Acoustics and optimisation of vibration attenuation of composite structures through banded behaviour

Noise and vibration transmission within payload and passenger compartments is a major issue for modern transport vehicles. To ensure the quality of their products, manufacturers in the transport industry are simultaneously trying to optimise the mechanical and the vibroacoustic performance of structural assemblies. In the same time, current research in most industries focuses on materials that offer low density along with superior dynamic and static performance. This goal has led to increasing use of sandwich structures and composite materials in general, whose high stiffness-to-weight ratio along with the tailoring of their properties make them quite appealing. This property, though, comes with a significant cost in their vibroacoustic behaviour, being responsible for high noise and displacement resonant vibrations. Prompted by that, elevated quality and quantity of research is about modelling the behaviour of these materials, along with conventional ones, using time and cost-efficient computational methods. These methods are used to reach the goal of enhanced stiffness, weight and vibration performance. Using efficient tools developed for the aforementioned methods, it has been demonstrated that judiciously designed periodic structures can induce vibration attenuation and stop-band behaviour in specific frequency ranges (so-called band gaps or stop bands). The mechanisms that generate band gaps, though, usually do not leave the structure's stiffness unaffected, which renders it incapable of bearing loads. In this work the potential of the application of banded schemes on load bearing structures is examined. Special care is taken so that the designs are easy-to-manufacture and plausible to be industrialised. For this to be achieved several steps are followed with each one preparing the ground for the next one. Firstly, the necessary computational tools are developed using periodic structure theory, wave finite element method and commercially available finite element software package and the effect of the pre-stress on acoustic and damping performance of composite monolithic and sandwich laminates is examined. It is shown that pre-stress (both tension/compression and pressure) must be taken into consideration when a structure is designed to achieve vibroacoustics targets and produce reliable predictions. Then, having developed the initial form of mathematical and programming tools, the banded behaviour of sandwich beams is calculated. Three different case studies are considered, with the first one being non-structural and the rest two being capable of bearing loads. This part of the work proves that unconventional but easy-to-manufacture core architectures can demonstrate banded behaviour, which makes it a lightweight and easily industrialised solution with possible high-volume applications. A rig is designed and manufactured, and a vibration experimental setup is developed consisting of an electrodynamic shaker and a single-point laser vibrometer. This setup is used to validate the numerical and finite element calculations. It is also demonstrated that band gaps can be achieved in load bearing structures, an observation which leads to the third part of the work. Having developed a more efficient version of computational tools, the master script, a way to examine numerous solutions is aimed for. Hence, a multi-disciplinary optimisation algorithm is developed which uses the master script and optimisation method to calculate the optimal solution for the geometry of a banded structure. An optimal geometry of a one-dimensional composite beam with additive manufactured stiffener is acquired, whose banded behaviour is experimentally verified. In brief, this research produces designs and manufactures specimens of one-dimensional band gap structures capable of being load-bearing part of a structure. This is achieved by developing efficient multi-disciplinary optimisation tools and experimentally validating the results by developing a vibration testing setup

Examination of acoustic insulation of composite metamaterials with negative stiffness elements inclusions

Damping is one of the most important properties of materials, being responsible for increased durability and conmfort of the structures. In this paper the damping enhancement of materials with internal oscillators is examined. Wave Finite Element (WFE) method is used to get the wave propagation of an two-dimensional layered mechanical material having embedded nonlinear internal oscillators acting as negative stiffness element. The acoustic properties of the panel are studied in a Statistical Energy Analysis (SEA) scheme

Wave propagation in pressurised composite structures with frequency band gap behaviour

In engineering there are numerous examples of structures, such as train rails and airplane fuselages, that present periodicity in their geometry or mechanical properties. It has been observed that this periodicity leads to banded frequency response after excitation. These band gaps can be engineered to isolate noise and vibrations of the periodic structure. In this paper an infinite composite sandwich beam with hollow and pressurised core cells as periodic band gap inducing factors was examined. Wave finite element (WFE) method was used to predict the effect of pressured core cells periodicity on wave propagation and band gaps generation. Three low order finite elements (FE) models were produced using commercially available FE software package. These models consisted of a small section of the simple sandwich beam with homogenous core, with hollow core and with pressurised hollow core

Mid-frequency band gap performance of sandwich composites with unconventional core geometries

In this work novel unconventional core architectures are presented which are able to induce flexural band gaps while not being detrimental for structural bending stiffness of the sandwich structures. Two different core schemes are examined with both of them exhibiting low-frequency stop bands. While unconventional, the designs of the core offer a novel solution which can be easily manufactured in high volume parts using two-dimensional automated cutting machine. A hybrid finite element and periodic structure theory scheme is employed for the calculation of the stiffness and mass matrices, and periodic structure theory is used to obtain the wave propagation of the beams. Having acquired the wave dispersion curves and the finite element analysis' results, two specimens are manufactured using carbon fibre cured plates and commercially available PVC foam as core material. Experimental measurements of the dynamic performance of the structures are conducted using a laser vibrometer and electrodynamic shaker setup

Parametric study of control of frequency banded behaviour of periodic pressurised composite structures

Periodic structures are very common in engineering, such as airplane fuselages and train rails. This periodicity has been observed to be the cause of banded frequency response after mechanical excitation. This response can be engineered so that noise and vibrations to be isolated or even annihilated. In addition to this, further methods of inducing band-gaps without weight penalty are of interest among the researchers. In this paper a parametric survey was conducted examining the impact of the core geometry and the pressure in the core cells on the suppression of the vibrations. An infinite composite sandwich beam with hollow and pressurised core cells as periodic band gap inducing factors was examined. The periodic theory was used to predict the effect of pressured core cells periodicity on wave propagation and band gaps generation. Three low order finite elements (FE) models were used in this survey, which consisted of a small section of the simple sandwich beam with homogeneous core, with hollow core and with pressurised hollow cor

Advanced carbon/flax/epoxy composite material for vehicle applications: vibration testing, finite elements modelling, mechanical and damping characterization.

Nowadays, research in automotive and construction industries focuses on materials that offer low density along with superior dynamic and static performance. This goal has led to increasing use of composites in general, and carbon fibre (CF) composites in particular. CF composites have been adopted widely in the space industry and motorsports. However, their high stiffness and low density leads to low damping performance, which is responsible for increased levels of noise and reduction in service life. On the other hand, natural fibres (NF) like flax fibres (FF) are capable of delivering a much better damping performance. A hybrid composite comprising of FF and CF can potentially deliver both on strength and higher damping performance. In this study the mechanical and damping properties of CF, FF and their hybrid composites were examined. Composites' anisotropic nature affects their response to vibrations and so traditional damping experimental setups used for metals had to be ruled out. A damping set up based on Centre Impedance Method (CIM) was adopted for the purpose of this study which was based on an ISO standard originally developed for glass laminates. Standard tensile and flexural tests were conducted in order to characterise the performance of the hybrid composite. The experimental work was accompanied by finite elements analysis (FEA). The experimental data and FEA were used to optimize the hybrid structure layup with respect to damping and structural response.Engineering and Physical Sciences (EPSRC)MSc by Researc

Acoustics and optimisation of vibration attenuation of composite structures through banded behaviour

Noise and vibration transmission within payload and passenger compartments is a major issue for modern transport vehicles. To ensure the quality of their products, manufacturers in the transport industry are simultaneously trying to optimise the mechanical and the vibroacoustic performance of structural assemblies. In the same time, current research in most industries focuses on materials that offer low density along with superior dynamic and static performance. This goal has led to increasing use of sandwich structures and composite materials in general, whose high stiffness-to-weight ratio along with the tailoring of their properties make them quite appealing. This property, though, comes with a significant cost in their vibroacoustic behaviour, being responsible for high noise and displacement resonant vibrations. Prompted by that, elevated quality and quantity of research is about modelling the behaviour of these materials, along with conventional ones, using time and cost-efficient computational methods. These methods are used to reach the goal of enhanced stiffness, weight and vibration performance. Using efficient tools developed for the aforementioned methods, it has been demonstrated that judiciously designed periodic structures can induce vibration attenuation and stop-band behaviour in specific frequency ranges (so-called band gaps or stop bands). The mechanisms that generate band gaps, though, usually do not leave the structure's stiffness unaffected, which renders it incapable of bearing loads. In this work the potential of the application of banded schemes on load bearing structures is examined. Special care is taken so that the designs are easy-to-manufacture and plausible to be industrialised. For this to be achieved several steps are followed with each one preparing the ground for the next one. Firstly, the necessary computational tools are developed using periodic structure theory, wave finite element method and commercially available finite element software package and the effect of the pre-stress on acoustic and damping performance of composite monolithic and sandwich laminates is examined. It is shown that pre-stress (both tension/compression and pressure) must be taken into consideration when a structure is designed to achieve vibroacoustics targets and produce reliable predictions. Then, having developed the initial form of mathematical and programming tools, the banded behaviour of sandwich beams is calculated. Three different case studies are considered, with the first one being non-structural and the rest two being capable of bearing loads. This part of the work proves that unconventional but easy-to-manufacture core architectures can demonstrate banded behaviour, which makes it a lightweight and easily industrialised solution with possible high-volume applications. A rig is designed and manufactured, and a vibration experimental setup is developed consisting of an electrodynamic shaker and a single-point laser vibrometer. This setup is used to validate the numerical and finite element calculations. It is also demonstrated that band gaps can be achieved in load bearing structures, an observation which leads to the third part of the work. Having developed a more efficient version of computational tools, the master script, a way to examine numerous solutions is aimed for. Hence, a multi-disciplinary optimisation algorithm is developed which uses the master script and optimisation method to calculate the optimal solution for the geometry of a banded structure. An optimal geometry of a one-dimensional composite beam with additive manufactured stiffener is acquired, whose banded behaviour is experimentally verified. In brief, this research produces designs and manufactures specimens of one-dimensional band gap structures capable of being load-bearing part of a structure. This is achieved by developing efficient multi-disciplinary optimisation tools and experimentally validating the results by developing a vibration testing setup

Examination of acoustic insulation of composite metamaterials with negative stiffness elements inclusions

Damping is one of the most important properties of materials, being responsible for increased durability and conmfort of the structures. In this paper the damping enhancement of materials with internal oscillators is examined. Wave Finite Element (WFE) method is used to get the wave propagation of an two-dimensional layered mechanical material having embedded nonlinear internal oscillators acting as negative stiffness element. The acoustic properties of the panel are studied in a Statistical Energy Analysis (SEA) scheme

Wave propagation in pressurised composite structures with frequency band gap behaviour

In engineering there are numerous examples of structures, such as train rails and airplane fuselages, that present periodicity in their geometry or mechanical properties. It has been observed that this periodicity leads to banded frequency response after excitation. These band gaps can be engineered to isolate noise and vibrations of the periodic structure. In this paper an infinite composite sandwich beam with hollow and pressurised core cells as periodic band gap inducing factors was examined. Wave finite element (WFE) method was used to predict the effect of pressured core cells periodicity on wave propagation and band gaps generation. Three low order finite elements (FE) models were produced using commercially available FE software package. These models consisted of a small section of the simple sandwich beam with homogenous core, with hollow core and with pressurised hollow core