Advanced Materials Research Vol. 65

Abstract: Failure of one upper anchorage element in a cable-stayed bridge and its consequent analysis concluded that the main cause of the failure was a deficient heat treatment that resulted in large micro structural grain size and low fracture toughness, vulnerable to fatigue damage. Previous research studies demonstrated that ultrasonic evaluation could provide some insight of the microstructural integrity by correlating the ultrasonic response to the grain size. Thus, this technique was used to inspect the 112 elements in service in the bridge and 16 were qualified as structurally deficient, without direct verification of the grain size, since these elements were partially embedded in the concrete structure. Late rehabilitation of the bridge considered the replacement of the 16 structural deficient anchorage elements, plus 4 elements qualified in good condition, to complete a reliability analysis for the remaining 92 elements from the statistical mechanical properties of the removed pieces. Rehabilitation made possible the confirmation of the initial diagnosis made by ultrasonic inspection and fatigue cracks were identified in some elements. This study demonstrated that the ultrasonic non destructive evaluation is highly reliable for structural integrity qualification of steel structural elements partially embedded in concrete.

Abstract: This work deals with rotating bending fatigue tests at high speed (150 Hz), carried out on aluminum alloy AISI-SAE 6061-T6 for which the highest experimental stress inside the specimen is close to the elastic limit of material. Simulation results are obtained by Visual Nastran software in order to determine the numerical stress and strain distributions inside the specimen; then, this information is used for the experimental set up. A general description of experimental test machine and experimental conditions are developed in first sections, following section present the experimental results and discussion about the observed failure origin related to discontinuities and the associated high stress zones. A principal contribution of this work is related to the analysis of fracture surfaces and its correlation with experimental fatigue endurance: a simple model is proposed for the prediction of fatigue life of this aluminum alloy under high speed rotating bending fatigue tests, based on the surface density of associated fatigue macro-plastic deformation zones close the crack initiation.

Abstract: Aircraft composite structures are mostly joined by mechanical fasteners like bolts, pins or screws. However, the effect of the presence of holes in the remaining strength of the composite structures is still being studied extensively. In this work, epoxy/glass laminates with drilled holes of different sizes were tensile tested and from these results, the residual strength was plotted. Strength vs. hole’s diameter at different fiber orientation was obtained. The fracture path and failure mechanism were identified by fractographic examination. The Point Stress Criterion (PSC) was used, in order to establish the stress intensification due to the presence of a drilled hole. A numerical model by Finite Element Method was carried out to verify the experimental results and the analytic failure predictions. A reduction of 50% in laminate strength was observed when diameter-width ratio was 0.12. The principal fracture mechanism observed in composite laminates was interface breakup. FEM results and analytic results by PSC show accuracy of 90% for predicting the damage in drilled composites.