Characterization and optimization of polymer membranes for acoustic applications

Gilbert Knapp

Research output: ThesisDoctoral Thesis

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Abstract

The rise of smartphones and their increased thinness has led to a significant increase of the cyclic loads in their miniature loudspeakers. Their polymer membranes have to fulfill two main tasks. An adequate damping performance is required in order to ensure an excellent sound quality and to prevent the membranes from high dynamic loads at resonance. Furthermore a high fracture toughness is needed to ensure a long service life. As a consequence the knowledge of these parameters is indispensable for a successful loudspeaker design. Since state of the art test methods are not capable of performing experiments at application-relevant conditions new approaches are presented in this thesis. In monotonic fracture mechanical tests it was found that miniature loudspeaker films exhibit predominantly brittle fracture modes and therefore linear elastic fracture mechanics (LEFM) seemed applicable. Since in monotonic tests also no correlation was found to component tests a fatigue test was setup at a high frequency of 100 Hz based on Wöhler and fatigue crack growth (FCG) tests. To verify the applicability of LEFM for the cyclic fatigue of thin loudspeaker films the crack tip similitude concept was applied by comparing the FCG behavior of double edge notched tension (DENT) and center cracked tension (CCT) specimens with different polymer films with thicknesses between 4.5 and 10 µm. For polyarylate (PAR), polyether sulfone (PESU) and polyethylene therephthalate (PET) films the applicability of the crack similitude concept was verified. Then effects due to temperature, the multilayer design and anisotropic material behavior were investigated. Temperature-dependent fatigue tests with PET films revealed a maximum of their fatigue strength below their glass transition at approximately 60 °C. Also it was observed that delamination processes in multilayer miniature speaker laminates only play a minor role in their fatigue behavior. Finally extrusion induced molecular orientations were identified as the cause for an anisotropic fatigue behavior of extruded films. Here crack growth was deflected by the molecular orientation in a direction of lower crack growth resistance. In order to characterize the damping behavior of miniature speaker laminates two methods based on forced frequency dynamic mechanical analysis (DMA) were established. In the so-called "speaker" mode the supports of loudspeakers were mimicked between the clamps of a modified dual cantilever DMA setup. With this setup the effects of the damping performance of both the damping layer itself and also effects due to different stiffness ratios between the stiff base and soft damping layers were characterizable. The second method is based on a DMA shear mode setup and is optimized for constrained layer damping laminates. Since both methods are limited in their frequency range the time temperature superposition principle (TTSP) was applied in order to enable extrapolations to application-relevant frequencies of up to 1000 Hz. Furthermore with the calculation of the integrated average of the mechanical loss factor tA in the application-relevant temperature range an factor was introduced which simplified the ranking of laminates regarding their damping performance. The applicability of the DMA "speaker" mode was checked by characterization of a monolayer, a FLD and CLD laminate. In concordance with the literature the CLD laminate exhibited the best damping performance, followed by the FLD laminate and the monolayer film. In a next step the influences due to different constraining layers and also the adhesion between the damping and the stiff outer layers to the damping performance in CLD laminates was investigated in DMA shear mode. Therefore laminates with different technical polymers and one laminate with release films as outer layers were characterized. Finally the effect of the damping layer thickness on the damping pe
Translated title of the contributionCharakterisierung und Optimierung von Polymermembranen für akustische Anwendungen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Friesenbichler, Walter, Assessor B (internal)
  • Pinter, Gerald Gerhard, Assessor A (internal)
  • Oreski, Gernot, Supervisor (internal)
Publication statusPublished - 2015

Bibliographical note

embargoed until 26-05-2020

Keywords

  • miniature loudspeakers
  • damping
  • fatigue strength

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