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The sensitivity of a temperature-compensating fiber Bragg grating (FBG) sensor which has different FBGs in one fiber line was analyzed for the real time measurement of mechanical normal strain in structures. Measurement of mechanical strains of the aluminum beam surface by the double FBG sensor was performed under various heating rates and temperature range. The results were compared with those of an electrical resistance strain gage. A considerable delay in the strain measurement by the double FBG sensor during the heating process was shown, which could be quantified for the strain compensation by the introduction of a strain retardation factor to the FBG sensor.

The hybrid composite/metal beam-joint structure used for this study modeled the actual joint of the side-wall composite panel and the steel under-frame in a low-floor bus body. The fracture behaviors of the joints under transverse loadings showed that the fracture site induced by the cyclic loading was completely different from that under static loadings. For the upward loading, the pulled-out fracture of the rivet joints was dominant in the static test while a fracture of the weld joint region appeared during the fatigue cyclic test. The downward loading induced a partial delamination along the adhesive layer of which the site was different according to the static and the cyclic tests. A statistical life prediction with a two-parameter Weibull distribution for transversely upward loads revealed a large increase in the estimated life with respect to the load decrease. This was in contrast with the case of transverse downward loadings. Probability stress life curves, obtained with a high confidence level, offered statistical life prediction data for the reliability of the hybrid beam-joints.

1. 서 론 복합재의 경우 피로시험 결과가 동일 조건 하에 서도 큰 편차를 보이기 때문에 많은 시편들을 시 험해야만 정확한 피로수명을 예측할 수 있다. 하 지만 시험비용이나 시간적 제한 등의 이유로 인해 한정된 시험편만을 사용할 수 밖에 없는 경우가 많다. 따라서 소량의 시험편으로 정확한 내구수명 데이터를 얻기 위해 피로시험 결과를 통계적으로 처리하여 파괴확률 또는 비파괴확률을 부품의 수 명과 관련시키려는 시도가 점차 증대되고 있 다. (1~4) 또한 신뢰성 해석에 응용하기 위해 확률분 포곡선을 이용하여 복합재나 금속의 피로수명을 예측하는 연구가 많이 수행 되어 왔다. (5~7) 피로수 명을 추정하는 방법에는 정규분포, 대수정규분포, 와이블(Weibull) 분포의 통계량을 이용한 방법 등 이 이용되고 있다. 예를 들어, Nishijima 등 (1) 은 피 로시험 데이터를 측정한 뒤에 가중치를 이용한 probit 해석을 통하여 소표본 시험편에 대한 통계 적 해석을 정량적으로 수행 하여 확률-응력-수명(probability stress life, P-S-N) 관계를 구했다. Kim 등 (3) 은 원공을 가진 CFRP 복합재를 대상으로 Markov chain model 을 도입하여 3 개의 하중레벨(S)에서 얻은 하중-수명(S-N) 곡선과 표준화정규변수, 피로 강도의 표준편차를 이용하여 P-S-N 관계를 제시하 였다.Abstract: The reliable fatigue life for hybrid composite joint structures was estimated by a statistical method for evaluating fatigue life; the results of the fatigue test varied widely. Cyclic bending tests were performed on a cantilever beam with a hybrid composite joint, which was developed for the body of a low-floor bus. In order to estimate the fatigue life of the hybrid composite joint structure by comparing the data obtained during the fatigue tests, the most suitable probabilistic density function among the normal, lognormal, and Weibull distributions was selected. The probabilistic-stress-life (P-S-N) curves calculated by using the selected Weibull distribution was suggested for process of statistical fatigue life estimation and reliability design. † Corresponding Author, nschoi@hanyang.ac.kr