Damage characterization of sandwich composites subjected to impact loading

Abstract

Advanced composite materials are usually optimized to achieve balance of properties for given range of applications. In recent times, researchers had worked on the sandwich composites by using different foam and metal honeycomb as a core material. In the current project, honeycomb core is prepared by using 3D printed technology. In this case of sandwich composites, cross-linked polyethylene foam and 3D-printed polylactic acid honeycomb as core and GFRP is used as face sheet. The comparison is made between polyethylene foam and 3D printed honeycomb core sandwich composite in the aspect of toughness, strength, and modulus. The present study is to characterize the damages in the sandwich structure for the amount of energy absorbed by the structures such as delamination, indentation, crushing of foams, and debonding of face sheets and core material subjected to free fall impact. The contact force versus time, contact force versus deflection of plates with respect to impact energy levels of 9.3, 16.5, and 25.7 J and impact energy versus time are determined. The current research helps in determination of core materials effecting/absorbing the damage and behavior of sandwich materials subjected to impact loads.

Keywords

3D printed honeycomb core impact sandwich composite damage

Introduction

Sandwich composite structures bonded with a thick core but less dense material between two strong but thin face sheet materials, structurally, in general, it represents an I-section. In these sandwich composite structures, face sheets are made by composite materials like glass fibers, carbon fibers, and Kevlar; Core materials used are sponge, foam, and honeycomb etc. Various research works have done on sandwich composite structures like impact, blasting, bending, and vibrational tests. Park et al. [1] impact response of sandwich composites was highly affected on the thickness of core and face sheet material. Tan and Akil [2] conducted drop weight impact test on polypropylene core sandwich composites with fiber metal laminates and concluded that severe damages observed by increasing the energy absorption in the specimen. Ji and Waas [3] explained that the impact damage of the sandwich specimen starts after the impactor reaches the maximum contact forces. Muzzy et al. [4] conducted drop weight impact test on sandwich composites with different core material. Impact behavior of polypropylene honeycomb, foam, and paper core sandwich specimens were not conclusive. He and Hu [5] studied on honeycomb sandwich structure to design it for achieve optimum mechanical properties and concluded that the satisfying weight of honeycomb core is 50%–66.7% of the total weight of honeycomb sandwich structure. Namala et al. [6] conducted low-velocity impact test on unidirectional and bidirectional E-glass/epoxy laminates and concluded that damages were occurred due to matrix cracking, delamination, and debonding of the fibers, and there was no fiber damage in laminates during the impact. Dogan and Arikan [7] conducted a low-velocity impact test on glass fiber-reinforced thermoset and thermoplastic sandwich composites with PVC foam core material and concluded that the thermoset sandwich composites have more stiffness than the thermoplastic sandwich composite specimens, and thermoplastic composites have high load carrying capacity. Ude et al. [8] conducted drop weight impact test on foam, core mat, and honeycomb core sandwich with natural silk/epoxy face sheet. Among all the specimens foam core sandwich specimens absorbed more energy. Russell et al. [9] conducted an impact test on a square-honeycomb core with carbon fiber face sheets sandwich composite, to study the impact behavior of the structure and concluded that, there was no visible damage at impulse of the impactor below 1.7 kPa s, delamination was initiated at impulse level above 2.7 kPa s, and total failure of the structure observed at impulse level above 3.18 kPa s. Ma et al. [10] conducted drop weight impact test on sandwich composite panels as per ASTM D 7136 and explained the damage formation with help of the graph during the impact. Constantin et al. [11] conducted low-velocity impact on both foam and honeycomb core sandwich composites by using thick and thin glass fiber reinforced polymer (GFRP) face sheets and concluded that impact forces for the honeycomb core sandwich specimens depends on the position of the impact in honeycomb-like cell wall or center of the cell. Meo et al. [12] investigated load distribution, and failure mechanism of sandwich honeycomb panels represents the fan cowl door during the impact and understand that damage area and dent depths are depends on the position and size of the impactor. Zhu and Chai [13] explained the types of damage mode maps for the sandwich panels during the low-velocity impact and concluded that damage initiation was independent of the boundary condition and stacking sequence of the sandwich panels. Atas and Sevim [14] investigated the impact behavior of sandwich panels with glass fiber by using balsa wood and PVC foam core. The damage behavior explained by using load versus deflection graphs and the damage formed as fiber breakage of the skin, delamination, and debonding of the core from the face sheets. Gustin et al. [15] identified the improvements in impact resistance of the sandwich structure by replacing the top face sheets with varying number of layers of carbon, Kevlar or hybrid fibers. The failure modes were shear and tensile failure observed. Low-velocity impact test on sandwich panels by using aluminum foam and concluded that aluminum foam cells crushing, pulverization of cells, and the face sheets were damaged by localized shear fracture [16, 17]. Impact test on Rohacell foam sandwich specimens and concluded that the modes of damages were matrix cracking, delamination, and yielding [18, 19]. Indentation behavior of sandwich panels subjected to impact loading studied by Hachemane et al. [20] and Rizov et al. [21]. Anderson and Madenci [22] investigated the impact behavior of sandwich composite by conducting low-velocity impact test, graphite/epoxy face sheet by varying different thickness, foam and honeycomb core taken as core material by varying the densities and concluded foam and honeycomb core sandwich specimens formed significant damage for residual indentation of 0.13 mm and 0.025 mm, respectively.

In this study, the sandwich composites cross-linked polyethylene foam and 3D printed polylactic acid (PLA) plastic honeycomb as core and GFRP used as face sheet. The comparison is made between polyethylene foam and 3D printed honeycomb core sandwich composite in the aspect of stiffness, strength, and modulus at impact energies 9.3, 16.5, and 25.7 J. The present study is to characterize the damages in the sandwich structure for the amount of energy absorbed by the structures for the damages such as delamination, crushing of foams, and debonding of face sheets and core material. The impact energy versus time, contact force versus time contact force versus deflection, and deflection versus time are determined.

Materials and methods

In this research, two types of sandwich composites are used with different core material with same face sheets. Bidirectional E-glass fiber is used for reinforcement material and epoxy as matrix material in the face sheets. Matrix is the combination of Lapox-C51 resin and curing agent (Hardener) K6 in the ratio of 10:1 by volume. Compression molding with hand layup method is used for preparation of face sheet with dimensions 175 mm × 175 mm × 2 mm. In the preparation of face sheets, 16 layers are one over the other by applying the resin between the layers and then compress the layers in the compression mold and then cured 8–10 h in the elevated temperature.

Two types of core materials one is polyethylene foam and other is 3D printed PLA honeycomb are used. For the preparation of 3D printed honeycomb layer manufacturing or 3D printing technology is used. For this first prepared honeycomb model in the CATIA software with the dimensions of 175 mm × 175 mm × 6 mm. This model converted in to .stl file format. This .stl format is the input for the 3D printing machine. MAKERBOT 3D printing machine and its software are used for printing of honeycomb. This machine taken .stl format as input and slices it in to number of layers and starts printing layer by layer. It has taken 6 h 30 min for completion of the total honeycomb mesh.

After preparation of honeycomb, it is bonded between the two face sheet plates. The final dimensions of foam and 3D printed honeycomb core sandwich composite specimens are 175 mm × 175 mm × 10 mm.

Tensile test

Tensile strength of foam core sandwich composite specimens tested according to ASTM C279. The dimension of specimen is 250 mm × 30 mm and the crosshead speed of the UTM during the test is 2 mm/min. The test results such as, load at yield point, yield stress, peak load, and tensile strength of the specimen are shown in Table 1.

Table 1.

Tensile test results.

Specimen no.	Thickness (mm)	Width (mm)	Load at yield point (kN)	Yield stress (MPa)	Peak load (kN)	Tensile strength (MPa)
1	12.58	31.38	40.47	102.51	46.71	118.32
2	12.36	30.8	39.93	104.88	45.09	117.03
3	12.45	32.69	43.02	105.70	48.58	119.36

Izod test

Izod test is conducted on the sandwich composite specimen with the dimension 75 mm × 10 mm. This test carried out on six specimens, three along the width direction and three along the thickness direction and amount energy absorbed by the specimens are shown in Tables 2 and 3.

Table 2.

Izod test results along the thickness direction.

Specimen no.	Thickness (mm)	Energy absorption (J)
1	12.26	17
2	12.26	16
3	12.26	16

Table 3.

Izod test results along the width direction.

Specimen no.	Width (mm)	Energy absorption (J)
1	10	15
2	10	16
3	10	15

Drop weight impact testing

An instrumented drop weight impact test machine is used to find the damage resistance of sandwich composite specimens during the impact according to ASTM D 7136. It is a low-velocity impact machine. After conducting this test, two types of damages appear, one is clearly visible impact damage and other is hardly visible impact damage, it can be evaluated by post-impact test like compression after impact test. Dynamic load cell or tup is attached with the impactor. It contains a strain gauge that evaluates the strains with respect to time when the impactor hits the sample. Data acquisition system is used to capture data for impact from the strain gauge.

This impact test machine actuated with pneumatic system by using this system 2.058 kg hemispherical impactor raised to certain height and compressed under the spring forces and released with different velocities such as 3, 4, and 5 m/s. A velocity flag and gate is fixed at zero position to determine the velocity of the impactor V= $\sqrt{2 g h}$ , when it touch the specimen before the conduct of experiment, where V = velocity of the impactor in (m/s), h = height of the impactor from the specimen in (m) and g = acceleration due to gravity in (m/s²).

Test setup

In this test, flat sandwich composite specimens with the dimensions 175 mm × 175 mm for foam and 3D printed honeycomb core are used. These sandwich specimens fix in the machine and tighten all the clamps along its edges. As the clamping is over the impactor raised to certain height and the impactor is locked into the place. The drop weight impact test machine set up as shown in Figure 1.

The test machine set into the correct configuration and all data acquisition software is running, the impactor is unlocked and allows to impact the specimen. Software loaded the strain values which are measured by tup, the data received from this experiment is to examined to see the resistance of the impact damage of the specimen. The graphs plotted as force, energy, and displacement versus time.

Results and discussions

In the current research, low-velocity impact tests are carried on 3D printed honeycomb and foam core sandwich composite specimens at 9.3, 16.5, and 25.7 J of energies. At each amount of energy, the sandwich specimens are tested. From the test results, characteristics of damage are compared between foam and honeycomb sandwich specimens. The behavior of damage in the specimens with respect to impactor is explained with the help of contact force versus time, energy versus time histories, contact force versus deflection, and deflection versus time.

Amount of energy absorption with respect to time

Initially, the impactor attains maximum energy during the first contact of the impactor. After reaching the maximum energy the impactor hit, fraction of the energy is transferred to the specimen from the impactor, fraction of energy is retained to rebound the impactor, and marginal rate of energy is lost in the friction.

Energy transferred to specimen is classified into elastic strain energy and plastic strain energy. The elastic strain energy retains the position of the plate, the plastic energy is the damage caused in the specimen. Hence, the drop in energy versus histories helps in understanding the energy absorbed by the plate. This amount energy absorption indicates the damage in the specimen.

The energy versus time histories of foam and honeycomb core sandwich composite specimens is shown in Figure 2. At all energy levels, foam core sandwich specimens (F C S specimen) follows similar path. At each energy level, time taken for maximum amount of energy absorption is 8 and 4.8 ms for foam and honeycomb core sandwich specimens (H C S specimen), respectively. Maximum amount of energy absorption for foam core sandwich specimens are 4, 8, and 20 J at energy levels 9.3, 16.5, and 25.7 J, respectively and for honeycomb core specimens are 7, 9, and 20 J at energy level 9.3, 16.5, and 25.7 J, respectively. At 8.3, 17.3, and 25.7 J of energies significant, moderate, and most visible damages were found [22].

At lower energy levels amount of energy absorption honeycomb core sandwich specimens is more in less time compared to foam core sandwich specimens, it indicates that the damage formed in the top face sheet of honeycomb sandwich specimens is more than the foam core sandwich specimen. In honeycomb sandwich specimens, impact energy absorption was depending on the position of the impact honeycombs, like cell wall and center of the cell [11]. At higher energy level, 25.7 J on an average same amount of energy absorbed by the all the specimens, this indicates that damage occurred in face sheet also in core material during the impact.

Contact force with respect to time

Contact force versus time at energy 9.3, 16.5, and 25.7 J for both foam and honeycomb core sandwich specimens are shown in Figure 3. The time taken to reach the maximum contact forces of the foam core sandwich specimen is higher than the honeycomb core sandwich specimen. By increasing the energy levels, the maximum contact force increased, this indicates that by increasing the energy level of the impactor the damage in the sandwich specimens also increased.

At lower energy levels 9.3 and 16.5 J smooth unloading of the curve obtained after the impactor reaches the maximum contact forces, it clarifies that the damage occurred only in the top face sheet. At higher energy level 25.7 J irregular path is formed in both foam and honeycomb core sandwich specimens. Similar behavior graphs and the maximum contact forces are almost correlated with same impact energy levels [22]. It represents that the indenter causes damage in the core material, at this stage, the damages are like core crushing, debonding of face sheet and core material and delamination of fibers in the face sheets are formed. The impact behavior of honeycomb sandwich composites with respect to damage area and pattern, as observed from the images from Figure 4. The dent is high for foam core sandwich composites than the honeycomb sandwich composites for the same impact test. Damages of the specimens after the impact test as shown in Figure 4.

At all energy levels honeycomb core sandwich specimen reaches high contact forces with less time compare to foam core sandwich composite specimen, it indicates that 3D printed honeycomb core sandwich composites having high load carrying capacity than compared to foam core sandwich composite specimens.

Contact force versus deflection curves

The stiffness of composite specimens is observed from the contact force versus deflection graphs. The contact force versus deflection histories at different energy levels for both foam and honeycomb core sandwich composites as shown in Figure 5. At every energy level, foam sandwich composites form same pattern compared to honeycomb sandwich composites.

Deflection of the specimens increased with increasing energy level of the impactor. At 9.3 and 16.5 J sudden load drop occurred in the impactor after reaching the maximum contact forces, this indicates that damage occurred only in the face sheet material. The deflections of the specimens are 1 and 2 mm at energy levels 9.3 and 16.5 J of the impactor, respectively. The damages occurred for foam and honeycomb sandwich specimens having similar type damages [22].

At 25.7 J, all specimens, there is a decrease in contact force after impactor reach the maximum contact force, and before the sudden drop in contact force on the impactor, it indicates that damage occurred in face sheet and core material. The deflection of the honeycomb core sandwich specimen less deflection compare to foam core sandwich specimen, it represents that honeycomb core sandwich specimen having high stiffness compared to foam core sandwich specimens. The dent depth for foam core sandwich specimens are 1.44, 1.76, and 4.8 mm and for honeycomb sandwich specimens are 1.17, 1.25, and 3 mm at 9.3, 16.5, and 25.7 J, respectively. The dent depth is not comparable due to different foam and honeycomb materials [22].

Area formed by graphs in contact force versus deflection curves is increased by increasing the energy levels of the impactor. The area of the foam core sandwich composite specimens is 70, 150, and 310 N-mm and for honeycomb core sandwich specimens are 130, 200, and 320 N-mm at the energies 9.3, 16.5, and 25.7 J of the impactor. The damage is observed in core of honeycomb sandwich structures is crushed under the point of impact, as observed for impact energy of 25.7 J. Figure 6 shows the damage of the 3D printed honeycomb core at 25.7 J of impact energy level. This damage is observed from backside after removing the bottom face sheet.

At lower energy levels area of honeycomb sandwich specimens is higher than the foam core sandwich specimens, it represents that honeycomb core sandwich specimens absorbed high energies than foam core specimens. At higher energies all specimens have proximately same area, it represents that all specimens absorbed same amount of energy.

Deflection with respect to time

Deflection versus time histories of both foam and honeycomb core sandwich composite specimen at 9.3, 16.5, and 25.7 J as shown in Figure 7. Deflection of the specimens increases with increasing energy levels of the impactor. At all energy levels, maximum deflection is at 4 and 2.8 ms for foam and honeycomb core sandwich specimens. In all the cases, honeycomb core specimens having less deflection with less time compare to foam core sandwich specimens, it indicates that honeycomb sandwich having high stiffness at all energy levels of the impactor.

Tensile test and Izod test

The average tensile strength of the sandwich composite specimen with the dimensions 250 mm × 30 mm is 118.23 ± 1.16 N/mm². In Izod test, the amount of energy absorbed by the specimens along the thickness and width directions are same, the average amount of energy absorbed by the specimens along the thickness and width directions are 16.33 ± 0.57 and 15.33 ± 0.57 J.

Conclusions

Foam core sandwich specimens absorbed more energy than honeycomb sandwich specimens for all impact energies.

The thresholds of contact force are observed in contact force versus time histories of 16.5 and 25.7 J in the case of 3D printed honeycomb core sandwich specimen. It indicates that core has failed. This is not observed in case of 9.3 J of impact energy.

The contact duration for honeycomb core sandwich specimens is less as compared to foam core sandwich specimens for all the impact energies at 9.3, 16.5, and 25.7 J.

Contact force is higher for honeycomb core sandwich specimens than foam core sandwich specimen, indicating that honeycomb core sandwich specimens is stiffer than foam core sandwich specimens and the deflections are more in foam core sandwich than honeycomb core sandwich specimens.

The contact duration of impactor in case of foam core sandwich specimens increasing with respect to impact energies 9.35 and 16.5 J, and it is higher as compared to honeycomb sandwich composite specimen, and similar increase is observed in case of honeycomb core sandwich specimens.

The damage is visible and increasing for higher energies as observed from contact force versus deflection curves and deflections higher as 1.4, 1.8, and 4.2 mm for impact energies 9.3, 16.5, and 25.7 J, respectively.

The impacted specimens with their energies are subjected to CAI according to ASTM D 7137 for determining the residual strength of the sandwich composites.

Footnotes

Acknowledgements

We are extremely thankful to Prof. Puneet Mahajan, Applied Mechanics Department, Indian Institute of Technology, Delhi for permitting the authors to conduct drop weight impact test, Impact Mechanics Laboratory, IIT Delhi.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by VNR Vignana Jyothi Institute of Engineering & Technology.

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