| Modern structural design technique accept higher values of working stresses in order to utilize the full load carrying capacity of the material; hence fatigue 'becomes one of the important failure criteria and the present thesis is an attempt to clarify t)ae significance of fatigue strength and behaviour of structural components from a civil engineering point of view, and as such deals with the three basic parameters which are the maximumnand minimum stress of stress cycles and corresponding life under repetitions of such stress cycles. Few other factors of interest, like the effect of stress concentrations (which cannot be avoided in a civil engineering structure), application of plastic loads, and cumulative damage effect have also been discussed. In order to study these aspects of fatigue some existing theories and relations have been selected and the particular metallurgical aspects which seem to be of relevance are also reviewed. The experimental part of the thesis consists of testing of beams and sheet specimens cut from the beam web of an al uminium alloy (HE 30 WP) and the discussions of the results. Apart from establishing S-N curves for beam and sheet specimens, results of cumulative damage tests showing the effect of sequence of loading and the effect of occassional plastic load applications on a. beam under bending fatigue has been reported. An attempt has also been made to establish a simple relation stating the dependence of life on the stress range. |
