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Eingeladener Vortrag
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Common air-coupled transducers for non-destructive testing consist of a piezocomposite material and several matching layers. Better acoustical matching to air is achieved by transducers based on charged cellular polypropylene (PP). This material has about hundred times lower acoustic impedance than any piezocomposite, having about the same piezoelectric coefficient. The piezoelectric properties of cellular PP are caused by the polarization of air cells. Alternatively, a ferroelectret receiver can be understood as a capacitive microphone with internal polarization creating permanent internal voltage. The sensitivity of the receiver can be increased by applying additional bias voltage. We present an ultrasonic receiver based on cellular PP including a high-voltage module providing bias voltage up to 2 kV. The application of bias voltage increased the signal by 12 to 15 dB with only 1 dB increase of the noise.
This receiver was combined with a cellular PP transmitter in through transmission to inspect several test specimens consisting of glass-fiber-reinforced polymer face sheets and a porous closed-cell PVC core. These test specimens were inspected before and after load. Fatigue cracks in the porous PVC core and some fatigue damage in the face sheets were detected. These test specimens were originally developed to emulate a rotor blade segment of a wind power plant. Similar composite materials are used in lightweight aircrafts for the general aviation. The other inspected test specimen was a composite consisted of glass-fiber-reinforced polymer face sheets and a wooden core. The structure of the wooden core could be detected only with cellular PP transducers, while commercial air-coupled transducers lacked the necessary sensitivity. Measured on a 4-mm thick carbon-fiber-reinforced polymer plate, cellular PP transducers with additional bias voltage achieved a 32 dB higher signal-to-noise ratio than commercial air-coupled transducers.
An intermediate-scale fire testing approach on the structural integrity of lightweight materials
(2013)
Carbon or glass über composites and Sandwich structures, the lightweight materials of choice for aviation, naval, offshore and construction show an enormous energy saving potential. Their combination of excellent specific mechanical properties, high corrosive resistance and thermal insulation properties in combination with various adoptable fabrication techniques leading to mass and fuel cost reduction. The most limiting single factor for a wider use of fibre reinforced plastics (FRP) in particular as elements for structural application is believed to be their fire behaviour (Mouritz and Gibson, 2006). FRPs promote burning by themselves consuming the stabilizing polymeric matrix while embedded fibers (glass, carbon) persisting the flame (Mouritz et ah, 2006). Already at elevated temperatures (100 - 200 °C) the matrix softens with a loss in mechanical properties (Perret et al., 2011, Mouritz and Gibson, 2006). For this reason the stability of the structural component is decreased severely. Fire behavior becomes the major hazard to worry about, increasingly demanding targetoriented investigation, suitable testing and tailored development.
Experimental approaches in the bench-scale have been proposed to investigate the structural integrity in the past (La Delfa et al., 2009, Gibson et al., 2010, Seggewiß, 2011, Mouritz and Gardiner, 2002, Schartel et al.). Ascribed to the small-scale neither the mechanical properties nor the effects of fire may be represented satisfactorily. Flence, the task is to perform more realistic investigations under adequate compressive loads in fully developed fires, based on suitable specimen sizes. Also (La Delfa et al., 2009)) have announced that it is evident that larger scale test of composites are needed. The aim of this study is to present a developed intermediate-scale test setup to perform more realistic investigations (Hörold et al.). Mechanical loading is generated by a column furnace in terms of compression due to a more severe response of specimens in fire tests (Seggewiß, 2011, Gibson et al., 2012, Feih et al., 2008, Feih et al., 2007). An oil burner used to determine the burnthrough resistance of thermal/acoustic insulation materials provides fire directly onto one side of the specimen (Federal Aviation Administration, 2003). Generating a fully developed fire the NexGen burner offers a homogenous heat flux of ~ 180 kW/m2. The intermediate-scale is addressed by specimen sizes either 500 x 500 mm or 1000 x 500 mm with a maximum thickness of 50 mm. The specimen attachment is realized by a compression device that was designed to apply the compressive loads, figure 1. The test setup for specimens with component like dimensions allows realistic investigations up to structural failure in absence and presence of fire load. A first test series was carried out with different levels of loading while the fire remained unchanged. Failure mechanisms, temperature distributions, diversity of FRPs regarding fiber, matrix, lay-up and core as well as flame retardant Systems are in the scope of investigation.
Near-infrared spectroscopy (NIR) turned out to be well suited for analyzing the degree of cure for epoxy systems. In contrast to dynamic scanning calorimetry (DSC), where the released heat of reaction determines the degree of epoxy conversion indirectly, NIR spectroscopy is able to determine the conversion directly by analyzing structural changes. Therefore, a new heatable NIR cell was equipped with an integrated thermocouple, which enables the real sample temperature to be controlled and monitored in situ during epoxy curing. Dynamic scans at different heating rates were used for kinetic modelling, to define kinetic parameters and to predict real curing processes. The kinetic models and their parameters were validated with an isothermal and a more complex multi-step curing scenario. Two available commercial epoxy systems based on DGEBA were used with an anhydride and with an amine hardener. NIR results were compared with DSC data. The simulated conversion predicted with a model fitted on the basis of NIR and DSC dynamic scans showed good agreement with the conversion measured in the isothermal curing validation test. Due to the proven reliability of NIR in measuring the reaction progress of curing, it can be considered a versatile measurement system for in situ monitoring of component production in the automotive, aerospace and wind energy sectors.
Assessing the structural integrity of carbon-fibre sandwich panels in fire: Bench-scale approach
(2019)
The fire resistance of lightweight sandwich panels (SW) with carbon fibre/epoxy skins and a poly(methacryl imide) (PMI) foam core is investigated in compression under direct application of a severe flame (heat flux=200 kW m−2). A bench-scale test procedure was used, with the sample held vertically. The epoxy decomposition temperature was quickly exceeded, with rapid flash-over and progressive core softening and decomposition.
There is a change in failure mode depending on whether the load is greater or less than 50% of the unexposed failure load, or in other words if one or two skins carry the load. At high loads, failure involved both skins with a single clear linear separation across each face. There is an inflection in the failure time relationship in the ∼50% load region, corresponding to the time taken for heat to be transmitted to the rear face, along with a change in the rear skin failure mode from separation to the formation of a plastic hinge. The integrity of the carbon front face, even with the resin burnt out, and the low thermal diffusivity of the core, both play key roles in prolonging rear face integrity, something to be borne in mind for future panel design. Intumescent coatings prolong the period before failure occurs. The ratio of times to failure with and without protection is proposed as a measure of their effectiveness. Apart from insulation properties, their adhesion and stability under severe fire impact play a key role.
Im Rahmen des Vortrags werden die neusten Ergebnisse aus dem Fachbereich 5.3 zur Beschreibung des Ermüdungsverhaltens von FKV mittels Mikromechanischer-Modelle präsentiert. Explizit wird der theoretische Ansatz am Beispiel von GFK unter thermomechanischer Beanspruchung hergeleitet und an Hand von Versuchsergebnissen verifiziert.