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Investigation of plate geometry on the crushing of flat composite plates
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 1639–1650 www.elsevier.com/locate/compscitech
Investigation of plate geometry on the crushing of flat composite plates S. Cauchi Savona, P.J. Hogg
*
Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom Received 25 May 2005; accepted 17 November 2005 Available online 4 January 2006
Abstract The aim of this work was to identify the energy absorbing capabilities of fibre reinforced composites manufactured using glass-fibre multiaxial warp-knit non-crimp fabrics (NCF) for use in low-cost, weight-critical energy absorbing structures. A variety of layups, using polyester and epoxy matrices, were evaluated using a plate crush rig. The plates were tested at varying unsupported widths to identify the stability of the different orientations. It was found that the energy absorbed was dependent upon the stability of the laminates and possible delamination of outer plies during the crush. The different resins appeared to have little influence in the specific crushing stresses (SSCS). Additionally, comparison of the results with the literature for tubes showed that the trends observed for the crushing of plates were comparable. Ó 2005 Published by Elsevier Ltd. Keywords: A. Polymer-matrix composites; B. Mechanical properties; C. Crushing
1. Introduction Recently, there has been considerable interest to incorporate composite crashworthy structures in mainstream automotives: some high-end, low volume sports cars already utilise composite crush structures. Crashworthiness is typically defined as the ability of a structure to absorb energy in a collision or an impact, and survive; in the case of a passenger vehicle, this would be the ability to ensure the survivability of the occupants. Well designed composite crush structures can absorb significantly more energy per unit mass than metals. However, unlike metals that crush by a progressive folding mechanism, and hence plastic deformation, the crushing of composites is more complex. For example, composites can crush by brittle fracturing, lamina bending and also in a progressive folding mode [1,2]. The ideal progressive crushing mechanism in composites is therefore a balance
*
Corresponding author. Tel.: +44 0 20 7882 5161; fax: +44 0 20 8981 9804. E-mail address: [email protected] (P.J. Hogg). 0266-3538/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2005.11.011
between brittle fracturing and lamina bending as this imparts the highest energy absorption. Such complex crushing mechanisms require careful design of the composite crush structures, and a significant amount of testing is therefore required to validate these materials. Most of the tests performed to date are typically on tubular structures, predominantly due to the self-stabilising nature of these structures. Furthermore, the data can be easily translated into designs for structural components. The downside is that tubes are expensive to manufacture such that the evaluation of various parameters becomes a costly exercise. Circular tubes are the preferred experimental crush structure, presumably as a result of the ease of machining triggers and testing these structures. The results from tube specimens have shown that the crushing energy is dependent on a wide variety of factors such as: trigger mechanism; fibres and matrix; fibre lay-up; cross-sectional geometry, specifically diameter to thickness ratio; as well as the crushing surface roughness. However, it has been noted that circular tubes are not really representative of actual composite structures [3], and hence significant research has also been performed on square tubes [4,5].
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The realisation that most structural composites are in the form of plate-like elements has sparked interest in the design of specific rigs that would be able to test the crushing characteristics of flat composite plates [6]. The main characteristic of such rigs is that they incorporate anti-buckling guides; these are essentially knife-edges that contact and hence stabilise the specimen during the crushing process. The knife-edges aim to simulate the end conditions of closed structures such as tubes, and promote tearing of the splayed fronds. Without these anti-buckling guides, once the crushing process is triggered, the plate would merely separate into two fronds with little energy absorbed in the process. Plate crush rigs have been used to evaluate triggers, the effect of off-axis loads [7,8] and dynamic crush testing [9]. Research on plate-specimens has shown that scaling up the baseline test-pieces has to be performed by sublaminate scaling rather than by simply increasing the ply thickness [2,10]. Such scaling involves the addition of further plies thereby increasing the interfaces between off-axis plies. Scaled-up components tend to exhibit a reduction in the total absorbed energy; yet, sublaminate scaling yields only a 9% reduction in energy as opposed to 25–50% for plylevel scaling. Furthermore, the ideal trigger for evaluating the crush energy absorption of plates has been found to be a steeple trigger [10] since this type of trigger is insensitive to plate thickness effects. The plate crush technique has also been utilised to identify a direct link between the interlaminar shear stress and the crushing characteristics of composites [11]. As most research focussed on filament wound tubes or the use of pre-pregs or woven fabrics, the purpose of this research was to evaluate the use of multiaxial warp-knitted non-crimp fabrics (henceforth referred to as NCFs) for use as precursors for structural crush elements. NCFs are ideal from a manufacturing perspective since they are manufac-
0 + 45
90 - 45
Fig. 1. Schematic of the arrangement of fibres in a multiaxial warpknitted non crimp fabric (NCF).
tured from continuous fibres arranged in a specific orientation and stitched together by a light polyester thread (Fig. 1). This keeps the fibres together without any of the crimping problems associated with conventional weaving technologies. The NCFs also allow for a quick assembly of dry-reinforcements with scaling permitted in a ply-level fashion. 2. Experimental The crush test specimens used in this research were flat plates manufactured from COTECHÒ multiaxial NCFs supplied by Brunswick Technologies Europe Ltd. (BTI Europe). The range of fabrics used include uniweave, biaxial, triaxial and quadriaxial: The reference names and areal weights of the fibres in each axis are detailed in Table 1. Table 2 lists the details of the resins and curing agents; the two resins were Crystic 272 isopthalic polyester resin from Scott Bader and Epikote 828 epoxy resin cured with Vantico HY-932 amine hardener. The properties of the resins, obtained from the manufacturersÕ data sheets are presented in Table 3.
Table 1 NCF fabric types used in this research Description
Reference
Weights in each axis (g/m2)
E-Glass E-Glass E-Glass E-Glass E-Glass E-Glass E-Glass
ELPb-567 ELT-566 EBX-602 ETLX-751 EQX-1034 EQX-1168 EQX-2336
567 283
0° [0° UD] biaxial [0/90] biaxial [±45] triaxial [+45, 45, 0] quadriaxial [45, 90, +45, 0] quadriaxial [45, 90, 45, 0] quadriaxial [45, 90, +45, 0]
283 283 283 567
45°
90
+45
50 283 301 234 234 301 601
283 283 567
301 234 234 301 601
Table 2 Resin types, curing agents and formulations Resin
Curing agent
Formulation details
Crystic 272 – polyester resin
Accelerator G and catalyst M Hardener HY-932
0.25% Accelerator G mixed into Crystic 272 followed by 1% Catalyst M
Shell Epikote 828 – epoxy resin
32 g of hardener are mixed into 100 g of epoxy heated to 80 °C (to lower resin viscosity)
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Table 3 Mechanical properties of the resins Resin
Modulus (GPa)
Strength (MPa)
Strain to failure (%)
Density (g/cm3)
Crystic 272 – polyester resin Shell Epikote 828 – epoxy resin
3.4 2.8
79 85
4.5 3.2
1.2 1.1
One of the problems with the triaxial and quadriaxial NCFs is that due to their design, it is impossible to manufacture symmetrical laminates unless an identical symmetrical fabric is also used. Since such fabrics are only made to order, all laminates manufactured with the triaxial and quadriaxial fabrics are balanced and are indicated by the subscript ÔbÕ; where the laminates are symmetrical, they are indicated by the subscript ÔsÕ. Furthermore, to distinguish between the different plies within an NCF, a comma Ô,Õ is used, whereas separate layers of NCF are differentiated by a forward slash Ô/Õ. 2.1. Specimen manufacturing The laminates were manufactured using a wet-layup process and were consolidated by cold pressing at 1.24 bar. For the polyester matrix specimens, the laminate was left to cure for 24 h at room temperature and then post-cured at 80 °C for 4 h. The epoxy matrix laminates were cured for 5 h at 130 °C and post-cured for 3 h 30 min at 180 °C. The volume fraction and the density of the laminate were estimated from the dry weight of the fibres and final weight of the laminates.
Plate specimens having a height of 90 mm and widths of 60, 70, 80 and 90 mm were cut from the test laminate using a water-cooled diamond saw. A 45° steeple chamfer was subsequently machined into the specimens. At least four specimens were tested for each series. Since the knife-edges had a thickness of 3 mm, this corresponded to plate unsupported widths of 54, 64, 74, and 84 mm, respectively. 2.2. Plate crush test Fig. 2 is a rendered image of the crush fixture used in this research. It is a modified version of the original plate crush fixture [6] that has movable knife-edges to allow for different plate widths and thicknesses up to 110 and 50 mm, respectively. The specimens were placed such that the trigger was in contact with the base and the knife-edges were tightened such that minimal friction was applied onto the plate and thus it could freely move. A loading block is placed on the top of the specimen, and loading is achieved through a rod attached to the load cell of an Instron 6025 electromechanical testing machine. The test was performed at a crosshead speed of 20 mm/min and the specimens were
Fig. 2. Rendered image of the crush rig used in this research.
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crushed over a distance of 50 mm; the data was digitally acquired through the Instron Merlin software.
Sections of the 70 mm wide crushed specimens were taken from the centre, polished and photographed first with a digital camera then under a Wild M8 stereomicroscope in order to gain a detailed view of the crushing region. Pencil lead was rubbed into the cracks in order to make the cracks identifiable for stereomicroscopy. Image acquisition, scaling and stitching was performed through a camera attached to the c-mount of the stereomicroscope using Image Pro Plus 5 image analysis software.
Crushing Load
2.3. Microscopy
Small drop after peak with consistent crush stress Large drop after peak with recovery of crush stress Large drop after peak with no recovery of crush stress
crush Stroke (mm)
3. Results A typical plot of the load/displacement data is shown in Fig. 3; in this, there are four distinct features that can be identified: (1) a first peak resulting from the collapse of the steeple chamfer tip; (2) a second peak resulting from the split of the chamfer to produce two fronds for crushing; (3) a drop in stress, the magnitude of which is determined by the distance that the split extends into the laminate; and (4) a region of sustained crushing with the splayed fronds tearing along the knife edges. Three different load/displacement traces were observed after the second peak. These are schematically illustrated in Fig. 4: (a) a small drop in load, after which the load recovers and remains consistent throughout the crushing stroke;
Fig. 4. Schematic of the typical modes of crushing for a composite plate.
(b) a large drop in load which it rises to a consistent crushing load after a certain crush stroke; or (c) a large drop in the load that does not recover over the remainder of the stroke. In all cases, the load drops following the second peak as this point triggers the crushing mechanisms. The length of the stroke required to reach a consistent crushing stress depends on material variables such as the length of the delamination created, which in turn tends to be related to the magnitude of the second peak. For example, in Fig. 5 it can be seen that where the load recovers (Fig. 5(a)–(c)), a crush stroke of at least 15 mm is required to achieve a stable crushing zone. 3.1. Calculation of the specific sustained crushing stress The load data is converted into crushing stress, and the average stress over the region of sustained crushing (Fig. 3 feature 4) is used to obtain the energy absorbed. This energy is then normalised to the density of the composite to obtain the Specific Sustained Crushing Stress (SSCS), which is sometimes also referred to as specific energy absorption (SEA). SSCS ¼
Fig. 3. Features of the plot of a typical crush for EQX-1034/EP (F6634d): (1) first peak; (2) second peak; (3) drop after initial split; and (4) specific sustained crushing stress.
average crushing stress r ¼ material density q
For easier comparison of the charts between the different materials, the load–displacement curves were additionally converted to charts of specific stress vs. crush stroke. The second peak load that initiates crushing is recorded and is also normalised to the density to provide the specific peak stress (SPS). The second peak is the one recorded since in some cases the initial peak is not very well defined (Fig. 5(b)). The ratio SSCS/SPS gives the crushing efficiency of a laminate. A crushing efficiency that is close to or greater than 1 (Fig. 4, solid line) is desirable in energy absorbing structures.
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a
b
c
d
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Fig. 5. Different types of peaks obtained for materials crushed with an aspect ratio 14.5–16 showing: (a) large second peak followed by a large drop with the crushing stress stabilising after a stroke of 20 mm; (b) smallish peak followed by a recovery of crushing stress after a stroke of 15 mm; (c) small drop after peak with sustained crushing achieved after a stroke of 15 mm; and (d) large drop after peak stress with crushing stress consistent throughout stroke.
3.2. Effect of aspect ratio on the crushing behaviour Testing the plates at different unsupported widths allows for evaluation of the stability of crushing behaviour. Varying the unsupported widths changes the aspect ratio of the specimens, and this aspect ratio is normally defined as the ratio of the unsupported plate width to its thickness. From initial evaluations [12], it was found that if the plates are slightly wider than the unsupported width, the energy absorbed was not significantly changed. It can be understood that the knife-edges act as both supports and constraints for the flat plate. As supports, they help prevent the plate from buckling, while as constraints, they prevent simple splitting and promote tearing once crushing has initiated. Crushing occurs almost entirely between the knife-edges such that the unsupported sections of the laminate can only contribute to the SSCS by tearing along the outer knifeedges. Therefore, in this research, the aspect ratio is termed as the ratio of knife-edge separation (KES) to the average laminate thickness (t) of that particular sample (referred
to as KES/t). As the aspect ratio is increased, it can be seen (Fig. 6) that the effective SSCS, represented by the horizontal lines, decreases.
Fig. 6. Crushing charts showing the change in the SSCS (horizontal lines) with an increasing aspect ratio (F670).
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3.3. Crushing characteristics
Fig. 7. Charts for triaxial laminates showing the possibility of stable or unstable crushing behaviour for the same aspect ratio (F668/EP).
3.3.1. Triaxial laminates In some cases (Fig. 7), the crushing stress could either rise to a high level or, during crushing, the laminate collapses to give a low crushing energy. This behaviour was especially noted at high aspect ratios, though for the [(45, +45, 0)3]b series based on ETLX-751 NCFs, this behaviour was observed at nearly all aspect ratios, producing a large scatter in the data. Comparisons between identical laminates with different resins (Fig. 8) shows that, on average, the UP based laminate exhibited a more stable crushing behaviour. The reason for this can be observed in the cross-sectional photos of the crushed specimens (Fig. 9) that shows considerably more delamination in the crushed fronds of the EP based laminates (Fig. 9(b)). In addition, the level of post-crush
a
b Fig. 8. Comparison of: (a) UP matrix and (b) EP matrix crushing patterns for triaxial NCF fabrics with increasing aspect ratio (F674 and F668, respectively).
Fig. 9. Photos of a section of crushed [(+45, 45, 0)3]b: (a) UP based (F674) and (b) EP based (F668) showing the initial extensive ply delamination that leads to the long crush stroke required to reach the sustained crushing, or the possibility of collapse during crushing.
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a Fig. 10. Charts comparing the effects of resin type on the SSCS of [±45/ 0]2s laminates for an aspect ratio of 13.5.
b
c Fig. 12. Comparison of: (a) high-energy (F663) and (b,c) low-energy crushing patterns for quadriaxial NCF fabrics (F662 and F532, respectively). Fig. 11. Photos of a section of crushed: (a) UP based (b) EP based [(±45/ 0)2]s showing the difference in crushing modes (F519 and F667, respectively).
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spring-back is higher in the EP based composites, indicating that the crushing radius of curvature was larger than for the UP based composites. Fig. 10 shows a different condition to Fig. 8 where the EP is superior to the UP resin. Fig. 11 shows that the crushing behaviour is probably most likely due to the quality of the laminate, where any significant delamination of the crushed fronds would result in a poor crush. In addition, where delamination of the outer layers occurs, the radius of curvature is quite large and the specimen springs back once removed from the rig (Fig. 11(a)). On the other hand, in the case of a high energy crush, the fracturing in the fronds is so severe that the specimen retains most of its shape once removed from the crushing rig (Fig. 9(a)).
For the [+45, 45/90, 0]s laminates, the SSCS depends on the resin, where the epoxy based laminates exhibit low values of SSCS for all specimen widths (Fig. 14(b)). In contrast, the polyester based laminates crush in a high energy mode. In the epoxy based laminates, crushing is not always between the central 0° plies, and tends to shift to between the 0° and 90° plies; furthermore, the outer 45° plies tend to delaminate (Fig. 15(b)). For all the epoxy based [+45, 45/90, 0]s specimens, the radius of curvature of the splayed fronds is quite high. 4. Discussion 4.1. Crushing of composite plates
3.3.2. Quadriaxial laminates Laminates manufactured from quadriaxial NCFs show that fabrics containing a higher ratio of 0° fibres to off-axis fibres, i.e. the EQX-1034 (Fig. 12(a)), lead to higher crushing energies than for fabrics containing lower ratios i.e. EQX-2336 and EQX-1168 (Fig. 12(b) and (c)). The crosssectional photos (Fig. 13) show that the difference is due to the delamination of the outer 45° plies coupled with post-crush specimen spring-back in the EQX2336.
In all composites, crushing initiates at the chamfer. At the first peak, the tip of the chamfer squashes and generates
a
b Fig. 13. Photos of a section of crushed EP based: (a) [(+45, 90, 45, 0)2]b and (b) [(+45, 90, 45, 0)]b showing the difference in crushing modes (F663 and F662, respectively).
Fig. 14. Change in crushing mode for EBX602/ELT566 laminates due to change in matrix type: (a) UP and (b) epoxy resin (F528 and F666, respectively).
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continues rising as the chamfer is fully squashed out. At this point, a second peak, which occurs at roughly the same displacement for all laminates, is visible as a large central crack is generated. This second peak is important since resistance to a crack propagating is useful for crush structures that are mainly designed to absorb energy in large impacts. However, when the second peak rises to a very large value, the central crack tends to be longer at the initiation of crush and a longer crush stroke is required to reach the sustained portion of crushing. High values of this sustained portion of crushing normally only occur when the central crack is short, therefore forcing the splayed fronds into low radii of curvature. The resistance to the growth of the central crack is provided by stable crack growth; so high Mode-I propagation values should
Fig. 15. Photos of a section of crushed [(±45/90, 0)2]s laminates: (a) UP matrix showing a small radius of curvature (F528) and (b) Epoxy matrix showing delaminated outer 45° layers (F666).
a small crack. Depending on the stiffness and possibly the Mode-I initiation values of the laminate, this first peak is either well defined or barely visible. After this, the load
Fig. 16. SSCS vs. KES/t for all laminates – closed symbols represent UP matrix laminates, while open symbols represent the equivalent laminate with an epoxy matrix.
Fig. 17. SSCS vs. KES/t for all data with showing the general behaviour of the data.
Fig. 18. Corrected SSCS vs. KES/t illustrating the possibility that the lower trend is merely due to delamination of outer fronds that cause an effective reduction in cross-sectional area and a decrease in the overall stability of the plate.
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be ideal, as was shown by Ramakrishna et al. [13] for carbon fibre/thermoplastic based tubes. Typical examples of a long central crack requiring a long stroke to stabilise are the triaxial laminates in Fig. 8, while examples of where the sustained crushing was achieved after a short stroke are the quadriaxial laminates in Fig. 12(a). After the central crack is generated, the laminate then splits into two fronds. The knife-edges restrain the lami-
nate and hence, for crushing to continue, the fronds need to tear. This tearing and the friction between the splayed fronds and the base of the crushing rig can be considered to be two of the main methods of energy absorption. 4.1.1. Relationship between SSCS and aspect ratio A convenient way to visualise the results is to plot the SSCS against the aspect ratio (Fig. 16). To allow for easy identification of the equivalent epoxy and polyester
Table 4 Crushing results for the polyester-based laminates Plate number
Lay-up
KES/t
Vf
SSCS (Nm/g)
SPS (Nm/g)
SSCS/ SPS
F537
(EQX-2336) [(+45,90,45,0)]b
19.3 22.1 25.5 28.7
54.3
35.70 29.72 29.34 29.11
(3.58) (1.47) (2.65) (1.41)
43.33 41.53 44.96 43.98
0.82 0.72 0.65 0.66
F532
(EQX-1168) [(+45,90,45,0)2]b
17.1 19.2 21.2 23.02
48.0
40.96 37.25 34.28 34.04
(3.42) (3.89) (3.22) (3.88)
41.19 42.38 41.45 40.45
0.99 0.88 0.83 0.84
F765
(ELPb-567) [(+45/90/45/0)]s
13.0 15.4 17.9
53.4
67.33 (2.36) 62.69 (3.73) 59.72 (2.60)
61.68 57.22 64.60
1.09 1.10 0.92
F569
(ELPb-567) [(90/+45/45/0)]s
15.2 20.2
43.5
66.65 (6.52) 50.53 (4.56)
61.33 59.86
1.09 0.84
F528
(EBX-602, ELT 566) [(±45/90,0)2]s
16.0 18.6 21.4 22.3
45.4
63.35 52.68 48.85 48.03
(10.16) (6.35) (7.11) (7.89)
64.11 63.71 58.94 60.02
0.99 0.83 0.83 0.80
14.1 15.0 19.2 19.7
45.0
67.93 60.87 51.14 54.10
(8.90) (3.63) (3.16) (2.36)
65.11 68.92 66.40 65.00
1.04 0.88 0.77 0.83
14.9 15.5
38.1
71.63 (12.32) 68.46 (3.36)
71.50 75.75
1.00 0.90
14.4 17.5 20.0 23.2
47.9
73.42 65.17 53.21 47.05
(8.52) (7.01) (1.95) (9.60)
79.03 64.58 61.79 62.48
0.93 1.01 0.86 0.75
F549
F522
(ETLX 751) [(+45,45,0)3]b
F674
F519
(EBX-602, ELPb-567) [(±45/0)2]s
13.6
39.0
46.16 (4.05)
47.01
0.98
F511
(ELPb-567) [0/90/0]s
18.9
45.4
35.19 (1.91)
61.14
0.58
F551
(EBX-602) [(±45)4]s
12.9
41.3
43.14 (6.78)
63.64
0.68
13.2 17.1
40.3
41.87 (5.72) 39.77 (2.58)
64.40 64.97
0.65 0.61
19.4
38.6
37.57 (2.61)
55.18
0.68
22.8
50.3
29.44 (2.30)
60.32
0.49
F553 F512 F507
(ELPb-567) [03]s
KES/t, aspect ratio; Vf, volume fraction of fibres; numbers in parentheses are standard deviation; SSCS/SPS, crushing efficiency.
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matrix laminates, the symbol shape and colour is the same for both resins. The polyester resin laminates are given by a solid symbol, while the hollow symbols refer to epoxy based laminates. The chart illustrates that the SSCS increases as the aspect ratio decreases. Two seemingly parallel trends can also be observed whereby the difference between them is due to differences in the energy absorbing mechanisms. In the upper trend, the specimens crush with a low radius of curvature and hence a large amount of fibre fragmentation, with the result that there is little specimen spring-back once the specimen is removed from the crushing rig. For the lower trend, there was not as much fibre fragmentation; outer plies tended to delaminate and such that the frond radius of curvature was larger. In most of the latter cases, the split fronds simply splayed out. Fig. 17 is a simplification of the data in Fig. 16 showing the scatter and the trendlines observed. Of particular significance is that at the upper boundary the results seem to exhibit a large degree of scatter at the lower aspect ratios. It is possible that this scatter is partly due to some materials exhibiting a mode of failure that is in between the lower bound curves and the upper bound. The delamination of the outer plies in the low energy crushing mode is effectively a reduction in the area of the material coupled with an increase in the aspect ratio. For this reason, estimates of the difference in thickness were
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measured from the micrographs, and the corrected SSCS and aspect ratios are plotted in Fig. 18 together with the upper boundary trend. This verifies that the main reason for composites failing in a low energy mode is due to delamination in favour of the more complex crushing mechanisms reported for composites. 4.1.2. Comparison between epoxy and polyester matrix laminates Fig. 16 seems to show that for the resins selected, there is apparently no performance gain when using epoxy resins over polyester resins. The reason for this could be that the different resins do not have significantly differing mechanical properties, and so as Farley suggested, the energy absorption mechanisms would not differ greatly for similar reinforcements [14]. However, the main difference is that, on average, the crushing of the epoxy plates shows a lower variation within a series than for the polyester based laminates; but the crushing efficiencies of the epoxy matrix laminates are typically worse than for those with a polyester matrix (Tables 4 and 5). This is due to the epoxy matrix laminates having a higher peak stress prior to the initiation of crushing. 4.1.3. Comparison to literature data The KES/t parameter can be compared to the wall thickness/diameter (t/d) parameter for tubes, and possibly
Table 5 Crushing results for the epoxy-based laminates Plate number
Lay-up
KES/t
Vf
SSCS (Nm/g)
SPS (Nm/g)
SSCS/SPS
F662
(EQX-2336) [(+45,90,45,0)]b
15.9 17.6 20.9 24.4
53.8
44.10 37.54 33.64 33.06
(2.59) (1.32) (1.56) (1.96)
63.70 54.91 52.89 51.28
0.69 0.68 0.64 0.66
F663
(EQX-1034) [(+45,90,45,0)2]b
14.6 16.8 19.9 23.1
52.5
67.33 57.79 55.08 50.43
(6.09) (3.02) (6.03) (5.95)
70.49 64.63 64.24 63.84
0.96 0.89 0.86 0.79
F670
(ELPb-567) [(+45/90/45/0)]s
12.6 15.0 17.3 20.0
45.0
68.54 61.50 53.00 50.10
(7.24) (5.60) (4.02) (4.55)
69.31 57.82 58.97 59.46
0.99 1.06 0.90 0.84
F666
(EBX-602 + ELT-566) [(±45/90,0)2]s
14.1 16.5 19.7 22.6
48.2
41.75 36.13 32.05 30.23
(2.00) (6.11) (1.91) (2.56)
64.63 57.30 57.56 59.21
0.65 0.63 0.56 0.51
F668
(ETLX-751) [(+45,45,0)3]b
14.8 16.6 19.9 22.9
46.9
67.72 53.37 41.50 41.61
(12.37) (12.62) (3.38) (4.21)
77.41 62.80 63.18 59.62
0.87 0.85 0.66 0.87
F667
(EBX-602 + ELPb-567) [(±45/0)2]s
13.5 15.7 18.2 20.9
48.6
61.29 57.04 51.93 44.37
(4.86) (4.58) (4.99) (3.62)
70.74 63.59 55.89 60.64
0.87 0.90 0.93 0.73
KES/t, aspect ratio; Vf, volume fraction of fibres; numbers in parentheses are standard deviation; SSCS/SPS, crushing efficiency.
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[17,18], however, in general the results seem to be following similar patterns. Acknowledgements The authors acknowledge funding from the Engineering and Physical Science Research Council (EPSRC) and the Overseas Research Scheme (ORS), and Saint Gobain BTI for supplying the NCF fabrics. References
Fig. 19. Chart showing the differences between the upper and lower trends compared to data from literature for tube-crushing – square or circular symbols refer to square tubes or circular tubes, respectively.
similar effects are present. To illustrate the similarity between the KES/t of plates and the t/d for tubes, data from various sources in the literature for circular [15–18] and square [4,19] tubes was superimposed for comparison purposes onto Fig. 19. 5. Conclusion From this analysis on the crushing of NCF based composite plates, it has been shown that, overall, the SSCS is not significantly influenced by the particular resins investigated, but by the fibre architecture. This fibre architecture includes the ratio of individual layers, and the type of NCF or UD fabric. The results indicate that the quadriaxial laminates result in better crushing efficiencies, and that the crushing is more consistent than for triaxial orientations. This is most likely due to the lower amount of 0° fibres within the entire laminate. In the triaxial laminates, due to the higher ratio of 0° fibres, the crushing stress requires a long stroke to stabilise due to the longer central crack formed after the peak stress. A long central crack can possibly destabilise the laminate if the Mode-I propagation properties are not large enough to arrest the crack growth. If the outer plies on the splayed fronds delaminate without any obvious damage, the SSCS of the composites decreases. When the crushing area is adjusted to remove the delaminated plies, the effective crushing stress increases and falls on standard upper trend. This implies that it is ideal to arrest any extensive delamination in the composite and the fronds to absorb the maximum energy. Finally, comparisons between the laminates tested and data obtained from the literature show (Fig. 19) that the results from the plate testing compare well with results from tube testing, i.e. as the aspect ratio of the structures increases, the energy absorbed decreases. In some cases, the results differ where the aim of the research was to improve the crushing stress of a particular aspect ratio
[1] Hull D. A unified approach to progressive crushing of fiber-reinforced composite tubes. Compos Sci Technol 1991;40(4):377–421. [2] Lavoie JA, Morton J, Jackson K. An evaluation of the energyabsorption of laminated composite plates. Proc Inst Mech Eng G 1995;209(3):185–94. [3] Lavoie JA, Meeks GA, Curtis PT, Morton J. Crush energy absorption of width-scaled composite plates. In: NATO advanced studies institute conference on crahsworthiness of transportation systems, Troia, Portugal; 1996. [4] Thornton PH. Energy absorption in composite structures. J Compos Mater 1979;13:247–62. [5] Thornton PH. The crush behavior of glass-fiber reinforced plastic sections. Compos Sci Technol 1986;27(3):199–223. [6] Lavoie JA, Morton J. A crush test fixture for investigating energyabsorption of flat composite plates. Exp Tech 1994;18(6):23–6. [7] Fleming DC, Vizzini AJ. Off-axis energy absorption characterization of composites for crashworthy rotorcraft design. J Am Helicopter Soc 1996;41(3):239–46. [8] Fleming DC, Vizzini AJ. The energy absorption of composite plates under off-axis loads. J Compos Mater 1996;30(18):1977–95. [9] Lavoie JA, Kellas S. Dynamic crush tests of energy-absorbing laminated composite plates. Composites Part A 1996;27(6):467–75. [10] Jackson K, Morton J, Lavoie JA, Boitnott R. Scaling of energy absorbing composite plates. J Am Helicopter Soc 1994;39(1): 17–23. [11] Daniel L, Hogg PJ, Curtis PT. The relative effects of throughthickness properties and fibre orientation on energy absorption by continuous fibre composites. Composites Part B 1999;30(3): 257–266. [12] Cauchi Savona S. Energy absorbing composites for crash energy management. Ph.D., Queen Mary, University of London, Department of Materials; 2003. [13] Ramakrishna S, Hamada H, Maekawa Z, Sato H. Energy-absorption behavior of carbon-fiber-reinforced thermoplastic composite tubes. J Thermoplast Compos Mater 1995;8(3):323–44. [14] Farley GL. Effect of fiber and matrix maximum strain on the energy – absorption of composite-materials. J Compos Mater 1986;20(4): 322–334. [15] Hamada H, Ramakrishna S, Maekawa Z, Nakamura M. Energy absorption characteristics of composite tubes with different crosssectional shapes. In: 10th annual ASM/ESD advanced composites conference, Dearborn, Michigan, USA; 1994. p. 523–34. [16] Gupta NK, Velmurugan R, Gupta SK. An analysis of axial crushing of composite tubes. J Compos Mater 1997;31(13):1262–86. [17] Karbhari VM, Falzon PJ, Herzberg I. Energy absorption characteristics of hybrid braided composite tubes. J Compos Mater 1997; 31(12):1164–86. [18] Warrior NA, Turner TA, Robitaille F, Rudd CD. Effect of resin properties and processing parameters on crash energy absorbing composite structures made by RTM. Composites Part A 2003;34(6):543–50. [19] Jimenez MA, Miravete A, Larrode E, Revuelta D. Effect of trigger geometry on energy absorption in composite profiles. Compos Struct 2000;48(1–3):107–11.
Investigation of plate geometry on the crushing of flat composite plates S. Cauchi Savona, P.J. Hogg
*
Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom Received 25 May 2005; accepted 17 November 2005 Available online 4 January 2006
Abstract The aim of this work was to identify the energy absorbing capabilities of fibre reinforced composites manufactured using glass-fibre multiaxial warp-knit non-crimp fabrics (NCF) for use in low-cost, weight-critical energy absorbing structures. A variety of layups, using polyester and epoxy matrices, were evaluated using a plate crush rig. The plates were tested at varying unsupported widths to identify the stability of the different orientations. It was found that the energy absorbed was dependent upon the stability of the laminates and possible delamination of outer plies during the crush. The different resins appeared to have little influence in the specific crushing stresses (SSCS). Additionally, comparison of the results with the literature for tubes showed that the trends observed for the crushing of plates were comparable. Ó 2005 Published by Elsevier Ltd. Keywords: A. Polymer-matrix composites; B. Mechanical properties; C. Crushing
1. Introduction Recently, there has been considerable interest to incorporate composite crashworthy structures in mainstream automotives: some high-end, low volume sports cars already utilise composite crush structures. Crashworthiness is typically defined as the ability of a structure to absorb energy in a collision or an impact, and survive; in the case of a passenger vehicle, this would be the ability to ensure the survivability of the occupants. Well designed composite crush structures can absorb significantly more energy per unit mass than metals. However, unlike metals that crush by a progressive folding mechanism, and hence plastic deformation, the crushing of composites is more complex. For example, composites can crush by brittle fracturing, lamina bending and also in a progressive folding mode [1,2]. The ideal progressive crushing mechanism in composites is therefore a balance
*
Corresponding author. Tel.: +44 0 20 7882 5161; fax: +44 0 20 8981 9804. E-mail address: [email protected] (P.J. Hogg). 0266-3538/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2005.11.011
between brittle fracturing and lamina bending as this imparts the highest energy absorption. Such complex crushing mechanisms require careful design of the composite crush structures, and a significant amount of testing is therefore required to validate these materials. Most of the tests performed to date are typically on tubular structures, predominantly due to the self-stabilising nature of these structures. Furthermore, the data can be easily translated into designs for structural components. The downside is that tubes are expensive to manufacture such that the evaluation of various parameters becomes a costly exercise. Circular tubes are the preferred experimental crush structure, presumably as a result of the ease of machining triggers and testing these structures. The results from tube specimens have shown that the crushing energy is dependent on a wide variety of factors such as: trigger mechanism; fibres and matrix; fibre lay-up; cross-sectional geometry, specifically diameter to thickness ratio; as well as the crushing surface roughness. However, it has been noted that circular tubes are not really representative of actual composite structures [3], and hence significant research has also been performed on square tubes [4,5].
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The realisation that most structural composites are in the form of plate-like elements has sparked interest in the design of specific rigs that would be able to test the crushing characteristics of flat composite plates [6]. The main characteristic of such rigs is that they incorporate anti-buckling guides; these are essentially knife-edges that contact and hence stabilise the specimen during the crushing process. The knife-edges aim to simulate the end conditions of closed structures such as tubes, and promote tearing of the splayed fronds. Without these anti-buckling guides, once the crushing process is triggered, the plate would merely separate into two fronds with little energy absorbed in the process. Plate crush rigs have been used to evaluate triggers, the effect of off-axis loads [7,8] and dynamic crush testing [9]. Research on plate-specimens has shown that scaling up the baseline test-pieces has to be performed by sublaminate scaling rather than by simply increasing the ply thickness [2,10]. Such scaling involves the addition of further plies thereby increasing the interfaces between off-axis plies. Scaled-up components tend to exhibit a reduction in the total absorbed energy; yet, sublaminate scaling yields only a 9% reduction in energy as opposed to 25–50% for plylevel scaling. Furthermore, the ideal trigger for evaluating the crush energy absorption of plates has been found to be a steeple trigger [10] since this type of trigger is insensitive to plate thickness effects. The plate crush technique has also been utilised to identify a direct link between the interlaminar shear stress and the crushing characteristics of composites [11]. As most research focussed on filament wound tubes or the use of pre-pregs or woven fabrics, the purpose of this research was to evaluate the use of multiaxial warp-knitted non-crimp fabrics (henceforth referred to as NCFs) for use as precursors for structural crush elements. NCFs are ideal from a manufacturing perspective since they are manufac-
0 + 45
90 - 45
Fig. 1. Schematic of the arrangement of fibres in a multiaxial warpknitted non crimp fabric (NCF).
tured from continuous fibres arranged in a specific orientation and stitched together by a light polyester thread (Fig. 1). This keeps the fibres together without any of the crimping problems associated with conventional weaving technologies. The NCFs also allow for a quick assembly of dry-reinforcements with scaling permitted in a ply-level fashion. 2. Experimental The crush test specimens used in this research were flat plates manufactured from COTECHÒ multiaxial NCFs supplied by Brunswick Technologies Europe Ltd. (BTI Europe). The range of fabrics used include uniweave, biaxial, triaxial and quadriaxial: The reference names and areal weights of the fibres in each axis are detailed in Table 1. Table 2 lists the details of the resins and curing agents; the two resins were Crystic 272 isopthalic polyester resin from Scott Bader and Epikote 828 epoxy resin cured with Vantico HY-932 amine hardener. The properties of the resins, obtained from the manufacturersÕ data sheets are presented in Table 3.
Table 1 NCF fabric types used in this research Description
Reference
Weights in each axis (g/m2)
E-Glass E-Glass E-Glass E-Glass E-Glass E-Glass E-Glass
ELPb-567 ELT-566 EBX-602 ETLX-751 EQX-1034 EQX-1168 EQX-2336
567 283
0° [0° UD] biaxial [0/90] biaxial [±45] triaxial [+45, 45, 0] quadriaxial [45, 90, +45, 0] quadriaxial [45, 90, 45, 0] quadriaxial [45, 90, +45, 0]
283 283 283 567
45°
90
+45
50 283 301 234 234 301 601
283 283 567
301 234 234 301 601
Table 2 Resin types, curing agents and formulations Resin
Curing agent
Formulation details
Crystic 272 – polyester resin
Accelerator G and catalyst M Hardener HY-932
0.25% Accelerator G mixed into Crystic 272 followed by 1% Catalyst M
Shell Epikote 828 – epoxy resin
32 g of hardener are mixed into 100 g of epoxy heated to 80 °C (to lower resin viscosity)
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Table 3 Mechanical properties of the resins Resin
Modulus (GPa)
Strength (MPa)
Strain to failure (%)
Density (g/cm3)
Crystic 272 – polyester resin Shell Epikote 828 – epoxy resin
3.4 2.8
79 85
4.5 3.2
1.2 1.1
One of the problems with the triaxial and quadriaxial NCFs is that due to their design, it is impossible to manufacture symmetrical laminates unless an identical symmetrical fabric is also used. Since such fabrics are only made to order, all laminates manufactured with the triaxial and quadriaxial fabrics are balanced and are indicated by the subscript ÔbÕ; where the laminates are symmetrical, they are indicated by the subscript ÔsÕ. Furthermore, to distinguish between the different plies within an NCF, a comma Ô,Õ is used, whereas separate layers of NCF are differentiated by a forward slash Ô/Õ. 2.1. Specimen manufacturing The laminates were manufactured using a wet-layup process and were consolidated by cold pressing at 1.24 bar. For the polyester matrix specimens, the laminate was left to cure for 24 h at room temperature and then post-cured at 80 °C for 4 h. The epoxy matrix laminates were cured for 5 h at 130 °C and post-cured for 3 h 30 min at 180 °C. The volume fraction and the density of the laminate were estimated from the dry weight of the fibres and final weight of the laminates.
Plate specimens having a height of 90 mm and widths of 60, 70, 80 and 90 mm were cut from the test laminate using a water-cooled diamond saw. A 45° steeple chamfer was subsequently machined into the specimens. At least four specimens were tested for each series. Since the knife-edges had a thickness of 3 mm, this corresponded to plate unsupported widths of 54, 64, 74, and 84 mm, respectively. 2.2. Plate crush test Fig. 2 is a rendered image of the crush fixture used in this research. It is a modified version of the original plate crush fixture [6] that has movable knife-edges to allow for different plate widths and thicknesses up to 110 and 50 mm, respectively. The specimens were placed such that the trigger was in contact with the base and the knife-edges were tightened such that minimal friction was applied onto the plate and thus it could freely move. A loading block is placed on the top of the specimen, and loading is achieved through a rod attached to the load cell of an Instron 6025 electromechanical testing machine. The test was performed at a crosshead speed of 20 mm/min and the specimens were
Fig. 2. Rendered image of the crush rig used in this research.
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crushed over a distance of 50 mm; the data was digitally acquired through the Instron Merlin software.
Sections of the 70 mm wide crushed specimens were taken from the centre, polished and photographed first with a digital camera then under a Wild M8 stereomicroscope in order to gain a detailed view of the crushing region. Pencil lead was rubbed into the cracks in order to make the cracks identifiable for stereomicroscopy. Image acquisition, scaling and stitching was performed through a camera attached to the c-mount of the stereomicroscope using Image Pro Plus 5 image analysis software.
Crushing Load
2.3. Microscopy
Small drop after peak with consistent crush stress Large drop after peak with recovery of crush stress Large drop after peak with no recovery of crush stress
crush Stroke (mm)
3. Results A typical plot of the load/displacement data is shown in Fig. 3; in this, there are four distinct features that can be identified: (1) a first peak resulting from the collapse of the steeple chamfer tip; (2) a second peak resulting from the split of the chamfer to produce two fronds for crushing; (3) a drop in stress, the magnitude of which is determined by the distance that the split extends into the laminate; and (4) a region of sustained crushing with the splayed fronds tearing along the knife edges. Three different load/displacement traces were observed after the second peak. These are schematically illustrated in Fig. 4: (a) a small drop in load, after which the load recovers and remains consistent throughout the crushing stroke;
Fig. 4. Schematic of the typical modes of crushing for a composite plate.
(b) a large drop in load which it rises to a consistent crushing load after a certain crush stroke; or (c) a large drop in the load that does not recover over the remainder of the stroke. In all cases, the load drops following the second peak as this point triggers the crushing mechanisms. The length of the stroke required to reach a consistent crushing stress depends on material variables such as the length of the delamination created, which in turn tends to be related to the magnitude of the second peak. For example, in Fig. 5 it can be seen that where the load recovers (Fig. 5(a)–(c)), a crush stroke of at least 15 mm is required to achieve a stable crushing zone. 3.1. Calculation of the specific sustained crushing stress The load data is converted into crushing stress, and the average stress over the region of sustained crushing (Fig. 3 feature 4) is used to obtain the energy absorbed. This energy is then normalised to the density of the composite to obtain the Specific Sustained Crushing Stress (SSCS), which is sometimes also referred to as specific energy absorption (SEA). SSCS ¼
Fig. 3. Features of the plot of a typical crush for EQX-1034/EP (F6634d): (1) first peak; (2) second peak; (3) drop after initial split; and (4) specific sustained crushing stress.
average crushing stress r ¼ material density q
For easier comparison of the charts between the different materials, the load–displacement curves were additionally converted to charts of specific stress vs. crush stroke. The second peak load that initiates crushing is recorded and is also normalised to the density to provide the specific peak stress (SPS). The second peak is the one recorded since in some cases the initial peak is not very well defined (Fig. 5(b)). The ratio SSCS/SPS gives the crushing efficiency of a laminate. A crushing efficiency that is close to or greater than 1 (Fig. 4, solid line) is desirable in energy absorbing structures.
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a
b
c
d
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Fig. 5. Different types of peaks obtained for materials crushed with an aspect ratio 14.5–16 showing: (a) large second peak followed by a large drop with the crushing stress stabilising after a stroke of 20 mm; (b) smallish peak followed by a recovery of crushing stress after a stroke of 15 mm; (c) small drop after peak with sustained crushing achieved after a stroke of 15 mm; and (d) large drop after peak stress with crushing stress consistent throughout stroke.
3.2. Effect of aspect ratio on the crushing behaviour Testing the plates at different unsupported widths allows for evaluation of the stability of crushing behaviour. Varying the unsupported widths changes the aspect ratio of the specimens, and this aspect ratio is normally defined as the ratio of the unsupported plate width to its thickness. From initial evaluations [12], it was found that if the plates are slightly wider than the unsupported width, the energy absorbed was not significantly changed. It can be understood that the knife-edges act as both supports and constraints for the flat plate. As supports, they help prevent the plate from buckling, while as constraints, they prevent simple splitting and promote tearing once crushing has initiated. Crushing occurs almost entirely between the knife-edges such that the unsupported sections of the laminate can only contribute to the SSCS by tearing along the outer knifeedges. Therefore, in this research, the aspect ratio is termed as the ratio of knife-edge separation (KES) to the average laminate thickness (t) of that particular sample (referred
to as KES/t). As the aspect ratio is increased, it can be seen (Fig. 6) that the effective SSCS, represented by the horizontal lines, decreases.
Fig. 6. Crushing charts showing the change in the SSCS (horizontal lines) with an increasing aspect ratio (F670).
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3.3. Crushing characteristics
Fig. 7. Charts for triaxial laminates showing the possibility of stable or unstable crushing behaviour for the same aspect ratio (F668/EP).
3.3.1. Triaxial laminates In some cases (Fig. 7), the crushing stress could either rise to a high level or, during crushing, the laminate collapses to give a low crushing energy. This behaviour was especially noted at high aspect ratios, though for the [(45, +45, 0)3]b series based on ETLX-751 NCFs, this behaviour was observed at nearly all aspect ratios, producing a large scatter in the data. Comparisons between identical laminates with different resins (Fig. 8) shows that, on average, the UP based laminate exhibited a more stable crushing behaviour. The reason for this can be observed in the cross-sectional photos of the crushed specimens (Fig. 9) that shows considerably more delamination in the crushed fronds of the EP based laminates (Fig. 9(b)). In addition, the level of post-crush
a
b Fig. 8. Comparison of: (a) UP matrix and (b) EP matrix crushing patterns for triaxial NCF fabrics with increasing aspect ratio (F674 and F668, respectively).
Fig. 9. Photos of a section of crushed [(+45, 45, 0)3]b: (a) UP based (F674) and (b) EP based (F668) showing the initial extensive ply delamination that leads to the long crush stroke required to reach the sustained crushing, or the possibility of collapse during crushing.
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a Fig. 10. Charts comparing the effects of resin type on the SSCS of [±45/ 0]2s laminates for an aspect ratio of 13.5.
b
c Fig. 12. Comparison of: (a) high-energy (F663) and (b,c) low-energy crushing patterns for quadriaxial NCF fabrics (F662 and F532, respectively). Fig. 11. Photos of a section of crushed: (a) UP based (b) EP based [(±45/ 0)2]s showing the difference in crushing modes (F519 and F667, respectively).
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spring-back is higher in the EP based composites, indicating that the crushing radius of curvature was larger than for the UP based composites. Fig. 10 shows a different condition to Fig. 8 where the EP is superior to the UP resin. Fig. 11 shows that the crushing behaviour is probably most likely due to the quality of the laminate, where any significant delamination of the crushed fronds would result in a poor crush. In addition, where delamination of the outer layers occurs, the radius of curvature is quite large and the specimen springs back once removed from the rig (Fig. 11(a)). On the other hand, in the case of a high energy crush, the fracturing in the fronds is so severe that the specimen retains most of its shape once removed from the crushing rig (Fig. 9(a)).
For the [+45, 45/90, 0]s laminates, the SSCS depends on the resin, where the epoxy based laminates exhibit low values of SSCS for all specimen widths (Fig. 14(b)). In contrast, the polyester based laminates crush in a high energy mode. In the epoxy based laminates, crushing is not always between the central 0° plies, and tends to shift to between the 0° and 90° plies; furthermore, the outer 45° plies tend to delaminate (Fig. 15(b)). For all the epoxy based [+45, 45/90, 0]s specimens, the radius of curvature of the splayed fronds is quite high. 4. Discussion 4.1. Crushing of composite plates
3.3.2. Quadriaxial laminates Laminates manufactured from quadriaxial NCFs show that fabrics containing a higher ratio of 0° fibres to off-axis fibres, i.e. the EQX-1034 (Fig. 12(a)), lead to higher crushing energies than for fabrics containing lower ratios i.e. EQX-2336 and EQX-1168 (Fig. 12(b) and (c)). The crosssectional photos (Fig. 13) show that the difference is due to the delamination of the outer 45° plies coupled with post-crush specimen spring-back in the EQX2336.
In all composites, crushing initiates at the chamfer. At the first peak, the tip of the chamfer squashes and generates
a
b Fig. 13. Photos of a section of crushed EP based: (a) [(+45, 90, 45, 0)2]b and (b) [(+45, 90, 45, 0)]b showing the difference in crushing modes (F663 and F662, respectively).
Fig. 14. Change in crushing mode for EBX602/ELT566 laminates due to change in matrix type: (a) UP and (b) epoxy resin (F528 and F666, respectively).
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continues rising as the chamfer is fully squashed out. At this point, a second peak, which occurs at roughly the same displacement for all laminates, is visible as a large central crack is generated. This second peak is important since resistance to a crack propagating is useful for crush structures that are mainly designed to absorb energy in large impacts. However, when the second peak rises to a very large value, the central crack tends to be longer at the initiation of crush and a longer crush stroke is required to reach the sustained portion of crushing. High values of this sustained portion of crushing normally only occur when the central crack is short, therefore forcing the splayed fronds into low radii of curvature. The resistance to the growth of the central crack is provided by stable crack growth; so high Mode-I propagation values should
Fig. 15. Photos of a section of crushed [(±45/90, 0)2]s laminates: (a) UP matrix showing a small radius of curvature (F528) and (b) Epoxy matrix showing delaminated outer 45° layers (F666).
a small crack. Depending on the stiffness and possibly the Mode-I initiation values of the laminate, this first peak is either well defined or barely visible. After this, the load
Fig. 16. SSCS vs. KES/t for all laminates – closed symbols represent UP matrix laminates, while open symbols represent the equivalent laminate with an epoxy matrix.
Fig. 17. SSCS vs. KES/t for all data with showing the general behaviour of the data.
Fig. 18. Corrected SSCS vs. KES/t illustrating the possibility that the lower trend is merely due to delamination of outer fronds that cause an effective reduction in cross-sectional area and a decrease in the overall stability of the plate.
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be ideal, as was shown by Ramakrishna et al. [13] for carbon fibre/thermoplastic based tubes. Typical examples of a long central crack requiring a long stroke to stabilise are the triaxial laminates in Fig. 8, while examples of where the sustained crushing was achieved after a short stroke are the quadriaxial laminates in Fig. 12(a). After the central crack is generated, the laminate then splits into two fronds. The knife-edges restrain the lami-
nate and hence, for crushing to continue, the fronds need to tear. This tearing and the friction between the splayed fronds and the base of the crushing rig can be considered to be two of the main methods of energy absorption. 4.1.1. Relationship between SSCS and aspect ratio A convenient way to visualise the results is to plot the SSCS against the aspect ratio (Fig. 16). To allow for easy identification of the equivalent epoxy and polyester
Table 4 Crushing results for the polyester-based laminates Plate number
Lay-up
KES/t
Vf
SSCS (Nm/g)
SPS (Nm/g)
SSCS/ SPS
F537
(EQX-2336) [(+45,90,45,0)]b
19.3 22.1 25.5 28.7
54.3
35.70 29.72 29.34 29.11
(3.58) (1.47) (2.65) (1.41)
43.33 41.53 44.96 43.98
0.82 0.72 0.65 0.66
F532
(EQX-1168) [(+45,90,45,0)2]b
17.1 19.2 21.2 23.02
48.0
40.96 37.25 34.28 34.04
(3.42) (3.89) (3.22) (3.88)
41.19 42.38 41.45 40.45
0.99 0.88 0.83 0.84
F765
(ELPb-567) [(+45/90/45/0)]s
13.0 15.4 17.9
53.4
67.33 (2.36) 62.69 (3.73) 59.72 (2.60)
61.68 57.22 64.60
1.09 1.10 0.92
F569
(ELPb-567) [(90/+45/45/0)]s
15.2 20.2
43.5
66.65 (6.52) 50.53 (4.56)
61.33 59.86
1.09 0.84
F528
(EBX-602, ELT 566) [(±45/90,0)2]s
16.0 18.6 21.4 22.3
45.4
63.35 52.68 48.85 48.03
(10.16) (6.35) (7.11) (7.89)
64.11 63.71 58.94 60.02
0.99 0.83 0.83 0.80
14.1 15.0 19.2 19.7
45.0
67.93 60.87 51.14 54.10
(8.90) (3.63) (3.16) (2.36)
65.11 68.92 66.40 65.00
1.04 0.88 0.77 0.83
14.9 15.5
38.1
71.63 (12.32) 68.46 (3.36)
71.50 75.75
1.00 0.90
14.4 17.5 20.0 23.2
47.9
73.42 65.17 53.21 47.05
(8.52) (7.01) (1.95) (9.60)
79.03 64.58 61.79 62.48
0.93 1.01 0.86 0.75
F549
F522
(ETLX 751) [(+45,45,0)3]b
F674
F519
(EBX-602, ELPb-567) [(±45/0)2]s
13.6
39.0
46.16 (4.05)
47.01
0.98
F511
(ELPb-567) [0/90/0]s
18.9
45.4
35.19 (1.91)
61.14
0.58
F551
(EBX-602) [(±45)4]s
12.9
41.3
43.14 (6.78)
63.64
0.68
13.2 17.1
40.3
41.87 (5.72) 39.77 (2.58)
64.40 64.97
0.65 0.61
19.4
38.6
37.57 (2.61)
55.18
0.68
22.8
50.3
29.44 (2.30)
60.32
0.49
F553 F512 F507
(ELPb-567) [03]s
KES/t, aspect ratio; Vf, volume fraction of fibres; numbers in parentheses are standard deviation; SSCS/SPS, crushing efficiency.
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matrix laminates, the symbol shape and colour is the same for both resins. The polyester resin laminates are given by a solid symbol, while the hollow symbols refer to epoxy based laminates. The chart illustrates that the SSCS increases as the aspect ratio decreases. Two seemingly parallel trends can also be observed whereby the difference between them is due to differences in the energy absorbing mechanisms. In the upper trend, the specimens crush with a low radius of curvature and hence a large amount of fibre fragmentation, with the result that there is little specimen spring-back once the specimen is removed from the crushing rig. For the lower trend, there was not as much fibre fragmentation; outer plies tended to delaminate and such that the frond radius of curvature was larger. In most of the latter cases, the split fronds simply splayed out. Fig. 17 is a simplification of the data in Fig. 16 showing the scatter and the trendlines observed. Of particular significance is that at the upper boundary the results seem to exhibit a large degree of scatter at the lower aspect ratios. It is possible that this scatter is partly due to some materials exhibiting a mode of failure that is in between the lower bound curves and the upper bound. The delamination of the outer plies in the low energy crushing mode is effectively a reduction in the area of the material coupled with an increase in the aspect ratio. For this reason, estimates of the difference in thickness were
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measured from the micrographs, and the corrected SSCS and aspect ratios are plotted in Fig. 18 together with the upper boundary trend. This verifies that the main reason for composites failing in a low energy mode is due to delamination in favour of the more complex crushing mechanisms reported for composites. 4.1.2. Comparison between epoxy and polyester matrix laminates Fig. 16 seems to show that for the resins selected, there is apparently no performance gain when using epoxy resins over polyester resins. The reason for this could be that the different resins do not have significantly differing mechanical properties, and so as Farley suggested, the energy absorption mechanisms would not differ greatly for similar reinforcements [14]. However, the main difference is that, on average, the crushing of the epoxy plates shows a lower variation within a series than for the polyester based laminates; but the crushing efficiencies of the epoxy matrix laminates are typically worse than for those with a polyester matrix (Tables 4 and 5). This is due to the epoxy matrix laminates having a higher peak stress prior to the initiation of crushing. 4.1.3. Comparison to literature data The KES/t parameter can be compared to the wall thickness/diameter (t/d) parameter for tubes, and possibly
Table 5 Crushing results for the epoxy-based laminates Plate number
Lay-up
KES/t
Vf
SSCS (Nm/g)
SPS (Nm/g)
SSCS/SPS
F662
(EQX-2336) [(+45,90,45,0)]b
15.9 17.6 20.9 24.4
53.8
44.10 37.54 33.64 33.06
(2.59) (1.32) (1.56) (1.96)
63.70 54.91 52.89 51.28
0.69 0.68 0.64 0.66
F663
(EQX-1034) [(+45,90,45,0)2]b
14.6 16.8 19.9 23.1
52.5
67.33 57.79 55.08 50.43
(6.09) (3.02) (6.03) (5.95)
70.49 64.63 64.24 63.84
0.96 0.89 0.86 0.79
F670
(ELPb-567) [(+45/90/45/0)]s
12.6 15.0 17.3 20.0
45.0
68.54 61.50 53.00 50.10
(7.24) (5.60) (4.02) (4.55)
69.31 57.82 58.97 59.46
0.99 1.06 0.90 0.84
F666
(EBX-602 + ELT-566) [(±45/90,0)2]s
14.1 16.5 19.7 22.6
48.2
41.75 36.13 32.05 30.23
(2.00) (6.11) (1.91) (2.56)
64.63 57.30 57.56 59.21
0.65 0.63 0.56 0.51
F668
(ETLX-751) [(+45,45,0)3]b
14.8 16.6 19.9 22.9
46.9
67.72 53.37 41.50 41.61
(12.37) (12.62) (3.38) (4.21)
77.41 62.80 63.18 59.62
0.87 0.85 0.66 0.87
F667
(EBX-602 + ELPb-567) [(±45/0)2]s
13.5 15.7 18.2 20.9
48.6
61.29 57.04 51.93 44.37
(4.86) (4.58) (4.99) (3.62)
70.74 63.59 55.89 60.64
0.87 0.90 0.93 0.73
KES/t, aspect ratio; Vf, volume fraction of fibres; numbers in parentheses are standard deviation; SSCS/SPS, crushing efficiency.
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S. Cauchi Savona, P.J. Hogg / Composites Science and Technology 66 (2006) 1639–1650
[17,18], however, in general the results seem to be following similar patterns. Acknowledgements The authors acknowledge funding from the Engineering and Physical Science Research Council (EPSRC) and the Overseas Research Scheme (ORS), and Saint Gobain BTI for supplying the NCF fabrics. References
Fig. 19. Chart showing the differences between the upper and lower trends compared to data from literature for tube-crushing – square or circular symbols refer to square tubes or circular tubes, respectively.
similar effects are present. To illustrate the similarity between the KES/t of plates and the t/d for tubes, data from various sources in the literature for circular [15–18] and square [4,19] tubes was superimposed for comparison purposes onto Fig. 19. 5. Conclusion From this analysis on the crushing of NCF based composite plates, it has been shown that, overall, the SSCS is not significantly influenced by the particular resins investigated, but by the fibre architecture. This fibre architecture includes the ratio of individual layers, and the type of NCF or UD fabric. The results indicate that the quadriaxial laminates result in better crushing efficiencies, and that the crushing is more consistent than for triaxial orientations. This is most likely due to the lower amount of 0° fibres within the entire laminate. In the triaxial laminates, due to the higher ratio of 0° fibres, the crushing stress requires a long stroke to stabilise due to the longer central crack formed after the peak stress. A long central crack can possibly destabilise the laminate if the Mode-I propagation properties are not large enough to arrest the crack growth. If the outer plies on the splayed fronds delaminate without any obvious damage, the SSCS of the composites decreases. When the crushing area is adjusted to remove the delaminated plies, the effective crushing stress increases and falls on standard upper trend. This implies that it is ideal to arrest any extensive delamination in the composite and the fronds to absorb the maximum energy. Finally, comparisons between the laminates tested and data obtained from the literature show (Fig. 19) that the results from the plate testing compare well with results from tube testing, i.e. as the aspect ratio of the structures increases, the energy absorbed decreases. In some cases, the results differ where the aim of the research was to improve the crushing stress of a particular aspect ratio
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