INTERLAMINAR REINFORCEMENT OF COMPOSTE LAMINATES WITH HEAT ACTIVATED SHRINKING MICROFIBERS

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1 INTERLAMINAR REINFORCEMENT OF COMPOSTE LAMINATES WITH HEAT ACTIVATED SHRINKING MICROFIBERS Eric S. Kim, Patrick C. Lee, Dryver R. Huston The Department of Mechanical Engineering, University of Vermont, Burlington, VT Abstract This paper describes an innovative through-thickness fiber reinforcement technology for laminate structures by using shrinking microfibers. Unlike incumbent passive fiber reinforcing technology, in-situ shrinking microfibers that respond to an external stimulus such as heat can induce pre-compression to matrix and create additional resistance from external loads. In this paper, Heat-Activated Shrinking (HAS) microfibers and Heat Passive (HP) microfibers made were used to investigate the interlaminar reinforcing effect of fiber shrinking mechanism. The specimens were reinforced by three different fiber geometries: (i) 1.27 cm (0.5 in) interval stitch of single microfibers, (ii) 2.54 cm (1 in) interval stitch of single microfibers, (iii) 2.54 cm (1 in) interval stitch of double microfibers, and then peel strengths were compared with control using T-peel tests. For Case (i), the reinforcing effect from HAS microfibers was shown by 47.2 % improvement compared to the specimens with HP microfibers. By comparing to control specimens, it was almost 2,883% improvement. For Cases (ii) and (iii), 27.7 % and 57.0 % increases in peel strengths were resulted respectively. Comparing the control specimens and the specimens with HAS microfiber, it was 2,191% and 3,741% improvements, respectively. Introduction Polymer laminates with two-dimensional reinforcement structures have been exploited in various applications such as aircrafts, automobiles, and constructions [1]. However, the lack of through-thickness reinforcing technology can be a disadvantage in terms of cost, ease of processing, mechanical performance and impact damage resistance [2]. To overcome the shortcomings of 2D laminates such as cost and the efficiency of manufacturing, 3D composites has been introduced with various manufacturing technologies such as weaving, braiding, stitching, and knitting [2]. composites mechanical performance have been reviewed with the challenge of developing more automated means to manufacture composite structures of optimized design [5]. The manufacturing processes of through-the-thickness reinforced composites using tufting method have been presented in Dell Anno s study, and the presence of tufts using carbon fiber showed the significant increase in the delamination crack growth resistance of the double-beam specimens [6]. The study of interlaminar as well as intralaminar reinforcement of polymer-matrix composites using carbon nanotubes has been conducted as well [7, 8]. The advantages and disadvantages of applying z-pins (i.e, pins that applied to through-thickness direction of the laminate) to polymer composite laminates have been examined [9]. The delamination resistance of thick woven glass fiber reinforced polyester specimens with different ply orientation has been studied [10]. The interlaminar stresses resulting from bending of thick cylindrical laminated shells has been analyzed using a Kantorovich method [11]. The effects of through-thickness stitching using natural fibers on the interlaminar fracture toughness and tensile properties of flax fibre/epoxy composite laminates have been studied [12]. The present and future applications for 3D textile composites made by weaving, braiding, stitching, and knitting have been reviewed [1]. However, it seems that there is little research exploiting fiber materials with shrinking characteristics to precompress and reinforce the through-thickness directional strength of the laminate structures. The contraction of microfibers under specific stimuli in the composite would have higher mechanical properties with internal pre-stress, which could withstand from higher external loads or torsions (Figure 1). The objective of this study is to present and evaluate polymer lay-up structures with interlaminar-reinforcement by heat activated in-situ shrinking microfibers. The microfiber shrinking ratios were measured. Also, T-peel test results of shrinking fiber reinforced composite specimens, and control specimens were compared to demonstrate the reinforcing effect of shrinking reinforcement fibers. There have been numerous studies and applications regarding interlaminar reinforcement of composite structures. The effects of through-thickness reinforcement to the in-plane mechanical properties have been studied [3, 4]. The main attractions of the current levels of major interlaminar reinforcing techniques, the ability to tailor the SPE ANTEC Anaheim 2017 / 656

Materials The polymer matrix used for this study was epoxy (System 3000 High Temp Epxy kit, part number 3000/3120), manufactured by Fibre Glast.

2 Microfiber Preparation Figure 1. Hypothesis of an interlaminar strengthening effect of HAS microfibers, (a) Undamaged laminate structure, (b) Shrinking behavior of HAS microfibers that withstands delamination. Materials The polymer matrix used for this study was epoxy (System 3000 High Temp Epxy kit, part number 3000/3120), manufactured by Fibre Glast. For lay-up fiber, fiberglass fabric (part number 241) was selected. For HAS through-thickness microfibers, heat shrink tubes from Buyheatshrink (2:1 Low Shrink HP, Clear color, AMS- DTL-23053/5-306) were used. The tubes that heat activate at the lowest temperature (e.g., 80 C) was selected to minimize the strength loss and shape change of epoxy matrix during heat shrinking activation. Table 1 details the properties of the epoxy, fiberglass, and heat activated tubes. Table 1. Properties of law materials. Epoxy (a) (b) Cured Hardness Shore D 88 D Tensile Strength >252 MPa Elongation at Break % Tensile Modulus 16.1 GPa Flexural Strength 363 MPa Flexural Modulus 20.7 GPa Fiberglass Weave Pattern Plain Weight 81 g/m " Thickness mm Breaking Strength 1.79 Kg/mm Heat shrinking tube Shrink ratio 2:1 Longitudinal shrinkage 5-10% Activation temperature 80 C Operating temperature -55 C to 125 C Inside diameter before activation (mm) 6.50 Inside diameter after activation (mm) 3.18 HAS Microfibers Heat activated shrinking tubes, commonly used to protect electrical wiring and connections, were used to create HAS microfibers. These tubes shrink to ½ of their original diameter when they are heat activated. They were cut in spiral shape that the longest dimension (the length) of microfibers would shrink the most (i.e., 50%, Figure 2). This particular shrink tubing was selected with a critical shrink temperature (80 C) to be less than the continuous service temperature of epoxy (121 C) to prevent any damage during the shrinkage of microfiber. The average cross sectional areas of HAS microfibers were 1.16 mm " (4 mm x 0.29 mm). It was controlled such as a way that the cross sectional areas of HP microfibers are almost same as those of HAS microfibers (Figure 2). HP Microfibers HP microfibers were prepared by shrink-activating HAS microfibers prior to applying into the lay-up composite. HAS microfibers were put in an oven for 2 minutes at 100 C. The average cross sectional areas of HP microfibers were 1.12 mm " (2 mm x 0.56 mm), similar to those of HAS microfibers. Also, shrinking ratios in three-dimensions were calculated. HAS microfibers were prepared as 4 mm in length and 1 mm in width, and half of them were heat activated. The lengths, widths, and heights of 10 randomly selected HAS and HP microfibers were measured. The results are presented in Figure 3. 5 mm Figure 2. HAS microfiber to be stitched to the laminate composite. SPE ANTEC Anaheim 2017 / 657

(b) Figure 3. Three-dimensional measurement results of microfibers. It was shown that the shrinking ratio were 45.9% in longitudinal, 6.4% in lateral direction. There was 94.

4 g) of fiberglass were sewn with a HAS or HP microfiber. The first stitch was sewn in the middle of the fiberglass. Seven gram of epoxy was prepared. Three grams of epoxy were applied to 15.

5 g of epoxy was applied on the bottom set of fiberglass. Another sheet of Teflon paper was put on the epoxy layer, and 0.5 g of epoxy was added on it.

3 (b) Figure 3. Three-dimensional measurement results of microfibers. It was shown that the shrinking ratio were 45.9% in longitudinal, 6.4% in lateral direction. There was 94.6% expansion in vertical direction. Laminate Composite Sample Preparation A fiberglass fabric was cut to a cm by 2.54 cm (6 inch by 1 inch) rectangular shape. Twelve sheets (5.4 g) of fiberglass were sewn with a HAS or HP microfiber. The first stitch was sewn in the middle of the fiberglass. Seven gram of epoxy was prepared. Three grams of epoxy were applied to cm by 2.54 cm rectangular shape on the Teflon paper. Fiberglass sewn with a HAS or HP microfiber was added on the epoxy. Fiberglass was separated in 6 sheets each, and 0.5 g of epoxy was applied on the bottom set of fiberglass. Another sheet of Teflon paper was put on the epoxy layer, and 0.5 g of epoxy was added on it. The other 6 sheets of fiberglass was layered on the epoxy-covered Teflon paper. 3 g of epoxy was poured on the fiberglass. Finally, the other Teflon paper sheet was added on the pre-cured epoxy-fiberglass composite, and pressed it with a weight. The composite were left for 20 hours of curing at room temperature. After 20 hours, the samples were put into the oven at 100 C for 3 minutes. After 3 minutes, the specimens were left for 12 hours for full curing. After the matrix was completely hardened, all the Teflon papers were taken away (Figure 4). Figure 4. Test specimens. (a) Layout, (b) Actual figures. Results and Discussions The T-peel tests were controlled by using a displacement control protocol (Figure 5). The load rate was fixed at 5 mm/min. The tests were conducted using a Family universal test machine (Model no , Test Resources Inc.). The device has a 4450 N (1000 lbf) capacity with 50 data points per second of data collecting frequency. The tests were conducted with three different Cases: (i) 1.27 cm interval stitch of single microfibers, (ii) 2.54 cm interval stitch of single microfibers, (iii) 2.54 cm interval stitch of double microfibers. (a) Figure 5. T-peel test using family universal test machine (Model no , Test Resources Inc.) with a load rate of 5mm/min. SPE ANTEC Anaheim 2017 / 658

4 Case (i): 1.27cm interval stitch of single microfibers Case (ii): 2.54cm interval stitch of single microfibers Figure 6. Case (i) of T-peel tests for laminate structures Figure 7. Case (ii) of T-peel tests for laminate structures The results in Figure 6 demonstrate that peel strengths for the Case (i): 1.27cm interval stitch using a single microfiber show that the specimens with HAS microfibers have higher peeling strengths than the specimens with HP microfibers. Compared to the specimens with HP microfibers, there was a 47.2 % improvement in the maximum peeling strengths. Compared to control specimens, there was a 2,883% improvement. The results confirm the hypothesis that introducing shrinking microfibers in composite structures can have a better interlaminate reinforcing strength than conventional passive fibers. Shrinking microfibers would reinforce the composites by inducing internal pre-stresses as well as physically closing the air pockets during shrinkage. Figure 7 shows that for the Case (ii): 2.54 cm interval stitch of a single microfiber, the specimens with HAS microfibers have higher maximum peel strengths than the specimens with HP microfibers in T-peel tests. Compared to HP microfiber reinforced specimens, the increase in the maximum strength of 27.7 % was observed. Compared to the control specimens, there was a 2,191% improvement. The results also indicate that shrinking microfibers in composite structures have a better reinforcing effect than conventional passive microfibers. The lower maximum peeling strengths for both HAS microfibers and HP microfibers compared to Case (i) indicate that interval of Case (i) (1.27 cm) would be closer to the optimum ratio to maximize the reinforcing effect of microfibers than Case (ii) (2.54 cm). SPE ANTEC Anaheim 2017 / 659

5 Case (iii): 2.54cm interval stitch of double microfibers Figure 8. Case (iii) of T-peel tests for laminate structures The T-peel test results of the specimens with HAS and HP microfibers for the Case (iii): 2.54 cm interval stitch of double microfibers indicate that the average maximum strengths of the specimens with HAS microfibers are 57.0 % higher than those with HP microfibers, as shown in Figure 8. By comparing to control specimens, there was a 3,741% improvement. Compared to Case (ii), the maximum strength increased by 70.7 %. As the volumetric ratio of through-thickness fiber increases, the matrix damage would be more likely to be caused, resulting that the peel strength would not be doubled as proportionally (Figure 9). Figure 9. Defects in matrix caused by through-thickness shrinking microfibers. Conclusion The study presented an innovative through-thickness fiber reinforcement technology for laminate structures by using in-situ heat activated shrinking microfibers in three different Cases with T-peel test results: (i) 1.27 cm interval stitch of single microfibers, (ii) 2.54 cm interval stitch of single microfibers, (iii) 2.54 cm interval stitch of double microfibers. For the Case (i), the reinforcing effect from HAS microfibers was shown by 47.2 % compared to the specimens with HP microfibers. Compared to the control specimens, it was 2,883%. For the Case (ii) and (iii), 27.7 % and 57.0 % increase in peeling strengths were resulted. By comparing to the control specimens, there were 2,191% and 3,741%, respectively. The purpose of this study is to investigate the interlaminar reinforcing effect of in-situ shrinking microfibers versus passive microfibers in laminate structures. The shrinking activation used in this study is triggered by external heat that could damage the polymeric matrix. Therefore, in real applications, selection of materials would need to be cautiously conducted considering the environment of specific applications. However, the result shown in the paper indicates that the exploitation of shrinking microfibers for through-thickness interlaminar reinforcement would be effective, and further study with shrinking microfibers with different stimulus would be worth being studied. As the further research, the effect of in-plane directional properties due to the throughthickness shrinking microfibers would be studied. Acknowledgement This research was supported in part by the University of Vermont Spark program. References 1. A. P. Mouritz, M. K. Bannister, P. J. Falzon, and K. H. Leong, Composites: Part A, 30, 1445 (1999) 2. L. Tong, A. P. Mouritz, M. K. Bannister, "3D fibre reinforced polymer composites", Esevier (2002) 3. A. P. Mouritz, and B. N. Cox, Composites: Part A, 31, 1 (2000) 4. A. P. Mouritz, B. N. Cox, Composites: Part A, 41, 709 (2010) 5. M. Bannister, Composites: Part A, 32, 901 (2001) 6. G. Dell'Anno, D. D. Cartie', I. K. Patridge, and A. Rezai, Composites: Part A, 38, 2366 (2007) 7. S. S. Wicks, R. G. D. Villoria, and B. L. Wardle, Composites Science and Technology, 70, 20 (2010) 8. E. J. Garcia, B. L. Wardle, and A. J. Hart, Composites: Part A, 39, 1065 (2008) 9. A. P. Mouritz, Composites: Part A, 38, 2383, (2007) 10. E. Triki, B. Zouari, and F. Dammak, Engineering Fracture Mechanics, 159, 63, (2016) 11. M. Tahani, A. Andakhshideh, and S. Maleki, Composite Part B, 98, 151, (2016) 12. M. Ravandi, W. S. Teo, L. Q. N. Tran, M. S. Yong, and T. E. Tay, Materials and Design, 109, 659, (2016) SPE ANTEC Anaheim 2017 / 660

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