Analysis of Synthetic Fiber Pull-Out From a Cement Matrix
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ANALYSIS OF SYNTHETIC FIBER PULL-OUT FROM A CEMENT MATRIX Youjiang Wang*, Victor C. Li**, and Stanley Backer* * Department of Mechanical Engineering; ** Department of Civil Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT Experiments were conducted on specimens containing nylon or polypropylene monofilaments embedded in a precracked matrix. During pull-out tests, it was generally observed that the pulling force continued to increase after one or both sides of the filament had begun to slip out, even though one or both of the embedded filament lengths were decreasing. This indicated that the fiber/matrix shear stress increased with the fiber slippage distance. Examination of the extracted filaments under a scanning electron microscope (SEM) revealed the increased shear resistance to be the result of fiber surface abrasion. The severity of abrasion was observed to increase with the fiber slippage distance before complete pull-out. A theoretical model has been developed to predict the pull-out force versus displacement relationship based on given fiber/matrix shear strength as an increasing function of the slippage distance. The model gives good prediction in comparison with experimental results. INTRODUCTION Fiber/matrix bond allows stress transfer between individual fibers and the matrix. Because of its important role in composite materials, the fiber/matrix interface has been studied extensively. Many researchers have investigated the effect of the interfacial bond on composite properties, the stress state at the interface, and experimental measurement of the fiber/matrix bond strength. Their work has been reviewed by Chamis [1] for fiber reinforced polymeric composites and by Bartos [2] for fiber reinforced concrete (FRC). Fibers often used in FRC are steel fiber, glass fiber, and various synthetic fibers (e.g. aramid, nylon, polypropylene). Most of the work on the fiber/matrix bond in FRC has been on steel and glass fibers, and the results have been summarized in [2]. Less information on the bond for synthetic fibers is available, although some work has been done on this subject [3,4,5,6]. Frequently, the fiber/matrix bond strength is obtained from fiber pull-out tests and is given by an average value over the fiber surface area. Such bond strength, of course, does not represent the real stress state at the fiber surface. Solutions to the fiber pull-out problem based on an elastic bond strength and a constant and uniform post-elastic frictional bond strength have been obtained
[7,8]. In all the theoretical models reported for FRC, the fiber/matrix bond is generally characterized by: (1) a uniform bond shear strength; or (2) elastic and frictional bond strength parameters, which have unique values for an FRC. This, however, is not necessarily the case for all fibers, particularly synthetics. Due to their highly oriented structures, synthetic fibers are easily abraded by cement particles during pull-out. As the fiber surface abrasion becomes more and more severe, the apparent fiber/
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