The low-velocity impact and compression after impact (CAI) behavior of foam core sandwich panels with shape memory alloy hybrid face-sheets

Abstract Due to the properties of shape memory effect and super-elasticity, shape memory alloy (SMA) is added into glass fiber reinforced polymer (GFRP) face-sheets of foam core sandwich panels to improve the impact resistence performance by many researchers. This paper tries to discuss the failure mechanism of sandwich panels with GF/ epoxy face-sheets embedded with SMA wires and conventional 304 SS wire nets under low-velocity impact and compression after impact (CAI) tests. The histories of contact force, absorbed energy and deflection during the impact process are obtained by experiment. Besides, the failure modes of sandwich panels with different ply modes are compared by visual inspection and scanning electron microscopy (SEM). CAI tests are conducted with the help of digital image correlation (DIC) technology. Based on the results, the sandwich panels embedded with SMA wires can absorb more impact energy, and show relatively excellent CAI performance. This is because the SMA wires can absorb and transmit the energy to the outer region of GFRP face-sheet due to the super-elasticity-behavior. The failure process and mechanism of the CAI test is also discussed.


Introduction
Due to the high physical and mechanical properties in term of high specific strength and stiffness, sandwich panels are being widely applied in various branches of engineering fields, including leading applications such as the aircraft, civil engineering and railway industries [1][2][3][4][5][6]. The sandwich panels with carbon fiber reinforced plastic (CFRP) face-sheets exhibited better mechanical performance lower weight compared to those with metal facesheets [7][8][9]. However, because of the poor toughness of the fiber reinforced plastic face-sheets, the face-sheets can be easily perforated due to the sandwich structures is sensitive to impact loading. Meanwhile, barely visible damage after low energy impact could lead to a significant reduction to the structural load bearing capability. The compressive strength of the sandwich structure can be reduced up to 50% after an impact event [10][11][12]. Although the impact damage is nearly invisible, such damage could lead to sharp reduction to the structural load-bearing capability, possibly causing catastrophic failure of the sandwich structure. Thus, the study on the low-velocity impact failure mechanism and CAI behavior of sandwich panels is deserved more attention. It is essential to study the damage mode of the sandwich structure with composite facesheets, and it is necessary to investigate the compression after impact (CAI) behavior of sandwich composite panels.
Based on previous research, the residual after impact has been adopted to evaluate the impact resistance of sandwich structures with different facesheets and core materials [12][13][14][15]. Two types of sandwich panels which are based on hybrid cores consisting of polymer resin reinforced by metallic millitubes have been developed and characterize by Ji [11]. The damage mechanism of sandwich panels with thin aluminum skins and a Nomex™honeycomb core under impact and CAI have been presented by Gilioli [13]. Giannopoulos [14] presented the interrogation of low-velocity impact and compression after impact test results on a woven fiber composite having a fire retardant, syntactic core, two phase epoxy matrix. The main failure modes of sandwich panels under impact load including face-sheet failure, matrix/core crush and interface delamination. It is evident that the face-sheet play an important role on the performance of sandwich panels under the impact load. Moreover, tough face-sheet could reduce the level of destruction in the internal structure including core material and interface delamination.
In order to improve the impact resistence performance of composite face-sheet, researchers have carried out some works. Research indicates that carbon fiber has excellent mechanical properties, but a relatively poor impact resistence performance compared to the glass/aramid fiber. This is due to the rigidity and fragility of the carbon fiber [16]. The influence of the matrix properties, type of fiber and fiber orientation on low-velocity impact behavior of the fabric reinforced hybrid composites with stratified filled epoxy matrix was investigated [17]. Reference [18] reported that high angle between neighboring layer increased the delamination area under the low-velocity impact. Liu et al. [19] designed a new type of hybrid laminate with five layers of woven carbon and unidirectional carbon fibers. In general, there are two main kinds of methods to improve the impact resistence performance of composite face-sheet of sandwich, hybrid reinforced fiber and lay-up designs. However, the low impact resistance is hard to improve fundamentally due to the fragility and rigidity of matrix and common fiber.
Compared to fiber reinforced plastic, metal has an excellent low-velocity impact performance due to its high ductility. Aluminium was added into fiber reinforced plastic to improve the performance of impact in aerospace field [20]. Moreover, steel wire nets are employed to improve the impact resistence performance of concrete slab [21][22][23]. In the civil engineering, steel wire nets are widely used in concrete-shotcrete. It is reported that [21] the use of fiber reinforcement in concrete/shotcrete can greatly enhance the punching shear capacity, flexural ductility and toughness. Reference [22] shows that the use of high-performance polypropylene fiber reinforcement in shotcrete, with sufficient fiber content, can greatly improve flexural ductility, toughness, and ultimate load capacity. Therefore, it can be used with steel mesh and steel fiber reinforcement, especially in tunnel applications. The experimental and numerical works conducted by Liu et al. [23] indicate that the steel wire nets enhance the toughness of concrete. Therefore, the metal wire nets can be a candidate to improve the impact resistence performance of conventional fiber reinforced plastic. Because of the properties of high failure strain, superelastic and shape recov-erable, shape memory alloy (SMA) has been added into the composite laminates to improve the low-velocity performance in many papers [24][25][26][27]. The impact response of composite laminates embedded with SMA wires in the thermal environment was investigated by Shariyat [24]. The effect of embedding SMA wires on the impact behavior of composite beam under different mass and initial incident energies was studied by Khalili [25] studied into the laminated composite beam on the impact behavior of the beam with different masses and initial incident energy. Eslami-Farsani [26] investigated the high-velocity impact behavior of composite laminates enhanced with SMA wires. In their works, the influences of volume fraction and pre-strain of SMA wires on the impact responses of composite laminates was investigated. However, based on the authors' knowledge, there is less attention paid to the advantages of SMA wires compared to the convention 304 SS wires in improving the impact resistance of composite laminates.
In this paper, low-velocity impact and CAI experiments are conducted to analyze the impact resistence performance of foam core sandwich panels with SMA hybrid GF/epoxy face-sheets which are designed with different orientations and numbers of SMA wires. In order to present the advantage of the sandwich panel hybrid with SMA wires, similar sandwich panels embedded with 304 SS wire nets are fabricated and the impact resistance is compared with the sandwich panels with SMA wires. The Instron/9250 HV low-velocity impact device is employed and the impact responses including contact force-time, absorbed-time and defection-time curves are recorded. Moreover, macro and micro failure morphology of sandwich panels with different ply modes are presented and compared. Digital image correlation (DIC) is used to assist to analyze the failure mechanism of designed sandwich panels during CAI tests. Finally, the advantage of SMA enhanced sandwich panels in CAI tests are presented and some conclusions explained this results are drawn.

Experimental details 2.1 Materials and manufacturing
As shown in Figure 1, the test specimens are fabricated by vacuum assisted resin infusion (VARI). Vinyl ester resin which can be cured at room temperature mixed with hardening and accelerating agent was employed as the matrix. The main of hardening and accelerating agents were Methyl Ethyl Ketone Peroxide (MEKP) and Dimethylani-

Low-velocity drop weight impact test
Instron /9250 HV low-velocity impact device is employed to carry out the drop-weight experiment at room temperature. As shown in Figure 1, the specimens with dimension of 100 mm × 100 mm were clamped by pneumatic press fixture. A circle area with a radius of 38.1 mm was located at the central area of the specimens. A hemispherical impactor with weight of 17.2 kg and a diameter of 12.7 mm was adopted in all tests. During the impact process, a load cell located at the above of the impactor bar was used to measure the contact force. The work of data storage and analysis was completed with the help of computer and software. Then, the impact history of absorbed energy and deflection were presented as well. In this paper, all type of specimens were tested at the incident energy of 14 J, 18 J and 22 J, respectively. For each type of ply codes under the same incident energy, the experiment was repeated three times.

Compression test after low-velocity impact
CAI tests are conducted to evaluate residual performance of the impacted sandwich panels with different ply codes based on ASTM D7137 standard [29]. This is because of the foam core/facesheet delamination and the facesheet failure induced by low velocity impact can make a significant reduction on the compressive strength of sandwich panels [30,31]. The set-up of the compressive test is shown in Figure 2. During the compressive test, the speed of 0.25mm/min was set to the load cross beam. Meanwhile, a 2-D digital image correlation (DIC) system was build up to measure and record the strain of the face-sheet. As shown in Figure 3, the DIC system is consist of specimen marked with white base and blackspot on one face-sheet, highresolution camera, spotlight and computer with analysis software.

Characterization technique
In order to analyze the failure mechanism of the sandwich panels with different ply code, visual inspection and scanning electron microscopy (SEM, Hitachi S-4300, Tokyo, Japan) were employed to observe failure modes such as fiber broken, matrix crush and delamination. Damage region including SMA wire is our main concern. This is because the performance of interface region could indicate the enhancement mechanism of the new type of hybrid sandwich panels.

Results and discussions
This study focuses on the impact response of sandwich panels consisting of seven different modes of face-sheets. Impact energy were chosen as 14, 18 and 22J.

Low-velocity impact response history of the designed hybrid sandwich panels
In Figure 4-6, contact force-time curves of different configurations for 14, 18 and 22J impact energies are presented, respectively. From the figures, it can be seen that contact force history of each ply mode shows a similar process under different incident energies. Figure 4 reveals that the maximum contact force increases with the adding of SMA in the hybrid composites. For example, the maximum contact force of ply I and VI under the incident energy of 14 J is 1.97 kN and 2.76 kN, respectively. And, for incident energies of 14J, 18J and 22J, the maximum contact force of ply VI shows an improvement of 40.1%, 37.7% and 27%, relative to ply I, respectively. For ply I, when the contact force is up to the maximum, it decreases sharply and the plates lose the load bearing capacity. Though second peak contact force was obtained soon, the value of second peak contact force increased sharply. However, two peak values of the contact force are observed in the contact force-time curves of the hybrid composites. This phenomenon indicates that hybrid sandwich panels can absorb considerable impact  energy sustainably after the contact force reaching the peak value. Thus, the impact resistance performance is improved by inserting SMA wires into the GFRP. This is because of the contact force of hybrid sandwich panels keeps a large level for a longer time than the sandwich with GFRP face-sheets. conventional laminate, the similar peak value of the contact force of Ply VI and S2 increases signally with the help of SMA and 304 SS wire net. Secondly, for Ply I impacted under the incident energy of 18 J, the contact force reaches the peak value twice in 22.3 ms. However, for Ply VI and Ply S2, the sandwich with hybrid face-sheet, two peak values of contact force appear during a shorter time interval of 2.55 ms and 0.29 ms. It indicates that the Ply VI sandwich can keep high value of contact force for a longer time than Ply S2. This is attribute to the superelastic property of SMA. Thus, Ply VI sandwich can absorb more impact energy. As shown in Figure 7, there are two peak forces shown in the contact force-time curve of Ply I. It is due to the impactor penetrates the upper face-sheet and resulting in damages the bottom face-sheet as well. However, for Ply VI and Ply S2, two peak forces are cause by the failure of GFRP and SMA/ 304 SS with relative high ductility. Moreover, in terms of the rebound energy of Ply VI and S2, the impactor rebounds before the impactor penetrates the upper face-sheet.

Failure mechanism of different types of sandwich panels under low-velocity impact with various incident energies
The impact damage morphology of different types of hybrid sandwich panels are presented in Figure 8. In general, the impact damage shows a circle shape in both two sides of the sandwich panels. With the addition of SMA and 304 SS wire net, there is less damage area on the front and rear face-sheet under the same incident energy. As shown in Figure 8, a yellow circle with the same diameter as the impactor separates the damage and intact region. And the failure region for both upper and bottom surfaces of ply code I is concentrated in a circular region, besides, the di- ameter of upper damage region is approximately equal to the size of the impactor for the impact energy of 22J. But for Ply II-VI and Ply S2, there is significant incomplete damaged region located between the central failure region and the outside intact face-sheet. Moreover, there is more incomplete damaged region on the impact faces of Ply II-VI relative to Ply S2. This phenomenon indicates that more face-sheet region is damaged and more impact energy is dissipated with the addition of SMA and 304 SS wire net. This is because of the high elongation of 304 SS and SMA absorbs and transmit the impact energy from the center to the outer region of face-sheets. The typical cross sections of low-velocity impact cases is shown in Figure 9. Shipsha's results [32] indicated that the typical damage morphology for foam-core sandwich panels under low-velocity impact mainly includes delamination at the interface, broken fiber, partial crushed core located around the impactor and the permanent inden-tation on the upper face-sheet. All four types of failure modes can be seen clearly for each Ply code in Figure 9. For Ply I, with increasing of the incident energy, there is no significant difference in the damage of front face-sheet among the cases with different incident energy, and, the foam core crushed and expands to the rear face-sheet. However, for Ply II-VI, the foam core and bottom face-sheet are both protected better and show no large difference among different Ply codes relative to Ply I. Besides, with the increasing of incident energies, the degree and scale of damage for upper face-sheet become worse. This phenomenon indicates that the face-sheet embedded with SMA can absorb more impact energy due to hyperelastic behavior. In terms of the orientation and number of embedded SMA, the fracture morphology of Ply III-V, enhanced with more numbers of SMA wire layers, is similar. Generally, the face-sheet enhanced with more layer number of SMA could be more intact under the same incident energy. Under the impact energy of 18 J, typical cross-section fracture morphology of Ply I, VI and S2, the GFRP facesheets hybrid with nothing, two layers orthogonality SMA wire and 304 SS wire nets, are compared in Figure 10. For Ply I, the central region of upper face-sheet was completely destroyed during the impact event. But for Ply S2, there is an obviously permanent indentation in the central region, and the fiber and matrix are relatively intact. This is due to the 304 SS wire nets absorbing more impact energy with the property of high elongation. Besides, the fiber breaking and the plastic deformation of 304 SS wire nets lead to the central permanent indentation. However, for Ply VI, the impact of permanent indentation shows not obviously. Moreover, there is more crushed core located around the impact central region and a continuous gap between the upper face-sheet and the crushed core can be observed. Thus, during the low-velocity process, the characteristic of hyperelastic and shape memory of SMA resulted in more core was compress crushed and the deformation recovery, and then, created the continuous gap.
Typical microstructure damage aroud the impact region of the SMA hybrid sandwich panels is shown in Figure 11 and the damage morphology of the SMA hybrid facesheet can be identified. As discussed in the reference [32], there are three primary categories, included matrix crushing, fiber breaking and delaminated at the interface between the SMA and matrix. However, the SMA wire near the crushed and delaminated matrix is found relatively intact and protect the left side face-sheet very well. The facesheet of the SMA hybrid sandwich panel is divided into the crushed and intact parts at the line of SMA wire location. According to the failure shown in SEM figure, the crack and crushed matrix extend along with the interface between SMA wire and matrix. Thus, the embedded SMA wire pre-

CAI behavior of different types model of sandwich panels
In the incident energy range from 18J-22J, penetrate and unpenetrated cases were both found in the low-velocity impact test. And, the effect of low-velocity impact on the compressive behavior could presented more clearly. The stressstrain curves of different CAI tests for various impact energies of 0 J, 18 J and 22 J, respectively, are shown in Figure 12.
Experimental results show that all the CAI stress firstly increases linearly with strain, and then increases gently. The failure strain and stress of different sandwich panels in CAI tests are summarized in Table 2. For both damaged and undamaged sandwich panels, the failure stress of Ply-VI panels is maximum. Compared with reference Ply I, the compressive strength of Ply-VI undamaged, impact energy of 18 J and 22J increase by up to 3.43, 13.36 and 2.45 times. Based on the results, sandwich panels with two layers of cross SMA wires has better CAI performance. Moreover, for Ply-III, IV and V, they all have two layers of unidirectional SMA wires, and perform better than Ply-I & II in CAI performance. Besides, the Ply-III & IV panels reach higher ultimate compressive stress. Based on the transverse fracture morphology shown in Figure 9, this is due to a lower level of damage for Ply III & IV under the same incident energy. The linear fitting curves of CAI strength versus impact energy for different sandwich panels are shown in Figure 13. As seen in the figure, the compressive strength of all cases decreases with the increasing of impact energy. According to the rate of decrease compressive strength from quickly to slowly, the order is Ply-II-VI, Ply-S2 and Ply-I successively. Thus, the compressive strength of sandwich panels embedded with SMA wires is sensitive to the lowvelocity impact load. Combined with the typical failure modes shown in Figure 10, this result is caused by the relatively wider matrix crush in the upper face-sheets. When the impact energy increases greater than 18 J, the CAI result of Ply-I shows a close value, which is because that the upper and bottom face-sheet of sandwich panels are pen-     with SMA or 304 SS wire fails with obvious delamination. Generally, delamination formation occurs at the interface between the form core and impaired face-sheet during the CAI event. Besides, as the propagation of interface delamination, the sandwich panels suffers irreversible damage and lose the carrying capacity of compression. This is because there is an already existing damage on the face-sheet and interface which are unstable. During the CAI process, the sandwich panels loses the carrying capacity of compression completely when the delamination reaches any side of the loading plate. DIC failure modes of the different sandwich are shown in Figure 16 & 17. As seen in the figures, all maximal strain along the load direction of the damage sandwich panels is located at the central region. It indicates that the low-velocity impact on the sandwich panels leads to the face-sheet week and this region fails firstly during the CAI test.

Conclusion
This study focuses on the low-velocity impact and compression-after-impact (CAI) performance of foam core sandwich panels incorporated with shape memory alloy (SMA) or 304 SS wires. The comparison of failure modes is also presented and discussed. Some conclusions can be drawn as follows: (a) After inserting superelastic SMA into the GFRP, the impact resistance performance can be improved up to 40.1%. The contact force of hybrid sandwich keeps a large level for a longer time than the sandwich panel with GFRP face-sheets and the sandwich panels embedded with 304 SS wire. Moreover, the sandwich panels embedded with SMA wires of cross configuration could dissipate more impact energy. (b) Due to the property of super-elasticity, the addition of SMA wires can dissipate and transfer the kinetic energy of impactor to the outer region of the upper face-sheet, which leads to a better impact resistance performance of the sandwich panels embedded with SMA. (c) In terms of both damaged and undamaged sandwich panels, the failure stress of Ply-VI panels showed maximum value. Compared with the reference Ply I, the compressive strength after impact increases up to 3.43, 13.36 and 2.45 times for the Ply-VI sandwich panels impacted by 0J, 18J and 22J, respectively. (d) During CAI test, the delamination of the sandwich panels between the foam core and the face-sheet first occur at the impact region. With the propagation of interface delamination, the sandwich panels suffer irreversible damage and lose the carrying capacity of compression. Besides, the DIC results show that the maximum strain is located at the damaged region of the face-sheet.