Evaluation of Compression Molding Simulation with Compression Properties of Carbon Fiber Prepreg

탄소 섬유 프리프레그의 압축 물성을 고려한 복합재 고온 압축 성형 해석 평가

Abstract

In order to optimize the prepreg compression molding (PCM) process, the forming simulation is required to cope with any problems that may be raised during the process. For the improvement of simulation accuracy, the input data of material property should be measured accurately. However, most studies assume that the compressive properties of the prepreg are identical to the tensile properties without quantifying them separately. Therefore, in this study, the in - plane compressive properties of the prepreg are presented to improve the accuracy of the forming simulation. As a result, the compressive modulus of the fibers was measured to be about $10^{-2}$ times lower than the tensile modulus. Also we designed a square-cup mold with a tilting angle of $110^{\circ}$ to simulate the prepreg formability during the high temperature compression mold process. Shear angles were measured at each corner, which were compared with the simulation results. It was observed that the simulation results using the accurate compressive properties of the prepreg showed a similar trend with the experimental results. It was confirmed that the measured data of the in-plane compression property improved the accuracy of the forming simulation results.

프리프레그 압축 성형(PCM, Prepreg Compression Molding) 공정을 최적화 하기 위해서 성형 해석을 통해 공정 시 나타날 문제를 사전에 예측할 필요가 있다. 해석 정확도를 높이기 위해서는 성형 물성을 구할 때 정확한 물성 측정이 필요하다. 그러나 대부분의 연구는 프리프레그의 압축 물성을 따로 구하지 않고 인장 물성과 동일하다고 가정하여 사용하고 있다. 따라서 본 연구에서는 성형 해석의 정확성을 높이기 위해 섬유의 면내 압축 물성 실험법을 제시했으며 측정 결과, 섬유의 압축 강성은 인장 강성에 비해 약 $10^{-2}$배 낮게 측정되었다. 실제 프리프레그의 성형성을 모사하기 위해 경사면($110^{\circ}$)을 갖는 정사각형 컵 금형을 설계 및 제작하였고 이를 이용한 프리프레그 고온 압축 성형성 평가를 수행하였다. 압축 물성 영향성 확인을 위해 금형 내 취약 지점으로 예상되는 각 코너 부근에서의 전단각을 측정하였으며 동일한 위치에서의 해석 결과와 실험 데이터를 비교하였다. 비교 결과 섬유의 압축 물성이 반영된 해석 결과에서 실험값과 유사한 패턴이 관찰되었으며 면내 압축 물성 반영이 성형 해석결과의 정확도를 향상시키는 것을 확인하였다.

Fig. 1. Thickness measurements for (a) UD and (b) PW carbon

Fig. 2. Viscosity for the fast-cure epoxy resin using dynamic scan

Fig. 3. Determination of the shear angle in Zone A from deformed angle (θ) during the tensile (F_i) and shear (N_i) load

Fig. 4. Measurement of bending properties of fabric proposed by ASTM D1388

Fig. 5. Experimental apparatus for coefficient of friction (COF)

Fig. 6. Measurement of in-plane compression properties of prepreg by DMA

Fig. 8. Non-linear stress-strain curves for (a) UD 0°, PW carbon and (b) UD 90° at 100°C

Fig. 9. Shear stress-shear strain curve for (a) PW carbon and (b) UD carbon

Fig. 10. In-plane compression test results for UD and PW carbon

Fig. 13. Magnified image (22.5 x) of deformed angle of the outer layer of prepreg laminate (PW carbon) at 2-d

Fig. 14. Predicted shear angles without compression properties of prepreg using the PAM-FORM simulation (From top view)

Fig. 15. Predicted shear angles with compression properties of prepreg using the PAM-FORM simulation (From top view)

Fig. 16. Comparison of measurement of shear angle at each position near the square-cup corner with simulation results

Fig. 7. (a) Cross-sectional view and (b) top view images of lower mold and (c) open and (d) closed stage of thermoforming experimental apparatus

Fig. 11. (a) Outside and (b) inside images and (c) enlarged corner images at outside of thermoformed product

Fig. 12. (a) Shear-dominated regions of the PW carbon prepreg at four different area in each corner of square-cup; (b) Enlarged images of each position in Area 1

Table 1. Physical properties of the different types of prepreg

Table 2. Calculation of bending stiffness using measured overhang length (O), thickness (t) and areal weight (W) for each prepreg

Table 3. Coefficient of friction (COF) of eight different patterns against our process parameters

Table 4. Comparison of tensile stiffness with compression stiffness of prepreg.

Table 5. Average shear angle at 4 different points of each corner of the square-cup