This article presents a numerical optimization procedure of continuous gradient porous layer properties to achieve perfect absorption under normal incidence. This design tool is applied on a graded porous medium composed of a periodic arrangement of ordered unit cells allowing to link the effective acoustic properties to its geometry. The best microgeometry continuous gradient providing the optimal acoustic reflection and/or transmission is designed via a nonlinear conjugate gradient algorithm. The acoustic performances of the so-designed continuous graded material are discussed with respect to the optimized homogeneous, i.e. non-graded, and monotonically graded material. The numerical results show a shifting of the perfect absorption peak to lower frequencies or a widening of the perfect absorption frequency range for graded materials when compared to uniform ones. The results are validated experimentally on 3D-printed samples therefore confirming the relevance of such gradient along with the efficiency of the control of the entire design process.

The acoustic behavior of 3D printed micro-lattices is investigated to assess the impact of defects induced by the Fused Deposition Modeling technique on the parameters of the equivalent uid medium. It is shown that the manufacturing process leads to three types of non-trivial defects: elliptical lament section, lament section shrinkage and lament surface rugosity. Not considering these defects may lead to acoustic predictions errors such as an underestimation of around 0.1 of the rigid backing absorption coecient. Inverse characterization of seven homogeneous samples allows to t the acoustic prediction model considering this kind of defects.

The purpose of this article is to experimentally study the damping of composite sandwich beams with lightweight honeycomb core. The top and bottom facesheets are made of carbon/epoxy layers with partial interleaved viscoelastic layers. A new damping approach consisting of selectively targeting the inflection points of the bending mode shapes is proposed. At the nodes, the shearing deformation in the beam is maximal, and so is the strain in the viscoelastic layers. The experimental investigation of damping is made by means of standard impact tests using an instrumented hammer performed on beam specimens. The nodes are determined experimentally by moving a small accelerometer along the beam axis and by measuring the amplitude of the acceleration at each point. This novel damping approach keeps the damping ratio as high as the ratio obtained with standard (full coverage) surface damping treatment while reducing the added mass by almost 50%. A comparison of the results obtained in this study with experimental and numerical results found in the literature leads to the conclusion that the most efficient way of damping this type of sandwich structure is to modify and/or improve the viscoelastic properties of the core.

No abstract

No abstract