Low Velocity Impact Response of the Filament Wound Anisogrid CFRP Cylindrical Shells


 This work presents the experimental and finite element simulations to investigate the behavior of both unstiffened and anisogrid composite lattice cylindrical shells under low velocity axial impact. Impact damage has been an epidemic problem for composite structures. Even subjected to a low velocity impact, composite may sacrifice its load carrying capacity considerably due to various modes of failure. The test coupons fabricated as per American Society for Testing of Materials (ASTM) standards were put through Infrared (IR) thermography to find the imperfections during fabrication. The test coupons without defects were only taken into account for material characterization. Finite Element simulations are carried out on both the unstiffened and anisogrid shell structures using LS-DYNA® for a series of low velocity impacts. Also these shell structures were subjected to impact load experimentally for the validation of the results. The results of these studies indicate that the anisogrid model presented in this work possess greater load carrying capacity than unstiffened shell under dynamic loading conditions, also the weight of the structure has been drastically reduced.


Introduction
Composite materials are substance consisting of two or more materials, insoluble in one another, which are combined to form a useful engineering material possessing certain properties than those of the individual components used alone. Because of the high strength to weight ratio and damage tolerance composite materials, especially carbon ber reinforced polymer (CFRP) shell structures are nding many applications in aerospace applications [1]. The loads which can occur during its service life must be taken into account for a stable and reliable structural design. Low-velocity impacts are considered potentially dangerous for a composite shell structures, even subjected to a low velocity Impact, these shell structures may sacri ce its load carrying capacity due to various modes of failure [2]. Several areas have been accounted which are not fully exploited and restrict potential advancement of weight-e ciency of composite structures namely, post-buckling capabilities, imperfection robustness and loading dynamics.
Many works concerned with the buckling response of thin cylindrical shells have been published and the different modes of failure [3][4][5][6][7], have been described. In this work low velocity impact response of composite cylindrical shell structures have been analyzed by means of experimental testing and numerical simulations Vasiliev et al investigated the history of the lattice structures [3] development and made the founding studies on fabrication techniques. They proposed that the modern unidirectional carbon composites, being loaded along the bers, are characterized with speci c strength and stiffness than the corresponding characteristics of aluminum alloys. The anisogrid composite lattice structures are thin walled cylindrical shells [4], [5] composed of both helical and circumferential ribs articulated in such a way that con ned defects in the grid do not grossly affect the universal behavior of the structure. Buragohain et al carried out a study of lament wound [6], [7] grid-stiffened composite cylindrical structures. The composite lattice structures have been considered as the excellent replacement for conventional solid and honeycomb structures.
Morozov et al have reported that Anisogrid composite shells [8] are used in various structural applications, such as rocket interstages, payload adapters for spacecraft launchers, fuselage components for aerial vehicles, and components of the deployable space antennas. Recently, Buragohain et.al reported the optimal design of lament wound grid-stiffened composite cylindrical structures. According to them the grid of stiffening ribs that are made by lament winding makes such a structure very highly e cient and reliable. Zhifeng Zhang et.al demonstrated a progressive failure [9] methodology to simulate the initiation and propagation of multi failure modes for advanced grid stiffened (AGS) composite plates/shells on the basis of a stiffened element model.
Polymer matrix composite laminates are prone to delamination when impacted, resulting in low damage tolerances, which is of great concern for load carrying applications. The impact behavior of composite materials has been investigated by many authors [11], [12]. Damages in composites are different from those in metals. Composite failure is a progressive accumulation of damage, including multiple damage modes and complex failure mechanisms [13], [14]. Our earlier investigations [15] proved that the anisogrid shell structure deliver more energy absorption than the unstiffened shell structure under static loading condition. Recently, the effects of impact energy levels, impact locations and changes in layer thickness of 3D printed thermoplastic plates were studied [30]. In addition to the available literature on anisogrid shell structures over the past decades, this paper present another more recent approach to the experimentation and numerical simulation of CFRP shell structures. The objective of this study is to compare the impact response of the CFRP shells plain cylinder and anisogrid, when they are dynamically loaded in axial direction. The developed numerical models were validated with the obtained experimental results. The present paper deals with the following phases of work: i. Infrared thermography Nondestructive testing (IRNDT) of the CFRP test coupons followed by material characterization as per ASTM standards for the estimation of unidirectional elastic and strength properties.
ii. Fabrication of CFRP cylindrical shell structures viz., unstiffened (plain cylinder) and anisogrid using newly developed lament winding machine.
iii. Experimental and Finite element analysis (FEA) of unstiffened and anisogrid CFRP cylindrical shell structures under axial impact.

Materials
The matrix and reinforcement selected for the fabrication of the CFRP test specimens and composite shell structures are epoxy and carbon ber respectively. Carbon ber is composed of carbon atoms bonded together to form a long chain [1]. Properties of carbon and epoxy used for the fabrication of CFRP composite shell structures are shown in Table 1. Epoxy resin possess better mechanical properties in tension, compression and impact than polyester resins hence they are preferred in aerospace applications. For enhancing the performance of resin (epoxy), hardener (HY 951) is used in ratio 1:10. For the fabrication of anisogrid composite shell structure polyurethane foams are used. Wooden and foam mandrels were prepared as per the structural geometry of anisogrid shell structures to perform lament winding process.

Structural Geometry
The actual dimensions of rocket interstage structure [7] are 10 m in diameter and 14.5 m in length with a thickness of 0.28 m. For performing FEA simulation the interstage structure is modeled with a scale of 1:70 and their speci cations are listed in the Table 2.
The scaled structure is modeled [16] in Solidworks® 2018 as shown in Fig. 2 & Fig. 3. The circumferential ribs are arranged in such a way that it passes through the centre of the helical ribs in order to reduce the deformation of the shell structure. The ribs are considered as the main load bearing members in the anisogrid shell structure.

Fabrication of Test Coupons
Fibers are commercially available in small diameter, ranging from 5 to 7 microns for carbon bers. Fibers (single lament) together are gathered in the form of a tow. The specimens were prepared by the combination of impregnation and hand layup process in which the impregnated carbon ber was wound over the wooden plate. Impregnation refers to partially cured mixture of ber and resin. The unidirectional impregnated bers can be cut and stacked to form the nal product. The ber and resin hardener mixture are taken in the ratio 1:1.
A wooden plate of appropriate dimensions was selected and pins were hammered in at the appropriate locations depending on the ber width and specimen size as shown in Fig. 4. Once the pins were placed on the mold a layer of Mylar sheet (release lm) was placed on to it in order to facilitate the easy removal of the sample after curing. Pressure rollers are used for the uniform distribution of the resin and to remove the excess content. After curing the rectangular plate is sized into ASTM standards (D3039 & D3518) by means of cutting tool as shown in Table 3.

Fabrication of unstiffened shell
Filament winding is a comparatively simple method for the fabrication of cylindrical shells using continuous tows. Carbon tows and epoxy resin mixture are taken in 1:1 ratio for the fabrication of the shell. A wooden mandrel was prepared according to the unstiffened shell dimensions and tted in the lament winding machine. Impregnated carbon tows are wrapped around a rotating mandrel and cured under room temperature to produce unstiffened cylindrical shell. Release lm is wrapped over the wooden mandrel for easy ejection and good surface nish. The impregnated carbon tows are wounded as per the required dimensions and allowed to cure in the room temperature to get the nal structure as shown in Fig. 5. The winding operation is achieved by manually rotating the handle as shown in Fig. 6 whereas the grooved mandrel is replaced by plain wooden mandrel. Totally four CFRP unstiffened shell structures are fabricated with the con guration shown in Table 4. The effect of pretension during the lament winding process on mechanical properties of nished component were studied earlier [29]. In addition there is an effect of the ber orientation on the behavior of continuous carbon ber composites [30], so that in this study the helical ribs angle was selected properly.

Fabrication of Anisogrid shell
The anisogrid shells are also fabricated by means of lament winding, where the plain wooden mandrel is replaced with a foam mandrel for easy removal after curing. The main objective of the fabrication is to control the con guration of the lattice ribs both helical and circumferential. A low density foam mandrel is grooved as per the anisogrid shell CAD model as shown in gure 7. Silicon spray is applied on the grooved surface for creating a non-sticky layer between the foam mandrel and impregnated carbon tows. Proper placement of the bers along the grooves cut on the mandrel surface is important in any lament winding activity to get the required structure ( gure 8). The guide wheels are used for aligning the carbon tow inside the grooves along the circumferential or helical direction of the mandrel. The tension of lament winding must be strictly controlled in order to minimize the buckling of the shell. The CFRP anisogrid shell con gurations are shown in Table 5. Recently, an innovative method is proposed for the production of three-dimensional cylindrical lattice structures using autoclave [28]. They have designed an out-of-autoclave compaction process which uses the compression of a heat-shrink tube during its shape recovery in oven.

Lock-In Thermography Experiments
Inspection of composite materials using Infrared thermography has been increasing in the recent years. IR thermography has the possibility of inspecting large surface areas rapidly.
Lock-in IR thermography technique has been chosen in this study for the inspection of CFRP test coupons for locating the defects like delamination, voids, debonds, wrinkles, inclusions, broken ber, and ber misalignment. The process of IR thermography testing of CFRP test coupons is based upon the principle of thermal imaging. It involves dynamic stimulation which can be applied by halogen lamp, ultrasonic and by mechanical means. Figure 9 and Fig. 10 describes the working principle of lock-in thermography. The specimens which are sized to ASTM standards are examined in order to locate any imperfections present in them. A uniform heat source using a hot air blower is applied on the bottom surface of the CFRP test coupon and the top surface is lled with water. So that any cracks or defects present in the specimen can be located as water inclusion areas or temperature variation zones using the IR camera (Fluke Thermography Model Ti32). Finally the digital data from the IR camera is acquired and stored on a personal computer for the subsequent analysis.

Material characterization
For the evaluation of unidirectional mechanical properties, stress transfer, distribution of load, and to govern the damage accumulation and propagation material characterization is essential. The main objective of mechanical testing of composite material was to determine the basic properties like longitudinal modulus, ultimate tensile strength, in-plane shear strength, shear modulus and Poisson's ratio for the nite element analysis of the shell structures. The mechanical testing was carried out using the FIE make UTE-40 Universal Testing Machine (UTM) with a capacity of 400kN and 5mm/min as feed rate. The test coupons fabricated as per ASTM (D3039 & D3518) standards were tted in the UTM as shown in Fig.  11 and an axial tensile load is applied. The measurement of displacement is based upon an electronic extensometer with a standard gauge length of 50 mm. In the UTM test setup, the carbon-bre-reinforced epoxy composite test coupon is aligned in such a way that the tensile loading direction is in accordance with the ber length direction for the estimation of unidirectional mechanical properties longitudinally. Poisson's ratio has been estimated by carrying out tensile tests with the assistance of electrical strain gauges employed at a distance of 25 mm wide.

Impact testing
Low velocity impact tests were performed on both the unstiffened and anisogrid shell structures by subjecting them to an axial impact load within a short interval of time. For low velocity impact testing, common features of impact loading are the vibratory load responses from the composite shell specimen, the impactor plate and the specimen supporting xture (base plate). A controlled impact is achieved by dropping a square shaped impactor plate (250mm×250mm×20mm) which is attached to an electromagnet from the prescribed impact heights as shown in Fig. 12. The impact parameters like resultant displacement, velocity and acceleration of the shell structures at the time of loading were recorded by means of electronic modules viz. digital instruments (height gauge, velocity meter, acceleration meter, displacement meter), analog modules (load cell ampli er, analog to digital converter), micro controller module(micro controller based data acquisition system) and data acquisition software. ohms, IPA Make) used for measuring the contact force history. The charges generated at the time of impact were converted into potential difference using the load cell ampli er in the data acquisition system.
The height gauge includes 20 magnetic read switches which are located along the vertical column at a distance of 10 cm wide. A microcontroller with an electromagnet was used for evaluating the height of the impactor. The actual velocity of the impactor before and after collision was evaluated by velocity meter arrangement. It comprises two inductive type proximity switches (30 mm diameter; sensing distance-15 mm) located approximately 30 mm above the specimen surface and are placed at a distance of 20 cm between them. The data acquisition system shown in Fig. 12 includes an ampli er, an analog to digital converter (ADC) and a micro controller (AT89C2051).

Impact analysis using LS-Dyna®
Both the anisogrid and unstiffened structure CAD models were imported and analysed individually using LS-Dyna®, explicit FEA software which is capable of predicting the crushing mode of thin-walled carbon/epoxy composite shells under axial impact loading.
Finite Element Analysis was implemented with the descriptions given in Table 6. The bottom plate (Base) and top plate (Impactor) as shown in Fig. 13 were given the properties of steel and for composite shell structures the unidirectional mechanical properties which are estimated in material characterization are used. Automatic topology mesh is chosen for the shell structures with an average mesh size of 7mm. The termination time for the motion of impactor plate varies for different drop velocity. The impact parameters such as displacement, velocity and acceleration were plotted with respect to time. The drop plate is given the impact velocity and an automatic surface to surface contact type is assigned for the shell and the drop plate.

Material characterization results
From the material characterization results given in Table 7

FE Analysis Results
Finite element modelling of Unstiffened and Anisogrid shell structures are carried out using LS-Dyna® in which each node is associated with three translational and rotational DOF. The impact parameters such as displacement, velocity and acceleration of the entire shell structures are plotted with respect to time. In case of unstiffened shell structure ( Fig. 16.a), when subjected to an axial impact the displacement of nodes at the centre region of the shell is high which will leads the structure to buckle easily. While in the case of Anisogrid shell structure ( Fig. 16.b) the displacement of the nodes in the helical ribs are controlled by circumferential ribs which reduces the tendency to buckle. Also the displacement, velocity and acceleration plot easily reveals that the rate of buckling was very much reduced in the case of Anisogrid shell structures. At the same time the weight of the structure can be reduced up to 51.27%. The entire displacement, acceleration, velocity and energy plot are not included in this paper just for the sake of brevity. Figure 17 reveals that for a drop velocity of 3m/s and 4m/s the drop plate gets in contact with the shell at a time of 155µsec and 204µsec respectively. It was clearly observed that the unstiffened shells deforms at a faster rate than that of anisogrid shell. The velocity plot (Fig. 18) shows that there is uctuation in deformation rate (velocity) of the unstiffened shell which leads to sudden failure of the shell structure. Similarly the acceleration of the unstiffened shell attains a peak value readily than that of the anisogrid counterparts which implies that the unstiffened shells are very prone to failure under impact loading. Moreover it is revealed that the deformation of helical ribs in anisogrid lattice structure meanwhile decreases because of the clamping effect of hoop ribs [26]. Earlier it was found that anisogrid pattern possess a higher local buckling strength in comparison to the triangular cells or isogrid pattern with the same design variables [27]. Now, within the scope of this work the next phase is to validate the developed numerical model in the light of comparison with experimental results.

Impact Results -Experimental and FEA
Impact experiments were carried out on both the unstiffened and anisogrid shell structures at four different drop velocities 3, 4, 5 and 6 m/s respectively.
The observations on the damaged shell structures after impact loading reveal that the shell damage consists of two modes of failure namely matrix cracking and bre breakage. The impact energy or the energy absorbed by the anisogrid shell structures is quiet better when compared to that of unstiffened shells as in the case of numerical simulations. The primary impact failure mechanisms are a very complex combination of energy absorption mechanisms such as delamination, which is followed by matrix  In light of these results, we can conclude that rate of deformation increases wit impact velocity in both unstiffened and anisogrid. Whereas the energy absorption rate has been improved in anisogrid pattern in comparison to the other. So that by incorporating more number of hoop ribs at speci c location the energy absorption rate can be further improved, at the same time weight reduction factor should be considered.
Here in this study the weight has been reduce to 51.27%. .

Conclusion
The deformation and energy absorption of unstiffened and anisogrid cylindrical shells are systematically studied in this paper. The results indicate that the deformation mechanisms of the anisogrid cylindrical shells are distinct from that of the unstiffened shells. The accuracy of the experimentation has been con rmed by numerical simulation at level of lattice and unstiffened cylindrical shell structures. This investigation reveals a number of remarkable features of anisogrid cylindrical shells, as summarized below: The overall manufacturing process of the unstiffened and anisogrid CFRP shells presented in this paper is made cheaper by means of manual lament winding manufacturing process.
IRNDT temperature contours infer that the infrared radiation is emitted inconsistently at the top surface due to the interruption of the ow of heat into the carbon/epoxy test sample by the defects.
Material characterization was made on the CFRP test coupons for the estimation of unidirectional elastic and strength properties.
FEA results demonstrate that for a drop velocity of 4m/s the maximum displacement of the anisogrid shell is 6 mm where in the case of unstiffened shell is 22.96 mm at an impact time of 204µsec without causing a signi cant damage in the shell structure.
The impact behaviors of the CFRP composite shells have been experimentally analyzed. It has been experimentally observed that the modes of collapse and shell compression heights for Anisogrid shells under impact loading are much lower than the unstiffened counterparts. Also, the energy absorption capabilities are quiet better when compared to that of unstiffened because of the interconnected circumferential and helical ribs.
The experimental and Finite Element procedures for impact loads described in this paper would be useful in the development of lattice shell structures in aerospace applications. From the above conclusions, it is clearly shown that thin-walled shell structures made out of CFRP material would be used for energy absorption in impact applications, which contributes to the growth and welfare of spacecraft industries.
Overall, the obtained results recommend the likelihood to explore more e cient design of grid structures whose energy absorption rate is still higher.     Lock-in Thermography Schematic sketch    Resultant Acceleration Plot At 4 & 6 m/s