No abstract

The good damping performance and inherent stability of viscoelastic materials in relatively broad frequency bands, besides cost effectiveness, offers many possibilities for practical engineering applications. However, for viscoelastic dampers subjected to dynamic loadings superimposed on static preloads, especially when good isolation characteristics are required at high frequencies, traditional design guidelines can lead to poor designs due to the rapidly increasing rate of temperature change inside the material. This paper is devoted to the numerical and experimental investigation in the degradation of the stiffness and capacity of a viscoelastic material induced by the thermal runaway phase, when it is subjected to dynamic and static loads simultaneously. After the theoretical background, the obtained results in terms of the temperature evolutions at different points within the volume of the material and the hysteresis loops for various static preloads are compared and the main features of the proposed study are highlighted.

The impedance-based structural health monitoring method has become a promising and attractive tool for damage identification and is considered a nondestructive evaluation technique. However, conventional impedance-based structural health monitoring studies have mainly focused on structural damage identification but not so much on statistical modeling approaches in order to determine a threshold for the decision making of the damage detection system. In this study, the impedance-based structural health monitoring technique is used in a damage detection problem considering temperature variation effects. For this aim, three aluminum 2024-T3 plates were instrumented with small lead zirconate titanate patches close to their borders, and damage was introduced in the central position of the plates, with temperature ranging from −10°C to 60°C. This article proposes a method to statistically determine a threshold for damage detection purposes using concepts of statistical process control, as well as confidence intervals and normality tests in order to obtain a diagnosis with a previously determined confidence level. Thus, this work presents a sensitivity evaluation of the impedance-based structural health monitoring technique as applied to aluminum plates under varying temperature. With the technique proposed, damage threshold levels are determined so that lead zirconate titanate patches placed approximately 280 mm from the damage inserted were able to detect saw cuts of approximately 7 mm long, with 95% confidence intervals inside the temperature range considered.

The prediction of chatter vibrations between the cutter and workpiece is important as a guidance to the machine tool user for an optimal selection of depth of cut and spindle rotation, resulting in maximum chip removal rate without this undesirable vibration. This can be done by some approaches. In this work, an analytical method is applied in which the time-varying directional dynamic milling forces coefficients are expanded in Fourier series and integrated in the width of cut bound by entry and exit angles. The forces in the contact zone between cutter and workpiece during the cut are evaluated by an algorithm using a mathematical model derived from several experimental tests with a dynamometer located between the workpiece and machine table. The algorithm results depend of the physical properties of the workpiece material and the cutter geometry. The modal parameters of the machine-workpiece-tool system like natural frequencies, damping and residues must also be identified experimentally. At this point, it is possible to plot the stability lobes to this dynamic system. These curves relates the spindle speed with axial depth of cut, separating stable and unstable areas, allowing the selection of cutting parameters resulting maximum productivity, with acceptable surface roughness and absence of chatter vibrations. Experimental face milling tests were performed in a knee-type machine, using a five inserts cutter. The results showed perfect agreement between chatter prediction and experimental tests

No abstract