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Impact behaviour of bombyx mori silk fibre composite face-sheet sandwiched coremat

Structural sandwich composite fabrication involves a number of layers of different materials, most often, but not exclusively, made of two identical thin and stiff skins, separated by a thick, lightweight and relatively weak core, bonded together with layers of adhesives. The structural concepts of any sandwich structure is made in a way that, the skins primary objective is to resist the in-plane and bending loads, with top side skin in compression and bottom side in tension. The core then corresponds to the web, which carries the transverse shear stresses and increases the stiffness of the structure by keeping the skin apart.1 The demand of sandwich structures has increased in many industries including, transportation and automotive, sports, aerospace and renewable energy in recent years. Superior structural ability to carry transverse loads, high stiffness, low weight ratio, excellent thermal insulation and acoustic damping are some of the properties which endeared sandwich composites to these industries. Structural members using sandwich construction are designed usually in terms of flexural stiffness and strength. However, some structures most are capable of sustaining large overloads, (e.g.): foreign object impact or impact with solid objects. In these cases, the non-linear (post-failure) behaviour of the structure is important and parameters of interest are the integrity of the structure before total collapse occurred.2−5


Introduction
Structural sandwich composite fabrication involves a number of layers of different materials, most often, but not exclusively, made of two identical thin and stiff skins, separated by a thick, lightweight and relatively weak core, bonded together with layers of adhesives. The structural concepts of any sandwich structure is made in a way that, the skins primary objective is to resist the in-plane and bending loads, with top side skin in compression and bottom side in tension. The core then corresponds to the web, which carries the transverse shear stresses and increases the stiffness of the structure by keeping the skin apart. 1 The demand of sandwich structures has increased in many industries including, transportation and automotive, sports, aerospace and renewable energy in recent years. Superior structural ability to carry transverse loads, high stiffness, low weight ratio, excellent thermal insulation and acoustic damping are some of the properties which endeared sandwich composites to these industries. Structural members using sandwich construction are designed usually in terms of flexural stiffness and strength. However, some structures most are capable of sustaining large overloads, (e.g.): foreign object impact or impact with solid objects. In these cases, the non-linear (post-failure) behaviour of the structure is important and parameters of interest are the integrity of the structure before total collapse occurred. 2−5 The complexity of composites structures and its susceptibility nature under impact loading/external objects has been viewed as a major setback to its structural applications. This phenomenon has triggered much research interest, to study the behaviour of composites under varied impact loading. 6−9 Among some of these researches reported earlier are, Daniel et al. 10 reported their experimental studies on the low-velocity impact behaviour of composite sandwich panel consisting of woven carbon/epoxy face-sheets and a PVC foam core on which they determined the effectiveness of predicting indentation behaviour of a sandwich panel. Hosur et al. 11 reported on the impact response of woven carbon/epoxy laminate under varied environmental condition; cold-dry and cold-moist. The focus of their research was the comparison of impact parameters like peak load, time to peak load, deflection at peak load, and absorbed energy of different types of hybrid face-sheets. Some other research works with similar interest are reported in. 12−21 Though significant research works have been carried out on the subject matter however, the combination of Bombyx mori with coremat is a novel idea. Fundamental investigations are needed to ascertain its capabilities in term of load carriability, energy attenuation and damage fragmentation. The current investigation present Coremat core which possess good mechanical properties and has been well investigated as the core material and woven Bombyx mori natural silk reinforced epoxy composite as face-sheet/skin. Woven Bombyx mori natural silk was considered because of its mechanical properties. 3,22−24 The focus of this investigation centred on understanding the damage progress through thickness of the sandwich material, by comparing the load carriability, energy capability and physical examinations of fractured cross-sections. This research work is informed by the quest to improve impact damage-resistance of sandwich structures.

Materials specification
Tables 1−3, showed the material property specification of the materials used in this research work. The materials used were: Resin was epoxy, type (DER 331); Curing agent was: Jointmine hardener, type (905-35); Core: was Coremat; and Bombyx mori plain woven natural silk fabric. All materials excerpt Bombyx mori were supplied by Dk Composites Maleka. Plain woven Bombyx mori natural silk fabric was supplied by Loxevi Silk Indonesia.

Sandwich Specimen Fabrication
The sandwich composite specimens were prepared via hand-layup method. This method provided high quality composite samples panel with minimal defects. To fabricate the sandwich samples, Coremat core materials was sandwiched in-between B.mori WNS/ Epoxy face-sheet of 1.5mm thicknesses. Special cares were taken to ensure that correct amount of epoxy was used in addition to being evenly spread out. The vacuum bagging was carefully spread over the sample; rubber squeeze was used to remove the extra epoxy and trapped air. The composite panels were placed in a hydraulic press at a room temperature and pressure of 10bar for 2hr. After being taken out from the hydraulic press, the panels were left to cure at a room temperature for 22h. Finally, the panels were cut into the standard drop weight specimen size of 100 x 100 using a diamond cuter Figures  1−3 showed the materials and schematics diagram of the lamination process.

Sample specimen
The actual size of the specimen was 100 by 100 (mm) square panel, shown in Figure 3. Three specimens were tested in each impact load category and the average values of the results were recorded.

Impact testing
The drop-weight impact testing was instrumented to measure impactor load and displacement during the impact event. This tests were performed using an instrumented drop weight testing system (Instron-Dynatup 9250 HV) Figure 4. This system is suitable for a wide variety of applications requiring low to high impact energies. The shape of the striker used was hemispherical and the size was 12.7 mm diameter. It was assumed to be perfectly rigid. The testing machine has a force transducer with capacity of 22.24 kN. The total mass of the impactor used was 5.1 kg and the impact velocity ranges from 1.95m/s-4.07m/s. The composite specimen with dimensions of 100 mm by 100 mm was clamped via a hydro operated clamp on a fixture along a circumference having a 76.2 mm Diameter. The main reason behind the choice of an instrumented drop weight impact testing machine for this study was based on its ability to simulate closer to real-life impact conditions.

Results and discussion
The response of composite panel on increasing impact load Figure 5 depicts the response of the composite panel to the increasing striking impact loads. The values and their corresponding locations were compared for 32J, 48J, and 64J impact load. Comparison of these values will help to establish fracture resistances and rate of energy attenuation as the impact load increases. From the displayed graph, all the profiles increased linearly to some points before change in directions were observed. This change in direction is believed to signify the initiation of internal damages on the composite material, this type of damages are usually matrix crack, or other forms of internal damages like delamination, which most often are invisible during physically examined. This kind of damages though may be unnoticed, causes reduction in the material stiffness and pose failure threats which may be catastrophic. Once there exists an internal damage or penetration in the specimen, absorption capability consequently reduced and non-linear progression is observed, the profiles showed different point of internal damage or pick load. The graph showed that the energy absorption capability of the composite reduced with increasing impact load.

Figure 5 Energy vs Time profile
The response of composite panels on impact load deflection   Figure 7 compares deflection/energy/load profiles. Energy absorbed by the specimen without failing, is the energy through to the peak load point. It is assumed that the energy up to the peak load point is absorbed through elastic deformation, all other energies absorbed beyond this point is assumed to be absorbed through the creation of damages. Therefore, total absorbed energy includes energies absorbed through creation of damages. The total absorbed energy under 32J impact load was 15.2J, under 48J was 12.8J and 64J impact loads was 10.8J respectively. The differences in structural absorbed energy i.e. (energy absorbed by the composite material before any changes in stiffness occurred) ranged from 1.0-5.0J.

Failure mechanism and damage fragmentation
Several damage modes have been observed under the three impact loads categories in this experiment. In Figure 8, specimen was stroked by 32J impact load, damage modes were detected as matrix cracks and fibre fracture. There was no penetration damage from the contact point (top surface), the cross-section photos showed the extent of damage that was incurred internally by the composite panel; this includes internal cracks and deformation. In Figure 9, the impact load was increased to 48J. The pattern of surface damage was a spherical shape; visible perforation damage was also noticed. Cross-sectional examination of the panel show increase in internal damage compared with the previous. In Figure 10 which has 64J impact load, the dominant damage was perforations accompanied by fibre breakage, matrix cracks and delaminations. In comparison, damage severity and fragmentation increased with increased impact load

Conclusion
An experimental investigation has been carried out on the impact response of woven natural fibre/epoxy sandwiched Coremat composite. Comparison of the load bearing capabilities, rate of energy absorption, damage fragmentation and failure mechanism of WNS/ Epoxy sandwich composite panels under varied drop weight impact load was assessed. From the results, these conclusions were drawn: absorption energy capability decreased with increasing impact load.
There was deflection on each impact load configuration, though the values between them were negligible. Visual examination of the composite specimen however, show that damage areas and severity increased with increase in impact load, suggesting that most of the energies absorbed under higher impact loads were absorbed through the formation of damages. Findings of this work are expected to be useful in understanding the impact behaviour of woven natural silk sandwiched Coremat composite panels under varied impact load.