Accepted Manuscript Design and Manufacturing of Long Fiber Thermoplastic Composite Helmet Insert Haibin Ning, Selvum Pillay, K. Balaji Thattaiparthasarathy, Uday K. Vaidya PII: DOI: Reference:
S0263-8223(16)30671-7 http://dx.doi.org/10.1016/j.compstruct.2017.02.077 COST 8299
To appear in:
Received Date: Revised Date: Accepted Date:
20 May 2016 3 January 2017 15 February 2017
Please cite this article as: Ning, H., Pillay, S., Thattaiparthasarathy, K.B., Vaidya, U.K., Design and Manufacturing of Long Fiber Thermoplastic Composite Helmet Insert, Composite Structures (2017), doi: http://dx.doi.org/10.1016/ j.compstruct.2017.02.077
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Design and Manufacturing of Long Fiber Thermoplastic Composite Helmet Insert Haibin Ninga*, Selvum Pillaya, K. Balaji Thattaiparthasarathy b, and Uday K. Vaidyac a Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294 b General Electric Global Research, 1 Research Cir, Niskayuna, NY 12309 c Department of Mechanical, Aerospace and Biomedical Engineering University of Tennessee, Knoxville, TN 37916 Abstract Long fiber thermoplastic composite (LFT) is one type of thermoplastic polymer matrix composites reinforced with discontinuous fibers above critical fiber length. It has been increasingly used in various applications due to its excellent specific strength and specific modulus in addition to its infinite shelf life, intrinsic recyclability, and high-volume processability even for complex geometries. In this work, long carbon fiber reinforced polyphenylene sulphide (LFT C/PPS) is used to prototype a helmet insert with high rigidity for stiffening a relatively soft ballistic shell. The helmet insert is designed to have a rim along its periphery in order to offer extra rigidity and facilitate a readily clip-on with the ballistic shell. Static structural analysis is carried out for evaluating the performance of the insert. Comparison was made among different patterns of the helmet insert in their rigidity. A compression molding tool is designed and machined and the helmet insert is prototyped using LFT C/PPS. A compression test is conducted for the ballistic shell integrated with the helmet insert to validate the stiffening capability of the LFT C/PPS helmet insert. Key words: Composites, Long fiber; thermoplastics; helmet insert; compression molding *Corresponding author: Haibin Ning; [email protected]
Introduction Long fiber thermoplastic composite (LFT) is one category of fiber reinforced polymer matrix composites that has been increasingly used because of its excellent mechanical properties and other characteristics such as ease of manufacturability, infinite shelf life and recyclabililty [1-7]. The LFT has shown its increasing market share  and it has been predicted that LFT will have 5% increase annually from year 2015 to and 2022 . Most of its use is in automotive or transportation applications for the main purpose of decreasing vehicle weight and enhancing fuel efficiency. Components such as dash boards, front ends, underbody panels, trunk lids, seats, or battery doors are prototyped or manufactured using LFT [5-7, 9]. The weight reduction has been reported to reach up to 50% for the LFT components compared to their metallic counterparts . LFT has also been used in other applications other than automotive or transportation. Vaidya et al  reported the design and manufacturing of a LFT tailcone with complex geometry for military applications. Long glass fiber reinforced nylon was used in this application because of its great impact resistance and heat resistance in short time. The LFT tailcone successfully passed the firing trials . Long carbon fiber reinforced nylon composite was also used to manufacture thin-walled aeroshell and baseplate with complex geometry for aviation application [11-12]. LFT is reinforced with discontinuous fibers that have typical length of 3-25 mm. The fibers with that length range have the fiber aspect ratio (length to diameter) of 300-2000 . The fiber length in LFT is above critical fiber length that is defined as
, where is critical
fiber length, is tensile stress acting on the fiber, is fiber diameter and is Interfacial shear strength . Mechanical properties of a fiber reinforced polymer matrix composite increases
with fiber length while its processability decreases. LFT composites possess great mechanical properties while maintain good processability that is suited for molding parts with complex geometry. The combination of its excellent mechanical properties and ease of processability for complicated geometries enables it an increasingly used composite material for lightweight semistructural or structural components. Polyphenylene sulphide (PPS) is a semi-crystallinity thermoplastic polymer with aromatic ring linked with sulfide group and has been reported to possess good modulus and strength and dimensional stability [14-18]. It has a tensile modulus of 3.3 GPa and tensile strength of 60 MPa . Long carbon fibers in the LFT C/PPS enhance the modulus and strength while minimally compromising its processability. LFT C/PPS has been used in automotive applications to manufacture components with complicated geometry such as throttle body, water pump, crankshaft flanges  where high stiffness and strength and good dimensional stability are desired. In this work, a helmet insert is designed based on a ballistic shell and prototyped. LFT C/PPS is used to manufacture the helmet insert to stiffen the ballistic shell and provide dimensional stability in a compression test. Different patterns of the insert are compared in their performance in stiffness using static structural modeling approach. Verification compression test is conducted to verify its performance in stiffness. Design and Modeling The main purpose of this work is to use a LFT insert to add stiffness to the ballistic shell while minimizing the weight penalty. Figure 1 shows the design concept of the ballistic shell (in green) being stiffened by a LFT insert (in gray). The ballistic shell has shown great ballistic properties
but unsatisfying stiffness. A combination of software, Creo and Altair Hyperworks, was used to accomplish the geometry design of the helmet insert. The main contour of the helmet insert, as shown in Figure 2, was designed based on the interior surface of the ballistic shell. Three versions of the insert design are shown in Figure 2 with the only difference in the rim along the insert periphery. Figure 1(a) shows the initial design without any rim along the insert periphery while the other two designs have rims with different depth. Figure 2(b) shows the insert with 8 mm deep rim and Figure 2 (c) the insert with 13 mm deep rim. Finite element analysis (FEA) was conducted using ANSYS Workbench with the material input of LFT C/PPS that has a tensile modulus of 18 GPa obtained from coupon tensile testing according to ASTM D3039- Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. Figure 3 shows the loading and boundary conditions of the compression testing. One side of the ear location was simply supported without any freedom of translation in the ear-to-ear direction and a load of 450 N was applied at the other ear location. Meshing density was varied to achieve convergence for the static structural analysis. All of the insert were assigned with the same conditions, including loading and boundary condition as shown in Figure 3, material properties, and a thickness of 3.2 mm. The analysis results show that the insert without rim has a max deflection of 10.8 mm, and the insert with the 8 mm rim has a max deflection of 5.2 mm, and the insert with the 13 mm rim has a max deflection of 3.7 mm, respectively. Figure 4 shows the deflection results from all of the helmet insert with the red colored area indicating the maximum deflection. It is obvious that the addition of the rim along the periphery of the insert has contributed significantly in reducing the deflection (or increasing the stiffness) of the helmet insert. The helmet insert with 13 mm rim reduced the max deflection by more than 60% compared to the helmet insert without rim. In addition, the rim provides a
feature that enables snap-on fit with the ballistic shell. It is decided that the helmet insert with the rim depth of 13 mm is used for the analysis below. More analysis was conducted on the insert with 13 mm rim to reduce its weight by machining out certain areas from the insert. Four different patterns were designed with the same surface area (or insert weight) as shown in Figure 5 and their performance in stiffness were evaluated using static structural analysis. The same boundary and loading conditions shown in Figure 3 were used for all of the four patterns (Figure 5). The same material properties and thickness (3.2 mm) were assigned to the patterns. The analysis results show that there is minimal difference in maximum deflection among these four patterns. Pattern A has 5.8 mm deflection and Pattern B has 5.9 mm deflection. Both Pattern C and D have 6.0 mm deflection. Considering that Pattern C has more balanced design with the stiffening ribs evenly distributed, Pattern C was selected for the following validation testing.
Manufacturing, Validation Testing and Discussion A mold for compression molding the helmet insert is designed as shown in Figure 6. The mold has the capability of being heated by oil through the channels running through the mold. 40 wt% C/PPS LFT pellets with 12 mm length from Celanese (Celstran PPS-CF40-P12) were used to mold the insert using extrusion-compression molding process. The process started with feeding the LFT pellets into a low shear extruder. The pellets were melted while being transported to the front end of the extruder to form an extrudite. The extrudite was then placed into the helmet insert mold and compression molded into a helmet insert. The processing parameters, mold temperature and molding pressure were optimized such that the filling of the mold cavity was
accomplished. The temperature of the mold (Figure 7) was set at a temperature of 150°C and the temperature of the low shear extruder was set as 310°C to ensure the adequate flow of the carbon fiber PPS extradite inside the mold while not degrading the PPS matrix. The molded insert is shown in Figure 8 (a) and Figure 8 (b) shows Pattern C after being machined from the molded insert. Figure 9 shows the microstructures at the ear location of the helmet insert. It is shown that the helmet insert has good consolidation with the fibers evenly distributed. Validation tests were conducted to demonstrate the efficiency of the stiffening capability from the C/PPS LFT insert. Figure 10 shows the setup of compression testing on a INSTRON T-5000 testing frame. The helmet insert was able to be snapped onto the ballistic shell as expected for compression testing. The loading rate was 1 mm/min and the load and cross head displacement were recorded. Figure 11 shows the comparison of the load and displacement behavior for the ballistic shell only, insert only, ballistic shell combined and insert. The testing results are averaged from 3 tests. It is seen that the addition of the C/PPS LFT insert has resulted in tremendous enhancement of the rigidity for the ballistic shell. The ballistic shell showed a deflection 9.1 mm at 450 N loading. With the insert being added to the ballistic shell, the deflection decreased to deflection 3.1 mm at the same loading of 450 N. The mass of each component was weighed. The helmet insert weighs 290 g and the ballistic shell weighs 1133 g. The total mass is 1423 g which has weight saving of 5% compared to current advanced combat helmet (size “large”) according to the manufacturer’s data . It is calculated that the insert mass added to the ballistic shell counts for 20% of the overall weight while it has resulted in a decrease of deflection (increase of rigidity) by 67%. In addition, the FEA results for insert only are plotted with the compression testing results in Figure 11 and it shows a good match with the compression testing results.
Conclusions A helmet insert was successfully designed and prototyped using long carbon fiber reinforced polyphenylene sulphide matrix composite. The rimmed helmet insert shows excellent stiffening capability compared to the insert without rim. The insert with a 13 mm rim has reduced the max deflection by more than 60% compared to the insert without rim. Several patterns were compared in their performance in stiffening the ballistic shell and it is found there is minimal difference in stiffening effect with the same insert weight. C/PPS LFT with 40 wt% fiber loading was used to prototype the helmet insert using extrusion-compression molding process. Compression testing was carried out on the helmet insert and the combined helmet insert and the ballistic shell. The testing results showed the insert mass added to the ballistic shell counts for 20% of the overall weight while it increases the rigidity by 67%.
Acknowledgements This work was sponsored by Army Research Laboratory (ARL) and the authors greatly acknowledge the financial support from ARL. References 1. Markarian, J. Long fibre reinforced thermoplastics continue growth in automotive. Plastics, Additives and Compounding, 9 (2007): 20-24. 2. Krause, W., Henning, F., Tröster, S., Geiger, O. and Eyerer, P., LFT-D—a process technology for large scale production of fiber reinforced thermoplastic components. Journal of Thermoplastic Composite Materials, 16 (2003): 289-302. 3. Henning, F., Heinrich E., and Richard B. LFTs for automotive applications. Reinforced Plastics. 49 (2005): 24-33. 4. Peijs, T., Composites for recyclability. Materials Today, 6 (2003): 30-35. 7
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Figure 1. The concept of adding the LFT insert (in gray) to the ballistic shell (in green) to increase structural stiffness: (a) disassembled view and (b) assembled view.
Figure 2. The design of the helmet insert (a) without rim; (b) with 8 mm deep rim; and (c) with 13 mm deep rim.
Figure 3. Loading and boundary conditions for all of the helmet insert designs in Figure 2.
Figure 4. (a) The deflection contour of the helmet insert without rim in side view and (b) isometric view (max deflection 10.8 mm); (c) The deflection contour of the helmet insert with 8 mm rim in side view and (d) isometric view (max deflection 5.2 mm); (e) The deflection contour of the helmet insert with 13 mm rim in side view and (f) isometric view (max deflection 3.7 mm).
Figure 5. Various design patterns, (a) Pattern A; two ribs (b) Pattern B; three ribs; (c) Pattern C; basket pattern; and (d) Pattern D. All of patterns have the same surface area (or insert weight).
Figure 6. Compression molding tool and the concept of the molded insert.
Figure 7. The helmet insert mold.
Figure 8. (a) As-molded C/PPS LFT insert and (b) the insert that machined into Pattern C.
Location 1 Location 2
Figure 9. (a) The side view of the helmet insert; (b) the microstructure at the ear location of the helmet insert highlighted in (a); the microstructure at (c) Location 1 and (d) Location 2 highlighted in (b).
Figure 10. Compression test for Pattern C helmet insert.
500 450 400
250 200 Ballistic Shell only-Testing Inserts only-Testing Ballistic Shell with inserts-Testing Inserts only-FEA
150 100 50 0 0
Displacement (mm) Figure 11. The comparison of the load vs. displacement relationship for the LFT insert (Pattern C) only, the ballistic shell only and combined LFT insert and ballistic shell from compression test and the FEA result for the insert. Note the tremendous increase of the rigidity after adding the LFT insert to the ballistic shell.