Controlled impact testing of woven fabric composites with and without reinforcing
SMA wires
Katerina Sofocleous
1
, Stephen L. Ogin
2
, Panos Tsakiropoulos
3
, Vassilis Draconakis
1
,
Charalabos Doumanidis
1
1
School of Engineering, University of Cyprus, Nicosia, 1678, Cyprus
2
Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, GU2
7XH, UK
3
Department of Engineering Materials, University of Sheffield, S1 3JD, UK
Correspondence: Katerina Sofocleous tel:+35799880038 fax: + 35722892254 katsof@ucy.ac.uk
Shape-memory alloy (SMA) composites are relatively new materials and their behaviour is not yet completely understood. The purpose of this work is to minimise the effect of impact damage on their structural performance. To do that, GFRP woven fabric reinforced epoxy composite panels, with and without additional superelastic
SMA wires, have been impacted at constant velocities using a servo-hydraulic testing machine, and a digital video camera has been used to monitor the impact event. Single, multiple and partial penetration impact tests have been carried out, and the energy absorption and damage development are similar in all cases for the same material. The benefit of using the superelastic SMA wires was seen only at high displacements and when the volume fraction of the wires was high.
Keywords: Glass fibres, Smart materials, Impact behaviour, Damage mechanics
1. INTRODUCTION
The central issue in work on impact damage of reinforced composites is how to minimise the effect of impact damage on their structural performance. Greenhalgh and
Hiley [1] provide a comprehensive review of currently available methods for improving the impact damage tolerance of a typical aerospace composite structural element (a
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stringer-stiffened panel). These methods include the use of tougher matrix systems, the use of textiles (woven fabrics, non-crimp fabrics, 3-dimensional fibre architectures), through-thickness stitching and Z-pinning, and the use of shape-memory alloy (SMA) reinforcement. The potential for SMA wires to improve the impact resistance of composite materials appears to have been investigated first by Paine and Rogers [2].
Many other potential benefits of SMA composites have been investigated since then, including the potential for SMA wires to improve the post-buckling response of composites, to improve the resistance to thermal buckling, and to modify the natural vibration frequency of composites (see the review by Wei et al [3]). Khalili et al [4] used SMA wires in composite material specimens subjected to low-velocity impact, which show improvement in the impact resistance of the composite. For improving impact resistance, SMA wires have the capability of either imparting a compressive strain to a structure, which can help to reduce the damage due to impact (e.g. Birman et al, [5], Meo et al, [6]); or can assist in dissipating impact energy through superelasticity or pseudoelasticity (Paine and Rogers [2]; Tsoi et al, [7], Aurrekoetxea et al,[8]). To date, all experiments on the beneficial effects of SMA wires in improving impact resistance have used drop-weight impact tests. In this work, an impact rig has been designed using a servohydraulic testing machine, which enables control of the impact velocity during the impact test, and easily permits observation of in situ damage development, due to the fact that the impactor can be easily stopped at any point inside the specimen.
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2. EXPERIMENTAL METHODS
The material investigated is a cross-ply (0/90/90/0) woven glass fabric reinforced composite laminate, with an epoxy resin matrix. The different kinds of laminate were eight-, four- and two-layer of woven fabric reinforced laminates with and without additional copper or superelastic SMA wire reinforcements. The fibre reinforcement was a Fothergill Engineered Fabrics Ltd., YO94 E-glass plain weave fabric. The composition of the resin was 100: 60: 4 (by weight) of Bisphenol A Epichlorohydrin resin 300, MNA2 hardener, Ancamine K61B catalyser respectively. This resin was chosen as it has a refractive index similar to that of the E-glass fibres, readily wets the glass, and produces a transparent composite material. The laminates were manufactured using an in-house wet lay-up process [9] . The laminates produced had a glass fibre volume fraction of approximately 0.5. Circular woven fabric panels of diameter 140 mm were cut from the resulting panels to be used as the impact specimens. For the specimens with additional SMA wire reinforcement, superelastic shape-memory alloy wires (NiTi alloy S from Memory-Metalle GmbH, straight annealed, 0.152 mm diameter, Af temperature of 16.50°C) were placed, between the last two woven glass fibre layers away from impact; this is because research shows that the best results are mostly obtained when the SMA material is embedded in the outer layers of the laminate away from the impactor [10]. The copper wire (plain pure copper) used for this work was obtained from Comax engineered wires with the same diameter as the SMA wires
(0.152mm). The wires were manually wound on a frame (420 mm x 420mm) in both 0° and 90° direction with a 1.5mm gap between each wire (as shown in Figure 1a) and then placed between the layers of woven fabric. The composite specimens having the SMA reinforcement had an overall wire volume fraction of about 0.8 per cent, 1.5 per cent
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and 2.4 per cent for eight-layers, four-layers and two-layers of woven fabric respectively.
A servo-hydraulic testing machine was used for the impact testing, and a schematic of the arrangement is shown in Figure 1b. A video camera was used to monitor the experiments and record the sequence of damage of the specimens. Testing was carried out in displacement control using an actuator speed of 4 mm/sec, giving quasi-static measurements for all the tests reported here, and both load and impactor displacement data were recorded. The impact rig consists of a cage with the circular composite specimen clamped at the base of the cage. The cage is gripped in the upper grip of the servo-hydraulic testing machine (which is connected directly to the load cell, in the usual way). The circular impact specimens were clamped over an annulus of 20 mm, so that the area exposed for impact is a circular panel with a diameter 100 mm. A digital video camera, fixed within the cage, recorded the impact event.
The impactor consisted of a glass sphere, 16 mm in diameter, mounted on a spigot
(glass has been used in order to minimise the shadow of the impactor in the video recording). The spigot is clamped in the lower grip and driven by the actuator at a constant velocity to produce an impact at the centre of the panel. Tests with impact velocities in the range 0.08mm/s to 200 mm/s have been carried out (although only those results at an impact velocity of 4 mm/s are presented here) with a maximum impact displacement of 18 mm. Three types of impact test were used. The first type is where the impactor is driven at constant velocity completely through the specimen
(“single impact test”). Generally, this required an impactor displacement of about
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18mm. The second type of test was a “multiple impact test”. During a multiple impact test the impactor was driven at constant velocity into the specimen to a certain displacement, e.g. 2mm from a datum where the impactor just contacted the specimen surface. Then, the impactor was withdrawn back to its initial position at the same velocity. The impactor was then driven to a higher displacement during the next impact, i.e. 5mm, and again returned to its initial position. This process was repeated with increasing impactor displacements of 2mm, 5mm, 8mm, 10mm, 12mm, 15mm and
18mm. The third type of impact test is called a “partial penetration impact test”. This test is similar to the multiple impact test, but instead of one specimen impacted at seven different displacements, a new specimen was used for each displacement. Using this type of test, it was possible to remove each specimen from the rig and examine the specimens by microscopic analysis to investigate damage development for different impact displacements. For each type of impact test, load-displacement graphs were recorded, showing the variation of load with impactor displacement during the experiments. From these graphs, the time that different types of damage occurred to the specimen can be estimated (since the impactor velocity is known) and these graphs can be linked with the video camera images of damage. For all the impact tests, the energy absorbed was found by calculating the area under the load-displacement curve up to the maximum displacement, using a Riemann integral. In the case of the multiple impact tests, overlapping areas need to be taken into account so that the energies are not
“double counted”. To identity the damage that occurred during the impact tests both macroscopic and microscopic studies were undertaken on impacted specimens.
Impacted specimens of two-layer woven fabric reinforced composites, with and without copper or SMA wires, were subjected to the partial penetration impact experiments,
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which generated different degrees of damage depending on the maximum depth of the impactor.
3. RESULTS
3.1 Eight and four layers
Thirteen specimens of eight-layer woven composites were tested with and without copper or SMA wires. Table 1 shows the thickness and the energy needed to break these specimens. A typical graph of the load-displacement response of a single impact test of an eight-layer woven fabric specimen without wires (W8N04S-01) is shown in Figure
2(a). The load showed a linear increase as the displacement increased up to 5mm where the slope decreased. After a displacement of 8mm where the peak load was reached, sudden drop of the load occurred due to macroscopic crack formation. The load remained approximately stable for a few millimetres, where it started decreasing as the specimen was penetrated by the impactor, until it reached zero load, where full penetration of the specimen had occurred. The average overall energy needed for fully penetrating the specimens was found by integrating the load-displacement curve, to be
34J. Similar results of load-displacement graphs of plain-weave E-glass fabric composite plates with epoxy resin can be found in the literature [11].Figure 2(b) shows load-displacement plots for a multiple impact test of an eight-layer woven fabric specimen without wires (W8N04M-02); during these multiple impact tests, seven cycles of impact loading were used and the arrows on Figure 2(b) show the load-displacement path followed for each cycle. For each cycle, the area of hysteresis was determined, and after all seven areas were found, the areas were added-up together to find the overall energy of penetration of the specimen; taking also into consideration the overlapping
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areas between cycles, which were subtracted from the overall energy. Apart from the first two cycles, when little damage occurred, significant hysteresis was found for each subsequent cycle and the area of the load-displacement hysteresis increases with increasing displacement.
Figure 3(a) shows a comparison of the load-displacement response of three eight-layer woven fabric specimens: without wires (W8N04S-01), with copper wires (W8C04S-
06H) and with SMA wires (W8S04S-03), impacted with single impact test, where the
SMA and copper wire volume fractions were the same (V f
=0.01). All three specimens started with very similar stiffness, but the peak loads of the specimens with wires were higher than the specimen without wires. No difference can be seen between the behaviour of the specimens with SMA or copper wire. To investigate any possible differences, the number of the layers of woven fabric was reduced from eight to four (so that the volume fraction of the wires will increase). Thirteen specimens of four-layer woven composites were tested with and without copper or SMA wires. Table 2 shows the thickness and the energy needed to break these specimens. These results are presented in Figure 3(b). Comparing the four-layer woven fabric specimens with and without copper or SMA wires, the effect of the wires can be recognised. Figure 3(b) shows this comparison between four-layer woven fabric specimens without wires,
(W4N04S-02), with copper wires, (W4C04S-03), with SMA wires, (W4S04S-02), all tested with single impact tests. The effect of the SMA wires can be seen after the sudden drop, where the values of load for specimen with SMA wires were kept higher than the ones of specimens with and without copper wires at the same displacement.
After the sudden drop of the load, the load decreases at a similar rate up to the point of
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full penetration where the specimens were fully penetrated. On average, specimens with
SMA wires needed slightly more energy to be fully penetrated than specimens with copper wires (15.3 J for specimens with SMA wires, 15 J for specimens with copper wires). Specimens without wires needed the least energy to be penetrated (14 J). Even though there was a slight increase in performance of specimens with SMA wires, the difference from specimens with and without copper wires was not clear enough.
Therefore the woven fabric layers were decreased to two layers (hence thickness of the specimen was decrease as well), so that the volume fraction of SMA and copper wires was increased.
3.2 Two-layer
Twenty specimens of two-layer woven composites were tested with and without copper or SMA wires. Table 3 shows the thickness and the energy needed to break these specimens. Typical load-displacement results for the single impact tests for two-layer woven fabric specimens are shown in Figure 4, which shows the results for a specimen without wires (W2N04S-02), with copper wires (W2C04S-03) and SMA wires
(W2S04S-03). It is evident that the SMA wires significantly change the specimen performance under impact. The peak load for the specimens with wires was about 20 per cent higher than for the specimen without wires. All specimens experienced a significant drop in load after the maximum value, but whereas the load in the specimens without wires continued to decrease after the peak load, the load for the specimen with
SMA wires remained at a constant range of loads for few millimetres before decreasing rapidly, and then following the load-displacement response of the specimen without wires. As a consequence of the higher peak load, and the constancy of the load after the
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initial load drop, the overall energy absorbed by the specimen with SMA wires is significantly higher (7.3J) than for the specimen without wires (5.5J).
The similarity in the load-displacement response for the single impact and multiple impact tests can be seen in Figure 5, where the responses are compared directly. Figure
5(a) shows the load-displacement response of two specimens; both with SMA wires, one impacted as a single impact test (W2S04S-06) and the second impacted as a multiple impact test (W2S04M-09). In each case, the curves have matching initial stiffness values and peak loads, and the overall shape of the load-displacement response is approximately the same. Similar results were found when comparing specimens without wires (Figure 5(b)), with average absorbed energies of 5.7 J (single impact) and
5.4 J (multiple impact). Comparing the behaviour of specimens with and without SMA wires, the results show an increase of about 2J for an overall wire density of 2.4 per cent, which is an increase of about 36 per cent in energy absorption for the specimens with wires.
The use of the video camera enabled images to be obtained of the specimens at different stages during the impact tests. Figure 6 shows the load-displacement response for a twolayer woven fabric specimen with SMA wires tested in a multiple impact test, together with images corresponding to different points in the load-displacement response. These images were taken at the peak displacement positions in each cycle of this multiple impact test. Figure 6(a) was taken at the maximum displacement of the first cycle; the impactor deflected the specimen by 2 mm, after which the specimen returned to its initial position without any damage formation, and hence there was no hysteresis loop
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formed. The image taken at the maximum displacement of the second cycle (5mm displacement) is shown in Figure 6(b), and whitened areas can be seen around the impacted area, and possibly a combination of matrix cracks and delaminations at crimp regions, although these cannot be seen clearly in these images. In the third cycle (Figure
6(c); 8 mm), penetration of the specimen had occurred, and the image records the specimen just after the major load drop had occurred. In Figure 6(d), macroscopic cracking of the specimen can be seen (the impactor has a displacement of 10 mm here) and this macroscopic cracking becomes increasingly clear in the following figures
(Figure 6(e) to 6(h)) as the impactor penetrates the specimen. Similar damage development can also be observed in single impact experiments. Figure 7 shows images from a single impact test of a specimen with superelastic wires at the same displacements as in the multiple impact test results shown in Figure 6, enabling a direct comparison between the single and multiple impact tests. The damage development in both cases is similar.
Two-layer woven fabric specimens with and without copper or SMA wires were tested using partial penetration impact tests. In these experiments, the impactor is driven into the specimen at a constant velocity to a certain displacement and then withdrawn. The displacements used were in the range of 2mm-18mm. Figure 8 shows the loaddisplacement responses for seven two-layer woven fabric specimens without wires, each one impacted at different displacements. The partial penetration impact experiments gave very similar results to the multiple impact experiments described earlier, confirming even more the reproducibility of the experiments.
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4. DAMAGE
Table 4 shows a summary of the macroscopic and microscopic observations of damage for the three different types of two-layer specimens at the different impact displacements. Specimens with and without copper or SMA wires showed similar damage when impacted at a displacement of 2mm. For better understanding of the damage, Figure 9 shows the stress-strain curves of the copper and SMA wires. It is immediately clear that the SMA wire dissipates substantially more energy to failure due to the higher stress for the same strain. Matrix cracks and delaminations were found in both the impact and exit layers. The copper and SMA wires seem to be unaffected by the impact (diameters of the wires stay the same), without any debonding between the wires and the surrounding matrix. The wires experience strains around 0.04% and stresses just a bit higher than their yield stress (Cu: 177MPa, SMA: 500MPa), which means that copper wire just started to permanently deform and SMA wire just enters its superelastic region. It is possible that stress whitening (visible white region around impact area) was macroscopically visible in the exit layer but not in the impact layer, because the exit layer was subjected to tension that led to the tows showing visible stress whitening, whereas the impact layer was subjected to compression.
In the same way to specimens impacted to 2mm, the different types of specimens showed similar damage when impacted to a displacement of 5mm. More extensive matrix cracks, delaminations and matrix cracks extending to the surface of both layers were found. No debonding was found around the copper wire and the surrounding matrix, whereas debonding was found around the SMA wire and the surrounding
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matrix. This is believed to be due to the expansion of the SMA wire and also the return of the wire to its original position after the load was removed (superelasticity of the wire), which caused friction between the SMA wire and the matrix.
Permanent deformation was observed on all types of specimens impacted to 8mm.
Microscopy suggests this is due to tow and matrix fracture at the centre of impact of the specimens. This penetration of the specimen is believed to be the cause of the sudden drop of load in the load-displacement plot. Circular cracks were found in the impact layer of all types of specimens. The radius of the inner circle was around 4mm and the cracks were about 1mm apart for specimens with and without copper wires. The inner circle radius for specimens with SMA wires was about 4.7mm and the cracks were about 2mm apart. Evidence of permanently deformed copper wires was found, but SMA wires appear to be unaffected. Copper and SMA wires had debonded from the surrounding matrix. Figures 10-12 show the cross-section at the centre of impact of two-layer woven fabric specimen without wires, with copper wires and with SMA wires. The impact and exit layer are heavily deformed. The arrows in the figures show the point of impact. The matrix region in the centre of impact was fractured in both layers. The warp tow in the impact layer was fractured and the tow in the exit region had suffered significant damage.
For impact displacements of 10mm, tow fractures, matrix cracks and delaminations in both layers of all specimens were found. All specimens showed through-thickness cracking. The circular cracks on the impact layer had an inner circle radius of about
4mm and the cracks were about 1mm apart for all types of specimens. Evidence of
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fractured copper wires was found, whereas no SMA wire failed, although some SMA wires were permanently deformed. The diameter of the wires was measured from specimens impacted at 2mm to specimens impacted at 10mm, and it was noticed that most of the copper wires (that were cut during impact) had a constant diameter of
0.152mm, whereas the diameter of the SMA wires decreased from 0.152mm to
0.144mm.
All types of specimens impacted at 12mm showed a cross-shaped through-thickness crack. Most of the copper wire in the damage area failed and the first evidence of the failure of SMA wires was found. At impact displacements of 15mm to 18mm, full penetration was reached for all specimens. All the copper wires in the damage area failed; but this was not the case for the SMA wires, where few of the wires failed and some of them deformed.
5. DISCUSSION
Forty-four specimens of glass-fibre woven fabric specimens with and without copper or
SMA wires were impacted tested. The energies absorbed by these specimens are shown in Tables 1-3. As the layers of glass fibres increase the energy absorbed increase as well due to the fact that more layers need more energy to be penetrated. Figure 13 shows a scatter plot of the distribution of these energies. The values of energies are consistent between each type of specimen. For eight-layer specimens, the percentage of scattering of the value of energies is around 20%, for four-layer specimens this value decreases to around 12% (except for specimens with copper wires which was 2.5%) and for twolayer woven fabric specimens the value decreased even more to around 6%. This
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increase of the percentage of scattering with the increase of number of layers of glass fibres is not unusual as the values of energy in the eight-layer specimens are much higher than the values of the two-layer composites. Figure 14 shows the load displacement response of the nine specimens impacted with single impact test. The specimens are eight-layer of woven fabric without wires (W8N04S-01), with copper wires (W8C04S-06H) and with SMA wires (W8S04S-03), four-layers of woven fabric without wires (W4N04S-02), with copper wires (W4C04S-03) and with SMA wires
(W4S04S-02); and two-layer of woven fabric without wires (W2N04S-03), with copper wires (W2C04S-03) and with SMA wires (W2S04S-02). The increase of load during impact of the four- and two-layer woven fabric specimens is non-linear, whereas for the eight-layer woven fabric specimens, is linear. This is because the eight-layer specimens are much stiffer that the four- or two-layer specimens due to the fact that they are much thicker than the other kinds of specimen. That is also why the eight-layer specimens experience higher loads compared to the other kinds of specimen for the same displacements. For the same reason four-layer specimens show higher value of load for the same displacement when compared with two-layer specimens. The peak load and the sudden drop of load occur at around the same displacement for all specimens, showing no obvious dependence on the thickness of the specimens. Table 5 shows the average values of the peak load, the penetration energy and the average thickness for the eight-, four- and two-layer woven fabric specimens with and without copper or SMA wires, tested with single and multiple impact tests.
Evaluating the differences that the wires made when they were inserted in the specimens, the following conclusions can be pointed out. The increase of peak load
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from two-layer woven fabric specimen without wires to the one with copper wires was
7.6 per cent, whereas for the four-layer woven fabric specimens was 20 per cent decrease, and for eight-layer woven fabric specimens was 23 per cent increase. The increase of energy for the two-layer woven fabric specimens without wires to specimens with copper wires was 16 per cent, i.e. twice that of the four-layer specimens (7.1 per cent), and close to the increase for eight-layer specimens (20 per cent). When comparing the increase on the energy values when SMA wires were introduced, the values of the eight-layer woven fabric specimens were lower than the ones when copper wires were inserted, i.e. 16 per cent for peak load and 12 per cent for energy. There was a decrease of 6.3 per cent of peak load, and an increase of 7.1 per cent of penetration energy was observed from four-layer woven fabric specimens without wires to fourlayer woven fabric specimens with SMA wires. A significant 33 per cent increase of penetration energy was observed from two-layer woven fabric specimens without wires compared to two-layer woven fabric specimens with SMA wires. The increase of peak load is 11 per cent. Looking at these values, it can be seen that the two-layer woven fabric specimens with SMA wires showed the higher improvement on energy.
Table 6 shows the average penetration energy of two-layer woven fabric specimens with and without copper or SMA wires. Although the specimens with wires are thicker than the specimens without wires and this can affect up to a point the value of energy absorption, specimens with copper or SMA wires have similar thickness and still the specimens with SMA wires show a higher energy absorption that the specimens with copper wires. To investigate whether this extra energy is due to the deformation and failure of the wires in the specimen, an attempt has been made to calculate the energy
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needed to fail the wires in the impact area. Figure 15 shows a schematic of a specimen and the impact area. The impactor has a diameter of 16mm. The intensive damage around the impactor is generally a circular area of approximate 20mm diameter. Around this intensive area is another region which apparently consists of stress whitening
(though, in fact, this is matrix cracking and delamination).
Several assumptions have been made in these calculations. First, it has been assumed that all the wires fail in the intensive damage region, which was taken to be a square with dimensions 20mm × 20mm. This is not exactly true because not all the wires in this region break, as it was shown in the macroscopic analysis, and the impact area is circular with 20mm diameter. However, knowing the area of the impact area and the spacing of the wires in the specimen (1.5mm) and the number of wires in the impact area (27 wires), the total volume of the wires can be found. Using the energy per unit volume to fracture the copper or SMA wires, calculated from the stress-strain curves shown in Figure 9, it was found that 0.4J and 1.6J were needed to fracture the copper and SMA wires in the impact area, respectively. Comparing these values with the values of enhanced energy in Table 6, it can be seen that not all the extra energy is solely due to deformation and failure of the wires. Presumably friction between the wires and the matrix makes an additional contribution, together with debonding of the wires from the matrix.
6. CONCLUSION AND FUTURE WORK
A method of controlled impact testing for GFRP woven fabric reinforced epoxy composite panels, with and without additional superelastic SMA wires, has been
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developed. A specially designed impact rig was installed within a servo-hydraulic testing machine with a spherical glass impactor that was driven into the specimens at a constant speed. A video camera enabled the development of damage within the transparent specimens to be observed. Three different types of impact tests were performed: single impact tests, multiple impact tests and partial penetration impact tests.
In general, the overall load-displacement response for a multiple impact test was very similar to a single impact or a partial penetration impact test. In all cases, the loaddisplacement response could be used to measure impact energy and absorbed energy during a test. Damage was found in all three kinds of specimens after impact displacements of only 2mm; the damage consisted of matrix cracks and delaminations.
The damage increased as the displacement increased and the through-thickness penetration of the specimens started from an impactor displacement of about 8mm.
Based on the impact results from tests on specimens with and without copper and SMA additional wire reinforcement, it has been concluded overall that the benefit of using superelastic SMA wires is achieved for impacts which produce through-thickness penetration of the specimens, when the large energy-absorbing characteristics of the wires are used, and when the volume of the SMA wires is high.
Present research underway addresses manufacture and testing of similar Carbon Fiber
Reinforced Polymer (CFRP) specimens made of carbon woven fabric, with and without
SMA wires similar to those in the GFRP panels above. The objective is to assess whether SMA wires can help the CFRP specimens absorb more impact energy as for the GFRP samples, and to compare the absorbed impact energy in the two fiber
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composite panels. Figure 16 shows preliminary results on such a control CFRP specimen, i.e. a four-layer woven fabric specimen (the plain weave fabric is C160P and the epoxy resin 921 with hardener 475524 all from Fibermax Composites) without
SMA wires impacted at (a) Single impact test and (b) multiple impact test, which correspond to the results of Figure 2 for the GFRP samples. These were measured under the same experimental procedures and conditions for impact testing as described above in this paper. Further results on the superelastic SMA-reinforced CFRP panels, and their performance comparison to the GFRP equivalent outcomes in this paper, is in progress and will be subsequently reported separately.
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Material Science, 33: 3763-3783.
4. Khalili SMR, Shokuhfar A, Ashenai Ghasemi F. (2007) Effect of smart stiffening procedure on low-velosity impact response of smart structures, Journal of Material
Processing Technology, 190: 142-152.
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5. Birman V, Chandrashekhara K, Sain S. (1996) An approach to optimization of shape memory alloy hybrid composite plates subjected to low-velocity impact, Composites,
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Matererial Structural, 20: 1-24
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ACKNOWLEDGEMENTS
Part of this work was co-funded by the European Regional Development Fund and the
Republic of Cyprus through the Cyprus Research Promotion Foundation (Project
ΔΙΔΑΚΤΩΡ/0609/33
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(a) (b)
Figure 1: (a) A schematic of the frame used to wind the wires. (b) A schematic of the servo-hydraulic testing machine that was used for the impact testing.
(a) (b)
Figure 2: (a) Load-displacement response of a single impact test of an eight-layer woven fabric specimen without wires. (b) Load-displacement plots for a multiple impact test of an eight-layer woven fabric specimen without wires.
(a) (b)
Figure 3: Comparison of the load-displacement response of three (a) eight-layer and (b) four-layer woven fabric specimens: without wires, with copper wires and with SMA wires, impacted with single impact test.
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Figure 4: Load-displacement results for the single impact tests for two-layer woven fabric specimens without wires, with copper wires and with SMA wires, impacted with single impact test.
(a) (b)
Figure 5: Comparison of the load-displacement response, impacted with a single impact test and with a multiple impact test of two-layer woven fabric specimens (a) with SMA wires and (b) without wires.
(b)
(a) 1.5
1
(c) (d)
(e)
(h)
0.5
0
0
-0.5
2 4
(g)
6 8 10 12 14 16
(f)
18
Figure 6: Load-displacement response for a two-layer woven fabric specimen with
SMA wires tested in a multiple impact test together with images corresponding to different points in the load-displacement response.
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(b)
(a)
1.5
(c) (d)
(e)
1
0.5
0
0 2 4 6 8 10 12 14 16 18
(h)
-0.5
(g) (f)
Figure 7: Load-displacement response for a two-layer woven fabric specimen with
SMA wires tested in a single impact test together with images corresponding to different points in the load-displacement response.
(a) (b)
(c) (d)
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(e) (f)
(g)
Figure 8: Load-displacement responses for seven specimens without wires impacted with partial penetration impact test at different displacements.
Figure 9: Stress-strain response of copper wire and SMA wire loaded until failure.
Figure 10: Cross-section at the centre of impact of two-layer woven fabric specimen without wires.
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Figure 11: Cross-section at the centre of impact of two-layer woven fabric specimen with copper wires.
Figure 12: Cross-section at the centre of impact of two-layer woven fabric specimen with
SMA wires.
Figure 13: Energy distribution of the eight-, four- and two-layer woven fabric specimens with and without copper or SMA wires impacted with single and multiple impact tests.
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Figure 14: Load displacement response of the eight-, four- and two-layer woven fabric specimens with and without copper or SMA wires impacted with single impact test.
impact
area
stress whitening
20mm
Figure 15 : A schematic of a specimen and the impact area.
Figure 16: Preliminary results on four-layer carbon woven fabric specimen without SMA wires impacted at (a) Single impact test and (b) multiple impact test.
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Name of
Specimen
W8N04S-01
W8N04M-02
W8N04M-03
W8N04S-04
W8N04S-05
W8N04M-06
W8C04M-05H
W8C04S-06H
W8C04M-07H
Type of specimen
No wires
No wires
No wires
No wires
No wires
No wires
Copper wires
Copper wires
Copper wires
Thickness Energy
(mm) (J)
2.69
2.75
2.92
2.78
2.58
2.53
2.63
2.75
2.79
34
37
37
34
31
30
41
40
41
W8S04M-01
W8S04S-02
W8S04S-03
W8S04M-04
SMA wires
SMA wires
SMA wires
SMA wires
2.97
2.91
2.96
2.89
37
38
39
38
Table 1: Values of the penetration energy and the thickness for the eight-layer woven fabric specimens with and without copper or SMA wires tested with single and multiple impact tests.
Name of
Specimen
W4N04M-01
W4N04S-02
W4N04M-03
W4N04S-04
W4C04M-01
W4C04S-02
W4C04S-03
W4C04M-04
W4C04S-05
W4C04M-06
W4S04M-01
W4S04S-02
W4S04S-03
Type of specimen
No wires
No wires
No wires
No wires
Copper wires
Copper wires
Copper wires
Copper wires
Copper wires
Copper wires
SMA wires
SMA wires
SMA wires
Thickness Energy
(mm) (J)
1.38
1.45
1.41
1.49
12
13
14
15
1.68
1.67
1.76
1.77
1.69
1.67
1.62
1.59
1.58
15
15
16
15
14
15
15
15
16
Table 2: Values of the penetration energy and the thickness for the four-layer woven fabric specimens with and without copper or SMA wires tested with single and multiple impact tests.
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Name of
Specimen
W2N04M-01
W2N04S-02
W2N04M-03
W2N04S-04
W2N04S-05
W2N04M-06
W2C04M-01
W2C04S-02
W2C04S-03
W2C04M-04
W2C04M-05
W2S04M-01
W2S04M-04
W2S04S-05
W2S04S-06
W2S04M-07
W2S04M-08
W2S04M-09
Type of specimen
No wires
No wires
No wires
No wires
No wires
No wires
Copper wires
Copper wires
Copper wires
Copper wires
Copper wires
SMA wires
SMA wires
SMA wires
SMA wires
SMA wires
SMA wires
SMA wires
Thickness Energy
(mm) (J)
0.95
1.03
0.99
0.99
1.02
1.06
1.02
0.97
1.04
0.76
0.82
0.77
0.8
0.82
0.75
1.04
0.97
0.98
6.6
6.9
7.3
7.4
7.2
7.1
7.4
6.9
7.3
5.4
5.7
5.5
5.4
6.2
4.9
6.2
6.3
6.1
Table 3: Values of the penetration energy and the thickness for the two-layer woven fabric specimens with and without copper or SMA wires tested with single and multiple impact tests.
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Displacement mm
2
5
8
10
12
15
without reinforcing wires
Damage on specimens
with reinforcing copper wires
Small amount of Small amount of delaminations & matrix delaminations & cracks in both layers. Matrix cracks.
Similar damage to N-02 and
C-02.
No debonding of wires.
No debonding of wires.
More damage than More damage than More damage than N-04 and
N-02. C-02. C-05.
Matrix cracks reached Matrix cracks reached Matrix cracks reached surface surface on both layers. surface on both layers. on both layers.
No damage on wires. Matrix region fractured.
Matrix cracks in warp tows.
Debonding around wires.
Surface of exit layer fractured.
Warp tow of exit layer fractured into pieces.
Circular cracks on impact layer.
with reinforcing
SMA wires
Partial penetration. Less damage than N-07 and
Damage around wires. C-08.
Wires deformed.
Warp tow of exit layer failed.
Circular cracks on
impact layer.
Matrix cracks reached surface on both layers.
Matrix region fractured.
Delamination around wires.
Circular cracks on impact layer
Deformation. Full penetration at the More damage than N-10 and
Warp tow of the impact centre of impact. layer fractured. Damage around wires.
C-10.
Full penetration at the centre
Weft tows separated. Debonding of tows and of impact.
Matrix region fractured. Matrix Damage around wires but no deformation.
Increased deformation Increased penetration. Increased deformation and penetration forming a cross of fractured tows.
Fractured wires. and penetration.
First fractured wires.
Almost penetrated. Almost penetrated.
All wires failed.
Almost penetrated.
More wire failed and others deformed.
18
Full penetration Full penetration.
All wires failed.
Full penetration
Not all wires failed.
Table 4: Summary of the main damage of the three different types of specimens at the seven different impact displacements investigated.
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Type of
Specimen
Eight-layer woven fabric specimens without wires
Eight-layer woven fabric specimens with copper wires
Eight-layer woven fabric specimens with SMA wires
Four-layer woven fabric specimens without wires
Four-layer woven fabric specimens with copper wires
Four-layer woven fabric specimens with SMA wires
Two-layer woven fabric specimens without wires
Two-layer woven fabric specimens with copper wires
Two-layer woven fabric specimens with SMA wires
Peak load
(kN)
3.7±0.5
4.6±0.1
4.35±0.09
2.4±0.7
1.9±0.1
2.2±0.1
1.05±0.07
1.13±0.02
1.17±0.08
Penetration Average
Energy thickness
(J) (mm)
34±3
40.7±0.6
38.0±0.8
14±1
15.0±0.6
15.3±0.6
5.5±0.4
6.4±0.3
7.3±0.3
2.7
± 0.1
2.81
± 0.05
2.93
± 0.04
1.43
1.71
1.60
±
±
±
0.05
0.04
0.02
0.79±0.02
1.0±0.3
1.04±0.05
Table 5: Average values of the peak load, the penetration energy and the average thickness for the eight-, four- and two-layer woven fabric specimens with and without copper or SMA wires tested with single and multiple impact tests.
Type of composite Energy (J) Increased of energy (J)
Two-layer woven fabric without wires
Two-layer woven fabric with copper wires
5.5
6.4
-
0.9
Two-layer woven fabric with SMA wires 7.3 1.8
Table 6: Energy of penetration of two layer woven fabric specimens with and without copper or
SMA wires, and increase of energy from specimens without wires to specimens with wires.
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