Ballistic impact analysis of balsa core sandwich composites

Composites: Part B 67 (2014) 160–169

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Ballistic impact analysis of balsa core sandwich composites N. Jover a, B. Shafiq b,⇑, U. Vaidya c a

Department of Civil Engineering, University of Puerto Rico, Mayagüez, PR 00681, United States Department of General Engineering, University of Puerto Rico, Mayagüez, PR 00681, United States c Department of Materials Science and Engineering Department, University of Alabama at Birmingham (UAB), Birmingham, AL 35294, United States b

a r t i c l e

i n f o

Article history: Received 12 September 2013 Received in revised form 1 July 2014 Accepted 6 July 2014 Available online 12 July 2014 Keywords: A. Wood B. Impact D. Non destructive testing E. Resin transfer molding (RTM) Sandwich composites

a b s t r a c t This work investigates balsa core sandwich composites with thin carbon fiber skins subjected to single and multi-site sequential ballistic impacts. For the sandwich composite composed of core and facesheet thicknesses of 0.95 cm and 0.25 mm respectively. The ballistic limit was determined to be 96 m s1, indicating that it was capable of withstanding impacts from small projectiles such as, secondary debris from blast, hurricane, tornado and foreign object debris from roads and runways. The visual inspection and non-destructive evaluation (NDE) tests showed that the damage was relatively localized. A neat perforation was observed on the strike side facesheet whereas the back facesheet indicated greater spread of damage. The effect of prior damage in terms of residual and absorbed energies became more pronounced as the number of impacts increased. Damage areas were measured in order to quantify the extent of visible failure and/or perforations, while NDE tap testing was implemented to extract the information regarding interfacial damage. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Functionally graded sandwich composites are increasingly becoming material of choice in military and civilian applications due to their lightweight, high strength and energy absorption capability [1–10]. Stiffer facesheets are typically responsible for the in-plane load bearing, whereas energy absorbing softer core serves to transfer shear between the facesheets when subjected to out of plane loading [8]. Therefore, the core properties are the key to fabricating impact resistant composites and naturally growing balsa wood offers some very attractive features in this regard. Although there is substantial literature on (i) the impact response of laminated [4,11–22] and foam and honeycomb core sandwich composites [1–10,23–25], (ii) low velocity impact in composites [1–4,6,8,11–15,19,26–29], (iii) multiple impacts on composites [2,3,5,12], and (iv) the mechanical and thermal aspects of balsa core [30–33]; studies examining single and multiple ballistic impact response of balsa wood as a core in sandwich composites are limited. As the quest for lighter and damage tolerant materials rises, it is important to investigate the impact response of this promising class of material, in particular in the ballistic range that would represent the effect of secondary blast, hurricane, tornado and foreign object debris from roads and runways at the intermediate level to ⇑ Corresponding author. Tel.: +1 787 832 4040x2094; fax: +1 787 265 3816. E-mail address: basir.shafi[email protected] (B. Shafiq). http://dx.doi.org/10.1016/j.compositesb.2014.07.002 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.

small firearms or explosive warhead fragments at the high end [4,22]. The ballistic limit (Vb) of a material is generally required in order to ensure safe operational life. This limit is commonly defined as the minimum velocity at which a projectile consistently and completely penetrates a target of given thickness and physical properties at a specific angle of obliquity [9,20,34]. The energy required for complete penetration of the target material (Up) is obtained from laws of conservation of energy [11]:

0:5mv 2i ¼ U P þ 0:5mv 2f

ð1Þ

where vi and vf represent the impact and residual velocities, respectively. Up typically depends on the velocity, shape and mass of the projectile and the composition of the impacted material, and indicates the extent of damage suffered during impact [1,3,4,14,23,34]. If the impact energy is below the threshold energy required for damage initiation, no damage occurs. With an increase in velocity above the threshold, a reduction in residual energy is accompanied by the spread of damage. In the ballistic range, a constant residual energy is observed as the impact energy exceeds the penetration threshold, thus localizing the damage to a neat perforation [9,10,16,20,22,23]. Discerning modes of failure under impact load is a significant aspect of reliability analysis. However, in comparison with other materials, damage in sandwich composites is difficult to detect, as it tends to remain hidden along the interfaces between the

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facesheets and the core [4,34]. Even after complete penetration of the composite, quantifying the extent of damage in the vicinity of the impact requires considerable effort as well as post impact NDE. Impact damage in foam core sandwich composites such as, fiber breakage, core crushing, rupture or shear, and interface failure caused by mismatch in facesheet, resin and core properties [4,9,14,16,34]) depends on the characteristics of the impactor and target material. In terms of the target material, a common observation is that an increase in the facesheet or core thickness/ density leads to higher energy absorption [4]. However, an increase in facesheet thickness if caused by increasing number of laminas can also create interlaminar shear cracks as well as local tensile cracks or compression induced wrinkling, [4,8,14,33]. Studies have also shown greater energy absorption by the core as well as an increase in fiber breakage, indentation and delamination of facesheets with increasing impact velocity [1,4,34]. In terms of the impactor shape, blunt projectiles tend to push the material inward whereas the sharp projectiles push the material sideways, resulting in discernible stress and damage signatures [4]. As a result, damage may be more visible for a sharp projectile due to

Table 1 ProBalsa data sheet supplied by the DIAB, Inc. manufacturer. Property

Method

Density Compressive strength Compressive modulus Tensile strength Shear strength Shear modulus

ASTM ASTM ASTM ASTM ASTM ASTM

C C C C C C

161

penetration ease that causes fewer disturbances to the surrounding material. However, the absorbed energy is greater for blunt projectiles due to large panel deflection [4,14,15,27–29]. Determination of the extent of damage due to multiple impacts is also of significant importance in the design of damage tolerant composite structures. Various studies are available on the multiple impacts, however, many are concerned with laminated composites and low velocity impacts [2,5,12,13]. Furthermore, the work that has been published on multiple impacts, target materials and projectile type vary substantially that extrapolating the results to other material systems is difficult. There is clearly a gap in the literature on the damage mechanisms under multiple impacts in balsa core sandwich composites particularly in the ballistic range, and the current study attempts at addressing this gap.

2. Experimental setup The sandwich composite panels consisting of 0.95 cm thick lightweight balsa core and one layer of 162 g, 3 K tow and 0.25 cm thick cross weave carbon fiber skin on each face were fabricated using the vacuum assisted resin transfer molding (VARTM) method [9,11]. Details of the target material are provided in Table 1 and Fig. 1. Tests ranging from 1 to 3 sequential impacts were

LD7 lightweight 271 365 365 297 273 273

90 kg/m3 5.4 MPa 1850 MPa 7 MPa 1.6 MPa 96 MPa

Fig. 1. Typical balsa core carbon fiber facesheet sandwich composite.

Fig. 2. Multiple impact gas gun apparatus: pressure vessel, pneumatic actuator, firing valve, capture chamber and chronographs location.

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Fig. 5. Illustration of the use of hammer.

Fig. 3. Schematic of the gas gun barrel configuration, dimensions and firing order as viewed from the capture chamber.

conducted with a spherical 0.30 caliber (3.8 mm radius), grade 25 alloyed steel ball bearings with a hardness of 63–67 Rockwell C, and a mass of 2 g. The test apparatus consisted of a single-stage

light-gas gun equipped with a regulated high pressure compressed fluid source (nitrogen or helium), a pressure transducer, pressure vessel, firing valve, barrel, velocity sensors, capture chamber, specimen fixture and projectile arrest. Further details of the assembly and its parts can be observed in Figs. 2 and 3 [9,35]. Because each barrel is independent, one or two of them can be closed in order to run single or multiple impact tests. The three barrels are breach loaded and connected to a single 63.5 mm diameter butterfly valve

Fig. 4. Typical mapping of single, double and triple impacts.

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via a common manifold assuring that the sabot assisted projectiles are subjected to the same consistent pressure. Before ballistic testing the fabricated materials were sized to 15  15 cm2 in order to fit into the test system of the machine. Both entering and exiting velocities were recorded with the use of stationary sensors. After the ballistic testing, visual inspection of the skins was carried out to identify the areas of visible damage and possible failure modes. To further classify the extent of damage, scaled images were measured and mapped using AutoCad software.

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Typical measurements are shown in Fig. 4. This inspection method provided a superficial estimate of the extent of damage. Barely visible damage (BVID) called for more extensive NDE, which was accomplished by ‘tap testing’. Tap testing is a viable damage detection technique to evaluate the extent and severity of damage in a composite panel. Tap test can also provide information regarding the overlapping damage areas in the case of multiple impacts, which has implications in the materials capacity to withstand the same load successively.

Fig. 6. Diagram of tap test grid of the facesheet (1.00 in2) and backsheet (0.25 in2).

Fig. 7. Representation of the impact event and the possible paths the projectile traveled when the sensor failed to detect it.

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60 1st Impact

Residual Energy (J)

50

2nd Impact 40

3rd Impact

30 20 10 0

0

10

20

30

40

50

60

70

Kinetic Energy (J) Fig. 8. Relationship between residual energy and kinetic energy of the projectile.

30 1st Impact

Absorbed Energy (J)

25 2nd Impact

20

3rd Impact

15

10

5

0

0

10

20

30

40

50

60

70

Tap testing was performed using a Wichitech Rapid Damage Detection Device (RD3), which consisted of two main components: (i) a tap hammer containing a high-frequency pulse vibration sensor, and (ii) a data receiver with a liquid crystal display (LCD) that displays the sensor’s reading (Fig. 5). The sensor in the hammer is activated upon a light tap and the pulse width (in microseconds) is recorded. A structurally sound material produces a short (narrow) force–time pulse width, whereas, the longer duration of the pulse represents the likelihood that there is a defect at or very near the point of impact. In order to analyze the tap test results, each test plate had a grid drawn on it as illustrated in Figs. 5 and 6. The top-left corner begins with the nomenclature A1 progressing down and right to F6. These designations correspond to grids in each test specimen. On the reverse side of each sample is a smaller, higher-resolution grid for each tap location, with the same grid notation. Side by side illustration of these grids is presented in Fig. 6. While tap testing can provide overall indication of the location of the defect, it is prone to human error. The tap hammer requires operator skill to ensure consistency of testing, (Fig. 5). Striking the sample too hard can result in an artificially short pulse, and striking too gently can yield no result. Even when used properly, the test itself is discrete with a fairly low resolution, as the hammer’s tapping surface is approximately a hemisphere of 6 cm diameter. Therefore, it contacts the sample at a relatively large surface, and cannot resolve small-area defects. Furthermore, relatively large sample errors can appear if there are small defects as striking such a defect head-on will result in a longer pulse than the area just around the defect. Despite its shortcomings, [36–38]), tap testing combined with visual inspection can offer valuable insight into the state of damage (see Fig. 7). 3. Results

Kinetic Energy (J)

3.1. Ballistic limit Fig. 9. Relationship between energy absorbed by the sandwich composite and kinetic energy of the projectile.

Nineteen panels were evaluated under ballistic impact for a consistent range of projectile velocity in this study for single,

Fig. 10. First impact response as a function of increasing velocity along with typical single impact image of facesheet and backsheet.

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double, and triple sequential ballistic impacts. Small variations in velocities were observed, however, the error was within commonly observed statistical variation of the impact testing [11–14]. Data collected revealed the ballistic limit for the material under study to be 96 m s1. According to the classifications of impact velocities, this ballistic limit is at the highest end of the intermediate velocity range (10–100 m s1) [4]. However, a direct comparison with literature is difficult due to lack of reports on balsa core sandwich composites and mismatch in the impactor and the host material used in other material systems. In terms of ballistic limit, it appears that the performance of glass fiber reinforced facesheet/balsa core sandwich composite is comparable to carbon fiber laminated composites, for example, 105 m s1 has been reported for a five layer Twill weave T300 carbon/epoxy laminate using a flat-ended projectile of 1.8 g; and 100 m s1 for 10 ply carbon/epoxy laminate under 1.73 g spherical projectile [16,22]. This would suggest that adding a balsa core can lead to substantial improvement in the impact resistance of the sandwich composite. 3.2. Single and sequential impacts Of the nineteen panels, at least five specimens were evaluated for each testing condition. All projectiles were consistently fired at a nominal pressure of 68 kPa (10 psi) with a gas gun for the first impact, and the pressure was increased proportionally up to 137 kPa (20 psi) for higher energy impacts [16]. The energy loss due to friction and heat conversion has been shown to be relatively small in comparison to the energy absorbed by the material or the projectile [9,34,39]. Hence, all energy dissipated during impact was attributed to energy absorbed by the specimens. As the impact energy increased, so did the residual energy (as seen in Fig. 8). The results indicate a linear relationship with each data set, indicating (i) highly localized damage, and (ii) after the ballistic limit is reached the energy required for penetration remains constant

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and independent of the initial velocity of the projectile [4,9,34]. The comparable intercepts of the linear regression in Fig. 8 suggest that the ballistic limit and the structural capacity to withstand impact remained similar at lower energy levels, even after prior impacts. However, as the kinetic energy increased the lines diverged suggesting that at higher energy levels a second or third impact can potentially result in a higher residual velocity (or less absorbed energy) and the possibility that the structure has experienced a reduction in its capacity to withstand impact. Fig. 9 represents the energy absorbed by the sandwich composite in relationship to the kinetic energy of the projectile. It can be seen that for the first and second impacts, a range between 4 J and 15 J was observed, while a lower range of less than 10 J is observed for the third impact. This indicates that for the third impacts the material was unable to absorb energy beyond that required to perforate the panel. The results also suggest that the material loses its ability to absorb energy as the number of impacts increase with a more clear difference observed between second and third impacts. The damage on the impact side remains localized regardless of the number of impacts. However, the damage on the back facesheet is cumulative in nature. Adjacent damage profiles intersect to form an accumulated damage zone with increasing number of impacts, as illustrated in Figs. 11 and 12. A multiple impact study on a composite with similar balsa core thickness and E-glass facesheet reports a gradual increase in energy absorption for the first six impacts at 10 J, and a constant variation beyond 10 J; even though the focus of this particular study was on low velocity impacts with a larger, much heavier projectile [32]. Literature on the dynamic response and energy dissipation characteristics of balsa wood suggests that it is dependent upon its microstructure and density [33]. Therefore, it is likely that the scatter observed in the absorbed and residual energies

Fig. 11. Second impact response as a function of increasing velocity along with typical double impact image of facesheet and backsheet. The images in the first two rows show each impact for clarity.

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Fig. 12. Third impact response as a function of increasing velocity along with typical triple impact image of facesheet and backsheet. The images in the first four rows show each impact for clarity.

Damage Area (mm2)

1000

Single Shot

1st shot

1st shot

2nd shot

2nd shot 3rd shot

800 600 400 200 0

Fig. 13. Backsheet damage area per sample and impact sequence.

(Figs. 8 and 9) are a function of the balsa wood’s microstructure at the impact location. Balsa is a naturally growing material, therefore, some variations are expected from panel to panel;

also the wood panels are formed out of glued segments of similar density but perhaps taken from different trees, which may lead to further disparities. Furthermore, VARTM manufacturing

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process of sandwich panels may introduce additional microscopic imperfections [9,11]. 3.3. Post impact analysis Figs. 10–12 show single, double and triple impact in sequence according to the impact velocity, however, beyond superficial visual inspection the extent of the damage remained hidden. Damage areas of only the backsheet were mapped on scaled images, according to Fig. 4 for typical single, double and triple impacts as the facesheets in general had a clear perforation. The measured area of damage per impact location is presented in Fig. 13. Measurements were compared to the kinetic energy of the projectile, as well as the energy absorbed by the sandwich composite, as illustrated in Figs. 14 and 15. It was expected that as the impact energy increased, the damage area would do so as well; Fig. 14 indicates increase in damage area as the absorbed energy increased. Similarly, an envelope around the data points in Fig. 15 suggests a tendency of the damage area to increase as the impact energy increased, up to impact energies of approximately 32 J, and then slightly decreased. It is worth noting that the responses for the second and third impacts fall within the envelope, which is mostly outlined by first impact markers. This could indicate that under the conditions studied, prior impact damage did not propagate enough to affect the material at the subsequent impact location. Also, damage areas were not observed to statistically vary between successive impacts of the same panel. This implies that the mate-

1000

Damage Area (mm2)

1st Impact 800

2nd Impact

rial retained its structural capacity to withstand successive impacts at the different locations. Damage areas were also categorized by the impact sequence. Of those impacted at least twice, 64% had larger damage area on the first impact rather than on the second. The difference in damage areas between the first and second impacts varied from 0.13 cm2 to 5.3 cm2. For those impacted third time, results between the first and third impact damage areas were mixed. Finally, 67% of the samples impacted three times experienced larger damage on the third impact location when compared to the second. The difference between second and third damaged areas ranged from 0.26 cm2 to 3.42 cm2. Localized damage at the impact site was observed for all facesheets, regardless of impact velocity. The unsupported condition of the backsheet, in contrast to the facesheet which had the core backing, was responsible for the formation of large out of plane strains (along 0°/90° directions of the woven fiber facesheet) that caused the development of interface failure, fiber breakage and matrix cracking [5]. Panels impacted at velocities lower than the ballistic limit exhibited damage in the form of fiber strain and separation, as well as, matrix cracking and debonding. In the ballistic range, backsheet ruptured and core walls sheared accompanied by a clear core push out, and cell buckling. Some reports on sandwich composites with balsa core provide similar findings and attribute it to the microstructural nature of the core [3,31–33]. Detailed stress analysis of balsa core under impact loading provided elsewhere suggests that due to the natural composition of the balsa wood, the core damage remains highly localized, which is consistent with the current observations [37]. Matrix damage could also be found in the vicinity of the fractured areas, but it is unclear if the damage was generated before or after fiber failure. 3.4. Post impact NDE

3rd Impact 600

400

200

0

0

3

6

9

12

15

Absorbed Energy (J) Fig. 14. Measured damaged area on the backsheet per impact location in relation to the energy absorbed.

1000

Damage Area (mm2)

167

800

600

400

1st Impact 200

0

2nd Impact 3rd Impact 0

10

20

30

40

50

60

70

Kinetic Energy (J) Fig. 15. Measured damaged area on the backsheet per impact location in relation to the kinetic energy of the projectile.

Beyond superficial assessment, visual observations could not provide the extent of damage. A host of damage detection techniques, such as, acoustic emission, ultrasound, thermal imaging and strain gages, have been implemented to evaluate sandwich composites, however, each technique has limitations and sheds light on only some specific aspect(s) of damage, as detailed elsewhere [36–38]. The current analysis was devoted to the use of Tap Testing which has recently shown promise in damage detection in sandwich composites [4,9,11]. However, just as with most of the other damage detection techniques, tap testing is sensitive to operator training and requires careful calibration, pre-testing and user care to obtain meaningful results. The tap testing results were recorded in a manner shown in Fig. 16, following the format provided in Fig. 6. At locations where data could not be collected because of the presence of a perforation the values were averaged from the vicinity of that point. From the strike-side of each panel, the shots were numbered clockwise as indicated in Fig. 3, whereas, from the back-side the impacts were oriented in a counterclockwise order. Also the backsheet grid was localized at the impact site, contrary to the facesheet, which covered the entire impact area (Fig. 6); meaning that the front and back grids cannot directly overlap. The pulse width from tap testing was observed to be 300 and 400 ls for the facesheet and 400 and 600 ls for the backsheet, for the single and sequential impacts, respectively. The mapped results of the facesheets suggest that there is a clear localized zone of damage, although in some instances it was found not to coincide with the impact location or visual inspections. The results indicated that the area around the impact location suffered more damage on the backsheet as compared to the strike side facesheet. This was expected considering the observations of the visual inspection. The ‘Tap Test’ results also suggested that damage on the strike side

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1

4 436 459 492 559 498 546

6

5 390 458 412 399 426 425

6 306 382 396 380 415 374

F 1

4 475 450 524 523

3 519 727 819 717

4 489 555 792 648

3 608 911 658 541

4 456 515 543 478

B C

2 481 558 590 476

References

D A

1 398 447 451 435

4

C

3 673 666 700 611

Backsheet 2nd shot. A B C D

3

B

A B C D

2 453 601 725 737

2

D C

2 551 707 766 535

B

1 425 516 609 515

A

Backsheet 3rd shot. A B C D

The authors acknowledge the support and guidance of Dr. Yapa Rajapakse, the ONR program Manager. The research was conducted under ONR grant #N000140611043. Acknowledgements are also due to Dr. Frederick Just and Dr. David Serrano for facilitating the work.

A

Backsheet 1st shot. 1 460 505 483 472

inspection a promising non-destructive ‘Tap Test’ provided a reasonable quantitative comparison of the extent of damage. Acknowledgements

E

3 477 472 500 548 491 503

5

D

2 431 442 427 418 444 429

4

C

1 427 373 409 427 436 357

3

B

A B C D E F

2

A

Facesheet all three shots.

D Fig. 16. Typical triple impact results of the facesheet and backsheet from the tap testing. Colors scheme and the accompanying tables represents the response time in ls (or the level of damage present) of the mean values of the 5 tap tests on the same sample and location. For comparison of the visual inspection and tap tests, see Fig. 12.

facesheet or backsheet was highly localized and may not have spanned more than 1.3 cm from the location of impact. This technique provided a useful methodology for damage assessment which otherwise remains hidden to the optical observations. However, the main drawback of this technique is the extreme caution required to produce consistent data. The tests could be useful for a qualitative/comparative assessment of the presence and extent of damage.

4. Conclusions Experimental results for the damage induced by single and multiple sequential impacts on balsa core sandwich composites were presented and discussed. The results indicated a ballistic limit of 96 m s1. Thus, the material could potentially be implemented for shielding against secondary debris from blast, hurricane, tornado and foreign objects on roads and runways. The effect of prior damage in terms of residual and absorbed energies became more pronounced as the number of impacts increased. However, this trend was not consistent with the damage areas or tap testing results; and was obscured by the large scatter observed. However, it appears that prior impact in this class of material may have only a limited influence on the global impact response, which is a significant outcome of this research. Apart from a post impact visual

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