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30 Figure 13. Cumulative damage curves for Fatigue Test 1. effects at the transverse bar locations. Figure 15(c) shows the the edge of the bar section, which is thought to have served as fracture surface of the bar, which is clearly indicative of a a crack initiator. fatigue failure. The failure of a bar at N = 155,005 under S = 45.5 ksi is very 2.4.5 Summary of Fatigue Tests close to the prediction, which is thought to be conservative. and Conclusions Therefore, an investigation of the fatigue failure surface using scanning electron microscopy (SEM) was conducted (see The adopted S-N relationship described in Appendix E Appendix E). The SEM revealed aluminum (Al) inclusions in and the two S-N pairs from the tests conducted are shown in the fracture surface and a significant silicon (Si) inclusion at Figure 16. Since both S-N pairs fall to the right of the S-N Figure 14. Cumulative damage curves for Fatigue Test 2 (fatigue failure occurred prior to obtaining the final data point).

OCR for page 30
31 (a) Location of Bar Fracture (Beam is Inverted, Cover Has Been Removed) (b) Fatigue Fracture of Bar 1 (c) Fracture Surface Figure 15. Fatigue failure of single bar in Fatigue Test 2. curve, it may be said that the specimen performance exceeded stress range to 24 ksi for the case of tension-tension fatigue that predicted by the curve (i.e., for a given stress range, S, the (i.e., fmin > 0). The adoption of Equation 10 is recommended fatigue life, N, was greater than predicted), although not by to address the unwarranted reduction in fatigue stress range a significant degree. Both tests serve as proof tests of the that results from the use of the present AASHTO recommen- AASHTO LRFD recommendations (Equation 9) and the dation (Equation 9) in conjunction with high-strength rein- proposed revision (Equation 10) that both limit the fatigue forcing steel. 70 60 Stress Range, S (ksi) 50 fatigue test #2 40 fatigue test #1 30 20 Equation E-2 10 0 10000 100000 1000000 10000000 100000000 Cycles, N Figure 16. Predicted and experimental S-N data.