamic properties for the composites with different fiber volume fractions Fig. 9 shows the stress vs. strain curves of DLC3 specimen (Vf = 55%) in the longitudinal compression at different high strain rates. It is found that the composites are strain-rate sensitive in spite of different fiber volume fractions. As seen from Fig. 9, the curve with a lower strain rate goes towards the bottom line. It surrounds a smaller area and the peak stress diminishes. The failure strain has a trend of increase with that of the strain rate. The strain rates of 675 and 1020 s−1 , correspond to the peak stress of the curves of 145.85 and 211.16 MPa and the failure strains of 0.0066 and 0.0076,
respectively. A fluctuation can also be observed at a low strain rate level during the softening phase. 5.4. Fracture morphology and dynamic failure mechanism Fig. 10 gives the fractographs of DLC1 specimen compressed at different strain rates. It is clear that the damage of composites is rate-sensitive. At the lower strain rate (378 s−1 ), only a small amount of tiny cracks spread irregularly on the surface. As the strain rate increases (582 s−1 ), the damage area of the specimen becomes larger. The surface is rougher and the broken fibers expose, showing the buckling fracture of the fibers and the matrix cracking. At a higher strain rate (1090 s−1 , 1355 s−1 ), stress wave propagation effects are more visible and the specimen is completely broken up into bunches of carbon fibers attached with a little resin. Fig. 11 shows the SEM Micrographs of damage morphology of DLC1 specimen compressed in the longitudinal direction at 582 s−1 . The matrix peels off sharply and serious interface damage occurs (see Fig. 11(a)). Fig. 11(b) shows the matrix deformed and yielded into rows of tile-shaped pieces with a larger crimp. At a higher strain rate (1355 s−1 ), the matrix has more serious damage and shows cleavage fracture (see Fig. 12(a) (b)). The size, shape and degree of convex are affected by the micro-stress, spacing between fibers and the interfacial bond strength. Fig. 12(b) shows the damage of the interface becoming more serious with plenty of matrix shocked off and with fibers broken off and debonded. In short, 3D braided composites have defects such as microcracks and porosity during manufacture. When the incident bar strikes on the surface of the specimen, the pressure wave produced will bring a local deformation around the defects due to the weak thermal conductivity of the resin matrix. Heat generated by the
Fig. 11. SEM micrographs of DLC1 specimen under 582 s−1 .
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Fig. 12. SEM micrographs of DLC1 specimen under 1355 s−1 .
local deformation under high strain rate loading cannot spread out instantly and thus results in a sharp increase of temperature in the local area softening the matrix. As a result, the bond ability of the interface declines and forms a large scale damage zone. Meanwhile, a stress wave against the impact direction transfers from fibers to the matrix and then to the nearby fibers producing a great pressure which debonds the fiber and matrix. The fibers migrate and expand along this direction and continuously absorb energy until the fiber structure becomes loose and destroyed. In addition, when the pressure wave propagates to the back of the specimen, the reflected wave will also rapidly produce the micro-cracks. Due to the short impact, the crack expands and merges fast and finally damages the specimen greatly. At a relatively low strain rate, where the intensity of the stress wave is not more than the local dynamic failure strength, only a number of small cracks can be seen on the surface of the specimen. If the intensity of the stress wave exceeds the local dynamic failure strength of the specimen, the composites will break up in local areas. When the strain rate is relatively high, due to very fast impact speed, a higher temperature and pressure is generated, resulting in more serious damage, and eventually causing overall specimen expansion and fragmentation. Therefore, the damage of 3D braided composites in the longitudinal dynamic compression is in the form of matrix cracking, yielding and falling off, the interface debonding, and the migration, local buckling and shear fracture of the fibers. 6. Conclusions The high strain rates (350–1600 s−1 ) compression properties of 3D five-directional carbon/phenolic braided composites in the longitudinal direction are determined with a split Hopkinson pressure bar (SHPB) apparatus. The dynamic compressive stress vs. strain curves exhibit the linear feature and a marked softening phenomenon. The composites show a high-strain-rate sensitivity. The peak stress and the corresponding failure strain increases as the strain rate increases; while the compressive modulus is not sensitive with the change of the strain rate.
The fracture morphology examinations indicate that the damage and failure patterns of composites vary with high strain rates. The composites exhibit more serious damage and fail in a more brittle mode with increasing strain rates. The damage and fracture of composites under the longitudinal dynamic compression is in the form of matrix cracking, yielding and falling off, interface debonding, together with the migrating, local buckling and shear fracture of fibers. Acknowledgements The authors of this paper acknowledge the financial supports of the Aeronautical Science Foundation of China (No. 04B51045), the Common Construction Project of the Education Committee of Beijing (No.XK100060522) and the Doctor Innovation Foundation of Beijing University of Aeronautics and Astronautics. The authors would like to thank the editors and anonymous reviewers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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