Filtern
Dokumenttyp
- Zeitschriftenartikel (1)
- Beitrag zu einem Tagungsband (1)
- Vortrag (1)
Schlagworte
- Micro cracks (3) (entfernen)
Eingeladener Vortrag
- nein (1)
Bauteile bzw. Tragwerke aus Hochleistungsbetonen müssen in der Regel gegen brandinduzierte Abplatzungen mit geeigneten Maßnahmen geschützt werden, um einen ausreichenden Feuerwiderstand im Brandfall zu gewährleisten. Die bisher wirtschaftlich und technologisch sinnvollste Methode zur Verhinderung von explosionsartigen Betonabplatzungen im Brandfall ist die Zugabe von Polypropylen-Fasern. Die Wirksamkeit der Fasern konnte zwar empirisch gezeigt werden, es stellt sich allerdings die Frage, welche Mechanismen zur Verhinderung der Abplatzungen führen. Der vorliegende Beitrag fasst bisherige Theorien zur Wirkungsweise von Polypropylen-Fasern in brandbeanspruchten Hochleistungsbetonen zusammen und stellt eine innovative Methodologie zur Erforschung der mikrostrukturellen Prozesse vor. Die Ergebnisse zeigen, dass die nach dem Schmelzen und Zersetzen der Polypropylen-Fasern frei werdenden Mikrokanäle durch eine gleichzeitig einsetzende Mikrorissbildung netzartig verbunden werden. Die Mikrorissbildung ermöglicht somit den Abbau von Eigen- und Zwangsspannungen im Beton (mechanischer Effekt) und die Entstehung eines Transportwegesystems für den ausströmenden Wasserdampf (Permeationseffekt).--------------------------------------------------------------------------- Structural members and bearing structures of high performance concrete generally have to be protected against explosive spalling due to fire exposure to guarantee a sufficient fire resistance. Up to now, the economically and technologically most worthwhile method to prevent explosive spalling is the addition of polypropylene fibres. Though the effectiveness of the fibres could be shown empirically, the mechanisms preventing explosive spalling are still debatable. The present article summarizes the existing theories concerning the mode of action of polypropylene fibres in fire exposed high performance concretes and presents an innovative methodology for analysing the micro structural processes. The results show that due to the thermal decomposition of the polypropylene fibres micro channels are created and simultaneously connected due to a netlike micro crack formation. This enables the relief of internal stresses (mechanical effect) and the formation of a permeable transport system for the escaping water vapour (permeation effect).
Nondestructive investigation of the VHCF-endurance on cyclically loaded CFRP by X-Ray-Refractography
(2016)
Carbon fiber-reinforced-plastics (CFRP) are already used under high-cycle- (HCF) and very-high-cycle-fatigue (VHCF) at relatively low loads not only in aerospace industry, but also in other growing markets like especially wind energy and increasingly also automotive industry. While the static strength of CFRP is satisfyingly predictable by means of layerwise-strenght-analysis and material data from single layers, it is more and more uncertain to predict residual strength or lifetime as the design lifetime increases. Previous studies showed that local minor cracks and inter-fiber-fracture (IFF) originate long before total failure of single layers. Reliable fatigue estimation for fiber-reinforced-plastics is not available at present. Subsequently safety-related primary structures made out of FRP are over dimensioned.
It is possible to detect the change of inner surface in a specimen by means of X-Ray-Refractography. This includes voids, intact fiber-matrix-interfaces as well as defective ones, cracks and all interfaces in plane with the X-Ray-beam. As the crack-density (including IFF) increases, the inner surface of the material correspondingly increases.
The borderline where even at high cycles no early cracks were detected was identified for selected laminates out of woven and non-crimped fabric with epoxy-matrix.
When no micro-cracks were detected up to 106 cycles, the specimens also lasted in the VHCF-regime. Furthermore for the epoxy resin LY556 in combination with carbon fiber it was distinguished that at a load minor to 50% IFF-load level, no micro cracks were detected and consequently the VHCF-endurance reached.
The integration of the set-up small-sized testing machine into the X-Ray buildup in order to achieve in-situ non-destructive detection of the crack initiation while loading the specimens (CFRP and GFRP- woven fabric and non-crimped-fabric) at the same time will increase the performance of the studies.
In earlier investigations at BAM it was shown, that the matrix has a strong influence on the micro-crack formation and finally on the total lifetime. In order to determine the influence of the matrix properties on the boarderline to infinite life, research is done on laminates while replacing the epoxy-based matrix with an equivalent one with different fracture mechanical properties.
With the evolution of early cracks and IFF monitored as well as the influence of the matrix on the crack propagation investigated we are working on an appropriate model to improve lifetime prediction in CFRP. This work is done within the priority program “infinite life” (SPP 1466) funded by Deutsche Forschungsgemeinschaft (DFG).