A new testing facility for fully reversed tension-torsion high cycle fatigue testing has been designed. The specimens used for the test programme were solid and made from a medium carbon steel. The test programme involved a tension-torsion multiaxial non-proportional loading sequence i.e. fully reversed torsion followed by a push-pull load interruption and then the continuation of the same torsion loading to failure. The push-pull load interruption represented a significantly low damage i.e. 4% damage according to Miner's linear damage theory, and was applied after different prior torsion cycle ratios. The tests were conducted with various interruption stress amplitudes all of which had fatigue lifetimes in the high cycle fatigue region. The torsion fatigue life was found to change significantly due to the application of push-pull load interruption which was considered to cause only a minor damage due to Miner's rule. Miner's linear damage theory cannot account for the predicted cumulative fatigue damage (Sigman/N[f]) for the push-pull interrupted torsion fatigue loading sequences used in the current test programme. The fatigue life was markedly enhanced when the interruption was applied at an early stage of torsion loading whilst the effect was less prominent when the interruption was applied at a later stage of torsion loading. At higher interruption stress amplitudes the torsion fatigue lifetime was reduced considerably and the damage summation was well below the unity predicted by the Miner's rule. The inability to predict damage accumulation by Miner's rule can be attributed to the complexity in the crack growth associated with the application of push-pull interruption. Crack growth equations to represent microstructural short crack (MSC) and the physically small crack (PSC) growth were determined for the material of the form;MSC - da/dN = C[m](d[i] - a)...(1) and PSC - da/dN = C[p]a-D ...(2). Material parameters for the models were derived using torsion and uniaxial constant amplitude fatigue S-N data, no crack coalescence, branching or re-initiation was considered. The crack growth model was able to predict the fatigue life in loading cases which were dominated by an uninterrupted crack growth. However, such a model was shown to significantly underestimate the torsion fatigue life in situations where the fatigue life was affected by secondary crack initiation due to the push-pull load interruption.

Doctor of Philosophy / Department of Civil Engineering / Hayder Rasheed / Most state highway agencies do not perform routine fatigue inspections on highway signs, luminaires, and traffic signals, thereby increasing the potential for unnoticed fatigue cracking. The Kansas Highway System utilizes over 450 sign trusses, most of which have been in service for 30-45 years. In addition, to aging support structures, the structural designs these signs and signals sometimes result in significant cyclical loading due to wind gust. This study conducted fatigue evaluations using nominal axial member-specific stress ranges corresponding to a wind speed database for a 45-year period, as well as, hundreds of structural analysis simulations. Potential fatigue failure was assessed for each member of the support structure by evaluating the ratio of consumed fatigue cycles to ultimate fatigue cycles using Miner’s rule to estimate finite life. If the ratio was close to zero after 45 years or any number of actual service years, the member was expected to have a practically infinite life. If the ratio was close to 1 after the service years, the member was expected to be at the end of its life. This information can help inspectors identify for critical spots that may have developed fatigue cracks that otherwise would be difficult to detect. Two approaches were hypothesized to account for fatigue life deterministically and probabilistically. Fatigue Life Simulator Software (FLSS) was developed to manage hundreds of simulations and determine the fatigue life of all members in a structure in specific areas of Kansas. FLSS is compatible and works simultaneously with STAAD Pro Software and Sign Truss Interface provided by KDOT, to generate results. Users apply the results to study the behavior of overhead structures and identify critical spots that should be physically inspected and potentially replaced. Results in Kanas indicated a range of structural fatigue life varying by city. Modifications were made to the output files of Sign Truss Interface to incorporate American Association of State Highway and Transportation Officials (AASHTO) load cases 1 and 2 and simulate wind speed into wind pressure using the effect of the two load cases. The modification also automatically incorporated 45-years of wind speed data into the Sign Truss Interface to simulate and generate structural models to determine corresponding stresses to the wind effect.