차체 구조용 CFRP 모자형 부재의 충돌 안전성능 평가

Abstract

The ability to protect passengers on an automobile accident depends on the condition of the collision, structural integrity, etc. The front-end members of vehicles must absorb the impact energy effectively to ensure passenger’s safety in front-end collision. Therefore, the designing vehicles should be more concerned on the aspect of securing safety performance, the while, it also should consider reducing weight of vehicle structural member. CFRP(Carbon Fiber Reinforced Plastics) of the advanced composite materials as structure materials for vehicles, has a widely application in lightweight structural materials of air planes, ships and automobiles because of high strength and stiffness. In the study, experimental investigations are carried out for The CFRP single hat shaped member and CFRP double hat shaped section member in order to study the effect of various stacking condition and shape of section. Static and impact collapse tests were performed with change of the stacking condition, such as fiber orientation angle, interface number. Collapse mode and energy absorption characteristics were analyzed. Following the above study, conclusions are drawn as below;

1. According to stacking angle, CFRP single hat shaped member and CFRP double hat shaped member show transverse shear, laminar bending, brittle fracture, localized collapsing, and its combination. Smaller CFRP stacking angle shows the significant energy absorption via laminar bending, and the energy absorption mode is changed to localized collapsing and transverse shear mode along with stacking angle.

2. For the static collapsing characteristics for single hat shaped member and double hat shaped CFRP member, energy absorption is increased when stacking angle is reduced from 90 degree to 15 degree. This can be elucidated by the energy absorption mode. When stacking angle is small, laminar bending and fiber fracture is the main mode of energy absorption. If angle is increased, transverse shear without fiber fracture will absorb high amount of energy. However, cracking of planar member and “ㄷ” shaped member in CFRP single hat shaped member is not observed for double hat shape of CFRP member. When the load is applied to CFRP single hat shaped member, cracking is occurred at the flange due to asymmetry of planar member. For CFRP double hat shaped member, symmetry of “ㄷ” shaped member prevent the cracking at flange. Therefore, CFRP single hat shaped member does not endure the sufficient amount of load because of the separation of “ㄷ” shaped member and planar one. Otherwise, CFRP double hat shaped member does not show the separation of “ㄷ” shaped member, and the better energy absorption characteristics because of flange area and increased number of edges.

3. For the member stacked in 0/90 degree and 90/9 degree, “ㄷ” shaped member shows the combination of collapsing modes, such as transverse shear and laminar bending, which can be found in the collapsing of CFRP single hat shaped member. However, cracking of planar member and “ㄷ” shaped one, which can be found in the member with single stacking angle, is not achieved. At the flange, combined collapsing mode of laminar buckling and basal fracture leads the better energy absorption characteristics, because the corresponding mode is not exhibited for the flange of CFRP single hat shaped member.

4. Impact collapsing properties of CFRP single and double hat-shaped members with 15 degree stacking shows the progress of collapsing along with fiber direction for “ㄷ” shaped member due to brittle fracture with transverse shear and laminar bending. At the flange, collapsing mode with inward and outward expansion is shown. This collapsing mode absorbs the energy from the friction of laminar buckling according to load face, the corresponding movement along with collapsing surface, and laminar bending from interlaminar cracks and intralaminar cracks. For the member with 45 degree stacking, the similar collapsing mode for 15 degree stacking is achieved. Member with 90 degree stacking is collapsed in crush mode due to basal fracture in transverse direction for “ㄷ” shaped member and flange. This collapsing mode absorbs the most of energy through basal fracture of laminar buckling in transverse shear mode.

5. For the member in 0/90 degree stacking, “ㄷ” shaped member is collapsed in brittle fracture mode combined with laminar bending and transverse shear, which is similar to the collapsing mode for CFRP single hat shaped member. In addition, flange area experiences laminar bending from the propagation of intralaminar and interlaminar crack and transverse mode with inward and outward expansion. Laminar buckling in 0 degree stacking causes the laminar bending to outward direction because of inter and intralaminar crack propagation. Fiber stacked in 90 degree from axial direction is fractured via blocking laminar bending of fiber buckling with 0 degree stacking. Therefore, fiber experiences bending, but closes to fracture. It shows the repeated cycle of crack propagation and bending/fracture of laminar buckling. Member with 90/0 degree stacking is collapsed in a way similar to the one from 0/90 degree stacking for “ㄷ” shaped member and flange. However, the laminar buckling, which is stacked with 0 degree stacking at the outermost layer, is expanded to outward direction. This collapsing mode is profound to be with laminar buckling with 0 degree stacking at the outermost layer. Fiber fracture, which is dominant for 0/90 degree stack is relatively minimal.

6. Energy absorption characteristics for static collapsing and shock collapsing test on CFRP single and double hat shaped member show the similar behavior. CFRP double hat shaped member shows 43% higher energy absorption compared with single hat shaped member according to the stacking angle variation during static collapsing test. The number of interface is also corresponded to the similar energy absorption (90 degree for outermost layer), which is CFRP double hat shaped member shows 62% higher energy absorption than the other counterpart. For 0 degree of stacking at the outermost layer, 64% higher energy absorption is achieved. For shock collapsing test, CFRP double hat shaped member shows 104% higher energy absorption with various tacking angles. Different number of interfaces also exhibits the similar results, 117% higher energy absorption for double hat shaped member (90 degree for the outermost layer stacking). For 0 degree of stacking at the outermost layer, 100% higher energy is absorbed for double hat shaped member.

7. During the shock collapsing test on CFRP double hat shaped member, 4 interfaces show the reduced energy absorption. This test specimen suffers from tearing at the edge, and “ㄷ” shaped member does not contain the sufficient amount of load compared with the others, and it induces the lower energy absorption characteristics.