Bestimmung eines Verbundgesetzes auf Basis von Einzelfaser‐Pull‐Out‐Versuchen

Abstract

In den letzten Jahren wurden viele Modelle zur analytischen Bestimmung des Verbundes zwischen Faser und Matrix aus Pull-Out-Versuchen entwickelt. Diese beruhen jedoch auf einer begrenzten Anzahl zu variierender materialspezifischer Parameter und erlauben somit keine freie mathematische Beschreibung des Verbundgesetzes. Im vorliegenden Beitrag wird ein analytisches Modell vorgestellt, das die direkte inverse Berechnung eines Verbundgesetzes, bestehend aus N linearen Teilstücken auf Basis von Last-Verformungs-Kurven eines Pull-Out-Tests ermöglicht, bei dem die Anzahl N nicht begrenzt ist. Das Verbundgesetz kann somit ohne mathematische Einschränkungen beschrieben werden. Die Anwendbarkeit des vorgestellten Modells wird anhand einer Auswertung von Stahlfaser-Pull-Out-Versuchsergebnissen demonstriert. Evaluation of a bond versus slip relation on basis of single fiber pull-out tests. An analytical simulation procedure for a pull-out test is proposed in the present study, which is based on an inverse boundary problem and allows the straightforward calculation of a N-piecewise linear bond law τ(s) with no limitation of N on basis of an experimentally determined load displacement distribution P(ω). The presented model is applied and validated using experimental results of pull-out tests, carried out on smooth and straight steel fiber/fine grained concrete systems. It is found that the determined bond stress versus slip relation τ(s) is a material parametersince it is not dependent on geometric factors of fiber and matrix, e. g. fiber diameter and embedded length. However, experimental tests not published in this study showed so called stochastic size effects, i. e. if the fiber diameter is small in regard to the maximum grain size used in the cement based matrix, the pull-out behavior is influenced more significantly in the post failure region due to e. g. abrasion effects. Nevertheless, with the presented model bond characteristics of different fiber/matrix systems can be compared much more in detail, because the complete bond stress versus slip relation is known. Furthermore, only if τ(s) is known, a tailoring of the interface properties in regard to an optimized mechanical performance of the composite material with enhanced characteristics is possible. Future work on this model includes e. g. the implementation of effects due to lateral pressure on the fiber, constant loading, and repeated load cycles.

Determination of the bond strength of treated wood strands embedded in a cement matrix by means of a pull-out test

“…The pull-out test in general is a quite common method to determine bond strength of fibres and cement (Banholzer et al 2004), but almost no literature is available about measuring bond strength of cement and wood particles like strands, excelsior or flakes. As there are no comparable results, the following work describes the effects of common chemical treatments and the use of an accelerator, namely ammonium chloride (NH 4 Cl).…”