本研究主旨在求得高拉力SD690鋼筋之S-N曲線圖，故以鋼筋混凝土梁進行疲勞實驗，記錄SD690鋼筋於不同應力差幅(fr)之反覆載重作用下，所對應之疲勞壽命，並於靜載重與疲勞載重的過程中，記錄梁中點撓度及混凝土應變，且參考梁之撓度、裂縫最大寬度及頂部混凝土壓應變，以探討其疲勞行為。 從文獻[5]可以得知鋼筋的強度較高者，疲勞極限(fr,limit)會相對地較大，代表高強度鋼筋具有較良好的抗疲勞特性。於目前實驗結果可得知，高拉力SD690鋼筋的疲勞特性與文獻[5]所提出之鋼筋疲勞壽命經驗式的計算結果相近。 試體梁之勁度會隨著反覆加載的次數增加而降低，尤其是在疲勞試驗初期，其勁度降低的幅度最為劇烈；而撓曲裂縫寬度也會隨著其反覆受力的次數增加而變寬，以及其最終疲勞斷裂位置，將位於撓曲裂縫寬度最大之處。另外，SD690鋼筋在發生疲勞斷裂之後，於斷面所產生之疲勞裂紋區域占全斷面的比例非常小，不同於SD420鋼筋之疲勞特性。 ;This study is to obtain the S-N curve of SD690 high-strength reinforcement. Therefore, the fatigue test is preformed to observe the fatigue behavior of RC beams with SD690 steel reinforcement. In the test, the cyclic fatigue loading, the mid-span deflection, concrete strain, the maximum width of concrete crack, and the compressive strain of top sectional concrete are measured for studying the fatigue characteristics of the New RC beams. The test results showed that the fatigue life of the higher strength reinforcement SD690 Bars can be predicted by the empirical equations proposed by the literature [5]. The SD690 bar has the higher fatigue limit(fr, limit), and the better fatigue resistance than SD420 reinforcement. The results also concluded that the stiffness of the New RC beams will decrease as the increase of the number of repeated fatigue loadings, especially in the about first one fifth of the fatigue cycles. That is, the reduction of RC beams stiffness is decreasing severely, and the width of flexural crack becomes wider as the number of repeated loadings increases. The final fatigue fracture locations of all the beam specimens were occurred at one of the top loading points. In addition, fracture area of the SD690 reinforcements is about 7% of total rebar sectional area, which is much smaller than the fatigue area of the SD420 reinforcement, about 50%.
