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Fluidelastic instability (FEI) remains the most important mechanism of flow-induced vibrations that can potentially cause large amplitude vibrations and early failure of nuclear steam generator tubes. In the present work, experimental tests are conducted to measure the unsteady and quasi-steady fluid forces acting on a normal triangular tube array of P/D=1.5 subjected to single-phase (water) cross-flow. The quasi-steady forces together with unsteady fluid forces are then used to estimate the time delay between the central tube motion and fluid forces on the tube itself. The time delay effect for the quasi-steady fluidelastic instability model is derived in the frequency domain in the form of an Equivalent Theodorsen Function. The results are compared to the first Equivalent Theodorsen function developed for rotated triangular array by Li and Mureithi [1]. Using the Equivalent Theodorsen Function, a stability analysis is carried out to predict the critical velocity for fluidelastic instability in a normal triangular array subjected to single-phase flow. The predicted stability threshold is consistent with previous published experimental data. The results show that the normal triangular array is more stable than the rotated triangular array.

Alloys such as the 354.1 (Al–9 wt% Si–1.8 wt% Cu–0.5 wt% Mg) alloys show a greater response to heat treatment as a result of the presence of both Cu and Mg. These alloys display excellent strength values at both low and high temperatures. Additions of Zr, Ni and Mn are expected to maintain the alloy performance at still higher temperatures. The present study was carried out to investigate the effects of these additions on the tensile properties of 354.1 alloy castings at ambient and at high temperature (250 °C), using different holding times at testing temperature. Alloys were prepared using 0.2 wt% Ti grain-refined 354.1 alloy, comprising alloy R (354.1 + 0.25 wt% Zr) considered as the base or reference alloy, alloy S (354.1 + 0.25 wt% Zr + 2 wt% Ni) and alloy U (354.1 + 0.25 wt% Zr + 0.75 wt% Mn), containing Ni and Mn. The latter two alloys perform better under high-temperature conditions following 1-h stabilization at 250 °C, displaying strength values of 226.64/230 MPa UTS and 225/226 MPa YS, respectively. For a prolonged stabilization time (200 h at 250 °C), however, the strength values are reduced to 111/96 MPa UTS and 90/78 MPa YS. The decrease in strength and increase in ductility observed after prolonged stabilization are related to alloy softening, which occurs because of the overaging conditions during which the equilibrium precipitates form, leading to loss of coherency strain between the precipitates and the matrix.