The characteristics of double taper shaped beam of the fibre reinforced plastic materials was analysed. The optimal taper ratio was proved to be 1/2, and the effect of fibre off-axis in the tapered region on the beam stiffness was negligible. On the basis of this, the longitudinal type, asymmetric, double tapered GRP leaf spring was designed for replacement of the four leaf steel spring in the rear suspension system of small passenger car. The 2-D and 3-D stress analysis showed that $\sigma_{11}$ is critical and that the unidirectional structure is optimal. Two kinds of prototype were fabricated with E-glass/epoxy and S2-glass/epoxy, using the dynamic dielectric analysis to optimize the curing process. The a.c field of $\approx 10 KHz$ was suitable for the monitoring of the changes of dielectric constant during the early stage of the curing process.
The weight saving of approximately 50% was possible with the prototype GRP leaf spring compared to the conventional steel spring of the same spring constant (2.3Kg/mm). The prototypes had the fatigue life more than 5 times than the steel spring under $265Kg±70mm$ cyclic load without hysteresis, and had fail safe characteristics. The stress wave factor measurement with the acousto-ultrasonic method during the road test of the prototypes indicated that the damage was not initiated during the 14,000 Km road test.
종형 비대칭형 판스프링의 수직하중 및 wind-up 모멘트 하에서의 거동과 이중테이퍼 FRP 판스프링의 섬유 off-axis에 따른 굽힘특성해석 및 2-D, 3-D 응력 해석을 하여, 최적 테이퍼비는 $\frac{1}{2}$이고 가장 critical한 응력 요소는 표면에서의 $σ_{11}$ 이라는 결론을 얻었다. 이에따라, optimal model을 설계하여, S2-Glass/Epoxy와 E-Glass/Epoxy로 prototype를 제작하였다. 제작시, 동적 유전성 측정으로 에폭시의 gelation time을 탐지함으로써 void 를 최소화하였고, 그 과정에서 교류 전압의 주파수는 10 KHz 정도가 가장 적합하다는 것을 관찰하였다.
Prototype는 같은 스프링상수 (2,3 kg/mm)를 갖는 철강재 겹판스프링 (8.44 kg)의 50% 이하의 무게로, 하중-변위곡선상에 hysteresis가 없었고, $265 kg ±70 mm$의 반복 하중 하에서 요구조건 (2×10^5 cycle)의 5배 이상의 피로수명을 갖고 있었으며, catastropic failure가 발생하지 않은 파손 안전특성을 갖고 있었다. 또한, 주행시험중 acousto-ultrasonic 방법으로 stress wave factor를 측정한 결과는 14,000 km의 주행후 damage가 생성하지 않음을 보여 주었다.
