Ferrous Fibre Network Materials for Jet Noise Reduction in Aeroengines Part II: Thermo‐Mechanical Stability

Abstract

As outlined in Part I, a promising lightweight material for jet noise attenuation is a highly porous metal, made by bonding metallic fibres into an open network. Results obtained in Part I indicated that major reductions in acoustic intensity, of the order of 5-10 dB in the frequency range of interest (1-10 kHz), are attainable by the introduction of such acoustic modules, without significant penalties in engine thrust or fuel consumption. Stainless steel grade 304 was chosen because of its combination of good high temperature mechanical properties and excellent oxidation resistance (scaling temperature ∼ 850°C). [1] The fibres are produced by melt extraction, a process which is economically attractive and creates fibres with fine microstructure and relatively high strength. Network material is produced by solid-state sintering. There is scope for controlling void content over a relatively wide range (∼ 70-95 %). Furthermore, selection of fibre diameter and length, and of consolidation conditions, gives good control over network architecture (distributions of fibre orientation and inter-joint length) and a capacity to generate gradients of structure, which have been shown to have potential for improvement of broadband noise attenuation. [2,3] In terms of acoustic performance, the choice of alloy is unimportant (although the network architecture may be significant). However, there are other important aspects to the component performance. In particular, the fibre network module must retain mechanical integrity when subjected to high temperature (∼ 650-800°C) and high velocity gas flows (∼ 200-300 ms -1 ), with superimposed fluctuations and cycles. This is demanding, since such conditions could lead to excessive plasticity, creep deformation and/or fatigue cracking. While it would presumably be possible to replace acoustic damping components at regular intervals, it is clear that rapid gross deformation or damage would be unacceptable. The thermal environment in a jet engine exhaust rules out all polymeric and macromolecular materials, but various metals and ceramics are stable at these temperatures. However, the thermo-mechanical stability requirements generated by exposure to thermal shock, thermal cycling, mechanical vibration and high velocity gas impact are quite demanding. Metals are much more likely to be able to meet these requirements than ceramics. Moreover, while many types of highly porous metal exhibit relatively poor toughness (under tensile loading) and mechanical durability, [4][5][6][7][8] metallic fibre network materials can be relatively strong and tough, provided that the fibre-fibre joints are well bonded together. [9][10][11][12][13][14] There are techniques available for the manufacture and processing of bonded fibre networks which facilitate the production of strong inter-fibre joints.The fracture behaviour (tensile strength and fracture energy) of metallic fibre network materials has been relatively little studied. Ducheyne et al. [15] measured the tensile strength of sintered 304 stainless ...

Compressive mechanical properties of metal fiber sintered sheets at different strain rates

“…Based on the stress wave theory, the regions separated by heavy lines can be divided into three types: (0, 2, 9), (1,5,6,10,13) and (3,4,7,8,11,12,14). 0, 2, 9 are regions with constant value and 1, 5, 6, 10, 13 are regions with simple waves, in which the value of the physical quantity increases or decreases monotonically along the across characteristic line direction.…”

“…0, 2, 9 are regions with constant value and 1, 5, 6, 10, 13 are regions with simple waves, in which the value of the physical quantity increases or decreases monotonically along the across characteristic line direction. 3, 4, 7, 8, 11, 12, 14 are regions consisting of Picard mixed problem regions (4,7,11,12) and Darboux boundary value problem regions (3,8,14).…”

Mixed-matrix membranes of zeolitic imidazolate framework (ZIF-8)/Matrimid nanocomposite: Thermo-mechanical stability and viscoelasticity underpinning membrane separation performance

“…10(a), except for the fact that fracture occurs at a considerably lower strain value compared to the unannealed counterparts. To be able to compare differential membrane toughness property, herein we propose to characterise the fracture energy (G f ) per unit volume of membrane material, using the following relationship [71], which literally sums up the area encompassed by the σ-ε curves:…”

Investigation of transport property of fibrous media: 3D virtual modeling and permeability calculation

“…In this work, the orientation parameter is depicted in spherical coordinates with a thickness angle θ (0 < θ < π/2), and a material plane angle φ (0 < φ < 2π), respectively. Analogous to most fibrous media [22,23], fibers in PMFSS are randomly distributed in the material plane with anisotropy observed in the through-thickness direction. Various methods have been developed to model the anisotropy of fiber orientation, such as the anisotropy tensor [6,18] and the specific distribution (e.g., uniform distribution) which have a constant mean value [28].…”